Nonthermal Dynamics of Dielectric Functions in a Resonantly Bonded Photoexcited Material

A fundamental question of whether it is thermal or nonthermal has often been argued, when the system undergoes an ultrafast phase change by femtosecond laser photoexcitation. One of the difficulties therein is how to consider the thermalization of a photoexcited system, in which the lattice and electron systems are decoupled from an equilibrium state just after photoexcitation. This work shows a methodology to identify a thermalization-time domain of a photoexcited semiconductor, where it can be detected by monitoring the time-resolved photoabsorption broadband spectrum and comparing that to thermally induced spectrum changes, by which the electron-lattice thermalization time is evaluated to be 12 ps for PbTe. Further analyzing the dynamic dielectric functions substantiates that thermalization time is related to the recovery time of the resonant bonding. Such a crucial determination of thermalization time would eliminate an uncertainty of thermal or nonthermal in ultrafast phase change of the resonantly bonded materials.