Light down-conversion with over 100% external quantum efficiency in bulk germanium

To bypass indirect bandstructure limitations in Ge and have it effectively emitting, we employ light down conversion process at elevated temperatures (emissivity enhancement technique). By applying short wavelength pump light ( lambda in, interband electron transitions), we enhance thermal emission output in the spectral range of intraband electron transitions ( lambda out, free carrier absorption band). We experimentally realized conditions ( lambda in = 1.9 mu m, lambda out = 2-25 mu m, T = 380 K) when Ge wafers demonstrated power conversion efficiency of 21% and external quantum efficiency (photon multiplication coefficient) of 130%. Advantages of Ge over Si and ways to realize Ge-based radiative cooler are discussed.