Enhancement of the absorption of intense visible femtosecond laser pulses in a silver film

The transmission coefficient of a 30-nm-thick silver film for single visible laser pulses with a wavelength of 515 nm and a duration of 300 fs decreases steadily with an increase in the peak intensity of the pulses, which means that the absorption coefficient of the material increases accordingly. Taking into account the high initial density of free s electrons in the material, the heating of the electron gas corresponds to a monotonic increase in the absorption coefficient of silver as a function of the electron temperature. At the same time, photoinjection of additional nonequilibrium free carriers ($\sim$10$^{22}$ cm$^{-3}$) caused by multiphoton $d$–$s$ interband transitions from quasibound $d$ states lying below the Fermi level can also be successfully invoked to qualitatively explain the dependence of the absorption coefficient on the peak intensity of the laser pulses. It is shown by the example of silver that, in order to describe changes in the optical properties of transition metals during an ultrashort excitation laser pulse, one has to take into account both the interband photoinjection of additional carriers and the characteristics of the resulting nonequilibrium states of the electron gas. The relative contributions from these two factors are largely determined by the relation between the width of the band gap between the Fermi level in the s band and the top of the d bands and the photon energy.