Increasing the yield of X-ray and THz radiation under the action of femtosecond laser pulses on a thin foil

We demonstrate experimentally that when a copper foil is exposed to femtosecond sub-relativistic laser pulses, the intensities characteristic X-rays (XRs), terahertz (THz) radiation, and the second optical harmonic increase simultaneously with decreasing foil thickness. The generation efficiency of XRs and THz radiation, as well as of the 3/2 harmonic, depends with different degrees of nonlinearity on the laser pulse intensity and duration. The general behaviour of the measured signals is determined by the two-plasmon decay instability, the contribution of hot electrons and their recirculation in the observed processes.