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Multitasking shared-aperture systems have initially emerged as radar phased array antennas. Recently, the shared-aperture concept has been suggested as a platform for multifunctional optical phased array antennas, accomplished by a reflective metasurface [1]. Metasurfaces consist of metallic or dielectric subwavelength nanoantennas, capable of manipulating light by controlling the local amplitude and phase of an incident electromagnetic wave [2-6]. An effective control of the electromagnetic response can be achieved by a geometric phase mechanism implemented within a metasurface, enabling spin-controlled phase modulation. Shared-aperture geometric phase metasurface (GPM) paves the way for multifunctional nano-optical device. Shared-aperture interleaved phased arrays are formed by the random interspersing of sub-arrays, thus resulting in a device with high flexibility in multifunctional wavefront generation and the angular resolution of the shared aperture. Each sub-array is associated with a specific phase function, sparsely sampled at randomly chosen lattice points. We presented multifunctional spin-dependent dielectric metasurfaces, and demonstrated multiple-beam technology for complete real-time control and measurement of the fundamental intrinsic properties of light, including frequency, polarization, and orbital angular momentum (OAM) [7].

Applying tensile strain with silicon nitride is demonstrated to improve the responsivity of germanium-tin (Ge 1−x Sn x ) PIN photodetectors at longer wavelengths. Such external stressor films show promise for extending the application of Ge 1−x Sn x optoelectronic devices into the mid-infrared range.

We present a new, CMOS-compatible platform for inducing a large, spatially homogeneous biaxial strain in Ge microdisks. This platform can deliver substantial performance improvements to biaxially strained Ge lasers for silicon-compatible optical interconnects.

We report uniaxial tensile strains up to 5.0% in lithographically patterned germanium nanowires, which is enough strain to make germanium a direct bandgap semiconductor. Theoretically, this strain can reduce a germanium laser's threshold by >16,000x.

We demonstrate electroluminescence (EL) from light-emitting diodes (LEDs) on highly strained germanium (Ge) membranes. Electrical measurements show an on-off ratio increase of one order of magnitude in membrane LEDs compared to bulk. The EL spectrum from the 0.76% strained Ge LED shows a 100nm redshift of the center wavelength. We also discuss the implications for highly efficient Ge lasers.

Solid-state dye-sensitized solar cells (ss-DSCs) are a type of solar cell that replaces the liquid electrolyte in a conventional DSC with a solid-state hole-transport material. SS-DSCs have already achieved power conversion efficiency over 6%, and they do not have problems with potential leakage and corrosion encountered by liquid electrolyte DSCs. However, current ss-DSCs are limited by both pore filling and electron-hole recombination such that the optimal thickness is around 2 μm, which is far too thin to absorb enough light. We show that the efficiency of ss-DSCs can be greatly enhanced by incorporation of plasmonic back reflectors, which consist of two-dimensional (2D) array of silver nanodomes. The plasmonic back reflectors can be fabricated by nanoimprint lithography. They enhance absorption through excitation of plasmonic modes and increased light scattering. SS-DSCs with plasmonic back reflectors show increased external quantum efficiency, particularly in the long wavelength region of the dye's absorption band. This approach is effective in increasing the efficiencies of ss-DSCs with normal thickness (2 μm) made with both ruthenium-complex sensitizers and strong-absorbing organic sensitizers, and the short-circuit photocurrents increased by 16% and 12%, respectively. They achieve power conversion efficiencies of 3.9% and 5.9%, on par with the world record for the devices with the same dyes. In addition to the device data, results on the theoretical modeling of plasmonic and photonic effects will also be presented.

We will present progress towards the use of plasmonic metal nanostructures to enhance the efficiency of solar fuel generation [1]. In the past, solar-to-fuel-efficiencies have been limited because of a large mismatch in the length scales for optical absorption and carrier extraction. Future generations of photoelectrodes must employ cheap, earth-abundant absorber materials in order to provide a large-scale source of clean energy. These materials will likely have relatively poor electrical properties, so progress must be made in optimizing their absorption properties [2, 3]. We chose iron oxide (β-Fe 2 O 3 ; hematite) [4] as a prototype system that shares many features with other candidate materials for future large-scale solar fuel production, and therefore anticipate that the results obtained in this study will be applicable to other materials systems as well. Hematite has relatively weak absorption in the 500–600 nm range (0.1 – 1 µm absorption length), very long compared to its minority carrier diffusion length on the order of 2–4 nm [5] or 20 nm [6].

Silicon nanowire Schottky junction solar cells have been fabricated using n-type silicon nanowire arrays and a spin-coated conductive polymer (PEDOT). The polymer Schottky junction cells show superior surface passivation and open-circuit voltages compared to standard diffused junction cells with native oxide surfaces. External quantum efficiencies up to 88% were measured for these silicon nanowire/PEDOT solar cells further demonstrating excellent surface passivation. This process avoids high temperature processes which allows for low-cost substrates to be used.

Group IV photonics has seen tremendous progress in the development of high performance light sources, waveguides, modulators, and detectors. Plasmonics is an exploding new field of science and technology in which the flow of light can be molded at the nanoscale using metallic nanostructures. This newly found ability is rapidly impacting every facet of optics and photonics and is enabling a myriad of exciting new technologies. As plasmonic devices can be fabricated using CMOS compatible nanofabrication techniques, it is worth exploring opportunities at the interface between group IV photonics and plasmonics. I will discuss recent progress in the development of hybrid devices that exploit the best of each technology and provide my personal assessment of the limitations and most exciting future opportunities for the field.

Plasmonics is an exploding new field of science and technology in which the flow of light can be molded at the nanoscale using metallic nanostructures. This newly found ability is rapidly impacting every facet of optics and photonics and is enabling a myriad of exciting new technologies.

We report on experimental measurements of the optical properties of thin films of poly-crystalline silicon carbide (poly-SiC) deposited by means of low pressure chemical vapor deposition (LPCVD). Measurements in the mid-IR region of the EM spectrum show strong dependence of both far field transmission and surface modes upon the deposition conditions including doping levels, ratio of dichlorosilane (DCS) to 1,3 disilabutane (DSB), and anneals performed on the film. We observe the link between the relative strength of near field phonon polariton resonances, the appearance of a polariton gap, and the achievement of extraordinary optical transmission (EOT).

Resonant photoluminescence of Si-nanocrystals is used to determine their free-carrier absorption cross-section (sigmaFCA) at visible/near-infrared wavelengths. Rate-equation modeling, including ensemble effects, yields sigmaFCA = 7:8 times 10-18 cm2 at 895 nm. This has implications for Si-based laser design at various wavelengths.

The optical properties of Si nanostructures of varying dimensionality, size and fabrication methods is discussed. The Auger coefficient of TiSi2-catalyzed Si nanowires is 2times10-13 cm3/s, lower than that of Si nanoparticles. To avoid ensembles artifacts, fabrication and optical analysis of single nanowires and nanoparticles is presented.

Auger recombination is an important process limiting stimulated optical emission in many semiconductor systems as it is especially prevalent at high excitation intensities. We have quantified the Auger rate in TiSi2-catalyzed Si nanowires by determining an Auger coefficient from time-resolved photoluminescence (PL) data. Above a threshold excitation intensity, the PL decay lifetime at 1.55 eV (800 nm) shortens with increasing excitation, suggesting that Auger recombination is the dominant non-radiative process at high excitation. The determined Auger rates are greater than that for bulk, but an order of magnitude less than that measured for nanoparticles.

Observation of Purcell enhancement, stimulated emission and nonlinear optical effects in microresonators depend critically on their quality factors (Q) and mode volumes (V). Ideally, a high value of Q/V ratio is desired. However, in most systems, there is a trade off between achieving a higher Q and reducing the size of the resonator (small V). For resonators containing Si nanocrystals (nc-Si), the Q-limiting optical loss mechanisms are: (a) the radiation loss resulting from low indices of refraction of the host material, (b) the sidewall scattering loss and (c) the material absorption. Based on these considerations, the aim of the present work is to experimentally characterize the quality factors and mode volumes of microdisk resonators containing luminescent silicon nanocrystals. This study helps us estimate the dimensions of optimal structures needed to observe the abovementioned effects (in particular, the Purcell effect) for nc-Si in the visible wavelength range. We report fabrication and characterization of micron sized, nc-Si coated, silicon nitride microdisk resonators with diameters ranging from 2 mum to 10 mum. Consistent with ray-optics and finite element simulations (FEM), small (diameter < 3 mum) microdisks appear to be limited by radiation loss whereas larger microdisks suffer from material absorption/sidewall scattering loss. These two competing mechanisms lead to a parameter window which achieves maximum Q/V for our system. We use these experimentally obtained values to predict the range of maximum Purcell enhancement achievable in our structures.

Si nanowires grown from TiSi2 catalysts exhibit near-infrared photoluminescence (PL) from quantum-confined excitons in the Si. Temperature dependent studies highlight the differences between Si nanocrystals and nanowires, which allow for exciton diffusion

We demonstrate coupling of room temperature photoluminescence (PL) emission from silicon nanocrystals (nc-Si) embedded in silicon-rich silicon oxide (SRSO) to whispering gallery modes (WGMs) of 10 mum radius microdisk resonators. The resonators were fabricated by wet chemical etching of circular patterns defined by electron beam lithography and display very smooth edges. We observe nearly degenerate counter-propagating WGMs from the microdisks and measure quality factors of Q>3500 for the fundamental mode at 850 nm - limited by the absorption loss from the nanocrystals

Metal-dielectric-metal structures support propagating electromagnetic modes that are strongly confined in the dielectric region. These modes can be used to create omni-directional absorbers, sub-wavelength waveguides, negative refraction lens, and high-dielectric-constant meta-materials