Short-Pulse Laser Absorption in Very Steep Plasma Density Gradients

We have measured the absorption of 1-ps laser pulses interacting with matter at intensities from ${10}^{10}$ to ${10}^{16}$ W/${\mathrm{cm}}^2$. The variations of absorption with incidence angle and polarization have been used to infer submicron plasma-density-gradient scale lengths. The results show a transition between a regime of laser interaction with sharply bounded dense cold matter ($I\le 5\times {10}^{12}$ W/${\mathrm{cm}}^2$), where absorption is by the usual skin depth effect, to a regime of interaction with a plasma of very steep density gradient ($\frac{L}{\lambda }\le 0.2$) ($5\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2\le I\le {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$).