Burton Neuner

Metamaterial-based integrated plasmonic absorber/emitter for solar thermo-photovoltaic systems

We present the concept of a solar thermo-photovoltaic (STPV) collection system based on a large-area, nanoimprint-patterned film of plasmonic structures acting as an integrated solar absorber/narrow-band thermal emitter (SANTE). The SANTE film concept is based on integrating broad-band solar radiation absorption with selective narrow-band thermal IR radiation which can be efficiently coupled to a photovoltaic (PV) cell for power generation. By employing a low reflectivity refractory metal (e.g., tungsten) as a plasmonic material, we demonstrate that the absorption spectrum of the SANTE film can be designed to be broad-band in the visible range and narrow-band in the infrared range. A detailed balance calculation demonstrates that the total STPV system efficiency exceeds the Shockley–Queisser limit for emitter temperatures above Te = 1200 K, and achieves an efficiency as high as 41% for Te = 2300 K. Emitter temperatures in this range are shown to be achievable under modest sun concentrations (less than 1000 suns) due to the thermal insulation provided by the SANTE film. An experimental demonstration of the wide-angle, frequency-selective absorptivity is presented.

Large-area, wide-angle, spectrally selective plasmonic absorber

Nanohmics, Inc. 6201 E. Oltorf, Suite 400, Austin, TX 78741(Dated: April 18, 2011)A simple metamaterial-based wide-angle plasmonic absorber is introduced, fabricated, and exper-imentally characterized using angle-resolved infrared spectroscopy. The metamaterials are preparedby nano-imprint lithography, an attractive low-cost technology for making large-area samples. Thematching of the metamaterial’s impedance to that of vacuum is responsible for the observed spec-trally selective “perfect” absorption of infrared light. The impedance is theoretically calculatedin the single-resonance approximation, and the responsible resonance is identified as a short-rangesurface plasmon. The spectral position of the absorption peak (which is as high as 95%) is exper-imentally shown to be controlled by the metamaterial’s dimensions. The persistence of “perfect”absorption with variable metamaterial parameters is theoretically explained. The wide-angle natureof the absorber can be utilized for sub-diffraction-scale infrared pixels exhibiting spectrally selectiveabsorption/emissivity.I. INTRODUCTION

Critically coupled surface phonon-polariton excitation in silicon carbide.

We observe critical coupling to surface phonon-polaritons in silicon carbide by attenuated total reflection of mid-IR radiation. Reflectance measurements demonstrate critical coupling by a double scan of wavelength and incidence angle. Critical coupling occurs when prism coupling loss is equal to losses in silicon carbide and the substrate, resulting in maximal electric field enhancement.

Measurements of the negative refractive index of sub-diffraction waves propagating in an indefinite permittivity medium.

An indefinite permittivity medium (IPM) has been fabricated and optically characterized in mid-infrared spectral range (10.7 µm-11.3 µm). Phase and amplitude transmission measurements reveal two remarkable properties of IPMs: (i) transmission of sub-diffraction waves (as short as λ/4) can exceed those of diffraction-limited ones, and (ii) sub-diffraction waves can propagate with negative refractive index. We describe a novel double-detector optical technique relying on the interference between sub-diffraction and diffraction-limited waves for accurate measurement of the transmission amplitude and phase of the former.

Efficient infrared thermal emitters based on low-albedo polaritonic meta-surfaces

A low-albedo all-semiconductor meta-surface with spectrally selective absorption peaks is demonstrated. By engineering the dimensions and shapes of the semiconductor antennas comprising the meta-surface, simultaneous reduction of reflectivity and enhancement of absorption are accomplished by controlling their electric and magnetic resonances. Thermal emissivity of the silicon carbide-based meta-surface is experimentally measured and found in agreement with both absorption measurements and theoretical predictions.

Optical properties of sub-wavelength hole arrays in SiC membranes

It is shown that perforated SiC membranes can be used for engineering optical properties of metamaterials in the infrared. The complex-valued frequency-dependent effective dielectric permittivityeff(ω) of a single membrane can be controlled by the size and spacing between the holes. Regions of the anomalous dispersion and strong absorption described by � eff(ω) have been identified and related to the excitation of even-parity surface phonon polaritons of a smooth SiC film. The effective permittivity description has been validated by comparing the transmittance and absorbance of the film obtained fromeff(ω) with that calculated using first principles electromagnetic simulations. Theoretical predictions of the enhanced transmission and absorption in the perforated film have been verified experimentally using FTIR micro-spectroscopy. For the first time, the dependence of enhanced transmission and absorption on the incidence plane of the incoming radiation has been studied.

The influence of impurities and planar defects on the infrared properties of silicon carbide films

Two cubic, single crystal silicon carbide (3C-SiC) films with similar thickness are shown to exhibit significantly different optical properties at mid-infrared wavelengths. Depth profiling by time-of-flight secondary ion mass spectroscopy indicates that these two films have substantially different n-type impurity concentrations that are responsible for the observed differences in optical absorption. The influence of impurities manifests as substantially different planar defect morphologies.

A novel method to optimize the wavelength for underwater free-space optical communications

Wirelessly transmitting large volumes of information at high data rates underwater is becoming increasingly important for such applications as environmental monitoring and petroleum exploration and maintenance. Underwater free-space optical (FSO) communication addresses the aforementioned need by providing wireless high-data-rate links. Visible light transmission through seawater typically peaks in the blue-green spectrum (475 nm–575 nm), but local clarity conditions, which are dynamic, strongly influence the actual maximum. We describe the development of a new laser-wavelength auto-selection algorithm and system for optimized underwater FSO communication. This system has the potential to improve underwater optical link reliability for high-data-rate communications. First, we describe the laser system and water tube setup for performing optical experiments. Next, we present research on recreating various seawater types (from clear to turbid) in the laboratory using particle suspensions and dye, which will enable wavelength-dependent transmission tests. Finally, we show experimental results from optical water tube tests, and describe the development of the autoselection algorithm.

Surface wave accelerator based on silicon carbide: theoretical study

Compact near‐field solid‐state accelerating structure powered by a carbon dioxide (CO2) laser is considered. The accelerating luminous transverse magnetic mode is excited in a few‐micron wide evacuated planar channel between two silicon carbide (SiC) films grown on silicon (Si) wafers. Laser coupling to this mode is accomplished through the properly designed Si gratings. Operating wavelength is dictated by the frequency‐dependent dielectric permittivity of SiC and the channel width. The geometric loss factor κ of the accelerating mode is computed. It is found that the unwanted excitation of the guided modes in Si wafers reduces the laser coupling efficiency and increases the fields inside the Si wafer.

Deployable wavelength optimizer for multi-laser sensing and communication undersea

This effort develops and tests algorithms and a user-portable optical system designed to autonomously optimize the laser communication wavelength in open and coastal oceans. In situ optical meteorology and oceanography (METOC) data gathered and analyzed as part of the auto-selection process can be stored and forwarded. The system performs closedloop optimization of three visible-band lasers within one minute by probing the water column via passive retroreflector and polarization optics, selecting the ideal wavelength, and enabling high-speed communication. Backscattered and stray light is selectively blocked by employing polarizers and wave plates, thus increasing the signal-to-noise ratio. As an advancement in instrumentation, we present autonomy software and portable hardware, and demonstrate this new system in two environments: ocean bay seawater and outdoor test pool freshwater. The next generation design is also presented. Once fully miniaturized, the optical payload and software will be ready for deployment on manned and unmanned platforms such as buoys and vehicles. Gathering timely and accurate ocean sensing data in situ will dramatically increase the knowledge base and capabilities for environmental sensing, defense, and industrial applications. Furthermore, communicating on the optimal channel increases transfer rates, propagation range, and mission length, all while reducing power consumption in undersea platforms.