Generation of strong coherent extreme ultraviolet radiation from the laser plasma produced on the surface of solid targets

We demonstrate the generation of high harmonics (up to the 65th order, lambda=12.24 nm) of a Ti:sapphire laser radiation after the propagation of femtosecond laser pulses through the low-excited plasma produced by a picosecond prepulse radiation on the surface of different targets. High-order harmonics generated from the surface plasma of most targets showed a plateau pattern. It is assumed that the harmonic generation in these conditions occurs due to the interaction of the femtosecond pulses with the ions. The conversion efficiencies at the plateau region were varied between 1x10(-7) to 8x10(-6), depending on the target. The main contribution to the limitation of harmonic generation efficiency and cutoff energy was attributed to the self-defocusing of main pulse. A considerable restriction of the 27th harmonic generation was observed at different focusing conditions in the case of chromium plasma. Our observation of the resonance-induced enhancement of a single harmonic (lambda=61.2 nm) at a plateau region with the efficiency of 8x10(-5) in the case of In plasma can offer some expectation that analogous processes can be realized in other plasma samples in the shorter wavelength range where the highest harmonics were achieved.