Conservation of photon rate in endothermic photoluminescence and its transition to thermal emission

Photoluminescence (PL) is a fundamental light–matter interaction that conventionally involves the absorption of an energetic photon, thermalization, and the emission of a redshifted photon. Conversely, in optical refrigeration, the absorption of a low-energy photon is followed by endothermic PL of an energetic photon. These two quantum processes are, in contrast to thermal emission, governed by photon-rate conservation. Thus far, both aspects of PL have been studied under thermal population that is far weaker than the photonic excitation, hiding the role of rate conservation when thermal excitation is significant. Here we theoretically and experimentally study endothermic PL at high temperatures. In contrast to thermal emission, we find that the PL photon rate is conserved with temperature increase, while each photon is blueshifted. Further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also demonstrate how endothermic PL generates orders of magnitude more energetic photons than thermal emission at similar temperatures. These new findings show that endothermic PL is an ideal optical heat pump. This opens the way for a proposed novel device that harvests thermal losses in photovoltaics with record efficiency.