Two-photon resonance absorption of relativistic-intensity laser pulses in steep overcritical plasmas

By means of particle-in-cell simulations, a mechanism of the two-photon resonance conversion of the laser energy into electron plasma waves in the interaction of relativistic-intensity laser pulses with overdense laser-induced plasmas is investigated. This conversion is a consequence of the periodic action of the ponderomotive (magnetic) component of Lorentzian force on plasma electrons. When the relativistic-intensity linearly-polarized laser pulse irradiates plasma with the about 4-times overcritical density and steep profile, this action is resonant with electron plasma oscillations at a double-laser frequency. As a result a part of the laser energy is transformed into the energy of plasma waves, hence into the energy of fast electrons. Simulations show that two-photon resonance absorption is efficient when the plasma density scale-length is about or less than the laser wavelength. Moreover, in the case of normal laser incidence the two-photon resonance absorption seems to be the dominant mechanism of overcritical plasma heating.