We imagine a world where diseases like cancer and Alzheimer's are cured with light, solar cells provide abundant clean energy, and cell phones compute at the speed of light. We then strive to make that future a reality through development of new nanomaterials and devices. Please browse below to learn more about our research:
| Energy Storage
|| Optical Tweezing
|| Nanophotonic Devices
In a just-accepted Nano Letters, Sassan Sheikholeslami, Hadiseh Alaeian, and Ai Leen Koh describe protein-directed synthesis of a metamaterial fluid. The fluid exhibits an emergent magnetic response at visible frequencies, despite being composed of entirely non-magnetic building blocks. Their metafluid could enable new liquid-based tunable refractive index media with positive, negative, and near-zero refractive indices.
Circular dichroism (CD) spectroscopy is a powerful technique in chemistry, molecular biology, and pharmacology, but it's sensitivity is limited. Chiral specimens exhibit a differential absorption of circularly polarized light that is orders of magnitude smaller than their absorption of unpolarized light. In a recent Physical Review B, Aitzol Garcia-Etxarri theoretically demonstrates how non-chiral nano-antennas can be used to enhance circular dichroism spectroscopy. The key is to use antennas that exhibit both electric and magnetic dipoles. This theory may form the basis for new solution or surface-enhanced CD spectroscopies with few-molecule sensitivity.
In the cover article of Advanced Optical Materials, Ashwin Atre, Aitzol Garcia-Etxarri, and Hadiseh Alaeian describe the mathematical design of the most broadband metamaterial to date, characterized by negative indices across hundreds of nanometers in the visible and near-infrared spectrum. Their results illustrate the power of transformation optics for designing new metamaterials and metasurfaces, and provide a foundation for future broadband superlenses, cloaks, or optical isolators.
In a new Journal of Applied Physics paper, Justin Briggs and Ashwin Atre develop a thermodynamic model of an upconverting solar cell considering a highly realistic narrow-band, non-unity-quantum-yield upconverter. Their calculations and case studies provide a framework optimizing future solar upconverter designs.
In a recently published Nano Letters article, Jon Scholl, Aitzol Garcia-Etxarri, and Ai Leen Koh have observed how electron tunneling between two metallic nanoparticles impacts their plasmonic modes. The technique relied on in-situ imaging, manipulation, and spectroscopy of particles in Stanford's abberation-corrected transmission electron microscope.
In a just accepted Nano Letters publication, Amr Saleh has extended optical trapping to a deeply subwavelength regime. While conventional optical traps are limited by diffraction, Amr has exploited the properties of surface plasmons to trap significantly smaller specimens. His theoretical work provides the foundation for trapping and manipulation of single proteins and small molecules.
Calculations by Hadiseh Alaeian demonstrate that lattices of Au nanoparticles can exhibit emergent optical properties distinct from the constituent particles, including optical-frequency magnetism. Her results could form th basis for new, bottom-up assembled metamaterials. Her results were published in the July 2 issue of Optics Express.
Jon Scholl and Ai Leen Koh demonstrate that metal nanoparticles with dimensions between one and ten nanometers exhibit plasmon resonances governed by quantum mechanical effects, as probed with electron microscopy and spectroscopy. Their results appear as the cover Article in the March 22 edition of Nature.
Recent calculations by Hadiseh Alaeian determine the ideal nanowire dimensions and lattice configuration for optimal photovoltaic conversion. Her results were published as an invited article in the Journal of Optics on January 12.
Recent calculations by Ahswin Atre indicate the potential for significant upconversion enhancements using plasmonic nanostructures. His results were published as an invited article in the Journal of Optics on January 12.
Recent calculations by Amr Saleh indicate the potential for gain-based plasmonic devices with low required threshold gains. Properly designed plasmonic structures can strongly enhance the gain factor of nonlinear materials, despite the higher losses usually associated with metals. His results, published in Physical Review B on January 5, will enable efficient nanoscale plasmon amplifiers, spasers, and lasers.
Recent calculations by Sassan Sheikholeslami and Aitzol Garcia demonstrate Fano-like interference effects between electric and magnetic modes in visible-frequency "metamolecules." Their results, pulished in Nano Letters on August 5, will enable exquisite spatial and temporal control of electromagnetic hotspots, with compelling applications for molecular and biosensing.
On subwavelength scales, photon-matter interactions are limited by diffraction. The diffraction limit restricts the size of optical devices and the resolution of conventional microscopes to wavelength-scale dimensions, severely hampering our ability to control and probe subwavelength-scale optical phenomena. Circumventing diffraction is now a principle focus of integrated nanophotonics. Surface plasmons provide a particularly promising approach to sub-diffraction-limited photonics. Surface plasmons are hybrid electron-photon modes confned to the interface between conductors and transparent materials. 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. For example, even x-ray wavelengths can be achieved at optical frequencies. Through their unique dispersion, surface plasmons provide access to an enormous phase space of refractive indices and propagation constants that can be readily tuned with material or geometry.
While SPs can occur at any metal-dielectric interface, particle and planar plasmon geometries have received particular attention. The figure above depicts two examples of these geometries with just one metal-dielectric interface. As seen in the schematic, particle plasmons resemble dipoles, with clouds of charge localized at the poles. At their resonant frequency, these localized surface plasmons will be characterized by a large scattering coefficient and a large extinction cross-section. Accordingly, particle-based geometries can act as optical nano-antennas, concentrating incident radiation to a subwavelength physical region. Such properties have been exploited in applications ranging from surface-enhanced Raman spectroscopy to photothermal tumor ablation. In contrast, planar plasmonic geometries are characterized by propagating charge-compression waves localized to the metal-dielectric interface. Depending on the excitation wavelength and the specific materials used, the field penetration into the dielectric can be subwavelength. Additionally, plasmon propagation lengths in planar geometries can approach centimeter scales.
Though surface plasmons have only recently been considered for optoelectronic applications, the properties of metallic nanostructures have been exploited for centuries. Both the vivid colors of stained-glass windows and the optical dishroism of the Lycergus cup, designed in the fourth-century, A.D. (shown above) derive from small metallic particles embedded in the glass. Since those early artisan studies, plasmonics has expanded to applications ranging from nanoscale optical waveguides and devices to enhanced light emission, improved photovoltaics and photocatalysts, and electromagnetic metamaterials that may enable "perfect lenses" and invisibility. The explosive growth of the field is reflected in the scientific literature: since 1990, the number of papers related to surface plasmons has doubled every five years. In conjunction with numerous US and internation groups investigating such nanoscale optical interactions, the Dionne group is hoping to make a significant contribution to the various technological challenges that will face our generation, using the power of plasmonics.
Negative index materials (NIMs) are characterized by an electric permittivity and magnetic permeability that are simultaneously negative, resulting in a negative index of refraction. Unlike naturally-occurring materials, the energy velocity of electromagnetic signals in a NIM is oriented opposite to the phase velocity. This antiparallel velocity flow gives rise to many unusual effects, including negative refraction at the interface between positive and negative index media, a reversed Doppler effect, and even negative radiation pressure (see the simulated image to the left from Dolling et al., Optics Express 2006). NIMs also serve as electromagnetic cloaks and amplify evanescent electromegnetic waves, creating the possibility for a "perfect lens" that can resolve arbitrarily-small feature sizes.
Traditional approaches to achieving a NIM involve fabricating subwavelength resonator elements to form a metamaterial. While two and three-dimensional metamaterials have been realized from microwave to near-infrared frequencies (see for example the beautiful work by X. Zhang at berkeley, V. Shalaev at Perdue, and D. Smith at Duke), scaling metamaterials down to visible frequencies has proved challenging. Achieving a NIM that operates over a broad range of visible-wavelengths remains a premier goal of the optics community, since many applications involve controlling electromagnetic radiation in the visible.
By exploiting the unique dispersion properties of surface plasmons, the first direct demonstration of all-angle negative refraction in the visible was achieved by Jen Dionne and colleagues (see the article "Negative Refraction at Visible Frequencies by Henri Lezec, Jen Dionne, and Harry Atwater in the publications section of this site). Rather than using discrete subwavelength resonators, our approach involves continuous metal-dielectric layers. For frequencies above the surface plasmon resonance but below the bulk plasma frequency, plasmons are characterized by oppositely-oriented energy and phase velocities and hence exhibit a negative refractive index. In our first experiments, negative refraction was observed across the interface between a positive index slot waveguide (similar to a fiber optic) and a negative-index metal-insulator-metal plasmonic waveguide. Negative refraction occured in the blue-green region of the spectrum, but calculations indicate that this range can be extended over the entire visible spectrum (see the article in Optics Express by Jen Dionne, Ewold Verhagen, Albert Polman, and Harry Atwater).
Current research is aimed at applying these negative index materials to optical imaging systems that overcome the Rayleigh limit, both focusing and magnifying subwavelength objects in the far-field. Research is also aimed at designing a visible-frequency electromagnetic cloak that exploits the index-tunability of modes in plasmon waveguides.
Colloidal nanocrystal are chemically-synthesized metallic or semiconducting clusters with sizes ranging from tens of nanometers down to a single nanometer. In one sense, these represent a near-ideal representation of the "particle in a box" model taught in introductory quantum mechanics. Due to their finite size, electrons and holes in these particles exhibit quantum confinement that shifts the band energies from the bulk values. Accordingly, their emission wavelengths can tuned from the ultraviolet to the infrared, simply by tuning the nanocrystal size. A variety of nanocrystal morphologies, including dots, rods, and tetrapods, can be readily synthesized, with heterojunction geometries achievable through core-shell coating and cation or annion exhange. Further, nanocrystals can be assembled into 'artifical molecules' through self-assembly or DNA-assisted assembly. Due to their vast tunability of emission and absorption wavelengths, their high degree of quantum confinement, and their low defect density, nanocrystals provide an ideal platform for subwavelength light-matter investigations. In addition, nanocrystals have been shown to exhibit a variety of unusual and exotic properties including strain-tuned photoluminescence, nonlinear processes, phonon-assisted upconversion, and multiexcitonic generation.
The Dionne Laboratories are located in Durand Hall and will soon include a full wet-laboratory and optics laboratory. The wet-lab is designed for device fabrication and nanocrystal synthesis and includes fume hoods, gloveboxes, an evaporation and sputtering chamber, spinners, centrifuges, dual-manifold schlenk lines, and characterization equipment. The optics lab will includes optical tables equipped with far-field and confocal fluorescence microscopes, a near-field microscope, a cryogenic stage, a low-temperature electro-optic probe station, and various CW and pulsed tunable laser sources. Additional characterization and fabrication facilities, including FIB, SEM, and TEM, are located in the SNL and CIS.
Research in the Dionne group is generously supported by the Gordon and Betty Moore Foundation, the Air Force Office of Scientific Research, the National Science Foundation, the Department of Energy, the Global Climate and Energy Project, the TomKat Center for Sustainable Energy, a Hellman Fellowship, a Federick E. Terman Fellowship (endowed by William Hewlett and David Packard) and a Robert N. Noyce Family Faculty Fellowship. Research support from Intel, Northrop Grumman, the Gabilan family, and Stanford University's School of Engineering is also acknowledged.