Nonlinear ionization mechanism dependence of energy absorption in diamond under femtosecond laser irradiation

We present first-principles calculations for nonlinear photoionization of diamond induced by the intense femtosecond laser field. A real-time and real-space time-dependent density functional theory with the adiabatic local-density approximation is applied to describe the laser-material interactions in the Kohn-Sham formalism with the self-interaction correction. For a certain laser wavelength, the intensity dependence of energy absorption on multiphoton and/or tunnel ionization mechanisms is investigated, where laser intensity regions vary from 10¹² W/cm² to 10¹⁶ W/cm². In addition, the effect of laser wavelength on energy absorption at certain ionization mechanism is discussed when the Keldysh parameter is fixed. Theoretical results show that: (1) at the fixed laser wavelength, the relationship between the energy absorption and laser intensity shows a good fit of E c_MI^N (N is the number of photons absorbed to free from the valence band) when multiphoton ionization dominates; (2) while when tunnel ionization becomes significant, the relationship coincides with the expression of E c_TIⁿ (n N).