Abstract: Excitations of a material using visible light pulses generate electronic heating, which results in transient electronic temperatures largely exceeding that of the underlying atomic lattice. In the case of metals, Allen showed that the subsequent equilibration between the “hot” electrons and “cold” lattice vibrations can be understood with a two-temperature (2T) picture, in which electrons and phonons remain in distinct thermal equilibria.
In this talk, I will show the limitations of this physical picture when applied to semiconductors and low-dimensional materials — materials with reduced dielectric screening, anisotropy, and, in some cases, higher lattice thermal conductivity. Based on first-principles calculations and the semiclassical Boltzmann transport equations for electrons and phonons, I will propose a generalized 2T model that captures the full thermal relaxation of hot electrons and holes, and I will discuss its consequences on measuring electron-phonon and phonon-phonon couplings from time-resolved spectroscopy experiments. Finally, I will show how such findings can be used to generate nonclassical electron-induced heat, which can be observed in experiments.