Veronika Stelmakh

High-temperature tantalum tungsten alloy photonic crystals: Stability, optical properties, and fabrication

We demonstrate tantalum-tungsten (Ta-W) solid solution alloy photonic crystals (PhCs) as spectrally selective components for high temperature energy conversion. The thermo-mechanical properties of the alloy are tuned by the Ta-W ratio. A 2D PhC was designed as a selective emitter, fabricated on a Ta3%W substrate, and optical properties and thermal stability were characterized. A thin layer of HfO2 was deposited for thermal stability. The PhCs show outstanding emittance selectivity, well preserved after annealing for 24h at 1200 °C. The structure is preserved as shown in cross-sectional images, demonstrating that the coating effectively prevents degradation due to surface diffusion at high temperatures.

Structural Colors from Fano Resonances

Structural coloration is an interference phenomenon where colors emerge when visible light interacts with nanoscopically structured material and has recently become a most interesting scientific and engineering topic. However, current structural color generation mechanisms either require thick (compared to the wavelength) structures or lack dynamic tunability. This report proposes a new structural color generation mechanism that produces colors by the Fano resonance effect on thin photonic crystal slab. We experimentally realize the proposed idea by fabricating the samples that show resonance-induced colors with weak dependence on the viewing angle. Finally, we show that the resonance-induced colors can be dynamically tuned by stretching the photonic crystal slab fabricated on an elastic substrate.

Photonic crystal enhanced silicon cell based thermophotovoltaic systems.

We report the design, optimization, and experimental results of large area commercial silicon solar cell based thermophotovoltaic (TPV) energy conversion systems. Using global non-linear optimization tools, we demonstrate theoretically a maximum radiative heat-to-electricity efficiency of 6.4% and a corresponding output electrical power density of 0.39 W cm(-2) at temperature T = 1660 K when implementing both the optimized two-dimensional (2D) tantalum photonic crystal (PhC) selective emitter, and the optimized 1D tantalum pentoxide - silicon dioxide PhC cold-side selective filter. In addition, we have developed an experimental large area TPV test setup that enables accurate measurement of radiative heat-to-electricity efficiency for any emitter-filter-TPV cell combination of interest. In fact, the experimental results match extremely well with predictions of our numerical models. Our experimental setup achieved a maximum output electrical power density of 0.10W cm(-2) and radiative heat-to-electricity efficiency of 1.18% at T = 1380 K using commercial wafer size back-contacted silicon solar cells.

Evolution of sputtered tungsten coatings at high temperature

Sputtered tungsten (W) coatings were investigated as potential high temperature nanophotonic material to replace bulk refractory metal substrates. Of particular interest are materials and coatings for thermophotovoltaic high-temperature energy conversion applications. For such applications, high reflectance of the substrate in the infrared wavelength range is critical in order to reduce losses due to waste heat. Therefore, the reflectance of the sputtered W coatings was characterized and compared at different temperatures. In addition, the microstructural evolution of sputtered W coatings (1 and 5 μm thick) was investigated as a function of anneal temperature from room temperature to 1000 °C. Using in situ x-ray diffraction analysis, the microstrain in the two samples was quantified, ranging from 0.33% to 0.18% for the 1 μm sample and 0.26% to 0.20% for the 5 μm sample, decreasing as the temperature increased. The grain growth could not be as clearly quantified due to the dominating presence of microstrai...

Sputtered Tantalum Photonic Crystal Coatings for High-Temperature Energy Conversion Applications

Thick sputtered tantalum (Ta) photonic crystal (PhC) coatings on Inconel were investigated as a potential replacement for bulk refractory metal substrates used for high-temperature emitters and absorbers in thermophotovoltaic energy conversion applications, where high-temperature stability and high reflectance of the surface in the infrared wavelength range are critical in order to sustain high operational temperatures and reduce losses due to waste heat. A selective emitter and solar absorber 2D PhC were fabricated in 8 and 30 micron sputtered Ta coatings, respectively, using standard semiconductor processes as a proof of concept. The fabricated PhCs showed high spectral selectivity in good agreement with the numerical simulations. The PhCs, coated with a thin HfO2 protective layer, sustained one hour anneals at 700, 900, and 1100°C with very little structural degradation or change in their optical properties. This study presents a promising alternative to bulk substrates as a relatively low-cost and easily integrated platform for nano-structured devices for high-temperature applications.

All-optical regenerator of multi-channel signals

One of the main reasons why nonlinear-optical signal processing (regeneration, logic, etc.) has not yet become a practical alternative to electronic processing is that the all-optical elements with nonlinear input–output relationship have remained inherently single-channel devices (just like their electronic counterparts) and, hence, cannot fully utilise the parallel processing potential of optical fibres and amplifiers. The nonlinear input–output transfer function requires strong optical nonlinearity, e.g. self-phase modulation, which, for fundamental reasons, is always accompanied by cross-phase modulation and four-wave mixing. In processing multiple wavelength-division-multiplexing channels, large cross-phase modulation and four-wave mixing crosstalks among the channels destroy signal quality. Here we describe a solution to this problem: an optical signal processor employing a group-delay-managed nonlinear medium where strong self-phase modulation is achieved without such nonlinear crosstalk. We demonstrate, for the first time to our knowledge, simultaneous all-optical regeneration of up to 16 wavelength-division-multiplexing channels by one device. This multi-channel concept can be extended to other nonlinear-optical processing schemes.Nonlinear optical processing devices are not yet fully practical as they are single channel. Here the authors demonstrate all-optical regeneration of up to 16 channels by one device, employing a group-delay-managed nonlinear medium where strong self-phase modulation is achieved without nonlinear inter-channel crosstalk.

Performance of tantalum-tungsten alloy selective emitters in thermophotovoltaic systems

A tantalum tungsten solid solution alloy, Ta 3% W, based 2D photonic crystal (PhC) was designed and fabricated for high-temperature energy conversion applications. Ta 3% W presents advantages compared to the non-alloys as it combines the better high-temperature thermomechanical properties of W with the more compliant material properties of Ta, allowing for a direct system integration path of the PhC as selective emitter/absorber into a spectrum of energy conversion systems. Indeed metallic PhCs are promising as high performance selective thermal emitters for thermophotovoltaics (TPV), solar thermal, and solar TPV applications due to the ability to tune their spectral properties and achieve highly selective emission. A 2D PhC was designed to have high spectral selectivity matched to the bandgap of a TPV cell using numerical simulations and fabricated using standard semiconductor processes. The emittance of the Ta 3% WPhC was obtained from near-normal reectance measurements at room temperature before and after annealing at 1200 °C for 24h in vacuum with a protective coating of 40 nm HfO2, showing high selectivity in agreement with simulations. SEM images of the cross section of the PhC prepared by FIB confirm the structural stability of the PhC after anneal, i.e. the coating effectively prevented structural degradation due to surface diffusion. The mechanical and thermal stability of the substrate was characterized as well as the optical properties of the fabricated PhC. To evaluate the performance of the selective emitters, the spectral selectivity and useful emitted power density are calculated as a function of operating temperature. At 1200 °C, the useful emitted irradiance is selectively increased by a factor of 3 using the selective emitter as compared to the non-structured surface. All in all, this paper demonstrates the suitability of 2D PhCs fabricated on polycrystalline Ta-W substrates with an HfO2 coating for TPV applications.

Photonic Crystal Emitters for Thermophotovoltaic Energy Conversion

This paper reports the design, fabrication, and characterization of 2D photonic crystal (PhC) thermal emitters for a millimeter-scale hydrocarbon TPV microgenerator as a possible replacement for batteries in portable microelectronics, robotics, etc. In our TPV system, combustion heats a PhC emitter to incandescence and the resulting radiation is converted by a low-bandgap TPV cell. The PhC tailors the photonic density of states to produce spectrally confined thermal emission that matches the bandgap of the TPV cell, enabling high heat-to-electricity conversion efficiency. The work builds on a previously developed fabrication process to produce a square array of cylindrical cavities in a metal substrate. We will present ongoing incremental improvements in the optical and thermo-mechanical properties, the fabrication process, and the system integration, as recently combined with fabrication using novel materials, such as sputtered coatings, to enable a monolithic system.

Enabling efficient heat-to-electricity generation at the mesoscale

We present a technology that efficiently harnesses the energy content of hydrocarbon fuels in a volume that is only a fraction of a cubic inch. A propane-fueled microcombustor heats a photonic crystal emitter to incandescence and the resulting spectrally-confined thermal radiation drives low-bandgap PV cells to generate electricity. We overcome the technical challenges that are currently limiting thermophotovoltaics in the following ways: we develop new fabrication processes; we adopt high-temperature alloys to improve the thermo-mechanical stability; we adopt commercial polycrystalline tantalum to fabricate large-area photonic crystals; and finally, we develop a passivation coating for improved thermo-chemical stability. We demonstrate unprecedented heat-to-electricity efficiencies exceeding 4%, greater than the 2–3% efficiencies that were previously thought to be the practical limit, and we predict that over 12% efficiency is achievable with only engineering optimization. For reference, a 1.5% efficiency corresponds to the energy density of lithium ion batteries. This work opens new opportunities to free portable electronics, robots, and small drones from the constraints of bulky power sources.

Thick sputtered tantalum coatings for high-temperature energy conversion applications

Thick sputtered tantalum (Ta) coatings on polished Inconel were investigated as a potential replacement for bulk refractory metal substrates used for high-temperature emitters and absorbers in thermophotovoltaic energy conversion applications. In these applications, high-temperature stability and high reflectance of the surface in the infrared wavelength range are critical in order to sustain operational temperatures and reduce losses due to waste heat. The reflectance of the coatings (8 and 30 μm) was characterized with a conformal protective hafnia layer as-deposited and after one hour anneals at 700, 900, and 1100 °C. To further understand the high-temperature performance of the coatings, the microstructural evolution was investigated as a function of annealing temperature. X-ray diffraction was used to analyze the texture and residual stress in the coatings at four reflections (220, 310, 222, and 321), as-deposited and after anneal. No significant changes in roughness, reflectance, or stress were observed. No delamination or cracking occurred, even after annealing the coatings at 1100 °C. Overall, the results of this study suggest that the thick Ta coatings are a promising alternative to bulk substrates and pave the way for a relatively low-cost and easily integrated platform for nanostructured devices in high-temperature energy conversion applications.