The list below highlights our recent peer-reviewed articles. Please scroll over the titles to open the abstract; access to full article content may require journal subscription. In addition to the scientific literature, our work has also been featured in popular science accounts, including Pickover's book "Archimedes to Hawking" and Kaku's book "Physics of the Impossible."

Identifying pathogens in complex samples such as blood, urine, and wastewater is critical to detect infection and inform optimal treatment. Surface-enhanced Raman spectroscopy (SERS) and machine learning (ML) can distinguish among multiple pathogen species, but processing complex fluid samples to sensitively and specifically detect pathogens remains an outstanding challenge. Here, we develop an acoustic bioprinter to digitize samples into millions of droplets, each containing just a few cells, which are identified with SERS and ML. We demonstrate rapid printing of 2 pL droplets from solutions containing S. epidermidis, E. coli, and blood; when they are mixed with gold nanorods (GNRs), SERS enhancements of up to 1500� are achieved. We then train a ML model and achieve greater than or equal to 99 percent classification accuracy from cellularly pure samples and greater than or equal to 87 percent accuracy from cellularly mixed samples. We also obtain greater than or equal to 90 percent accuracy from droplets with pathogen:blood cell ratios less than 1. Our combined bioprinting and SERS platform could accelerate rapid, sensitive pathogen detection in clinical, environmental, and industrial settings.

View article

Optical metasurfaces offer unprecedented flexibility in light wave manipulation but suffer weak resonant enhancement. Tackling this problem, we experimentally unveil a new phase gradient metasurface platform made entirely from individually addressable high quality factor (high-Q) silicon meta-atoms. Composed of pairs of nearly identical nanoblocks, these meta-atoms support dipolar-guided-mode resonances that, due to the controlled suppression of radiation loss, serve as highly sensitive phase pixels when placed above a mirror. A key novelty of this platform lies in the vanishingly small structural perturbations needed to produce universal phase fronts. Having fabricated elements with Q-factor approximately 380 and spaced by lambda/1.2, we achieve strong beam steering, up to 59 percent efficient , to angles 32.3, 25.3, and 20.9 degrees, with variations in nanoantenna volume fractions across the metasurfaces of less than or equal to 2.6%, instead of greater than 50% required by traditional versions. Aside from extreme sensitivity, the metasurfaces exhibit near-field intensity enhancement over 1000x. Taken together, these properties represent an exciting prospect for dynamic and nonlinear wave shaping.

View article

The ideal catalyst for energy-intensive reactions such as CO2 reduction and waste treatment is non-toxic and sustainable and leaves no unwanted by-products. Solvated electrons, while traditionally prepared by ionizing radiation or by dissolving alkali metals in harsh solvents, are otherwise ideal reducing agents. For the past 50 years, it has been proposed that solvated electrons could be produced by photoexcitation of roughened metal electrodes, but to date no study has demonstrated a clear mechanism for their generation, hindering the optimization and use of solvated electrons as climate-friendly catalysts. Here, we establish this missing link using a combination of electrochemical detection, spectroscopy, structural characterization, and simulations to show that plasmons create solvated electrons in the most sustainable solvent - water. Solvated electrons are powerful reducing agents capable of driving some of the most energetically expensive reduction reactions. Their generation under mild and sustainable conditions remains challenging though. Using near-ultraviolet irradiation under low-intensity one-photon conditions coupled with electrochemical and optical detection, we show that the yield of solvated electrons in water is increased more than 10 times for nanoparticle-decorated electrodes compared to smooth silver electrodes. Based on the simulations of electric fields and hot carrier distributions, we determine that hot electrons generated by plasmons are injected into water to form solvated electrons. Both yield enhancement and hot carrier production spectrally follow the plasmonic near-field. The ability to enhance solvated electron yields in a controlled manner by tailoring nanoparticle plasmons opens up a promising strategy for exploiting solvated electrons in chemical reactions....

View article

Researchers have demonstrated a new electron field emitter with unprecedented brightness and spectral purity, promising a breakthrough in electron microscope spectroscopy.

View article

Dynamically reconfigurable metasurfaces promise compact and lightweight spatial light modulation for many applications, including LiDAR, AR/VR, and LiFi systems. Here, we design and computationally investigate high-quality-factor silicon-on-lithium niobate metasurfaces with electrically driven, independent control of its constituent nanobars for full phase tunability with high tuning efficiency. Free-space light couples to guided modes within each nanobar via periodic perturbations, generating quality factors exceeding 30,000 while maintaining a bar spacing of View article

Upconverting nanoparticles (UCNPs) are an emerging platform for mechanical force sensing at the nanometer scale. An outstanding challenge in realizing nanometer-scale mechano-sensitive UCNPs is maintaining a high mechanical force responsivity in conjunction with bright optical emission. This Letter reports mechano-sensing UCNPs based on the lanthanide dopants Yb3+ and Er3+, which exhibit a strong ratiometric change in emission spectra and bright emission under applied pressure. We synthesize and analyze the pressure response of five different types of nanoparticles, including cubic NaYF4 host nanoparticles and alkaline-earth host materials CaLuF, SrLuF, SrYbF, and BaLuF, all with lengths of 15 nm or less. By combining optical spectroscopy in a diamond anvil cell with single-particle brightness, we determine the noise equivalent sensitivity (GPa/?Hz) of these particles. The SrYb0.72Er0.28F@SrLuF particles exhibit an optimum noise equivalent sensitivity of 0.26 � 0.04 GPa/?Hz. These particles present the possibility of robust nanometer-scale mechano-sensing.

View article

Palladium’s strong reactivity and absorption affinity to H2 makes it a prime material for hydrogen-based technologies. Alloying of Pd has been used to tune its mechanical stability, catalytic activity, and absorption thermodynamics. However, atomistic mechanisms of hydrogen dissociation and intercalation are informed predominantly by theoretical calculations, owing to the difficulty in imaging dynamic metal–gas interactions at the atomic scale. Here, we use in situ environmental high resolution transmission electron microscopy to directly track the hydrogenation-induced lattice expansion within AgPd triangular nanoprisms. We investigate the thermodynamics of the system at the single particle level and show that, contrary to pure Pd nanoparticles, the AgPd system exhibits α/β coexistence within single crystalline nanoparticles in equilibrium; the nanoparticle system also moves to a solid-solution loading mechanism at lower Ag content than bulk. By tracking the lattice expansion in real time during a phase transition, we see surface-limited β phase growth, as well as rapid reorientation of the α/β interface within individual particles. This secondary rate corresponds to the speed with which the β phase can restructure and, according to our atomistic calculations, emerges from lattice strain minimization. We also observe no preferential nucleation at the sharpest nanoprism corners, contrary to classical nucleation theory. Our results achieve atomic lattice plane resolution─crucial for exploring the role of crystal defects and single atom sites on catalytic hydrogen splitting and absorption.

View article

Nanoparticle photocatalysts are essential to processes ranging from chemical production and water purification to air filtration and surgical instrument sterilization. Photochemical reactions are generally mediated by the illumination of metallic and/or semiconducting nanomaterials, which provide the necessary optical absorption, electronic band structure, and surface faceting to drive molecular reactions. However, with reaction efficiency and selectivity dictated by atomic and molecular interactions, imaging and controlling photochemistry at the atomic scale are necessary to both understand reaction mechanisms and to improve nanomaterials for next-generation catalysts. Here, we describe how advances in plasmonics, combined with advances in electron microscopy, particularly optically coupled transmission electron microscopy (OTEM), can be used to image and control light-induced chemical transformations at the nanoscale. We focus on our group’s research investigating the interaction between hydrogen gas and Pd nanoparticles, which presents an important model system for understanding both hydrogenation catalysis and hydrogen storage. The studies described in this Account primarily rely on an environmental transmission electron microscope, a tool capable of circumventing traditional TEM’s high-vacuum requirements, outfitted with optical sources and detectors to couple light into and out of the microscope. First, we describe the H2 loading kinetics of individual Pd nanoparticles. When confined to sizes of less than ∼100 nm, single-crystalline Pd nanoparticles exhibit coherent phase transformations between the hydrogen-poor α-phase and hydrogen-rich β-phase, as revealed through monitoring the bulk plasmon resonance with electron energy loss spectroscopy. Next, we describe how contrast imaging techniques, such as phase contrast STEM and displaced-aperture dark field, can be employed as real-time techniques to image phase transformations with 100 ms temporal resolution. Studies of multiply twinned Pd nanoparticles and high aspect ratio Pd nanorods demonstrate that internal strain and grain boundaries can lead to partial hydrogenation within individual nanoparticles. Finally, we describe how OTEM can be used to locally probe nanoparticle dynamics under optical excitation and in reactive chemical environments. Under illumination, multicomponent plasmonic photocatalysts consisting of a gold nanoparticle “antenna” and a Pd “reactor” show clear α-phase nucleation in regions close to electromagnetic “hot spots” when near plasmonic antennas. Importantly, these hot spots need not correspond to the traditionally active, energetically preferred sites of catalytic nanoparticles. Nonthermal effects imparted by plasmonic nanoparticles, including electromagnetic field enhancement and plasmon-derived hot carriers, are crucial to explaining the site selectivity observed in PdHx phase transformations under illumination. This Account demonstrates how light can contribute to selective chemical phenomena in plasmonic heterostructures, en route to sustainable, solar-driven chemical production.

View article

Deep neural networks and other sophisticated machine learning models are widely applied to biomedical signal data because they can detect complex patterns and compute accurate predictions. However, the difficulty of interpreting such models is a limitation, especially for applications involving high-stakes decision, including the identification of bacterial infections. In this paper, we consider fast Raman spectroscopy data and demonstrate that a logistic regression model with carefully selected features achieves accuracy comparable to that of neural networks, while being much simpler and more transparent. Our analysis leverages wavelet features with intuitive chemical interpretations, and performs controlled variable selection with knockoffs to ensure the predictors are relevant and non-redundant. Although we focus on a particular data set, the proposed approach is broadly applicable to other types of signal data for which interpretability may be important.

View article

Lanthanide-doped nanophosphors have emerged as promising optical labels for high-resolution, “multicolor” electron microscopy. Here, we develop a library of 11 unique lanthanide-doped nanophosphors with average edge lengths of 15.2 ± 2.0 nm (N = 4284). These nanophosphors consist of an electron-stable BaYF5 host lattice doped at 25% atomic concentration with the lanthanides Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Ce3+, Ho3+, Er3+, Tm3+, and Yb3+. Under ∼100 pA/nm2 beam current in a transmission electron microscope, each nanophosphor species exhibits strong cathodoluminescence spectra with sharp characteristic emission lines for each lanthanide. The bright emission and stability of these nanoparticles enable not only ensemble, but also single-particle cathodoluminescence spectroscopy, which we demonstrate with BaYF5:Ln3+, where Ln3+ = Tb3+, Ho3+, Er3+, Sm3+, Eu3+, or Pr3+. Single-particle cathodoluminescence corresponds directly with HAADF intensity across nanoparticles, confirming high spatial localization of the measured cathodoluminescence signal of lanthanide-doped nanophosphors. Our synthesis and characterization of sub-20 nm, electron-stable nanophosphors provides a robust material platform to achieve single-molecule labeled correlative cathodoluminescence electron microscopy, a critical foundation for high-resolution correlation of single molecules within the context of cellular ultrastructure.

View article

Molecular imaging is a crucial technique in clinical diagnostics but it relies on radioactive tracers or strong magnetic fields that are unsuitable for many patients, particularly infants and pregnant women. Ultra-high-frequency radio-frequency acoustic (UHF-RF-acoustic) imaging using non-ionizing RF pulses allows deep-tissue imaging with sub-millimetre spatial resolution. However, lack of biocompatible and targetable contrast agents has prevented the successful in vivo application of UHF-RF-acoustic imaging. Here we report our development of targetable nanodroplets for UHF-RF-acoustic molecular imaging of cancers. We synthesize all-liquid nanodroplets containing hypertonic saline that are stable for at least 2 weeks and can produce high-intensity UHF-RF-acoustic signals. Compared with concentration-matched iron oxide nanoparticles, our nanodroplets produce at least 1,600 times higher UHF-RF-acoustic signals at the same imaging depth. We demonstrate in vivo imaging using the targeted nanodroplets in a prostate cancer xenograft mouse model expressing gastrin release protein receptor (GRPR), and show that targeting specificity is increased by more than 2-fold compared with untargeted nanodroplets or prostate cancer cells not expressing this receptor.

View article

Mark Stockman was a founding member and evangelist for the plasmonics field for most of his creative life. He never sought recognition, but fame came to him in a different way. He will be dearly remembered by colleagues and friends as one of the most influential and creative contributors to the science of light from our generation.

View article

Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Nevertheless, the dynamic structural origins of nonradiative relaxations in such materials are not understood. Here, femtosecond electron diffraction measurements corroborated by atomistic simulations uncover transient lattice deformations accompanying radiationless electronic processes in colloidal semiconductor nanocrystals. Investigation of the excitation energy dependence in a core/shell system shows that hot carriers created by a photon energy considerably larger than the bandgap induce structural distortions at nanocrystal surfaces on few picosecond timescales associated with the localization of trapped holes. On the other hand, carriers created by a photon energy close to the bandgap of the core in the same system result in transient lattice heating that occurs on a much longer 200 picosecond timescale, dominated by an Auger heating mechanism. Elucidation of the structural deformations associated with the surface trapping of hot holes provides atomic-scale insights into the mechanisms deteriorating optoelectronic performance and a pathway towards minimizing these losses in nanocrystal devices.

View article

The light sources that power photonic networks are small and scalable, but they also require the incorporation of optical isolators that allow light to pass in one direction only, protecting the light source from damaging backreflections. Unfortunately, the size and complex integration of optical isolators makes small-scale and densely integrated photonic networks infeasible. Here, we overcome this limitation by designing a single device that operates both as a coherent light source and as its own optical isolator. Our design relies on high-quality-factor dielectric metasurfaces that exhibit intrinsic chirality. By carefully manipulating the geometry of the constituent silicon metaatoms, we design three-dimensionally chiral modes that act as optical spin-dependent filters. Using spin-polarized Raman scattering together with our chiral metacavity, we demonstrate Raman lasing in the forward direction, while the lasing action is suppressed by over an order of magnitude for reflected light. Our high-Q chiral metasurface design presents a new approach toward compactly isolating integrated light sources by directly tailoring the emission properties of the light source itself.

View article

Phase gradient metasurfaces have revolutionized modern optical components by significantly reducing the path length of bulk optics while maintaining high performance. However, their geometric design makes dynamic modulation challenging, with devices facing a trade-off between the modulation range and efficiency. Here, we introduce Silicon-on-Lithium Niobate (LNO) high-Quality-factor (high-Q) metasurfaces for efficient electro-optic wavefront shaping and modulation. Periodic perturbations within Si metasurface elements allow incident light to be weakly coupled into guided modes, generating high-Q resonances that manifest in the far-field diffraction spectrum. The near field of each Si element spatially overlaps with the LNO substrate, enabling electrically tunable modulation of the resonant frequency. Using full-field simulations, we demonstrate near-infrared, dynamically tunable beamsteering, and beamsplitting metasurfaces. First, we demonstrate beamsteering metasurfaces whose +1st order diffracted intensity can be modulated from 70% to 7% absolute efficiency near the resonant frequency with applied electric fields of order V/μm. Next, we design a tunable beam splitter, switching between direct, 0th order transmission and beamsplitting with the application of 30 V across the metasurface. Our high-Q electro-optic metasurfaces provide a foundation for efficient, time-dependent transfer functions in subwavelength footprints.

View article

Nanoparticle surface structure and geometry generally dictate where chemical transformations occur, with higher chemical activity at sites with lower activation energies. Here, we show how optical excitation of plasmons enables spatially modified phase transformations, activating otherwise energetically unfavorable sites. We have designed a crossed-bar Au-PdHx antenna-reactor system that localizes electromagnetic enhancement away from the innately reactive PdHx nanorod tips. Using optically coupled in situ environmental transmission electron microscopy, we track the dehydrogenation of individual antenna-reactor pairs with varying optical illumination intensity, wavelength, and hydrogen pressure. Our in situ experiments show that plasmons enable new catalytic sites, including dehydrogenation at the nanorod faces. Molecular dynamics simulations confirm that these new nucleation sites are energetically unfavorable in equilibrium and only accessible through tailored plasmonic excitation.

View article

The structural arrangement of fibrous tissue is linked to the onset and progression of Alzheimer’s disease, heart disease, fibrosis, and cancer, yet its visualization remains challenging with conventional optical microscopy. Here, we design a guided-mode-resonant dielectric metasurface to detect the presence and orientation of fibrous tissue, modeled as a linearly birefringent anisotropic medium, by colorimetric readout. The metasurface consists of nanoscale Si3N4 (215 nm) and SiO2 (75 nm) layers on a SiO2 substrate, acting as a broadband antireflection coating, patterned with subwavelength-periodic rhombohedral perturbations, which result in guided-mode resonances with sub-10 nm bandwidth. Using full-field simulations, we show how transition from air to tissue in the dielectric environment at the metasurface interface results in a red-to-green change in reflected structural color, while the birefringence and orientation of an anisotropic medium manifests as a green-to-blue change. Importantly, the birefringence-based tuning of the guided mode resonances is spectrally separated from refractive-index-based displacements, allowing quantitative discrimination between both the index and structural arrangement of anisotropic media. We numerically simulate the application of our metasurface to cancer tissue diagnostics, predicting how changes in reflected structural color at the tumor margin can distinguish localized, early stage from metastasized, late-stage cancers. The quantitative, colorimetric mapping of tissue orientation angle marks an improved performance in comparison to polarized light microscopy, where multiple orientation angles yield an identical response. Our guided-mode-resonant metasurface provides a foundation for all-optical, label-free, and quantitative colorimetric visualization of fibrous biological media on a single, clinically compatible chip, promising improvements in staging and treatment decisions.

View article

Increasing the sensitivity of chiral spectroscopic techniques such as circular dichroism (CD) spectroscopy is a current aspiration in the research field of nanophotonics. Enhancing CD spectroscopy depends upon of two complementary requirements: the enhancement of the electromagnetic fields perceived by the molecules under study and the conservation of the helicity of those fields, guaranteed by duality symmetry. In this work, we introduce a systematic method to design nanostructured dual periodic photonic systems capable of enhancing molecular CD spectroscopy resonantly. As an illustration, we engineer a dual 1D silicon nanoparticle array and show that its collective optical modes can be efficiently employed to resonantly enhance by two orders of magnitude the local density of optical chirality and, thus, the CD signal obtained from a given molecular sample on its vicinity.

View article

Densely interconnected, nonlinear, and reconfigurable optical networks represent a route to high-performance optical computing, communications, and sensing technologies. Dielectric nanoantennas are promising building blocks for such architectures since they can precisely control optical diffraction. However, they are traditionally limited in their nonlinear and reconfigurable responses owing to their relatively low-quality factor (Q-factor). Here, we highlight new and emerging design strategies to increase the Q-factor while maintaining control of optical diffraction, enabling unprecedented spatial and temporal control of light. We describe how multipolar modes and bound states in the continuum increase Q and show how these high-Q nanoantennas can be cascaded to create almost limitless resonant optical transfer functions. With high-Q nanoantennas, new paradigms in reconfigurable wavefront-shaping, low-noise, multiplexed biosensors and quantum transduction are possible.

Read the article.

Lanthanide nanoparticles (LNPs) are promising sensors of chemical, mechanical, and temperature changes; they combine the narrow-spectral emission and long-lived excited states of individual lanthanide ions with the high spatial resolution and controlled energy transfer of nanocrystalline architectures. Despite considerable progress in optimizing LNP brightness and responsiveness for dynamic sensing, detection of stimuli with a spatial resolution approaching that of individual nanoparticles remains an outstanding challenge. Here, we highlight the existing capabilities and outstanding challenges of LNP sensors, en-route to nanometer-scale, single particle sensor resolution. First, we summarize LNP sensor read-outs, including changes in emission wavelength, lifetime, intensity, and spectral ratiometric values that arise from modified energy transfer networks within nanoparticles. Then, we describe the origins of LNP sensor imprecision, including sensitivity to competing conditions, interparticle heterogeneities, such as the concentration and distribution of dopant ions, and measurement noise. Motivated by these sources of signal variance, we describe synthesis characterization feedback loops to inform and improve sensor precision, and introduce noise-equivalent sensitivity as a figure of merit of LNP sensors. Finally, we project the magnitudes of chemical and pressure stimulus resolution achievable with single LNPs at nanoscale resolution. Our perspective provides a roadmap for translating ensemble LNP sensing capabilities to the single particle level, enabling nanometer-scale sensing in biology, medicine, and sustainability.

Read the article.

Strong enhancement of molecular circular dichroism (CD) has the potential to enable efficient asymmetric photolysis, a method of chiral separation that has conventionally been impeded by insufficient yield and low enantiomeric excess. Here, we study experimentally how predicted enhancements in optical chirality density near resonant silicon nanodisks boost CD. We use fluorescence-detected circular dichroism (FDCD) spectroscopy to measure indirectly the differential absorption of circularly polarized light by a monolayer of optically active molecules functionalized to silicon nanodisk arrays. Importantly, the molecules and nanodisk antennas have spectrally coincident resonances, and our fluorescence technique allows us to deconvolute absorption in the nanodisks from the molecules. We find that enhanced FDCD signals depend on nanophotonic resonances, in good agreement with simulated differential absorption and optical chirality density, while no signal is detected from molecules adsorbed on featureless silicon surfaces. These results verify the potential of nanophotonic platforms to be used for asymmetric photolysis with lower energy requirements.

Read the article.

All‐optical control and detection of magnetic states for high‐density recording necessitate nanophotonic approaches to amplify local light intensity below the diffraction limit. Sculpting the near‐field phase and polarization can additionally strengthen magneto‐optical effects that rely on circularly polarized pulses, such as all‐optical helicity‐dependent switching, imaging, and spin‐wave excitation. Here, high‐refractive‐index dielectric nanoantennas illuminated with circularly polarized light resonantly enhance local electric field rotation by more than sixfold within [Pt/Co]N thin films. Sub‐wavelength arrays of amorphous Si nanodisks, or metasurfaces, patterned on perpendicularly magnetized films support Mie‐type resonances that modulate reflection and transmission dissymmetry by >±2% in experiments. Spatial and spectral interference between dipolar modes, proximity effects, and gain are evaluated by varying disk aspect ratio, metasurface–metal separation, and magnetic film thickness, respectively. Simulated enhancements in magnetic circular birefringence and differential absorption are correlated with amplified local field rotation at electric dipolar modes. Greater achievable amplifications are shown via simulations with single‐crystalline Si metasurfaces exhibiting lower losses, including a 12‐fold strengthened electric field rotation within ferromagnetic layers. The metasurface design rules established here could enable nanoscale localization of all‐optical magnetic switching with lowered laser fluence thresholds, as well as enhanced magneto‐optical responses for light‐assisted reading in spintronic devices.

Read the article.

Upconverting nanoparticles (UCNPs) are promising candidates for photon‐driven reactions, including light‐triggered drug delivery, photodynamic therapy, and photocatalysis. Herein, we investigate the NIR‐to‐UV/visible emission of sub‐15 nm alkaline‐earth rare‐earth fluoride UCNPs (M1−xLnxF2+x, MLnF) with a CaF2 shell. We synthesize 8 alkaline‐earth host materials doped with Yb3+ and Tm3+, with alkaline‐earth (M) spanning Ca, Sr, and Ba, MgSr, CaSr, CaBa, SrBa, and CaSrBa. We explore UCNP composition, size, and lanthanide doping‐dependent emission, focusing on upconversion quantum yield (UCQY) and UV emission. UCQY values of 2.46 % at 250 W cm−2 are achieved with 14.5 nm SrLuF@CaF2 particles, with 7.3 % of total emission in the UV. In 10.9 nm SrYbF:1 %Tm3+@CaF2 particles, UV emission increased to 9.9 % with UCQY at 1.14 %. We demonstrate dye degradation under NIR illumination using SrYbF:1 %Tm3+@CaF2, highlighting the efficiency of these UCNPs and their ability to trigger photoprocesses.

Read the article.

Surface-enhanced Raman spectroscopy (SERS) is a promising cellular identification and drug susceptibility testing platform, provided it can be performed in a controlled liquid environment that maintains cell viability. We investigate bacterial liquid-SERS, studying plasmonic and electrostatic interactions between gold nanorods and bacteria that enable uniformly-enhanced SERS. We synthesize five nanorod sizes with longitudinal plasmon resonances ranging from 670-860 nm and characterize SERS signatures of Gram-negative Escherichia coli and Serratia marcescens, and Gram-positive Staphylococcus aureus and Staphylococcus epidermidis bacteria in water. Varying the concentration of bacteria and nanorods, we achieve large-area SERS enhancement that is independent of nanorod resonance and bacteria type; however, bacteria with higher surface charge density exhibit significantly higher SERS signal. Using cryo-electron microscopy and zeta potential measurements, we show that higher signal results from attraction between positively-charged nanorods and negatively-charged bacteria. Our robust liquid-SERS measurements provide a foundation for bacterial identification and drug testing in biological fluids.

Read the article.

Dielectric microcavities with quality factors (Q-factors) in the thousands to billions markedly enhance light–matter interactions, with applications spanning high-efficiency on-chip lasing, frequency comb generation and modulation and sensitive molecular detection. However, as the dimensions of dielectric cavities are reduced to subwavelength scales, their resonant modes begin to scatter light into many spatial channels. Such enhanced scattering is a powerful tool for light manipulation, but also leads to high radiative loss rates and commensurately low Q-factors, generally of order ten. Here, we describe and experimentally demonstrate a strategy for the generation of high Q-factor resonances in subwavelength-thick phase gradient metasurfaces. By including subtle structural perturbations in individual metasurface elements, resonances are created that weakly couple free-space light into otherwise bound and spatially localized modes. Our metasurface can achieve Q-factors >2,500 while beam steering light to particular directions. High-Q beam splitters are also demonstrated. With high-Q metasurfaces, the optical transfer function, near-field intensity and resonant line shape can all be rationally designed, providing a foundation for efficient, free-space-reconfigurable and nonlinear nanophotonics.

Read the article.

We propose and theoretically analyze a vibrational spectroscopy imaging technique, termed, termed electron- and light-induced stimulated Raman (ELISR) scattering, that combines the high spatial resolution of electron microscopy with the molecular sensitivity of surface-enhanced Raman spectroscopy. With ELISR, electron-beam excitation of plasmonic nanoparticles is utilized as a spectrally-broadband but spatially-confined Stokes beam in the presence of a diffraction-limited pump laser. To characterize this technique, we develop a numerical model and conduct full-field electromagnetic simulations to investigate two distinct nanoparticle geometries, nanorods, and nanospheres, coated with a Raman-active material. Our results show the significant stimulated Raman enhancement that is achieved with dual electron and optical excitation of these nanoparticle geometries. Importantly, the spatial resolution of this vibrational spectroscopy for electron microscopy is solely determined by the nanoparticle geometry and the plasmon mode volume. Our results highlight the promise of ELISR for simultaneous high-resolution electron microscopy with sub-diffraction-limited Raman spectroscopy, complementing advances in super-resolution microscopy, correlated light and electron microscopy, and vibrational electron energy loss spectroscopy.

Read the article.

In a pandemic era, rapid infectious disease diagnosis is essential. Surface-enhanced Raman spectroscopy (SERS) promises sensitive and specific diagnosis including rapid point-of-care detection and drug susceptibility testing. SERS utilizes inelastic light scattering arising from the interaction of incident photons with molecular vibrations, enhanced by orders of magnitude with resonant metallic or dielectric nanostructures. While SERS provides a spectral fingerprint of the sample, clinical translation is lagged due to challenges in consistency of spectral enhancement, complexity in spectral interpretation, insufficient specificity and sensitivity, and inefficient workflow from patient sample collection to spectral acquisition. Here, we highlight the recent, complementary advances that address these shortcomings, including (1) design of label-free SERS substrates and data processing algorithms that improve spectral signal and interpretability, essential for broad pathogen screening assays; (2) development of new capture and affinity agents, such as aptamers and polymers, critical for determining the presence or absence of particular pathogens; and (3) microfluidic and bioprinting platforms for efficient clinical sample processing. We also describe the development of low-cost, point-of-care, optical SERS hardware. Our paper focuses on SERS for viral and bacterial detection, in hopes of accelerating infectious disease diagnosis, monitoring, and vaccine development. With advances in SERS substrates, machine learning, and microfluidics and bioprinting, the specificity, sensitivity, and speed of SERS can be readily translated from laboratory bench to patient bedside, accelerating point-of-care diagnosis, personalized medicine, and precision health.

Read the article.

Metasurface lenses provide an ultrathin platform in which to focus light, but weak light–matter interactions limit their dynamic tunability. Here we design submicron-thick, ultrahigh quality factor (high-Q) metalenses that enable dynamic modulation of the focal length and intensity. Using full-field simulations, we show that quality factors exceeding 5000 can be generated by including subtle, periodic perturbations within the constituent Si nanoantennas. Such high-Q resonances enable lens modulation based on the nonlinear Kerr effect, with focal lengths varying from 4 to 6.5 μm and focal intensities decreasing by half as input intensity increases from 0.1 to 1 mW/μm2. We also show how multiple high-Q resonances can be embedded in the lens response through judicious placement of the perturbations. Our high-Q lens design, with quality factors 2 orders of magnitude higher than existing lens designs, provides a foundation for reconfigurable, multiplexed, and hyperspectral metasurface imaging platforms.

Read the article.

Defects in hexagonal boron nitride (hBN) exhibit high-brightness, room-temperature quantum emission, but their large spectral variability and unknown local structure challenge their technological utility. Here, we directly correlate hBN quantum emission with local strain using a combination of photoluminescence (PL), cathodoluminescence (CL) and nanobeam electron diffraction. Across 40 emitters, we observe zero phonon lines (ZPLs) in PL and CL ranging from 540 to 720 nm. CL mapping reveals that multiple defects and distinct defect species located within an optically diffraction-limited region can each contribute to the observed PL spectra. Local strain maps indicate that strain is not required to activate the emitters and is not solely responsible for the observed ZPL spectral range. Instead, at least four distinct defect classes are responsible for the observed emission range, and all four classes are stable upon both optical and electron illumination. Our results provide a foundation for future atomic-scale optical characterization of colour centres.

View Article

Chirality in Nature can be found across all length scales, from the subatomic to the galactic. At the molecular scale, the spatial dissymmetry in the atomic arrangements of pairs of mirror-image molecules, known as enantiomers, gives rise to fascinating and often critical differences in chemical and physical properties. With increasing hierarchical complexity, protein function, cell communication, and organism health rely on enantioselective interactions between molecules with selective handedness. For example, neurodegenerative and neuropsychiatric disorders including Alzheimer’s and Parkinson’s diseases have been linked to distortion of chiral-molecular structure. Moreover, d-amino acids have become increasingly recognized as potential biomarkers, necessitating comprehensive analytical methods for diagnosis that are capable of distinguishing l- from d-forms and quantifying trace concentrations of d-amino acids. Correspondingly, many pharmaceuticals and agrochemicals consist of chiral molecules that target particular enantioselective pathways. Yet, despite the importance of molecular chirality, it remains challenging to sense and to separate chiral compounds. Chiral-optical spectroscopies are designed to analyze the purity of chiral samples, but they are often insensitive to the trace enantiomeric excess that might be present in a patient sample, such as blood, urine, or sputum, or pharmaceutical product. Similarly, existing separation schemes to enable enantiopure solutions of chiral products are inefficient or costly. Consequently, most pharmaceuticals or agrochemicals are sold as racemic mixtures, with reduced efficacy and potential deleterious impacts. Recent advances in nanophotonics lay the foundation toward highly sensitive and efficient chiral detection and separation methods. In this Account, we highlight our group’s effort to leverage nanoscale chiral light–matter interactions to detect, characterize, and separate enantiomers, potentially down to the single molecule level. Notably, certain resonant nanostructures can significantly enhance circular dichroism for improved chiral sensing and spectroscopy as well as high-yield enantioselective photochemistry. We first describe how achiral metallic and dielectric nanostructures can be utilized to increase the local optical chirality density by engineering the coupling between electric and magnetic optical resonances. While plasmonic nanoparticles locally enhance the optical chirality density, high-index dielectric nanoparticles can enable large-volume and uniform-sign enhancements in the optical chirality density. By overlapping these electric and magnetic resonances, local chiral fields can be enhanced by several orders of magnitude. We show how these design rules can enable high-yield enantioselective photochemistry and project a 2000-fold improvement in the yield of a photoionization reaction. Next, we discuss how optical forces can enable selective manipulation and separation of enantiomers. We describe the design of low-power enantioselective optical tweezers with the ability to trap sub-10 nm dielectric particles. We also characterize their chiral-optical forces with high spatial and force resolution using combined optical and atomic force microscopy. These optical tweezers exhibit an enantioselective optical force contrast exceeding 10 pN, enabling selective attraction or repulsion of enantiomers based on the illumination polarization. Finally, we discuss future challenges and opportunities spanning fundamental research to technology translation. Disease detection in the clinic as well as pharmaceutical and agrochemical industrial applications requiring large-scale, high-throughput production will gain particular benefit from the simplicity and relative low cost that nanophotonic platforms promise.

View Article

Chiral-optical spectroscopies, such as circular dichroism, are critical in the biomedical, pharmaceutical, and agrochemical industries for revealing structural information about molecules and determining the purity of chemical samples. Emerging nanophotonic platforms have been shown to increase the intrinsically weak interaction between circularly polarized light and chiral molecules through the concentration of the local density of optical chirality, C. However, enhancements in C have been limited to infrared and visible frequencies, while the chiral absorption features of most small molecules are in the ultraviolet. Furthermore, achievable C enhancements in nanophotonic systems remain relatively low, especially when averaged across the sample volume. Here, we use full-field simulations to design a high quality factor (high Q) diamond metasurface that enhances C by over 3 orders of magnitude in the ultraviolet regime. The diamond nanostructures enable ultraviolet Mie resonances while a biperiodic disk lattice activates high Q resonances that significantly increase the electromagnetic field intensities. When a high Q electric dipole and magnetic dipole mode are spatially and spectrally overlapped, a Kerker-like condition emerges that enables uniform sign C enhancements that are locally as high as 1130-fold. Even when averaged across the unit cell and 40 nm away from the surface, enhancements in C exceed 100-fold. We show how the quality factor and C can be further tuned by adjusting the structural asymmetry via the diameter offset in the biperiodic lattice. Our results pave the way for ultrasensitive chiral spectroscopy and efficient light-mediated enantiomer separation.

View Article

Raman optical spectroscopy promises label-free bacterial detection, identification, and antibiotic susceptibility testing in a single step. However, achieving clinically relevant speeds and accuracies remains challenging due to weak Raman signal from bacterial cells and numerous bacterial species and phenotypes. Here we generate an extensive dataset of bacterial Raman spectra and apply deep learning approaches to accurately identify 30 common bacterial pathogens. Even on low signal-to-noise spectra, we achieve average isolate-level accuracies exceeding 82% and antibiotic treatment identification accuracies of 97.0±0.3%. We also show that this approach distinguishes between methicillin-resistant and -susceptible isolates of Staphylococcus aureus (MRSA and MSSA) with 89±0.1% accuracy. We validate our results on clinical isolates from 50 patients. Using just 10 bacterial spectra from each patient isolate, we achieve treatment identification accuracies of 99.7%. Our approach has potential for culture-free pathogen identification and antibiotic susceptibility testing, and could be readily extended for diagnostics on blood, urine, and sputum.

View Article

Upconverting nanoparticles provide valuable benefits as optical probes for bioimaging and Förster resonant energy transfer (FRET) due to their high signal-to-noise ratio, photostability, and biocompatibility; yet, making nanoparticles small yields a significant decay in brightness due to increased surface quenching. Approaches to improve the brightness of UCNPs exist but often require increased nanoparticle size. Here we present a unique core–shell–shell nanoparticle architecture for small (sub-20 nm), bright upconversion with several key features: (1) maximal sensitizer concentration in the core for high near-infrared absorption, (2) efficient energy transfer between core and interior shell for strong emission, and (3) emitter localization near the nanoparticle surface for efficient FRET. This architecture consists of β-NaYbF4 (core) @NaY0.8–xErxGd0.2F4 (interior shell) @NaY0.8Gd0.2F4 (exterior shell), where sensitizer and emitter ions are partitioned into core and interior shell, respectively. Emitter concentration is varied (x = 1, 2, 5, 10, 20, 50, and 80%) to investigate influence on single particle brightness, upconversion quantum yield, decay lifetimes, and FRET coupling. We compare these seven samples with the field-standard core–shell architecture of β-NaY0.58Gd0.2Yb0.2Er0.02F4 (core) @NaY0.8Gd0.2F4 (shell), with sensitizer and emitter ions codoped in the core. At a single particle level, the core–shell–shell design was up to 2-fold brighter than the standard core–shell design. Further, by coupling a fluorescent dye to the surface of the two different architectures, we demonstrated up to 8-fold improved emission enhancement with the core–shell–shell compared to the core–shell design. We show how, given proper consideration for emitter concentration, we can design a unique nanoparticle architecture to yield comparable or improved brightness and FRET coupling within a small volume.

View Article

Plasmon-coupled circular dichroism has emerged as a promising approach for ultrasensitive detection of biomolecular conformations through coupling between molecular chirality and surface plasmons. Chiral nanoparticle assemblies without chiral molecules present also have large optical activities. We apply single-particle circular differential scattering spectroscopy coupled with electron imaging and simulations to identify both structural chirality of plasmonic aggregates and plasmon-coupled circular dichroism induced by chiral proteins. We establish that both chiral aggregates and just a few proteins in interparticle gaps of achiral assemblies are responsible for the ensemble signal, but single nanoparticles do not contribute. We furthermore find that the protein plays two roles: It transfers chirality to both chiral and achiral plasmonic substrates, and it is also responsible for the chiral three-dimensional assembly of nanorods. Understanding these underlying factors paves the way toward sensing the chirality of single biomolecules.

View Article

The inherently weak nature of chiral light–matter interactions can be enhanced by orders of magnitude utilizing artificially-engineered nanophotonic structures. These structures enable high spatial concentration of electromagnetic fields with controlled helicity and chirality. However, the effective design and optimization of nanostructures requires defining physical observables which quantify the degree of electromagnetic helicity and chirality. In this perspective, we discuss optical helicity, optical chirality, and their related conservation laws, describing situations in which each provides the most meaningful physical information in free space and in the context of chiral light–matter interactions. First, an instructive comparison is drawn to the concepts of momentum, force, and energy in classical mechanics. In free space, optical helicity closely parallels momentum, whereas optical chirality parallels force. In the presence of macroscopic matter, the optical helicity finds its optimal physical application in the case of lossless, dual-symmetric media, while, in contrast, the optical chirality provides physically observable information in the presence of lossy, dispersive media. Finally, based on numerical simulations of a gold and silicon nanosphere, we discuss how metallic and dielectric nanostructures can generate chiral electromagnetic fields upon interaction with chiral light, offering guidelines for the rational design of nanostructure-enhanced electromagnetic chirality.

View Article

Time reversal symmetry stands as a fundamental restriction on the vast majority of optical systems and devices. The reciprocal nature of Maxwell’s equations in linear, time-invariant media adds complexity and scale to photonic diodes, isolators, circulators and also sets fundamental efficiency limits on optical energy conversion. Though many theoretical proposals and low frequency demonstrations of nonreciprocity exist, Faraday rotation remains the only known nonreciprocal mechanism that persists down to the atomic scale. Here, we present photon-spin-polarized stimulated Raman scattering as a new nonreciprocal optical phenomenon which has, in principle, no lower size limit. Exploiting this process, we numerically demonstrate nanoscale nonreciprocal transmission of free-space beams at near-infrared frequencies with a 250 nm thick silicon metasurface as well as a fully-subwavelength plasmonic gap nanoantenna. In revealing all-optical spin-splitting, our results provide a foundation for compact nonreciprocal communication and computing technologies, from nanoscale optical isolators and full-duplex nanoantennas to topologically-protected networks.

View Article

Upconverting nanoparticles (UCNPs) are promising tools for background-free imaging and sensing. However, their usefulness for in vivo applications depends on their biocompatibility, which we define by their optical performance in biological environments and their toxicity in living organisms. For UCNPs with a ratiometric color response to mechanical stress, consistent emission intensity and color are desired for the particles under nonmechanical stimuli. Here, we test the biocompatibility and mechanosensitivity of α-NaYF4:Yb,Er@NaLuF4 nanoparticles. First, we ligand-strip these particles to render them dispersible in aqueous media. Then, we characterize their mechanosensitivity (∼30% in the red-to-green spectral ratio per GPa), which is nearly 3-fold greater than those coated in oleic acid. We next design a suite of ex vivo and in vivo tests to investigate their structural and optical properties under several biorelevant conditions: over time in various buffers types, as a function of pH, and in vivo along the digestive tract of Caenorhabditis elegans worms. Finally, to ensure that the particles do not perturb biological function in C. elegans, we assess the chronic toxicity of nanoparticle ingestion using a reproductive brood assay. In these ways, we determine that mechanosensitive UCNPs are biocompatible, i.e., optically robust and nontoxic, for use as in vivo sensors to study animal digestion.

View Article

Light has emerged as a promising new reagent in chemical reactions, especially in enhancing the performance of metal nanoparticle catalysts. Certain metal nanoparticles support localized surface plasmon resonances (LSPRs) which convert incident light to strong electromagnetic fields, hot carriers, or heat for directing and improving chemical reactions. By combining plasmonically active metals with traditionally catalytic metals, bimetallic nanostructures promote simultaneous light conversion and strong molecular adsorption, expanding the library of light-controlled reactions. In this review, we cover three bimetallic geometries: antenna–reactor, core-shell, and alloyed nanoparticle systems. Each geometry hosts its own set of intermetallic interactions which can affect the photocatalytic response. While antenna–reactor systems rely exclusively on optical coupling between the plasmonic and catalytic metal to enhance reactivity, core-shell and alloy architectures introduce electronic interactions in addition to optical effects. These electronic interactions usually dampen the plasmonic response but also offer the potential for enhanced reactivity and product specificity. We review both state-of-the-art bimetallic photocatalysts as well as emerging research opportunities, including leveraging quantum effects, new computational methods to understand and predict photocatalysts, and atomic-scale architecting of materials.

View Article

The optical efficiency of lanthanide-based upconversion is intricately related to the crystalline host lattice. Different crystal fields interacting with the electron clouds of the lanthanides can significantly affect transition probabilities between the energy levels. Here, we investigate six distinct alkaline-earth rare-earth fluoride host materials (M1–xLnxF2+x, MLnF) for infrared-to-visible upconversion, focusing on nanoparticles of CaYF, CaLuF, SrYF, SrLuF, BaYF, and BaLuF doped with Yb3+ and Er3+. We first synthesize ∼5 nm upconverting cores of each material via a thermal decomposition method. Then we introduce a dropwise hot-injection method to grow optically inert MYF shell layers around the active cores. Five distinct shell thicknesses are considered for each host material, resulting in 36 unique, monodisperse upconverting nanomaterials each with size below ∼15 nm. The upconversion quantum yield (UCQY) is measured for all core/shell nanoparticles as a function of shell thickness and compared with hexagonal (β-phase) NaGdF4, a traditional upconverting host lattice. While the UCQY of core nanoparticles is below the detection limit (<10–5%), it increases by 4 to 5 orders of magnitude as the shell thickness approaches 4–6 nm. The UCQY values of our cubic MLnF nanoparticles meet or exceed the β-NaGdF4 reference sample. Across all core/shell samples, SrLuF nanoparticles are the most efficient, with UCQY values of 0.53% at 80 W/cm2 for cubic nanoparticles with ∼11 nm edge length. This efficiency is 5 times higher than our β-NaGdF4 reference material with comparable core size and shell thickness. Our work demonstrates efficient and bright upconversion in ultrasmall alkaline-earth-based nanoparticles, with applications spanning biological imaging and optical sensing.

View Article

Circularly polarized light (CPL) exhibits an enantioselective interaction with chiral molecules, providing a pathway toward all-optical chiral resolution. High index dielectric nanoparticles have been shown to enhance this relationship, but with a spatially varying sign (or enantiospecificity) that yields a near zero spatially averaged enhancement. Using full field electromagnetic simulations, we design metasurfaces consisting of high index dielectric disks that provide large-volume, uniform-sign enhancements in both the optical density of chirality, C (the figure of merit for sensing and spectroscopy), and Kuhn’s dissymmetry factor, g (the figure of merit for separation). By varying disk radius, we achieve local enhancements in C and g up to 138-fold and 15-fold, respectively, as well as volumetric enhancements of 30-fold and 4.2-fold. The uniform-sign enhancements in C occur near the first Kerker condition, where overlapping electric and magnetic modes maximize field strength and preserve the π/2 phase lag between the electric and magnetic fields of CPL; in contrast, uniform-sign enhancements in g occur with spectrally separated modes, where fields and phase remain optimal without reduced molecular absorption. Using first-order kinetics of the molecule thiocamphor, we show how this optically enantiopure metasurface could enable 20% enantiomeric excesses with a >2000-fold increase in yield for a photoionization reaction compared to CPL alone.

View Article

Electron microscopy has been instrumental in our understanding of complex biological systems. Although electron microscopy reveals cellular morphology with nanoscale resolution, it does not provide information on the location of different types of proteins. An electron-microscopy-based bioimaging technology capable of localizing individual proteins and resolving protein?protein interactions with respect to cellular ultrastructure would provide important insights into the molecular biology of a cell. Here, we synthesize small lanthanide-doped nanoparticles and measure the absolute photon emission rate of individual nanoparticles resulting from a given electron excitation flux (cathodoluminescence). Our results suggest that the optimization of nanoparticle composition, synthesis protocols and electron imaging conditions can lead to sub-20-nm nanolabels that would enable high signal-to-noise localization of individual biomolecules within a cellular context. In ensemble measurements, these labels exhibit narrow spectra of nine distinct colours, so the imaging of biomolecules in a multicolour electron microscopy modality may be possible.

View Article

This book offers a comprehensive review of the state-of-the-art theoretical and experimental advances in linear and nonlinear parity-time-symmetric systems in various physical disciplines, and surveys the emerging applications of parity-time (PT) symmetry. PT symmetry originates from quantum mechanics, where if the Schrodinger operator satisfies the PT symmetry, then its spectrum can be all real. This concept was later introduced into optics, Bose-Einstein condensates, metamaterials, electric circuits, acoustics, mechanical systems and many other fields, where a judicious balancing of gain and loss constitutes a PT-symmetric system. Even though these systems are dissipative, they exhibit many signature properties of conservative systems, which make them mathematically and physically intriguing. Important PT-symmetry applications have also emerged. This book describes the latest advances of PT symmetry in a wide range of physical areas, with contributions from the leading experts. It is intended for researchers and graduate students to enter this research frontier, or use it as a reference book.

Book Information

Plasmonic nanoparticle catalysts offer improved light absorption and carrier transport compared to traditional photocatalysts. However, it remains unclear how plasmonic excitation affects multi-step reaction kinetics and promotes site-selectivity. Here, we visualize a plasmon-induced reaction at the sub-nanoparticle level in-situ and in real-time. Using an environmental transmission electron microscope combined with light excitation, we study the photocatalytic dehydrogenation of individual palladium nanocubes coupled to gold nanoparticles with sub-2 nanometer spatial resolution. We find that plasmons increase the rate of distinct reaction steps with unique time constants; enable reaction nucleation at specific sites closest to the electromagnetic hot spots; and appear to open a new reaction pathway that is not observed without illumination. These effects are explained by plasmon-mediated population of excited-state hybridized palladium-hydrogen orbitals. Our results help elucidate the role of plasmons in light-driven photochemical transformations, en-route to design of site-selective and product-specific photocatalysts.

View Article

Control of the polarization state of light is essential for many technologies, but is often limited by weak light-matter interactions that necessitate long device path lengths or significantly reduce the signal intensity. Here, we investigate a nanoscale plasmonic aperture capable of modifying the polarization state of far-field transmitted light without loss in the probe signal. The aperture is a coaxial resonator consisting of a dielectric ring embedded within a metallic film; parity-time (PT) -symmetric inclusions of loss and gain within the dielectric ring enable polarization control. Since the coaxial aperture enables near-thresholdless PT symmetry breaking, polarization control is achieved with realistic levels of loss and gain. Exploiting this sensitivity, we show that the aperture can function as a tunable waveplate, with the transmitted ellipticity of circularly polarized incident light changing continuously with the dissipation coefficient from ?/2 to 0 (i.e., linear polarization). Rotation of linearly polarized light with unity efficiency is also possible, with a continuously tunable degree of rotation. This compact, low-threshold, and reconfigurable polarizer may enable next-generation, high-efficiency displays, routers, modulators, and metasurfaces.

View Article

Noble metal nanoparticles support localized surface plasmon resonances (LSPRs), which are light-driven oscillations of free electrons. Thanks to their strong dependence on the metal’s electron density, these resonances can be used to optically probe the equilibration of photogenerated charge carriers at metal/semiconductor interfaces, a process of paramount importance in energy conversion and sensing applications. We show that the observed optical response can be fully accounted for by an accumulation of photogenerated electrons in the TiO2 shells and that charge transfer to the metal cores is negligible.

View Article

Surface faceting in nanoparticles can profoundly impact the rate and selectivity of chemical transformations. Here, we investigate solute intercalation in individual palladium hydride nanoparticles with distinct surface terminations. Using a combination of diffraction, electron energy loss spectroscopy and dark-field contrast in an environmental transmission electron microscope, we compare the thermodynamics and directly visualize the kinetics of 40-70 nm {100}-terminated cubes and {111}-terminated octahedra with approximately 2 nm spatial resolution. Our results highlight the importance of low-coordination number sites and strain, more so than surface faceting, in governing solute-driven reactions..

View Article

Lanthanide-doped nanoparticles are an emerging class of optical sensors, exhibiting sharp emission peaks, high signal-to-noise ratio, photostability, and a ratiometric color response to stress. The same centrosymmetric crystal field environment that allows for high mechanosensitivity in the cubic-phase (α), however, contributes to low upconversion quantum yield (UCQY). In this work, we engineer brighter mechanosensitive upconverters using a core–shell geometry. Sub-25 nm α-NaYF4:Yb,Er cores are shelled with an optically inert surface passivation layer of ∼4.5 nm thickness. Using different shell materials, including NaGdF4, NaYF4, and NaLuF4, we study how compressive to tensile strain influences the nanoparticles’ imaging and sensing properties. All core–shell nanoparticles exhibit enhanced UCQY, up to 0.14% at 150 W/cm2, which rivals the efficiency of unshelled hexagonal-phase (β) nanoparticles. Additionally, strain at the core–shell interface can tune mechanosensitivity. In particular, the compressive Gd shell results in the largest color response from yellow-green to orange or, quantitatively, a change in the red to green ratio of 12.2 ± 1.2% per GPa. For all samples, the ratiometric readouts are consistent over three pressure cycles from ambient to 5 GPa. Therefore, heteroepitaxial shelling significantly improves signal brightness without compromising the core’s mechano-sensing capabilities and further, promotes core–shell cubic-phase nanoparticles as upcoming in vivo and in situ optical sensors.

View Article

Nanorods are promising components of energy and information storage devices that rely on solute-driven phase transformations, due to their large surface-to-volume ratio and ability to accommodate strain. Here we investigate the hydrogen-induced phase transition in individual penta-twinned palladium nanorods of varying aspect ratios with ~3 nm spatial resolution to understand the correlation between nanorod structure and thermodynamics. We find that the hydrogenated phase preferentially nucleates at the rod tips, progressing along the length of the nanorods with increasing hydrogen pressure. While nucleation pressure is nearly constant for all lengths, the number of phase boundaries is length-dependent, with stable phase coexistence always occurring for rods longer than 55 nm. Moreover, such coexistence occurs within individual crystallites of the nanorods and is accompanied by defect formation, as supported by in situ electron microscopy and elastic energy calculations. These results highlight the effect of particle shape and dimension on thermodynamics, informing nanorod design for improved device cyclability.

View Article

Outreach should be part of any scientist's curriculum, as Jen explains in this Nature Nanotechnology perspective.

View Article

We demonstrate a facile method to improve upconversion quantum yields in Yb,Er-based nanoparticles via emission dye-sensitization. Using the commercially available dye ATTO 542, chosen for its high radiative rate and significant spectral overlap with the green emission of Er3+, we decorate the surfaces of sub-25 nm hexagonal-phase Na(Y/Gd/Lu)0.8F4:Yb0.18Er0.02 upconverting nanoparticles with varying dye concentrations. Upconversion photoluminescence and absorption spectroscopy provide experimental confirmation of energy transfer to and emission from the dye molecules. Upconversion quantum yield is observed to increase with dye sensitization, with the highest enhancement measured for the smallest particles investigated (10.9 nm in diameter); specifically, these dye-decorated particles are more than 2× brighter than are unmodified, organic-soluble nanoparticles and more than 10× brighter than are water-soluble nanoparticles. We also observe 3× lifetime reductions with dye adsorption, confirming the quantum yield enhancement to result from the high radiative rate of the dye. The approach detailed in this work is widely implementable, renders the nanoparticles water-soluble, and most significantly improves sub-15 nm nanoparticles, making our method especially attractive for biological imaging applications.

View Article

Plasmonics is a rapidly developing field at the boundary of physical optics and condensed matter physics. It studies phenomena induced by and associated with surface plasmons—elementary polar excitations bound to surfaces and interfaces of good nanostructured metals. This Roadmap is a concise but authoritative overview of modern plasmonics, encompassing both fundamental applications and such selected applications as plasmonic waveguiding, practical applications of plasmonics enabled by novel materials, thermo-plasmonics, plasmonic-induced photochemistry and photo-catalysis.

View Article

Metasurfaces enable almost complete control of light through ultrathin, subwavelength surfaces by locally and abruptly altering the scattered phase. To date, however, all metasurfaces obey time-reversal symmetry, meaning that forward and backward traveling waves will trace identical paths when being reflected, refracted, or diffracted. Here, we use full-field calculations to design a passive metasurface for nonreciprocal transmission of both direct and anomalously refracted near-infrared light over nanoscale optical path lengths. The metasurface consists of a 100 nm-thick, periodically patterned Si slab. Owing to the high-quality-factor resonances of the metasurface and the inherent Kerr nonlinearities of Si, this structure acts as an optical diode for free-space optical signals. This structure also exhibits nonreciprocal anomalous refraction with appropriate patterning to form a phase gradient metasurface. Compared to existing schemes for breaking time-reversal symmetry, our platform enables subwavelength nonreciprocity for arbitrary free-space optical inputs and provides a straightforward path to experimental realization. The concept is also generalizable to other metasurface functions, providing a foundation for one-way lensing and holography.

View Article

We theoretically demonstrate a non-Hermitian metamaterial exhibiting broadband and wide-angle nonreciprocity. The metamaterial consists of planar metal-dielectric layers with a parity-time(PT)symmetric distribution of loss and gain. With increasing loss and gain, the band structure and band gap are strongly modified; further, the PT potential leads to distinct internal field distributions when illuminated from different sides. Including nonlinearities arising from natural loss and gain saturation leads to nonreciprocal transmission in the visible over a 50-nm wavelength and ±60∘ angular range.

View Article

Chameleons are masters of light, expertly changing their color, pattern, and reflectivity in response to their environment. Engineered materials that share this tunability can be transformative, enabling active camouflage, tunable holograms, and novel colorimetric medical sensors. While progress has been made in creating artificial chameleon skin, existing schemes often require external power, are not continuously tunable, and may prove too stiff or bulky for applications. Here, a chemically tunable, large-area metamaterial is demonstrated that accesses a wide range of colors and refractive indices. An ordered monolayer of nanoresonators is fabricated, then its optical response is dynamically tuned by infiltrating its polymer substrate with solvents. The material shows a strong magnetic response with a dependence on resonator spacing that leads to a highly tunable effective permittivity, permeability, and refractive index spanning negative and positive values. The unity-order index tuning exceeds that of traditional electro-optic and photochromic materials and is robust to cycling, providing a path toward programmable optical elements and responsive light routing.

View Article

Cellular signalling is governed in large part by mechanical forces and electromagnetic fields. Mechanical forces play a critical role in cell differentiation, tissue organization and diseases such as cancer and heart disease; electrical fields are essential for intercellular communication, muscle contraction, neural signalling and sensory perception. Therefore, quantifying a biological system's forces and fields is crucial for understanding physiology and disease pathology and for developing medical tools for repair and recovery. This Review highlights advances in sensing mechanical forces and electrical fields in vivo, focusing on optical probes. The emergence of biocompatible optical probes, such as genetically encoded voltage indicators, molecular rotors, fluorescent dyes, semiconducting nanoparticles, plasmonic nanoparticles and lanthanide-doped upconverting nanoparticles, offers exciting opportunities to push the limits of spatial and temporal resolution, stability, multi-modality and stimuli sensitivity in bioimaging. We further discuss the materials design principles behind these probes and compare them across various metrics to facilitate sensor selection. Finally, we examine which advances are necessary to fully unravel the role of mechanical forces and electrical fields in vivo, such as the ability to probe the vectorial nature of forces, the development of combined force and field sensors, and the design of efficient optical actuators.

View Article

In Pixar’s Inside Out, the character Joy proclaims, “Do you ever look at someone and wonder what’s going on inside?” Driven by similar curiosity, the scientific community has developed remarkable in situ characterization tools to visualize the inner workings of complex, dynamic systems, elucidating their functions and enabling next-generation technologies. This article describes our research developing plasmonic techniques to visualize dynamic chemical transformations in situ with nanometer-scale resolution. As a model system, we investigated the hydrogenation and dehydrogenation of palladium nanocrystals. Using environmental electron microscopy and spectroscopy, we monitored this reaction with sub-2-nm spatial resolution and millisecond time resolution. Particles of different sizes, shapes, and crystallinities exhibit distinct thermodynamic and kinetic properties, highlighting several important design principles for next-generation catalysts and energy-storage devices.

View Article

Circularly polarized light (CPL) exerts a force of different magnitude on left- and right-handed enantiomers, an effect that could be exploited for chiral resolution of chemical compounds as well as controlled assembly of chiral nanostructures. However, enantioselective optical forces are challenging to control and quantify because their magnitude is extremely small (sub-piconewton) and varies in space with sub-micrometre resolution. Here, we report a technique to both strengthen and visualize these forces, using a chiral atomic force microscope probe coupled to a plasmonic optical tweezer. Illumination of the plasmonic tweezer with CPL exerts a force on the microscope tip that depends on the handedness of the light and the tip. In particular, for a left-handed chiral tip, transverse forces are attractive with left-CPL and repulsive with right-CPL. Additionally, total force differences between opposite-handed specimens exceed 10 pN. The microscope tip can map chiral forces with 2 nm lateral resolution, revealing a distinct spatial distribution of forces for each handedness.

View Article

Manipulating the frequency of electromagnetic waves forms the core of many modern technologies, ranging from imaging and spectroscopy to radio and optical communication. The process of converting photons from higher to lower energy is easily accomplished and technologically widespread. However, upconversion, which is the process of converting lower-energy photons into higher-energy photons, is still a growing field of study with nascent applications and burgeoning interest. Here, we experimentally demonstrate a new photon upconversion technique mediated by hot carriers in plasmonic nanostructures. Hot holes and hot electrons generated via plasmon decay in illuminated metal nanoparticles are injected into an adjacent semiconductor quantum well where they radiatively recombine to emit higher-energy photons. Using GaN/InGaN quantum wells decorated with gold and silver nanoparticles, we show photon upconversion from 2.4 to 2.8 eV. The process scales linearly with illumination power and enables both geometry- and polarization-based tunability. The conversion of plasmonic losses into upconverted optical emission has the potential to impact bioimaging, on-chip wavelength conversion, and high-efficiency photovoltaics.

View Article

Mechanical forces affect a myriad of processes, from bone growth to material fracture to touch-responsive robotics. While nano- to micro-Newton forces are prevalent at the microscopic scale, few methods have the nanoscopic size and signal stability to measure them in vivo or in situ. Here, we develop an optical force-sensing platform based on sub-25 nm NaYF4 nanoparticles (NPs) doped with Yb3+, Er3+, and Mn2+. The lanthanides Yb3+ and Er3+ enable both photoluminescence and upconversion, while the energetically coupled d-metal Mn2+ adds force tunability through its crystal field sensitivity. Using a diamond anvil cell to exert up to 3.5 GPa pressure or ∼10 μN force per particle, we track stress-induced spectral responses. The red (660 nm) to green (520, 540 nm) emission ratio varies linearly with pressure, yielding an observed color change from orange to red for α-NaYF4 and from yellow–green to green for d-metal optimized β-NaYF4 when illuminated in the near infrared. Consistent readouts are recorded over multiple pressure cycles and hours of illumination. With the nanoscopic size, a dynamic range of 100 nN to 10 μN, and photostability, these nanoparticles lay the foundation for visualizing dynamic mechanical processes, such as stress propagation in materials and force signaling in organisms.

View Article

Many energy storage materials undergo large volume changes during charging and discharging. The resulting stresses often lead to defect formation in the bulk, but less so in nanosized systems. Here, we capture in real time the mechanism of one such transformation?the hydrogenation of single-crystalline palladium nanocubes from 15 to 80?nm?to better understand the reason for this durability. First, using environmental scanning transmission electron microscopy, we monitor the hydrogen absorption process in real time with 3?nm resolution. Then, using dark-field imaging, we structurally examine the reaction intermediates with 1?nm resolution. The reaction proceeds through nucleation and growth of the new phase in corners of the nanocubes. As the hydrogenated phase propagates across the particles, portions of the lattice misorient by 1.5%, diminishing crystal quality. Once transformed, all the particles explored return to a pristine state. The nanoparticles? ability to remove crystallographic imperfections renders them more durable than their bulk counterparts.

View Article

Separation of enantiomers is crucial to the pharmaceutical and chemical industries, but prevailing chemical methods are economically costly and time-consuming. Illumination with circularly polarized light (CPL) provides a potentially cost-effective and versatile alternative but can achieve only 2 percent enantiomeric excesses with substantial yield. Here, we theoretically show that high-index dielectric nanoparticles can increase enantiomeric excesses 7 times beyond CPL in free space. Mie theory and a local optimization algorithm indicate that magnetic multipolar Mie resonances supported by submicrometer silicon spheres increase Kuhn?s dissymmetry factor 7-fold, compared to CPL in free space. Further, the circular dichroism signal can be enhanced 170-fold. Importantly, these local enhancements maintain the total molecular absorption rate, enabling efficient selective photoexcitation. Even greater enhancements in Kuhn?s dissymmetry factor can be achieved with lower loss and higher refractive index nanoparticles. Our results provide a path toward more efficient all-optical chiral resolution techniques.

View Article

Subwavelength plasmonic apertures have been foundational for direct optical manipulation of nanoscale specimens including sub-100 nm polymeric beads, metallic nanoparticles and proteins. While most plasmonic traps result in two-dimensional localization, three-dimensional manipulation has been demonstrated by integrating a plasmonic aperture on an optical fiber tip. However, such 3D traps are usually inefficient since the optical mode of the fiber and the subwavelength aperture only weakly couple. In this paper we design more efficient optical-fiber-based plasmonic tweezers combining a coaxial plasmonic aperture with a plasmonic grating coupler at the fiber tip facet. Using full-field finite difference time domain analysis, we optimize the grating design for both gold and silver fiber-based coaxial tweezers such that the optical transmission through the apertures is maximized. With the optimized grating, we show that the maximum transmission efficiency increases from 2.5% to 19.6% and from 1.48% to 16.7% for the gold and silver structures respectively. To evaluate their performance as optical tweezers, we calculate the optical forces and the corresponding trapping potential on dielectric particles interacting with the apertures. We demonstrate that the enahncement in the transmission translates into an equivalent increase in the optical forces. Consequently, the optical power required to achieve stable optical trapping is significantly reduced allowing for efficient localization and 3D manipulation of sub-30 nm dielectric particles.

View Article

Lanthanide-based upconverting nanoparticles exhibit significant promise for solar energy generation, biological imaging, and security technologies but have not seen widespread adoption due to the prohibitively low efficiencies of current materials. Weak transition dipole moments between 4f orbitals hinder both photon absorption and emission. Here, we introduce a novel way to increase the radiative transition rates in Yb,Er-based upconverting nanoparticles based on local symmetry distortion. Beginning from a host matrix of the well-studied hexagonal-phase NaYF4, we incrementally remove Y3+ ions and cosubstitute for them a 1:1 mixture of Gd3+ and Lu3+. These two ions act to expand and contract the lattice, respectively, inducing local-level distortion while maintaining the average host structure. We synthesize a range of hexagonal-NaYGdLuF4:Yb_0.18Er_0.02 nanoparticles and experimentally confirm that particle size, phase, global structure, and Yb3+and Er3+ concentrations remain constant as the concentration (x) of Lu and Gd is varied. Upconversion quantum yield is probed as the degree of cosubstitution is varied from x = 0 to x = 0.24. We achieve a maximum quantum yield value of 0.074% under 63 W/cm2 of excitation power density, representing a 1.6x enhancement over the unmodified particles and the highest measured value for near-infrared-to-visible upconversion in sub-25 nm unshelled nanoparticles. We also investigate upconversion emission at the single-particle level and report record improvements in emission intensity for sub-50 nm particles. Radiative rate enhancements are confirmed by measuring excited-state lifetimes. The approach described herein can be used in combination with more established methods of efficiency improvement, such as adding passivating shells or coupling to plasmonic nanoattenas, to further boost the upconversion quantum yield.

View Article

For decades, progress in the field of optical (including solar) energy conversion was dominated by advances in the conventional concentrating optics and materials design. In recent years, however, conceptual and technological breakthroughs in the fields of nanophotonics and plasmonics combined with a better understanding of the thermodynamics of the photon energy-conversion processes reshaped the landscape of energy-conversion schemes and devices. Nanostructured devices and materials that make use of size quantization effects to manipulate photon density of states offer a way to overcome the conventional light absorption limits. Novel optical spectrum splitting and photon-recycling schemes reduce the entropy production in the optical energy-conversion platforms and boost their efficiencies. Optical design concepts are rapidly expanding into the infrared energy band, offering new approaches to harvest waste heat, to reduce the thermal emission losses, and to achieve noncontact radiative cooling of solar cells as well as of optical and electronic circuitries. Light-matter interaction enabled by nanophotonics and plasmonics underlie the performance of the third- and fourth-generation energy-conversion devices, including up- and down-conversion of photon energy, near-field radiative energy transfer, and hot electron generation and harvesting. Finally, the increased market penetration of alternative solar energy-conversion technologies amplifies the role of cost-driven and environmental considerations. This roadmap on optical energy conversion provides a snapshot of the state of the art in optical energy conversion, remaining challenges, and most promising approaches to address these challenges. Leading experts authored 19 focused short sections of the roadmap where they share their vision on a specific aspect of this burgeoning research field. The roadmap opens up with a tutorial section, which introduces major concepts and terminology. It is our hope that the roadmap will serve as an important resource for the scientific community, new generations of researchers, funding agencies, industry experts, and investors.

View Article

We theoretically investigate a nanoscale mode-division multiplexing scheme based on parity-time- (𝒫𝒯) symmetric coaxial plasmonic waveguides. Coaxial waveguides support paired degenerate modes corresponding to distinct orbital angular momentum states. 𝒫𝒯-symmetric inclusions of gain and loss break the degeneracy of the paired modes and create new hybrid modes without definite orbital angular momentum. This process can be made thresholdless by matching the mode order with the number of gain and loss sections within the coaxial ring. Using both a Hamiltonian formulation and degenerate perturbation theory, we show how the wave vectors and fields evolve with increased loss/gain and derive sufficient conditions for thresholdless transitions. As a multiplexing filter, this 𝒫𝒯-symmetric coaxial waveguide could help double density rates in on-chip nanophotonic networks.

View Article

Strain and defects can significantly impact the performance of functional nanomaterials. This effect is well exemplified by energy storage systems, in which structural changes such as volume expansion and defect generation govern the phase transformations associated with charging and discharging. The rational design of next-generation storage materials therefore depends crucially on understanding the correlation between the structure of individual nanoparticles and their solute uptake and release. Here, we experimentally reconstruct the spatial distribution of hydride phases within individual palladium nanocrystals during hydrogen absorption, using a combination of electron spectroscopy, dark-field imaging, and electron diffraction in an environmental transmission electron microscope. We show that single-crystalline cubes and pyramids exhibit a uniform hydrogen distribution at equilibrium, whereas multiply twinned icosahedra exclude hydrogen from regions of high compressive strains. Our technique offers unprecedented insight into nanoscale phase transformations in reactive environments and can be extended to a variety of functional nanomaterials.

View Article

Enantiomer separation is a critical step in many chemical syntheses, particularly for pharmaceuticals, but prevailing chemical methods remain inefficient. Here, we introduce an optical technique to sort chiral specimens using coaxial plasmonic apertures. These apertures are composed of a deeply subwavelength silica channel embedded in silver and can stably trap sub-20 nm dielectric nanoparticles. Using both full-field simulations and analytic calculations, we show that selective trapping of enantiomers can be achieved with circularly polarized illumination. Opposite enantiomers experience distinct trapping forces in both sign and magnitude: one is trapped in a deep potential well, while the other is repelled with a potential barrier. These potentials maintain opposite signs across a range of chiral polarizabilities and enantiomer–aperture separations. Our theory indicates that the interaction of chiral light and chiral specimens can be mediated by achiral plasmonic apertures, providing a possible route toward all-optical enantiopure syntheses.

View Article

CMOS-compatible fabrication of plasmonicmaterials and devices will accelerate the development of integrated nanophotonics for information processing applications. Using low-temperature plasma-enhanced atomic layer deposition (PEALD), we develop a recipe for fully CMOS-compatible titanium nitride (TiN) that is plasmonic in the visible and near infrared. Films are grown on silicon,silicon dioxide, and epitaxially on magnesium oxide substrates. By optimizing the plasma exposure per growth cycle during PEALD, carbon and oxygen contamination are reduced, lowering undesirable loss. We use electron beam lithography to pattern TiN nanopillars with varying diameters on silicon in large-area arrays. In the first reported single-particle measurements on plasmonic TiN, we demonstrate size-tunable darkfield scattering spectroscopy in the visible and near infrared regimes. The optical properties of this CMOS-compatible material, combined with its high melting temperature and mechanical durability, comprise a step towards fully CMOS-integrated nanophotonic information processing.

View Article

Plasmonic multinanoparticle systems exhibit collective electric and magnetic resonances that are fundamental for the development of state-of-the-art optical nanoantennas, metamaterials, and surface-enhanced spectroscopy substrates. While electric dipolar modes have been investigated in both the classical and quantum realm, little attention has been given to magnetic and other ?dark? modes at the smallest dimensions. Here, we study the collective electric, magnetic, and dark modes of colloidally synthesized silver nanosphere trimers with varying inter-particle separation using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). This technique enables direct visualization and spatially selective excitation of individual trimers, as well as manipulation of the interparticle distance into the subnanometer regime with the electron beam. Our experiments reveal that bonding electric and magnetic modes are significantly impacted by quantum effects, exhibiting a relative blueshift and reduced EELS amplitude compared to classical predictions. In contrast, the trimer?s electric dark mode is not affected by quantum tunneling for even Angstrom-scale interparticle separations. We employ a quantum-corrected model to simulate the effect of electron tunneling in the trimer which shows excellent agreement with experimental results. This understanding of classical and quantum-influenced hybridized modes may impact the development of future quantum plasmonic materials and devices, including Fano-like molecular sensors and quantum metamaterials.

View Article

From the photoinduced transport of energy that accompanies photosynthesis to the transcontinental transmission of optical data that enables the Internet, our world relies and thrives on optical signals. To highlight the importance of optics to society, the United Nations has designated 2015 The International Year of Light and Light-based Technologies. Although conventional optical technologies are limited by diffraction, plasmons - collective oscillations of free electrons in a conductor - allow optical signals to be tailored with nanoscale precision. Following decades of fundamental research, several plasmonic technologies have now emerged on the market and numerous industrial breakthroughs are imminent. This article highlights recent industrially-relevant advances in plasmonics, including plasmonic materials and devices for energy; for medical sensing, imaging, and therapeutics; and for information technology. Some of the most exciting industrial applications include solar-driven water purifiers, cell-phone Raman spectrometers, high-density holographic displays, photothermal cancer therapeutics, and nanophotonic integrated circuits. We describe the fundamental scientific concepts behind these and related technologies, as well as the successes and challenges associated with technology transfer.

View Article

Plasmonic materials and metamaterials allow light to be controlled with nanoscale precision, enabling development of on-chip lasers, modulators, and detectors; novel medical therapeutics; efficient molecular sensors; sub-diffraction-limited optical microscopies; and improved photovoltaic and photocatalytic cells, among other extraordinary applications. However, a key challenge faced by the field is materials. Current plasmonic devices predominately employ noble metals, which exhibit high optical loss and limited tunability. Additionally, they pose challenges in standard semiconductor fabrication and integration, preventing full CMOS compatibility and wide-spread utilization. The goal of this special issue is to highlight novel materials that could replace traditional plasmonic metals. These materials not only address application-specific challenges but also reveal new physics and enable new functional devices that can readily integrated with existing technologies. The 28 papers of this feature issue focus on emerging materials and fabrication technologies for plasmonics, and encompass recent advances in both passive and active components as well as linear and nonlinear plasmonic devices.

View Article

We propose a novel scheme of photon upconversion based on harnessing the energy of plasmonic hot carriers. Low-energy photons excite hot electrons and hot holes in a plasmonic nanoparticle, which are then injected into an adjacent semiconductor quantum well where they radiatively recombine to emit a photon of higher energy. We theoretically study the proposed upconversion scheme using Fermi-liquid theory and determine the upconversion quantum efficiency to be as high as 25% in 5 nm silver nanocubes. This upconversion scheme is linear in its operation, does not require coherent illumination, offers spectral tunability, and is more efficient than conventional upconverters.

View Article

Broadly tunable photonic crystals in the near- to mid-infrared region could find use in spectroscopy, non-invasive medical diagnosis, chemical and biological sensing, and military applications, but so far have not been widely realized. We report the fabrication and characterization of three-dimensional tunable photonic crystals composed of polymer nanolattices with an octahedron unit-cell geometry. These photonic crystals exhibit a strong peak in reflection in the mid-infrared that shifts substantially and reversibly with application of compressive uniaxial strain. A strain of ~40% results in a 2.2um wavelength shift in the pseudo-stop band, from 7.3um for the as-fabricated nanolattice to 5.1um when strained. We found a linear relationship between the overall compressive strain in the photonic crystal and the resulting stopband shift, with a ~50nm blueshift in the reflection peak position per percent increase in strain. These results suggest that architected nanolattices can serve as efficient three-dimensional mechanically tunable photonic crystals, providing a foundation for new opto-mechanical components and devices across infrared and possibly visible frequencies.

View Article

We investigate the effect of parity-time (PT) symmetric optical potentials on the radiation of achiral and chiral dipole sources. Two properties unique to PT-symmetric potentials are observed. First, the dipole can be tuned to behave as a strong optical emitter or absorber based on the non-Hermiticity parameter and the dipole location. Second, exceptional points give rise to new system resonances that lead to orders-of-magnitude enhancements in the dipolar emitted or absorbed power. Utilizing these properties, we show that enantiomers of chiral molecules near PT-symmetric metamaterials exhibit a 4.5-fold difference in their emitted power and decay rate. The results of this work could enable new atom-cavity interactions for quantum optics, as well as all-optical enantioselective separation.

View Article

Tomography has enabled the characterization of the Earth's interior, visualization of the inner workings of the human brain, and three-dimensional reconstruction of matter at the atomic scale. However, tomographic techniques that rely on optical excitation or detection are generally limited in their resolution by diffraction. Here, we introduce a tomographic technique - cathodoluminescence spectroscopic tomography - to probe optical properties in three dimensions with nanometre-scale spatial and spectral resolution. We first obtain two-dimensional cathodoluminescence maps of a three-dimensional nanostructure at various orientations. We then use the method of filtered back-projection to reconstruct the cathodoluminescence intensity at each wavelength. The resulting tomograms allow us to locate regions of efficient cathodoluminescence in three dimensions across visible and near-infrared wavelengths, with contributions from material luminescence and radiative decay of electromagnetic eigenmodes. The experimental signal can be further correlated with the radiative local density of optical states in particular regions of the reconstruction. We demonstrate how cathodoluminescence tomography can be used to achieve nanoscale three-dimensional visualization of light-matter interactions by reconstructing a three-dimensional metal-dielectric nanoresonator.

View Article

NaYF4:Yb3+,Er3+ nanoparticle upconverters are hindered by low quantum efficiencies arising in large part from the parity-forbidden nature of their optical transitions and the nonoptimal spatial separations between lanthanide ions. Here, we use pressure-induced lattice distortion to systematically modify both parameters. Although hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion emission, cubic-phase particles experience a nearly 2-fold increase in efficiency. In-situ X-ray diffraction indicates that these emission changes require only a 1% reduction in lattice constant. Our work highlights the intricate relationship between upconversion efficiency and lattice geometry and provides a promising approach to modifying the quantum efficiency of any lanthanide upconverter.

View Article

Non-Hermitian parity-time (PT)-symmetric optical potentials have led to a new class of unidirectional photonic components based on the spatially symmetric and balanced inclusion of loss and gain. While most proposed and implemented PT-symmetric optical devices have wavelength-scale dimensions, no physical constraints preclude development of subwavelength PT-symmetric components. We theoretically demonstrate a nanoscale PT-symmetric, all-optical plasmonic modulator capable of phase-controlled amplification and directional absorption. The modulator consists of two deeply subwavelength channels composed of either gain or loss dielectric material, embedded in a metallic cladding. When illuminating on-resonance by two counter-propagating plane waves, the aperture's total output can be modulated by changing the phase offset between the two waves. Modulation depths are greater than 10?dB, with output power varying from less than one half of the incident power to more than six times amplification. Off-resonance, the aperture possesses strong phase-controlled directionality with the output from one side varying from perfect absorption to strong scattering and transmission. The device design provides a platform for nanoscale all-optical modulators with gain while potentially enabling coherent perfect absorption and lasing in a single, compact structure.

View Article

The field of plasmonics has revolutionized the ability to control nanoscale light-matter interactions with applications ranging from high-efficiency photovoltaic modules to ultrasensitive biodetectors, electromagnetic cloaks, and subwavelength integrated photonic circuits. This article summarizes my group?s efforts to contribute to this burgeoning field, with emphasis on our research in quantum plasmonics and optical-frequency magnetism. First, we explore the plasmon resonances of individual nanoparticles as they transition from a classical to a quantum-influenced regime. We then utilize these results to directly monitor hydrogen absorption and desorption in individual palladium nanocrystals. Subsequently, using real-time manipulation of plasmonic particles, we investigate plasmonic coupling between pairs of particles separated by nanometer- and angstrom-scale gaps. For sufficiently small separations, we observe the effects of quantum tunneling between particles on their plasmonic resonances. Finally, using the properties of coupled metallic nanoparticles, we demonstrate the colloidal synthesis of an isotropic metafluid or "metamaterial paint" that exhibits a strong optical-frequency magnetic response and the potential for negative permeabilities and negative refractive indices.

View Article

The sun is the most abundant source of carbon-free sustainable energy, but current solar cells can only capture energy from a limited portion of the solar spectrum?specifically those photons with energies above the solar cell bandgap. Here, we explore a promising approach to reduce such transmission losses: placing an upconverter behind the solar cell. Upconverting materials can absorb low-energy photons and emit higher-energy photons back toward the solar cell, thereby improving the cell?s photocurrent. An ideal upconverter can increase the efficiency of an ideal solar cell from 30% to 44%. Existing upconverting materials offer much smaller improvements, though a boost in their upconverter efficiency could yield an absolute increase in solar cell efficiency of over 2% . We discuss several schemes to improve upconverter efficiency, focusing on use of nanophotonic and plasmonic antennas to increase the absorption and emission of upconverters. These schemes can in theory enable orders-of-magnitude improvement in upconverter efficiency, positioning the technology for significant commercial applications.

View Article

Upconversion, the conversion of photons from lower to higher energies, is a process that promises applications ranging from high-efficiency photovoltaic and photocatalytic cells to background-free bioimaging and therapeutic probes. Existing upconverting materials, however, remain too inefficient for viable implementation. In this Perspective, we describe the significant improvements in upconversion efficiency that can be achieved using plasmon resonances. As collective oscillations of free electrons, plasmon resonances can be used to enhance both the incident electromagnetic field intensity and the radiative emission rates. To date, this approach has shown upconversion enhancements up to 450x. We discuss both theoretical underpinnings and experimental demonstrations of plasmon-enhanced upconversion, examining the roles of upconverter quantum yield, plasmonic geometry, and plasmon spectral overlap. We also discuss nonoptical consequences of including metal nanostructures near upconverting emitters. The rapidly expanding field of plasmon-enhanced upconversion provides novel fundamental insight into nanoscale light?matter interactions while improving prospects for technological relevance.

View Article

Many energy- and information-storage processes rely on phase changes of nanomaterials in reactive environments. Compared to their bulk counterparts, nanostructured materials seem to exhibit faster charging and discharging kinetics, extended life cycles, and size-tunable thermodynamics. However, in ensemble studies of these materials, it is often difficult to discriminate between intrinsic size-dependent properties and effects due to sample size and shape dispersity. Here, we detect the phase transitions of individual palladium nanocrystals during hydrogen absorption and desorption, using in situ electron energy-loss spectroscopy in an environmental transmission electron microscope. In contrast to ensemble measurements, we find that palladium nanocrystals undergo sharp transitions between the alpha and beta phases, and that surface effects dictate the size dependence of the hydrogen absorption pressures. Our results provide a general framework for monitoring phase transitions in individual nanocrystals in a reactive environment and highlight the importance of single-particle approaches for the characterization of nanostructured materials.

View Article

We theoretically investigate the optical properties of parity-time (PT) symmetric three-dimensional metamaterials composed of strongly-coupled planar plasmonic waveguides. By tuning the loss-gain balance, we show how the initially isotropic material becomes both asymmetric and unidirectional. Investigation of the band structure near the material's exceptional point reveals several intriguing optical properties, including double negative refraction, Bloch power oscillations, unidirectional invisibility, and reflection and transmission coefficients that are simultaneously equal to or greater than unity. The highly tunable optical dispersion of PT-symmetric metamaterials provides a foundation for designing an entirely new class of three-dimensional bulk synthetic media, with applications ranging from lossless sub-diffraction-limited optical lenses to non-reciprocal nanophotonic devices.

View Article

Non-Hermitian Hamiltonians arising from parity-time PT-symmetric potentials have been extensively explored in optical systems, owing to their ability to generate asymmetric and even non-reciprocal light propagation. In this paper, we investigate such PT-potentials in plasmonic systems, demonstrating asymmetric optical propagation in deeply subwavelength waveguides. In particular, we investigate a five layer plasmonic waveguide composed of metallic layers separated by dielectric media containing either loss or gain in equal quantities. Through an analytic solution of Maxwell's equations, we identify the four lowest order modes of the waveguide, including two positive index modes and two negative index modes, and investigate their evolution with increasing but balanced gain and loss, K. Both the exact analytic approach and an approximate one based on Rayleigh-Schrodinger perturbation theory demonstrate eigenvalue merging and state coalescence with increasing K, unlike the familiar energy-level splitting observed in conventional coupled systems. The state coalescence always occurs between modes of opposite parity. Also, by changing the coupling between the waveguide layers, state coalescence can occur between modes with opposite refractive indices, resulting in the merging of a positive index mode with a negative index mode at the exceptional point. We use dispersion diagrams and field profiles to illustrate the asymmetric plasmon propagation properties with increasing K. We also show that at the system's exceptional point, the modal power varies quadratically along the waveguide. To our knowledge, this study represents the first spectral analysis of deeply sub wavelength PT-symmetric plasmonic and multi-modal photonic waveguides, and provides a foundation for designing novel asymmetric and unidirectional nanophotonic devices.

View Article

Advances in the field of metamaterials have enabled unprecedented control of lightmatter interactions. Metamaterial constituents support high-frequency electric and magnetic dipoles, which can be used as building blocks for new materials capable of negative refraction, electromagnetic cloaking, strong visible-frequency circular dichroism, and enhancing magnetic or chiral transitions in ions and molecules. While all metamaterials to date have existed in the solid-state, considerable interest has emerged in designing a colloidal metamaterial or ‘metafluid.’ Such metafluids would combine the advantages of solution-based processing with facile integration into conventional optical components. Here we demonstrate the colloidal synthesis of an isotropic metafluid that exhibits a strong magnetic response at visible frequencies. Protein-antibody interactions are used to direct the solution-phase self-assembly of discrete metamolecules comprised of silver nanoparticles tightly packed around a single dielectric core. The electric and magnetic response of individual metamolecules and the bulk metamaterial solution are directly probed with optical scattering and spectroscopy. This colloidal metafluid can be synthesized in large-quantity and high-quality, and may accelerate development of advanced nanophotonic and metamaterial devices.

View Article

We theoretically investigate light matter interactions for chiral molecules in the presence of non-chiral nanoantennas. Isotropic nanostructures supporting optical-frequency electric or magnetic dipoles are sufficient to locally enhance the excitation of a molecule's chiral polarizability and thus its circular dichroism spectrum. However, simultaneous electric and magnetic dipoles are necessary to achieve a net, spatially-averaged enhancement. Our contribution provides a theoretical framework to understand chiral light-matter interactions at the nanoscale and sets the necessary and sufficient conditions to enhance circular dichroism spectroscopy in the presence of nanoantennas. The results may lead to new, field-enhanced, chiral spectroscopic techniques.

View Article

A broadband metamaterial presenting negative indices across hundreds of nanometers in the visible and near-infrared spectral regimes is demonstrated theoretically, using transformation optics to design the metamaterial constituents. The approach begins with an infinite plasmonic waveguide that supports a broadband but dark (i.e, not easily optically accessed) negative index mode. Conformal mapping of this waveguide to a finite split-ring-resonator-type structure transforms this mode into a bright (i.e, efficiently excited) resonance composed of degenerate electric and magnetic dipoles. A periodic array of such resonators exhibits negative refractive indices at optical frequencies in multiple regions exceeding 200 nm in bandwidth. The metamaterial response is confirmed through simulations of plane-wave refraction through a metamaterial prism. These results illustrate the power of transformation optics for new metamaterial designs and provide a foundation for future broadband metamaterial devices.

View Article

Upconversion of sub-bandgap photons is a promising approach to exceed the Shockley-Queisser limit in solar technologies. Calculations have indicated that ideal, upconverter-enhanced cell efficiencies can exceed 44% for non-concentrated sunlight, but such improvements have yet to be observed experimentally. To explain this discrepancy, we develop a thermodynamic model of an upconverter-cell considering a highly realistic narrow-band, non-unity-quantum-yield upconverter. As expected, solar cell efficiencies increase with increasing upconverter bandwidth and quantum yield, with maximum efficiency enhancements found for near-infrared upconverter absorption bands. Our model indicates that existing bimolecular and lanthanide-based upconverters will not improve cell efficiencies more than 1%, consistent with recent experiments. However, our calculations show that these upconverters can significantly increase cell efficiencies from 28% to over 34% with improved quantum yield, despite their narrow bandwidths. Our results highlight the interplay of absorption and quantum-yield in upconversion, and provide a platform for optimizing future solar upconverter designs.

View Article

This News and Views describes results from the article "Tailoring and imaging the plasmonic local density of states in crystalline nanoprisms," by Sviatlana Viarbitskaya, Alexandre Teulle, Renaud Marty, Jadab Sharma, Christian Girard, Arnaud Arbouet, and Erik Dujardin, Nature Materials (2013). Their results describe how two-photon luminescence in metallic nanostructures provides a unique signature of the number of plasmonic modes per unit energy and volume, paving the way for more efficient plasmonic sources, detectors and sensors.

View Article

The plasmon resonances of two closely spaced metallic particles have enabled applica- tions including single-molecule sensing and spectroscopy, novel nanoantennas, molecular rulers, and nonlinear optical devices. In a classical electrodynamic context, the strength of such dimer plasmon resonances increases monotonically as the particle gap size de- creases. In contrast, a quantum mechanical framework predicts that electron tunneling will strongly diminish the dimer plasmon strength for sub-nanometer-scale separations. Here, we directly observe the plasmon resonances of coupled metallic nanoparticles as their gap size is reduced to atomic dimensions. Using the electron beam of a scanning transmis- sion electron microscope (STEM), we dynamically manipulate pairs of 10-nm-diameter spherical silver nanoparticles on a substrate, controlling their convergence and eventual coalescence into a single nanosphere. We simultaneously employ electron energy-loss spectroscopy (EELS) to observe the dynamic plasmonic properties of these dimers before and after particle contact. As separations are reduced from 7 nm, the dominant dipolar peak exhibits a redshift consistent with classical calculations. However, gaps smaller than 0.5 nm cause this mode to exhibit a reduced intensity consistent with quantum theories that incorporate electron tunneling. As the particles overlap, the bonding dipolar mode disappears and is replaced by a dipolar charge transfer mode. Our dynamic imaging, manipulation, and spectroscopy of nanostructures enables therst full spectral mapping of dimer plasmon evolution, and may provide new avenues for in-situ nanoassembly and analysis in the quantum regime.

View Article

Optical trapping using focused laser beams has emerged as a powerful tool in the biological and physical sciences. However, scaling this technique to nanosized objects remains challenging due to the diffraction limit of light and the high power levels required for nanoscale trapping. In this paper, we propose plasmonic coaxial apertures as low-power optical traps for nanosized specimens. The illumination of a coaxial aperture with a linearly polarized plane wave generates a dual optical trapping potential well. We theoretically show that this potential can stably trap dielectric particles smaller than 10 nm in diameter while keeping the trapping power level below 20 mW. By tapering the thickness of the coaxial dielectric channel, trapping can be extended to sub-2-nm particles. The proposed structures may enable optical trapping and manipulation of dielectric particles ranging from single proteins to small molecules with sizes previously inaccessible.

View Article

Electrons and photons can coexist as a single entity called a surface plasmon — an elementary excitation found at the interface between a conductor and an insulator. Because of their hybrid electric and photonic nature, plasmons allow photons to be precisely controlled on the nanoscale. Plasmons are evident in the vivid hues of rose windows, which derive their color from small metallic nanoparticles embedded in the glass. They also provide the basis for color-changing biosensors (such as home pregnancy tests), photothermal cancer treatments, improved photovoltaic cell efficiencies, and nanoscale lasers. While surface plasmons were first identified nearly 55 years ago, many of their exciting applications are yet to come. This issue of MRS Bulletin reviews the progress and promise of plasmonics—from the characterization tools that have allowed nanometer-scale probing of plasmons to the new materials that may enable low-loss, active, and quantum plasmonics. Within reach are applications ranging from integrated plasmonic circuits for nanophotonic computation to plasmonic optical tweezers for manipulation of nano-sized particles and proteins.

View Article

Nanocrystal superlattices have emerged as a new platform for bottom-up metamaterial design, but their optical properties are largely unknown. Here, we investigate their emergent optical properties using a generalized semi-analytic, full-field solver based on rigorous coupled wave analysis. Attention is given to superlattices composed of noble metal and dielectric nanoparticles in unary and binary arrays. By varying the nanoparticle size, shape, separation, and lattice geometry, we demonstrate the broad tunability of superlattice optical properties. Superlattices composed of spherical or octahedral nanoparticles in cubic and AB2 arrays exhibit magnetic permeabilities tunable between 0.2 and 1.7, despite having non-magnetic constituents. The retrieved optical parameters are nearly polarization and angle-independent over a broad range of incident angles. Accordingly, nanocrystal superlattices behave as isotropic bulk metamaterials. Their tunable permittivities, permeabilities, and emergent magnetism may enable new, bottom-up metamaterials and negative index materials at visible frequencies.

View Article

Collective electron excitations in metals, called plasmons, can play an important role in second harmonic generation of light. This Viewpoint describes results from the article "Nonlinear plasmon-photon interaction resolved by k-space spectroscopy," by Nicolai B. Grosse, Jan Heckmann, and Ulrike Woggonthe, Physical Review Letters (2012)

View Physics Article

View PRL Article

The plasmon resonances of metallic nanoparticles have received considerable attention for their applications in nanophotonics, biology, sensing, spectroscopy and solar energy harvesting. Although thoroughly characterized for spheres larger than ten nanometres in diameter, the plasmonic properties of particles in the quantum size regime have been historically difficult to describe owing to weak optical scattering, metal–ligand interactions, and inhomogeneity in ensemble measurements. Such difficulties have precluded probing and controlling the plasmonic properties of quantum-sized particles in many natural and engineered processes, notably catalysis. Here we investigate the plasmon resonances of individual ligand-free silver nanoparticles using aberration-corrected transmission electron microscope (TEM) imaging and monochromated scanning TEM electron energy-loss spectroscopy (EELS). This technique allows direct correlation between a particle’s geometry and its plasmon resonance. As the nanoparticle diameter decreases from 20 nanometres to less than two nanometres, the plasmon resonance shifts to higher energy by 0.5 electronvolts, a substantial deviation from classical predictions. We present an analytical quantum mechanical model that describes this shift due to a change in particle permittivity. Our results highlight the quantum plasmonic properties of small metallic nanospheres, with direct application to understanding and exploiting catalytically active and biologically relevant nanoparticles.

View Article

Reducing reflection and transmission losses in photovoltaic devices is essential for realizing highly efficient power conversion. Here, we theoretically investigate arrays of radial junction silicon wires to determine the optimal geometry for maximized light absorption. Using a generalized rigorous coupled wave analysis, we calculate the scattering spectra of arrays of varying wire radii, length, and lattice filling factors. Near unity absorption, far exceeding that of conventional thin film devices, is calculated for a square array of 20 µm long wires with radii of 200 nm and a filling fraction of 30%. These results suggest a potentially cost-effective route toward high efficiency solar cells.

View Article

Upconversion of sub-bandgap photons can increase the maximum efficiency of a single-junction solar cell from 30% to over 44%. However, upconverting materials often have small absorption cross-sections and poor radiative recombination efficiencies that limit their utility in solar applications. Here, we show that the efficiency of upconversion can be substantially enhanced with a suitably designed plasmonic nanostructure. The structure consists of a spherical nanocrescent composed of an upconverter-doped dielectric core and a crescent-shaped metallic shell. Using numerical techniques, we calculate a greater than 10-fold absorption enhancement for a broad range of sub-bandgap wavelengths throughout the entire upconverting core. Further, this nanocrescent enables a 100-fold increase in above-bandgap power emission toward the solar cell. Our results provide a framework for achieving low-power solar upconversion, potentially enabling a single-junction solar cell with an efficiency exceeding the Shockley–Queisser limit.

View Article

Loss is one of the most substantial impediments to integrated plasmonics. In this paper, we present a theoretical analysis of active cylindrical plasmon slot waveguides, including their modal characteristics, gain spectra, and lasing threshold. Particular attention is given to two classes of waveguide geometries composed of various core/channel/cladding materials: a dielectric/dielectric/metal (DDM) waveguide and a metal/dielectric/metal (MDM) waveguide. Using empirically determined optical constants, we systematically study the dispersion, propagation length, threshold gain, modal gain, and confinement factor of these slot structures. For DDM waveguides, we show that introducing the gain in the channel rather than the core is of paramount importance for reduced threshold gain and increased modal gain. Confinement factor enhancement is even more pronounced in MDM waveguides, where modal gain can exceed threshold gain by 10× to 100× across visible and near-infrared frequencies. By carefully tuning the core/channel relative dimensions along with the lasing frequency, we show that threshold gain as low as 500 inverse cm is achievable in cylindrical plasmonic devices with overall diameters less than 200 nm. Our results indicate the promise of plasmonic slot structures for low-loss optical networking, and provide a roadmap for the design of optimized nanoscale plasmonic laser cavities.

View Article

Assemblies of strongly coupled plasmonic nanoparticles can support highly tunable electric and magnetic resonances in the visible spectrum. In this letter, we theoretically demonstrate Fano-like interference effects between the fields radiated by the electric and magnetic modes of symmetric nanoparticle trimers. Breaking the symmetry of the trimer system leads to a strong interaction between the modes. The near and far field electromagnetic properties of the broken symmetry trimer are tunable across a large spectral range. We exploit this Fano-like effect to demonstrate spatial and temporal control of the localized electromagnetic hotspots in the plasmonic trimer.

View Article

Upconverting materials can be used to increase the energy conversion efficiency of a solar cell. Such materials convert low-energy transmitted photons to higher-energy photons that can be absorbed by the cell, substantially reducing the spectral mismatch between the cell and the solar spectrum. Previously, the performance enhancements achievable with an ideal upconverter-solar cell system were theoretically investigated. Here, we perform a comprehensive analysis to determine the effects of non-ideal cell and upconverter systems, accounting for non-ideal absorption and radiative recombination. We also allow for realistic nonradiative relaxation within the upconverter. The system is modeled using a detailed balance approach, with the upconverter treated as a series connection of two small-bandgap solar cells and a large-bandgap light emitting diode. We demonstrate that significant improvements in efficiency are possible even for nonconcentrated light, as long as the upconverter includes a small nonradiative relaxation pathway. Furthermore, we show that the existence of a nonradiative relaxation event in the upconverter is necessary for improved power conversion when cell absorption efficiency is low. Our results indicate that the efficiencies of both conventional-Si and thin film photovoltaic cells can be substantially improved with upconverting materials, even including non-idealities.

View Article

In this work, H2 absorption and desorption in faceted, crystalline Au/Pd core/shell nanocrystals and their interaction with a SiOx/Si support were studied at the single-particle level. Dark-field microscopy was used to monitor the changing optical properties of these Au/Pd nanoparticles (NPs) upon exposure to H2 as reversible H2 uptake from the Pd shell proceeded. Analysis of the heterogeneous ensemble of NPs revealed the H2 uptake trajectory of each nanocrystal to be shape-dependent. Differences in particle uptake trajectories were observed for individual particles with different shapes, faceting, and Pd shell thickness. In addition to palladium hydride formation, the single-particle trajectories were able to decipher specific instances where palladium silicide formation and Au/Pd interdiffusion occurred and helped us determine that this was more frequently seen in those particles within an ensemble having thicker Pd shells. This noninvasive, plasmonic-based direct sensing technique shows the importance of single-particle experiments in catalytically active systems and provides a foundation for studying more complex catalytic processes in inhomogeneous NP systems.

View Article

Silicon-based photonic devices dissipate substantially less power and provide a significantly greater information bandwidth than electronic components. Unfortunately, large-scale integration of photonic devices has been limited by their large, wavelength-scale size and the weak optical response of Si. Surface plasmons may overcome these two limitations. Combining the high localization of electronic waves with the propagation properties of optical waves, plasmons can achieve extremely small mode wavelengths and large local electromagnetic field intensities. Si-based plasmonics has the potential to not only reduce the size of photonic components to deeply-subwavelength scales, but also to enhance the emission, detection, and manipulation of optical signals in Si. In this article, we discuss recent advances in Si-based plasmonics, including subwavelength interconnects, modulators, and emission sources. From scales spanning slab waveguides to single nanocrystals, we show that Si-based plasmonics can enable optical functionality competitive in size and speed with contemporary electronic components.

Electronic Thesis

We report a method for filtering white light into individual colors using metal−insulator−metal resonators. The resonators are designed to support photonic modes at visible frequencies, and dispersion relations are developed for realistic experimental configurations. Experimental results indicate that passive Ag/Si3N4/Au resonators exhibit color filtering across the entire visible spectrum. Full field electromagnetic simulations were performed on active resonators for which the resonator length was varied from 1−3 μm and the output slit depth was systematically varied throughout the thickness of the dielectric layer. These resonators are shown to filter colors based on interference between the optical modes within the dielectric layer. By careful design of the output coupling, the resonator can selectively couple to intensity maxima of different photonic modes and, as a result, preferentially select any of the primary colors. We also illustrate how refractive index modulation in metal−insulator−metal resonators can yield actively tunable color filters. Simulations using lithium niobate as the dielectric layer and the top and bottom Ag layers as electrodes indicate that the output color can be tuned over the visible spectrum with an applied field.

Realization of chip-based all-optical and optoelectronic computational networks will require ultracompact Si-compatible modulators, ideally comprising dimensions, materials, and functionality similar to electronic complementary metal−oxide−semiconductor (CMOS) components. Here we demonstrate such a modulator, based on field-effect modulation of plasmon waveguide modes in a MOS geometry. Near-infrared transmission between an optical source and drain is controlled by a gate voltage that drives the MOS into accumulation. Using the gate oxide as an optical channel, electro-optic modulation is achieved in device volumes of half of a cubic wavelength with femtojoule switching energies and the potential for gigahertz modulation frequencies.

We present a theoretical analysis of planar plasmonic waveguides that support propagation of positive and negative index modes. Particular attention is given to the modes sustained by metal-insulator-metal (MIM), insulator-metal-insulator (IMI), and insulator-insulator-metal (IIM) geometries at visible and near-infrared frequencies. We find that all three plasmonic structures are characterized by negative indices over a finite range of visible frequencies, with figures of merit approaching 20. Moreover, using finite-difference time-domain simulations, we demonstrate that visible-wavelength light propagating from free space into these waveguides can exhibit negative refraction. Refractive index and figure-of-merit calculations are presented for Ag/GaP and Ag/Si3N4 - based structures with waveguide core dimensions ranging from 5 to 50 nm and excitation wavelengths ranging from 350 nm to 850 nm. Our results provide the design criteria for realization of broadband, visible-frequency negative index materials and transformation-based optical elements for two-dimensional guided waves. These geometries can serve as basic elements of three-dimensional negative-index metamaterials.

A diffusion bonding method has been developed that enables layer transfer of single crystal lithium niobate thin films to silicon substrates. A silver film was deposited onto both the silicon and lithium niobate surfaces prior to bonding, and upon heating, a diffusion bond was formed. Transmission electron microscopy confirms the interface evolution via diffusion bonding which combines interfacial diffusion, power law creep, and growth of (111) silver grains to replace the as-bonded interface by a single polycrystalline silver film. The transferred film composition was the same as bulk lithium niobate.

A nanoscale gap between two metal surfaces can confine propagating surface plasmon polaritons (SPPs) to very small dimensions, but this geometry makes it inherently difficult to image SPP propagation at high resolution. We demonstrate the near-field probing of these SPPs, propagating within a 50 nm thick Si3N4 waveguide with Ag cladding layers for frequencies ranging from the blue to the near-infrared. Using near-field SPP interferometry, we determine the wave vector, showing that the wavelength is shortened to values as small as 156 nm for a free-space wavelength of 532 nm.

Nanofabricated photonic materials offer opportunities for crafting the propagation and dispersion of light in matter. We demonstrate an experimental realization of a two-dimensional negative-index material in the blue-green region of the visible spectrum, substantiated by direct geometric visualization of negative refraction. Negative indices were achieved with the use of an ultrathin Au-Si3N4-Ag waveguide sustaining a surface plasmon polariton mode with antiparallel group and phase velocities. All-angle negative refraction was observed at the interface between this bimetal waveguide and a conventional Ag-Si3N4-Ag slot waveguide. The results may enable the development of practical negative-index optical designs in the visible regime.

We report experimental realization of subwavelength slot waveguides that exhibit both micrometer-range propagation and high spatial confinement of light. Attention is given to rectangular waveguides with a Si3N4 core and Ag cladding; core thicknesses of 50−100 nm and widths of 250 nm − 10 μm are explored. Propagation lengths of 5λ are achieved with light confined to lateral and transverse dimensions of λ/5 and λ/2, respectively. This unique combination of light localization and propagation is achieved via interacting surface plasmons, which produce short modal wavelengths and strong field confinement at each metal/dielectric interface.

We present a numerical analysis of surface plasmon waveguides exhibiting both long-range propagation and spatial confinement of light with lateral dimensions of less than 10% of the free-space wavelength. Attention is given to characterizing the dispersion relations, wavelength-dependent propagation, and energy density decay in two-dimensional Ag∕SiO2∕Ag structures with waveguide thicknesses ranging from 12 nm to 250 nm. As in conventional planar insulator-metal-insulator (IMI) surface plasmon waveguides, analytic dispersion results indicate a splitting of plasmon modes—corresponding to symmetric and antisymmetric electric field distributions—as SiO2 core thickness is decreased below 100 nm. However, unlike IMI structures, surface plasmon momentum of the symmetric mode does not always exceed photon momentum, with thicker films (d∼50 nm) achieving effective indices as low as n=0.15. In addition, antisymmetric mode dispersion exhibits a cutoff for films thinner than d=20 nm, terminating at least 0.25 eV below resonance. From visible to near infrared wavelengths, plasmon propagation exceeds tens of microns with fields confined to within 20 nm of the structure. As the SiO2 core thickness is increased, propagation distances also increase with localization remaining constant. Conventional waveguiding modes of the structure are not observed until the core thickness approaches 100 nm. At such thicknesses, both transverse magnetic and transverse electric modes can be observed. Interestingly, for nonpropagating modes (i.e., modes where propagation does not exceed the micron scale), considerable field enhancement in the waveguide core is observed, rivaling the intensities reported in resonantly excited metallic nanoparticle waveguides.

A numerical analysis of surface plasmon dispersion, propagation, and localization on smooth lossy films is presented. Particular attention is given to determining wavelength-dependent behavior of thin Ag slab waveguides embedded in a symmetric SiO2 environment. Rather than considering Ag as a damped free electron gas, the metal is defined by the experimentally determined optical constants of Johnson and Christy and Palik. As in free electron gas models, analytic dispersion results indicate a splitting of plasmon modes—corresponding to symmetric and antisymmetric field distributions—as film thickness is decreased below 50 nm. However, unlike free electron gas models, the surface plasmon wave vector remains finite at resonance with the antisymmetric-field plasmon converging to a pure photon mode for very thin films. In addition, allowed excitation modes are found to exist between the bound and radiative branches of the dispersion curve. The propagation characteristics of all modes are determined, and for thin films (depending upon electric field symmetry), propagation distances range from microns to centimeters in the near infrared. Propagation distances are correlated with both the field decay (skin depth) and energy density distribution in the metal and surrounding dielectric. While the energy density of most long-range surface plasmons exhibits a broad spatial extent with limited confinement in the waveguide, it is found that high-field confinement does not necessarily limit propagation. In fact, enhanced propagation is observed for silver films at ultraviolet wavelengths despite strong field localization in the metal. The surface plasmon characteristics described in this paper provide a numerical springboard for engineering nanoscale metal plasmon waveguides, and the results may provide a new avenue for integrated optoelectronic applications.

Since the development of the light microscope in the 16th century, optical device sizeand performance have been limited by diffraction.Optoelectronic devices of today aremuch bigger than the smallest electronic devices for this reason.Achieving control oflight–material interactions for photonic device applications at the nanoscale requiresstructures that guide electromagnetic energy with subwavelength-scale modeconfinement.By converting the optical mode into nonradiating surface plasmons,electromagnetic energy can be guided in structures with lateral dimensions of less than10% of the free-space wavelength.A variety of methods—including electron-beamlithography and self-assembly—have been used to construct both particle and planarplasmon waveguides.Recent experimental studies have confirmed the strongly coupledcollective plasmonic modes of metallic nanostructures.In plasmon waveguidesconsisting of closely spaced silver rods, electromagnetic energy transport over distancesof 0.5m has been observed.Moreover, numerical simulations suggest the possibility ofmulti-centimeter plasmon propagation in thin metallic stripes.Thus, there appears to beno fundamental scaling limit to the size and density of photonic devices, and ongoingwork is aimed at identifying important device performance criteria in the subwavelengthsize regime.Ultimately, it may be possible to design an entire class of subwavelength-scale optoelectronic components (waveguides, sources, detectors, modulators) thatcould form the building blocks of an optical device technology—a technology scalable to molecular dimensions, with potential imaging, spectroscopy, and interconnectionapplications in computing, communications, and chemical/biological detection.

By the mid-17th century, numerous scientists—notably including Hooke and Gallileo—had developed transparent ground lenses and applied them in the construction of compound optical microscopes. This development revolutionized the contemporary understanding of the natural world by, for example, enabling the imaging of blood cells and microbes. Ever since this now bygone era of fantastic development of optics principles and instrumentation, the size and performance of photonic devices has been largely limited by diffraction. Photonic devices of today are generally composed of dielectric materials with modest dielectric constants, and are much bigger than the smallest electronic devices (e.g., transistors in silicon integrated circuits) for this reason.