Generation of extreme-ultraviolet and x-ray light from a propagating nanometer electron layer in few-cycle laser interaction with solid targets

A new generation mechanism of coherent extreme-ultraviolet (XUV) and x-ray radiation presents in the few-cycle laser interaction with solid density plasma. Two-dimensional simulations show that the XUV and x-rays intensities do not have power law or exponential law dependence on the frequency which is followed by the high harmonic spikes generated through the coherent-wake-emission or relativistically oscillating mirror processes. The XUV and x rays are actually nonlinearly scattered by the successively propagating nanometer electron layers formed in the combined effects of the ponderomotive forces of the incident laser pulse and the electric force due to the charge separation. The nanometer electron layers move in the laser field and the characteristic electrons have their directions bended several times due to the change of the laser's magnetic field. At every bending point, these electrons emit strong synchrotron radiation along a direction that deviates from the reflected laser, even if the normalized drive laser amplitude is at ${a}_{0}\ensuremath{\approx}1$. The simulation results for two cases with and without the density profile truncation indicate that the efficiency of this mechanism strongly depends on the preplasma.