  Photoluminescence (PL) is a fundamental light-matter interaction, which
conventionally involves the absorption of energetic photon, thermalization and
the emission of a red-shifted photon. Conversely, in optical-refrigeration the
absorption of low energy photon is followed by endothermic-PL of energetic
photon. Both aspects were mainly studied where thermal population is far weaker
than photonic excitation, obscuring the generalization of PL and thermal
emissions. Here we experimentally study endothermic-PL at high temperatures. In
accordance with theory, we show how PL photon rate is conserved with
temperature increase, while each photon is blue shifted. Further rise in
temperature leads to an abrupt transition to thermal emission where the photon
rate increases sharply. We also show how endothermic-PL generates orders of
magnitude more energetic photons than thermal emission at similar temperatures.
Relying on these observations, we propose and theoretically study thermally
enhanced PL (TEPL) for highly efficient solar-energy conversion, with
thermodynamic efficiency limit of 70%.
