Absorption of an ultrashort laser pulse in very steep plasma density gradients

The authors present calculations of the absorption of an ultrashort laser pulse in various plasma density profiles, with density scale lengths less than or comparable to the wavelength, for various angles of incidence, and for both s and p polarizations. This simple model is used to infer submicron plasma-density-gradient scale lengths from measurements of the variation of absorption with angle and polarization, for data obtained with plasmas created by 1-ps, 1.06-μm laser pulses. These experimental results, obtained with a short pulse (1 ps) incident on solid matter, are analyzed between 5×10¹² W/cm² and 5×10¹⁴ W/cm². It is found that in this intensity range, where the interaction is with a plasma of finite but very steep density gradient and where prepulse and nonlinear effects are weak or negligible, 3×10^-2⩽L/λ⩽0.2