Can thermal emission from time-varying media be described semiclassically?

Time-varying media, i.e., materials whose properties dynamically change in time, have opened new possibilities for thermal emission engineering by lifting the limitations imposed by energy conservation and reciprocity, and providing access to nonequilibrium dynamics. In addition, quantum effects, such as vacuum amplification and emission at zero temperature, have been predicted for time-varying media, reopening the debate on the quantum nature of thermal emission. Here, we derive a semiclassical theory to thermal emission from time-varying media based on fluctuational electrodynamics, and compare it to the quantum theory. Our results show that a quantum theory is needed to correctly capture the contribution from quantum vacuum amplifications effects, which can be relevant even at room temperature and mid-infrared frequencies. Finally, we propose corrections to the standard semiclassical theory that enable the prediction of thermal emission from time-varying media with classical tools.