Numerical pump-probe experiments of laser-excited silicon in nonequilibrium phase

We calculate the dielectric response of crystalline silicon following irradiation by a high-intensity laser pulse, modeling the dynamics by the time-dependentKohn-Sham equations in the presence of the laser field. Pump-probe measurements of the response are numerically simulated by including both pump and probe externals fields in the simulation. As expected, the excited silicon shows features of an electron-hole plasma of nonequilibrium phase in its response, characterized by a negative divergence in the real part of the dielectric function at small frequencies. The response to the probe pulse depends on the polarization of the pump pulse. We also find that the imaginary part of the dielectric function can be negative, particularly for the parallel polarization of pump and probe fields. We compare the calculated response with a simple Drude model. The real part of the dielectric function is well fitted by the model, treating the effective mass as a fitting parameter while taking electron density from the calculation. The fitted effective masses are consistent with carrier-band dispersions.