Enhanced high harmonic generation and the phase effect in double-sided relativistic laser-foil interaction

High harmonic generation (HHG) from relativistic laser-foil interaction is investigated analytically and through particle-in-cell simulations. Previous work has shown that when two counter-propagating circularly polarized (CP) laser pulses interact with a thin foil, electrons can be well confined spatially to form a high density layer. The layer electrons oscillate in certain transversal direction and radiate intense high order harmonics. It is demonstrated here that there is a critical foil thickness, only below which can high harmonics be generated efficiently. Furthermore, to enhance the intensity in higher order region, the third linearly polarized (LP) short-wavelength laser pulse with much lower intensity is introduced. Analysis and simulations both show that the enhancement is determined by the relative phase delta phi between the driving CP laser pulses and LP pulse. The enhancement at high order is quite considerable and very sensitive to the relative phase delta phi, thus offering not only a way to efficiently produce HHG but also a new method to measure the phase of intense high-frequency laser pulses. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4796090]