Gradient metasurfaces are two-dimensional optical elements capable of manipulating light by imparting local, space-variant phase changes on an incident electromagnetic wave. These surfaces have thus far been constructed from nanometallic optical antennas, and high diffraction efficiencies have been limited to operation in reflection mode. We describe the experimental realization and operation of dielectric gradient metasurface optical elements capable of also achieving high efficiencies in transmission mode in the visible spectrum. Ultrathin gratings, lenses, and axicons have been realized by patterning a 100-nanometer-thick Si layer into a dense arrangement of Si nanobeam antennas. The use of semiconductors can broaden the general applicability of gradient metasurfaces, as they offer facile integration with electronics and can be realized by mature semiconductor fabrication technologies.

Photovoltaic (PV) cells can serve as a virtually unlimited clean source of energy by converting sunlight into electrical power. Their importance is reflected in the tireless efforts that have been devoted to improving the electrical and structural properties of PV materials. More recently, photon management (PM) has emerged as a powerful additional means to boost energy conversion efficiencies. Here, we demonstrate an entirely new PM strategy that capitalizes on strong broad band optical antenna effects in one-dimensional semiconductor nanostructures to dramatically enhance absorption of sunlight. We show that the absorption of sunlight in Si nanowires (Si NWs) can be significantly enhanced over the bulk. The NW's optical properties also naturally give rise to an improved angular response. We propose that by patterning the silicon layer in a thin film PV cell into an array of NWs, one can boost the absorption for solar radiation by 25% while utilizing less than half of the semiconductor material (250% increase in the light absorption per unit volume of material). These results significantly advance our understanding of the way sunlight is absorbed by one-dimensional semiconductor nanostructures and provide a clear, intuitive guidance for the design of efficient NW solar cells. The presented approach is universal to any semiconductor and a wide range of nanostructures; as such, it provides a new PV platform technology.

Empowering silicon (Si) with optical functions constitutes a very important challenge in photonics. The scalable fabrication capabilities for this earth-abundant, environmentally friendly material are unmatched in sophistication and can be unleashed to realize a plethora of high-performance photonic functionalities that find application in information, bio-, display, camouflage, ornamental, and energy technologies. Nanofashioning represents a general strategy to turn Si into a useful optical material and Si structures have already been engineered to enable light emission, optical cloaking, waveguiding, nonlinear optics, enhanced light absorption, and sensing. Here, we demonstrate that a wide spectrum of colors can be generated by harnessing the strong resonant light scattering properties of Si nanostructures under white light illumination. The ability to engineer such colors in a predetermined fashion through a choice of the structure size, dielectric environment, and illumination conditions opens up entirely new applications of Si and puts this material in a new light.

On-chip optical interconnection is considered as a substitute for conventional electrical interconnects as microelectronic circuitry continues to shrink in size. Central to this effort is the development of ultracompact, silicon-compatible, and functional optoelectronic devices. Photodetectors play a key role as interfaces between photonics and electronics but are plagued by a fundamental efficiency-speed trade-off. Moreover, engineering of desired wavelength and polarization sensitivities typically requires construction of space-consuming components. Here, we demonstrate how to overcome these limitations in a nanoscale metal-semiconductor-metal germanium photodetector for the optical communications band. The detector capitalizes on antenna effects to dramatically enhance the photoresponse (>25-fold) and to enable wavelength and polarization selectivity. The electrical design featuring asymmetric metallic contacts also enables ultralow dark currents (approximately 20 pA), low power consumption, and high-speed operation (>100 GHz). The presented high-performance photodetection scheme represents a significant step toward realizing integrated on-chip communication and manifests a new paradigm for developing miniaturized optoelectronics components.

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Fano resonances with a characteristic asymmetric line shape can be observed in light scattering, transmission and reflection spectra of resonant optical systems. They result from interference between direct and indirect, resonance-assisted pathways. In the nanophotonics field, Fano effects have been observed in a wide variety of systems, including metallic nanoparticle assemblies, metamaterials and photonic crystals. Their unique properties find extensive use in applications, including optical filtering, polarization selectors, sensing, lasers, modulators and nonlinear optics. We report on the observation of a Fano resonance in a single semiconductor nanostructure, opening up opportunities for their use in active photonic devices. We also show that Fano-resonant semiconductor nanostructures afford the intriguing opportunity to simultaneously measure the far-field scattering response and the near-field energy storage by extracting photogenerated charge. Together they can provide a complete experimental characterization of this type of resonance.

Subwavelength, high-refractive index semiconductor nanostructures support optical resonances that endow them with valuable antenna functions. Control over the intrinsic properties, including their complex refractive index, size, and geometry, has been used to manipulate fundamental light absorption, scattering, and emission processes in nanostructured optoelectronic devices. In this study, we harness the electric and magnetic resonances of such antennas to achieve a very strong dependence of the optical properties on the external environment. Specifically, we illustrate how the resonant scattering wavelength of single silicon nanowires is tunable across the entire visible spectrum by simply moving the height of the nanowires above a metallic mirror. We apply this concept by using a nanoelectromechanical platform to demonstrate active tuning.

Systems of coupled resonators manifest a myriad of exciting fundamental physical phenomena. Analogous to the synthesis of molecules from single atoms, the construction of photonic molecules from stand-alone optical resonators represents a powerful strategy to realize novel functionalities. The coupling of high quality factor (Q) dielectric and semiconductor microresonators is by now well-understood and chipscale applications are abound. The coupling behavior of low-Q nanometallic structures has also been exploited to realize high-performance plasmonic devices and metamaterials. Although dense arrays of semiconductor nanoparticles and nanowires (NWs) find increasing use in optoelectronic devices, their photonic coupling has remained largely unexplored. These high refractive index nano-objects can serve as low-Q optical antennas that can effectively receive and broadcast light. We demonstrate that the broad band antenna response of a pair of NWs can be tuned significantly by engineering their optical coupling and develop an intuitive coupled-mode theory to explain our observations.