Conventional optical devices use geometry and refractive index of materials as design tools. The real part of the refractive index is often the only significant part used in designs. But, the imaginary index is also an equally important design parameter. Here, we demonstrate a novel thermal light source using the imaginary refractive index as a design parameter. Employing non-Hermitian physics, we demonstrate a directional thermal emitter that suppresses thermal emission on one side of the metasurface in a desired spectral window for enhancing thermal imaging through it. In another case, we build an extremely spectrally selective thermal emitter for thermophotovoltaic conversion of heat to electricity. We demonstrate that the non-Hermitian thermal emitter greatly enhances conversion efficiency.
Aligned carbon nanotubes (CNTs) make a promising platform for thermal radiation applications due to their broadband IR hyberbolic dispersion and their ability to withstand high temperatures. However, their temperature dependent optical properties remain to be explored. Previously, the thermal stability of CNTs have been studied in helium and hydrogen atmospheres and vacuum, yet ambient air is yet to be explored. Here, we study optical properties of aligned CNTs at high temperatures in ambient air. We show that these films can withstand high temperatures when coated with a thin layer of dielectric and exhibit broadband IR hyperbolic dispersion at elevated temperatures.
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