Intensity-dependent resonance absorption in relativistic laser-plasma interaction

The effect of light intensity on resonance absorption of p-polarization laser pulses in plasma with steep density gradients is studied by particle-in-cell simulations. For given incident angle and density scale length, the absorption rate decreases with an increase of the laser intensity. This can be attributed to a relativistic reduction of the local plasma frequency and thus detuning of the driving laser field from the excited electron plasma waves near the critical surface. With a further increase of the laser intensity, the absorption rate begins to increase since the laser field can now further penetrate into the plasma up to higher densities and drive density cavities and peaks there. As a result, multiple-resonant absorption is found at different locations.