Thermally Tunable Angular Selectivity of Broadband Directional Thermal Emission.

Tuning the spatial extent of directional thermal emission across arbitrary spectral bandwidths is a challenging but critically important capability for a range of emerging applications such as thermophotovoltaics, thermal imaging, and radiative cooling. However, previous experimental demonstrations have been limited to narrow bandwidths, with the wavelengths of high emissivity themselves changing significantly as the directional response is reconfigured. Here, we present a thermally tunable photonic platform using InAs-based gradient epsilon-near-zero (ENZ) materials, functioning as a broadband directional thermal emitter whose angular extent of "thermal beaming", and thus its angular selectivity, can be gradually reconfigured while maintaining the spectral response over a prescribed emissivity bandwidth via thermal free-carrier effects. We experimentally demonstrate two emitters achieving an 8° and 14° increase in the angular extent of their directional emissivity in the p-polarization across a fixed, broad wavelength range of operation (12.5 to 15 μm), for moderate temperatures below 450 K.