Direct holographic imaging of ultrafast laser damage process in thin films.

Dynamic process of femtosecond laser-induced damage formation in dielectric thin films is reconstructed from a series of time-resolved images. Ta2O5 single-layer coatings of four different thicknesses have been investigated in transmission mode by means of time-resolved off-axis digital holography. Different processes overlapped in time were found to occur; namely, the Kerr effect, free-electron generation, ultrafast lattice heating, and shockwave generation. The trends in contribution of these effects are qualitatively reproduced by numerical models based on electron-rate equations and Drude theory, which take into account transient changes in the films and interference effects of the pump and probe pulses.