Non-equilibrium dynamics during warm dense matter formation

The ultrafast absorption of laser energy in condensed matter results in strongly out-of-equilibrium material conditions, which evolve into warm dense matter (WDM). Understanding the fundamental processes of ultrafast energy relaxation and structural evolution in these extreme systems is crucial for a wide range of fields, from laser nano-surgery to laser-fusion research. The generally accepted concept for the response of systems irradiated by femtosecond laser pulses is that the optical pulse directly excites electrons, which quickly thermalize, establishing a finite electronic temperature in a few tens of femtoseconds, while the lattice remains cold. The two subsystems then equilibrate through electron-phonon coupling, which is the basic premise of the two-temperature model (TTM). This framework has been widely applied to calculate optical and thermophysical properties of laser-irradiated matter, develop advanced models for thermal and nonthermal melting, and interpret data from various experiments. However, the electronic system's detailed dynamics might be more complex than described by the simple TTM.