Controlling thermal emission with refractory epsilon-near-zero metamaterials via topological transitions.

Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO2 metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero frequency of a metamaterial and the connected optical topological transition (OTT) are adjusted to selectively enhance and suppress the thermal emission in the near-infrared spectrum, crucial for improved TPV efficiency. The near-omnidirectional and spectrally selective emitter is obtained as the emission changes due to material properties and not due to resonances or interference effects, marking a paradigm shift in thermal engineering approaches. We experimentally demonstrate the OTT in a thermally stable metamaterial at high temperatures of 1,000 °C.

Tungsten band edge absorber/emitter based on a monolayer of ceramic microspheres.

We report on a band edge absorber/emitter design for high-temperature applications based on an unstructured tungsten substrate and a monolayer of ceramic microspheres. The absorber was fabricated as a monolayer of ZrO(2) microparticles on a tungsten layer with a HfO(2) nanocoating. The band edge of the absorption is based on critically coupled microsphere resonances. It can be tuned from visible to near-infrared range by varying the diameter of the microparticles. The absorption properties were found to be stable up to 1000°C.

Diamond photonic crystals for the IR spectral range.

2D photonic crystals formed inside monocrystalline diamond to operate in the IR spectral range are reported. The photonic structures consisting of 150-µm-long graphitized wires arranged in a square matrix with a period of 4 µm were produced by laser writing with ultrashort pulses. Transmittance spectra (λ=1-14 µm) measured for the structures with increasing thickness demonstrate the occurrence of few minima being different for TM and TE polarization modes. Complex refraction index of the laser-modified material was evaluated for the first time in order to be used in computer simulation of the structures.