Spectral narrowing of sub-bandgap absorbance and emissivity in highly doped silicon

Doping-engineered silicon is a vertatile material, well-suitable for exploring new optical properties and applications in infrared spectral region below its bandgap. In this paper, we report on a doping-induced spectral narrowing phenomenon in silicon's sub-bandgap absorbance and emissivity. By measuring silicon samples with doping concentration varied between <10(12) cm(-3) and 4.4 x 10(19) cm(-3), we reveal that, besides the increased amplitude in absorption and emissivity, doping induces a spectral narrowing feature in both mid-infrared absorption and emission, which indicates a trade-off existing for silicon doping level in consideration of efficiency and bandwidth. The spectral narrowing effect occurs around the plasma wavelength, where the silicon changes from dielectric-like property to metal-like property with a negative permittivity. These results are helpful in further understanding free-carrier responses in silicon, and are potentially useful for developing silicon-based materials for mid-infrared applications, such as broadband bolometric sensing, thermal energy harvesting and radiative cooling etc.