Tunable narrowband and diffuse metasurface thermal emitters based on doped semiconductors

Narrowband and diffuse thermal emitters are promising in applications like infrared gas sensing and thermophotovoltaics. However, current designs for narrowband and diffuse thermal emitters are typically limited to narrow wavelength ranges and cannot be broadly tuned. In this work, we propose a two-dimensional metasurface emitter that demonstrates a narrowband and diffuse emissivity profile, whose resonance wavelength is tunable in a relatively wide spectral range, by using doped semiconductors. More specifically, we have designed a metal-insulator-metal (MIM)-type metasurface composed of square arrays of indium-doped cadmium oxide (In:CdO) nanopatches. As the doped semiconductor exhibits metallic behavior with its permittivity described by the Drude model, the excitation of localized surface plasmon polaritons in these nanopatches deposited on a dielectric thin film, combined with an In:CdO substrate, can lead to a desirable behavior of narrowband and diffuse thermal emission with a high emissivity peak. By changing the doping concentration of carriers in the doped semiconductor, we show the resonance wavelength of emissivity can be flexibly tuned from around 3.5 to 5.5 mu m, without modifying the structural parameters. Moreover, the metasurface can always maintain a diffusive emission profile, whose emissivity peak can persist up to incident angle around 80, at different doping concentrations. Our proposal of broadly tunable narrowband and diffuse thermal emitters can find important applications in the development of novel devices for infrared gas sensing, thermophotovoltaics, radiative cooling , thermal camouflage and infrared detection.