Reflectivity of steep-gradient plasmas in intense subpicosecond laser pulses

The wave equation for laser light incident on a static, steep-gradient plasma has been solved numerically, with the inclusion of an intensity-dependent electron-ion collision rate, and incorporating spatial dispersion and wave-breaking effects. At moderate intensities (∼${10}^{14}$ W/${\mathrm{cm}}^2$), the nonlinearity has little effect on the calculated reflectivity, but at higher intensities the reflectivity is greater than that predicted by a linear model. The discrepancy is small for a steplike plasma (with a density gradient scale length L∼0.01${\lambda }_0$), but increases for longer scale lengths (L∼${\lambda }_0$). In the case of p-polarized light, the angular dependence of reflectivity is sharpened at higher intensities, and resonance effects play an important role even for very short scale lengths.