Nonlinear upconverted thermal emission through difference frequency generation.

Thermal radiation management is of critical importance in energy, sensing, and heat transfer. According to Planck's law, objects at room temperature predominantly emit thermal radiation within the mid- and far-infrared bands. Here, we demonstrated the upconversion of the mid- and far-infrared thermal radiation emitted by second-order nonlinear material to the easily-detectable visible band through a difference frequency process. This nonlinear broad-spectrum upconversion is facilitated by the random quasi-phase-matching technique in the nanoparticle system. Furthermore, we show the temperature measurement of thermal spots using such nonlinear thermal radiation. This scheme paves the way for applications in thermal management and sensing.