Efficient tenfold up-conversion through steady-state non-thermal-equilibrium excitation

Frequency up-conversion of a few low-energy photons into a single high-energy photon contributes to imaging, light sources, and detection. However, the up-converting of many photons exhibits negligible efficiency. Up-conversion through laser heating is an efficient means to generate energetic photons, yet the spectrally broad thermal-emission and the challenge of operating at high temperatures limit its practicality. Heating specific modes can potentially generate narrow up-converted emission; however, so far such "hot-carriers" have been observed only in down-conversion processes and as having a negligible lifetime, due to fast thermalization. Here we experimentally demonstrate up-conversion by excitation of a steady-state non-thermal-equilibrium population, which induces steady, narrow emission at a practical bulk temperature. Specifically, we used a 10.6um laser to generate 980nm narrow emission with 4% total efficiency and up-converted radiance that far exceeds the device s possible black-body radiation. This opens the way for the development of new light sources with record efficiencies.