Nonthermal phase transitions in semiconductors induced by a femtosecond extreme ultraviolet laser pulse

In that paper, we presented a novel theoretical approach, which allowed the study of nonequilibrium dynamics of both electrons and atoms/ions within free-electron laser excited semiconductors at femtosecond time scales.

© 2013, New Journal of Physics

The approach consisted of Monte-Carlo method treating photoabsorption, high-energy-electron and core-hole kinetics and relaxation processes. Low-energy electrons localized within the valence and conduction bands of the target were treated with a temperature equation, including source terms, defined by the exchange of energy and particles with high-energy electrons and atoms. We followed the atomic motion with the molecular dynamics method on the changing potential energy surface. The changes of the potential energy surface and of the electron band structure were calculated at each time step with the help of the tight-binding method. Such a combination of methods enabled investigation of nonequilibrium structural changes within materials under extreme ultraviolet (XUV) femtosecond irradiation. Our analysis performed for diamond irradiated with an XUV femtosecond laser pulse predicted for the first time in this wavelength regime the nonthermal phase transition from diamond to graphite. Similar to the case of visible light irradiation, this transition took place within a few tens of femtoseconds and was caused by changes of the interatomic potential induced by ultrafast electronic excitations. It thus occurred well before the heating stimulated by electron-phonon coupling started to play a role. This allowed us to conclude that this transition was nonthermal and represented a general mechanism of the response of solids to ultrafast electron excitation.

New J. Phys. 15 (2013) 015016