Dependence of the two-photon absorption coefficient of steel on the pulse duration during its ablation by femto- and picosecond laser pulses

The dependences of the depth of single-pulse ablation of the steel surface on the energy density of ultrashort laser pulses (0.3-12 ps) with a wavelength of 515 nm have the form different from the usual logarithmic one for linear absorption of radiation. Since the density of states of free conduction electrons in the $s$ band is low, and the $d$ band of strongly bound electrons with a high density of states is located around the Fermi level, these dependences have been approximated within the two-photon absorption model. The two-photon absorption coefficient increases almost linearly with the duration of laser pulses, which indicates the saturation of absorption and allows estimating the corresponding parameters of the electronic subsystem.