Efficient 10-fold upconversion through steady-state non-thermal-equilibrium excitation

Frequency upconversion of a few low-energy photons into a single high-energy photon contributes to imaging, light sources, and detection. However, the upconverting of many photons exhibits negligible efficiency. Upconversion 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 upconverted emission; however, so far such "hot-carriers" have been observed only in downconversion processes and as having negligible lifetime, due to fast thermalization. Here we experimentally demonstrate upconversion by excitation of a steady-state non-thermal-equilibrium population, which induces steady, narrow emission at a practical bulk temperature. Specifically, we used a 10.6 μm laser to generate 980 nm narrow emission with 4% total efficiency and upconverted radiance that far exceeds the device's possible blackbody radiation. This opens the way for the development of new light sources with record efficiencies.