Theory for laser-induced ultrafast phase transitions in carbon

The response of carbon to femtosecond laser pulses of arbitrary form, different durations, and different intensities is studied theoretically. We perform molecular dynamics simulations based on a microscopic electronic Hamiltonian. We include in our model the theoretical description of the pulse form, the electron thermalization, and diffusion effects explicitly. We apply our method to diamond and C₆₀ crystals. For the diamond case, we show that a femtosecond laser pulse induces a nonequilibrium transition to graphite, which takes place for a wide range of pulse durations and intensities. This ultrafast collective motion of the atoms occurs within a time scale shorter than 100 fs. The laser-induced melting of a C₆₀ crystal under pressure is also analyzed. In this case, an ultrafast melting of the system occurs. We discuss the mechanisms underlying these nonequilibrium phase transitions.