The nascent field of high-temperature nanophotonics could potentially enable many important solid-state energy conversion applications, such as thermophotovoltaic energy generation, selective solar absorption, and selective emission of light. However, special challenges arise when trying to design nanophotonic materials with precisely tailored optical properties that can operate at hightemperatures (>1,100 K). These include proper material selection and purity to prevent melting, evaporation, or chemical reactions; severe minimization of any material interfaces to prevent thermomechanical problems such as delamination; robust performance in the presence of surface diffusion; and long-range geometric precision over large areas with severe minimization of very small feature sizes to maintain structural stability. Here we report an approach for high-temperature nanophotonics that surmounts all of these difficulties. It consists of an analytical and computationally guided design involving high-purity tungsten in a precisely fabricated photonic crystal slab geometry (specifically chosen to eliminate interfaces arising from layer-by-layer fabrication) optimized for high performance and robustness in the presence of roughness, fabrication errors, and surface diffusion. It offers nearultimate short-wavelength emittance and low, ultra-broadband long-wavelength emittance, along with a sharp cutoff offering 4∶1 emittance contrast over 10% wavelength separation. This is achieved via Q-matching, whereby the absorptive and radiative rates of the photonic crystal's cavity resonances are matched. Strong angular emission selectivity is also observed, with shortwavelength emission suppressed by 50% at 75°compared to normal incidence. Finally, a precise high-temperature measurement technique is developed to confirm that emission at 1,225 K can be primarily confined to wavelengths shorter than the cutoff wavelength. E ver since photonic bandgaps were predicted to exist in appropriately designed periodic subwavelength structures (1-3)-i.e., photonic crystals (PhCs), significant interest has garnered in recent years to exploit this property. Coupled with the recent advancements in nanofabrication techniques, many applications have been made possible, ranging from room-and cryogenic-temperature optoelectronic devices for development of all-optical integrated circuits (4), to highly sensitive sensors (5), low-threshold lasers (6), and highly efficient light emitting diodes (7). PhCs also enable us to accurately control spontaneous emission by virtue of controlling the photonic bandgap (1, 8). In particular, metallic PhCs have been shown to possess a large bandgap (9-12) and consequently superior modification of the intrinsic thermal emission spectra is readily achievable. This is extremely promising for many unique applications, especially high-efficiency energy conversion systems encompassing hydrocarbon and radioisotope fueled thermophotovoltaic (TPV) energy conversion (13,14) as well as solar selective absorbers and emitters for the...

We develop a model for predicting the thermal emission spectrum of a 2D metallic photonic crystal for arbitrary angles based on coupled-mode theory. Calculating the appropriate coupled-mode parameters over a range of geometrical parameters allows one to tailor the emissivity spectrum to a specific application. As an example, we design an emitter with a step-function cutoff suppressing long-wavelength emission, which is necessary for high-efficiency thermophotovoltaic systems. We also confirm the accuracy of the results of our model with finite-difference time-domain simulations.

Selective solar absorbers generally have limited effectiveness in unconcentrated sunlight, because of reradiation losses over a broad range of wavelengths and angles. However, metamaterials offer the potential to limit radiation exchange to a proscribed range of angles and wavelengths, which has the potential to dramatically boost performance. After globally optimizing one particular class of such designs, we find thermal transfer efficiencies of 78% at temperatures over 1,000°C, with overall system energy conversion efficiencies of 37%, exceeding the Shockley-Quiesser efficiency limit of 31% for photovoltaic conversion under unconcentrated sunlight. This represents a 250% increase in efficiency and 94% decrease in selective emitter area compared to a standard, angular-insensitive selective absorber.PACS: 42.70.Qs; 81.05.Xj; 78.67.Pt; 42.79.Ek