thermochemical, and solar thermophotovoltaics. There exist a range of solutions with high absorptivity for low and intermediate temperatures. [1][2][3] However, for many applications, high operating temperatures (>700 K) are advantageous to achieve higher system effi ciencies. Conventional absorbers are unsuitable at these high operating temperatures since there are more considerations to be taken into account. [ 4 ] Firstly, the materials and structures need to be thermally stable and to maintain their optical properties at these high temperatures. Refractory metals are most advantageous due to their high melting point and low vapor pressure. Secondly, it is crucial that the absorber exhibits spectrally selective absorptance; namely high absorptivity in the shorter wavelength range to absorb most of the solar spectrum and low absorptivity (i.e., emissivity) in the longer wavelength range to minimize losses due to re-emission. Furthermore, this selectivity, i.e., the spectral range of high and low absorptivity, has to be tailored for the specifi c system operating conditions to achieve maximum system effi ciency.It is therefore advantageous to use PhCs which offer the possibility to tailor the spectral absorptance [ 5,6 ] and thus optimize system effi ciency. Several absorbers based on 1D multilayer stacks, [ 7,8 ] 2.5D structures such as pyramids, [9][10][11] 3D PhCs in refractory metals, [ 12,13 ] as well as metamaterials [14][15][16] have been proposed. Here, we demonstrate the suitability of a 2D PhC comprising a square lattice of cylindrical cavities etched into a Ta substrate as a highly effi cient and selective absorber at high temperatures, i.e., above 1000 K. While all of the above approaches achieve good spectral selectivity, the 2D PhC design is a compact and thermally robust structure, minimizing the number of interfaces as compared to multilayer or 3D PhC approaches which is crucial for high temperature stability. At the same time, fabrication is simple and scalable and can be achieved by standard semiconductor processes. In this 2D PhC design, the absorptivity of the material is selectively enhanced by the introduction of cavity modes and the spectral range of enhancement, i.e., high absorptivity, can be tuned A high-temperature stable solar absorber based on a metallic 2D photonic crystal (PhC) with high and tunable spectral selectivity is demonstrated and optimized for a range of operating temperatures and irradiances. In particular, a PhC absorber with solar absorptance α α = = 0.86 and thermal emittance ε ε = 0.26 at 1000 K, using high-temperature material properties, is achieved resulting in a thermal transfer effi ciency more than 50% higher than that of a blackbody absorber. Furthermore, an integrated double-sided 2D PhC absorber/ emitter pair is demonstrated for a high-performance solar thermophotovoltaic (STPV) system. The 2D PhC absorber/emitter is fabricated on a double-side polished tantalum substrate, characterized, and tested in an experimental STPV setup along with a fl at Ta absorber...

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Articles you may be interested inFabrication of transferrable, fully suspended silicon photonic crystal nanomembranes exhibiting vivid structural color and high-Q guided resonanceThe authors present highly selective emitters based on two-dimensional tantalum (Ta) photonic crystals, fabricated on 2 in. polycrystalline Ta substrates, for high-temperature applications, e.g., thermophotovoltaic energy conversion. In this study, a fabrication route facilitating large-area photonic crystal fabrication with high fabrication uniformity and accuracy, based on interference lithography and reactive ion etching is discussed. A deep reactive ion etch process for Ta was developed using an SF 6 =C 4 F 8 based Bosch process, which enabled us to achieve $8:5 lm deep cavities with an aspect ratio of $8, with very steep and smooth sidewalls. The thermal emitters fabricated by this method show excellent spectral selectivity, enhancement of the emissivity below cut-off approaching unity, and a sharp cut-off between the high emissivity region and the low emissivity region, while maintaining the low intrinsic emissivity of bare Ta above the cut-off wavelength. The experimental results show excellent agreement with numerical simulations.

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 HfO 2 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. V

A two-dimensional superlattice metallic photonic crystal (PhC) and its fabrication by nanoimprint lithography on tantalum substrates are presented. The superior tailoring capacity of the superlattice PhC geometry is used to achieve spectrally selective solar absorbtion optimized for high temperature and high efficiency solar energy conversion applications. The scalable fabrication route by nanoimprint lithography allows for a highthroughput and high-resolution replication of this complex pattern over large areas. Despite the high fill factor, the pattern of polygonal cavities is accurately replicated into a resist that hardens under ultra-violet radiation over an area of 10 mm 2 . In this way, cavities of 905 nm and 340 nm width are achieved with a period of 1 µm. After pattern transfer into tantalum via a deep reactive ion etching process, the achieved cavities are 2.2 µm deep, separated by 85-95 nm wide ridges with vertical sidewalls. The room temperature reflectance spectra of the fabricated samples show excellent agreement with simulated results, with a high spectral absorptance approaching blackbody absorption in the range from 300-1900 nm, and a steep cut-off. The calculated solar absorptivity of this superlattice PhC is 96% and its thermal transfer efficiency is 82.8% at an operating temperature of 1500 K and an irradiance of 1000 kW/m2. © 2015 Optical Society of America In the last decade, the field of high-temperature photonics is growing rapidly due to an increased scientific interest as well as emerging applications, especially in the field of energy conversion. In this field, the challenges photonic components have to meet are high temperature stability over long lifetimes, high tailoring capacity of the optical properties, and economic fabrication over large areas. In this study, a superlattice PhC consisting of polygonal cavities gave superior control over the spectral properties to achieve a highly selective solar absorber with low thermal emission for high temperature energy conversion applications. For the first time, a metallic superlattice PhC absorber was fabricated by nanoimprint lithography (NIL) as a highthroughput, high-resolution technique paving the way for complex high-temperature photonic components on a large scale. Selective thermal absorbers and emitters are critical components for high temperature and high-efficiency energy conversion applications, such as thermophotovoltaics (TPV), solar TPV, and solar thermal systems. TPV is a thermal-to-electrical energy conversion scheme where thermal emission from a hot radiation source (emitter) drives a suitable photovoltaic cell, promising low maintenance, scalability, high power densities as well as flexibility regarding the employed fuel. In solar TPV (STPV), the irradiation from the sun on an absorber is converted into narrow-band thermal radiation on the emitter side. 1 To reach the high efficiencies predicted by theoretical studies 2-4 it is crucial to (1) reach high operating temperatures (>1000 K) and (2) to employ spectrally select...