Nonlinear inverse bremsstrahlung in solid-density plasmas

The laser intensity dependence of the inverse bremsstrahlung absorption coefficient in a strongly coupled plasma is investigated, where the electron quiver velocity (${\mathit{v}}_0$=${\mathit{eE}}_0$/m${\mathrm{\omega }}_0$) and excursion length (${\mathit{r}}_0$=${\mathit{eE}}_0$/m${\mathrm{\omega }}_0^2$) in the laser electric field are finite in comparison with the electron thermal velocity and shielding distance, respectively. In plasmas produced by an ultrashort pulse laser, the electron and ion temperatures are not in thermal equilibrium, and the electron temperature is higher than the ion temperature. Since ions are strongly coupled in such plasmas, the ion-ion correlation is described by the hypernetted-chain equation. On the other hand, the electron-ion correlation is described by linear-response theory. By using those correlation functions, the high-frequency conductivity and the laser absorption coefficients are evaluated. The results are applied to a recent experiment on reflectivity [Phys. Rev. Lett. 61, 2364 (1988)]. The analysis discloses the laser intensity dependence of the ultrashort-pulse-laser absorption rate.