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 ($I\ensuremath{\le}5\ifmmode\times\else\texttimes\fi{}{10}^{12}$ 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 ($\frac{L}{\ensuremath{\lambda}}\ensuremath{\le}0.2$) ($5\ifmmode\times\else\texttimes\fi{}{10}^{12}\mathrm{W}/{\mathrm{cm}}^{2}\ensuremath{\le}I\ensuremath{\le}{10}^{15}\mathrm{W}/{\mathrm{cm}}^{2}$).