Media 2: Dielectric based resonant guided wave networks

Originally published in Optics Express on 07 May 2012 (oe-20-10-10674

Media 1: Dielectric based resonant guided wave networks

Originally published in Optics Express on 07 May 2012 (oe-20-10-10674

Media 3: Dielectric based resonant guided wave networks

Originally published in Optics Express on 07 May 2012 (oe-20-10-10674

Nanophotonic Heterostructures for Efficient Propulsion and Radiative Cooling of Relativistic Light Sails

Light sails propelled by radiation pressure from high-power lasers have the potential to achieve relativistic spaceflight. In order to propel a spacecraft to relativistic speeds, an ultrathin, gram-sized light sail will need to be stably accelerated by lasers with ∼MW/cm² intensities operating in the near-infrared spectral range. Such a laser-driven sail requires <i>multiband</i> electromagnetic functionality: it must simultaneously exhibit very low absorptivity in the (Doppler-broadened) laser beam spectrum in the near-infrared and high emissivity in the mid-infrared for efficient radiative cooling. These engineering challenges present an opportunity for nanophotonic design. Here, we show that designed thin-film heterostructures could become multifunctional building-block elements of the light sail, due to their ability to achieve substantial reflectivity while maintaining low absorption in the near-infrared, significant emissivity in the mid-infrared, and a very low mass. For a light sail carrying a payload, we propose a relevant figure of meritthe reflectivity adjusted area densitythat can capture the trade-off between sail mass and reflectivity, independent of other quantities such as the incident beam power, phased array size, or the payload mass. Furthermore, we present designs for effective thermal management via radiative cooling and compare propulsion efficiencies for several candidate materials, using a general approach that could apply to a broad range of high-power laser propulsion problems

Color Imaging <i>via</i> Nearest Neighbor Hole Coupling in Plasmonic Color Filters Integrated onto a Complementary Metal-Oxide Semiconductor Image Sensor

State-of-the-art CMOS imagers are composed of very small pixels, so it is critical for plasmonic imaging to understand the optical response of finite-size hole arrays and their coupling efficiency to CMOS image sensor pixels. Here, we demonstrate that the transmission spectra of finite-size hole arrays can be accurately described by only accounting for up to the second nearest-neighbor scattering-absorption interactions of hole pairs, thus making hole arrays appealing for close-packed color filters for imaging applications. Using this model, we find that the peak transmission efficiency of a square-shaped hole array with a triangular lattice reaches ∼90% that of an infinite array at an extent of ∼6 × 6 μm², the smallest size array showing near-infinite array transmission properties. Finally, we experimentally validate our findings by investigating the transmission and imaging characteristics of a 360 × 320 pixel plasmonic color filter array composed of 5.6 × 5.6 μm² RGB color filters integrated onto a commercial black and white 1/2.8 in. CMOS image sensor, demonstrating full-color high resolution plasmonic imaging. Our results show good color fidelity with a 6-color-averaged color difference metric (Δ<i>E</i>) in the range of 16.6–19.3, after white balancing and color-matrix correcting raw images taken with f-numbers ranging from 1.8 to 16. The integrated peak filter transmission efficiencies are measured to be in the 50% range, with a FWHM of 200 nm for all three RGB filters, in good agreement with the spectral response of isolated unmounted color filters

Solar Cell Light Trapping beyond the Ray Optic Limit

In 1982, Yablonovitch proposed a thermodynamic limit on light trapping within homogeneous semiconductor slabs, which implied a minimum thickness needed to fully absorb the solar spectrum. However, this limit is valid for geometrical optics but not for a new generation of subwavelength solar absorbers such as ultrathin or inhomogeneously structured cells, wire-based cells, photonic crystal-based cells, and plasmonic cells. Here we show that the key to exceeding the conventional ray optic or so-called ergodic light trapping limit is in designing an elevated local density of optical states (LDOS) for the absorber. Moreover, for any semiconductor we show that it is always possible to exceed the ray optic light trapping limit and use these principles to design a number of new solar absorbers with the key feature of having an elevated LDOS within the absorbing region of the device, opening new avenues for solar cell design and cost reduction

DFT Study of Water Adsorption and Decomposition on a Ga-Rich GaP(001)(2×4) Surface

We investigate the adsorption and decomposition states of a water molecule on a Ga-rich GaP(001)(2×4) surface using the PBE flavor of density functional theory (DFT). We selected the GaP(001)(2×4) mixed dimer surface reconstruction model to represent the Ga-rich GaP(001)(2×4) surface. Because our focus is on reactions between a single water molecule and the surface, the surface water coverage is kept at 0.125 ML, which corresponds to one water molecule in the (2×4) unit cell. We report here the geometries and energies for an exhaustive set of adsorption and decomposition states induced by a water molecule on the (2×4) unit cell. Our results support a mechanism in which (1) the first step is the <i>molecular adsorption</i>, with the water molecule forming a Lewis acid–Lewis base bond to the sp² Ga atom of either the first-layer Ga–P mixed dimer or the second layer Ga–Ga dimers using an addition reaction, (2) which is followed by dissociation of the adsorbed H₂O to form the <i>HO/H decomposition state</i> in which the hydroxyl moiety bonds with surface sp² Ga atoms, while the hydrogen moiety binds with the first-layer P atom, (3) which is followed by the <i>O/2H decomposition state</i>, in which the oxygen moiety forms bridged Ga–O–Ga structures with surface Ga dimers while one H bonds with the first-layer P atom and the other to surface sp² Ga atoms. (4) We find that driving off the hydrogen as H₂ leads to the <i>surface oxide state</i>, bridged Ga–O–Ga structures. This surface oxide formation reaction is exothermic relative to the energy of H₂O plus the reconstructed surface. These results provide guidelines for experiments and theory to validate the key steps and to obtain kinetics data for modeling the growth processes

Excitonic Effects in Emerging Photovoltaic Materials: A Case Study in Cu₂O

Excitonic effects account for a fundamental photoconversion and charge transport mechanism in Cu₂O; hence, the universally adopted “free carrier” model substantially underestimates the photovoltaic efficiency for such devices. The quasi-equilibrium branching ratio between excitons and free carriers in Cu₂O indicates that up to 28% of photogenerated carriers during photovoltaic operation are excitons. These large exciton densities were directly observed in photoluminescence and spectral response measurements. The results of a device physics simulation using a model that includes excitonic effects agree well with experimentally measured current–voltage characteristics of Cu₂O-based photovoltaics. In the case of Cu₂O, the free carrier model underestimates the efficiency of a Cu₂O solar cell by as much as 1.9 absolute percent at room temperature

Design of Nanostructured Solar Cells Using Coupled Optical and Electrical Modeling

Nanostructured light trapping has emerged as a promising route toward improved efficiency in solar cells. We use coupled optical and electrical modeling to guide optimization of such nanostructures. We study thin-film n-i-p a-Si:H devices and demonstrate that nanostructures can be tailored to minimize absorption in the doped a-Si:H, improving carrier collection efficiency. This suggests a method for device optimization in which optical design not only maximizes absorption, but also ensures resulting carriers are efficiently collected

Profiling Photoinduced Carrier Generation in Semiconductor Microwire Arrays via Photoelectrochemical Metal Deposition

Au was photoelectrochemically deposited onto cylindrical or tapered p-Si microwires on Si substrates to profile the photoinduced charge-carrier generation in individual wires in a photoactive semiconductor wire array. Similar experiments were repeated for otherwise identical Si microwires doped to be n-type. The metal plating profile was conformal for n-type wires, but for p-type wires was a function of distance from the substrate and was dependent on the illumination wavelength. Spatially resolved charge-carrier generation profiles were computed using full-wave electromagnetic simulations, and the localization of the deposition at the p-type wire surfaces observed experimentally correlated well with the regions of enhanced calculated carrier generation in the volumes of the microwires. This technique could potentially be extended to determine the spatially resolved carrier generation profiles in a variety of mesostructured, photoactive semiconductors