Anomalies in high-order harmonic generation at relativistic intensities

High-order harmonic generation from a solid target surface has been investigated using femtosecond laser pulses focused to intensities greater than {10}^{18}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.The experiments show that the harmonics are very intense, with a conversion efficiency that is one or two orders of magnitude larger than that of harmonics generated in gases. Beside the observation of presently the shortest wavelength harmonics from...