Second harmonic emission from a picosecond laser-produced plasma

Several kinds of experiments have been done relating to the second harmonic emission from a plasma produced by a 1 μm laser pulse of 30 psec. Complicated spectra of the second harmonic have been observed above ∼1013 W/cm2. It has been concluded that their production mechanisms are dependent on the self-focusing of the laser beam. In the absence of the self-focusing of the laser beam, it was determined that the complicated spectrum was caused by the parametric decay instability of an electron plasma wave excited by the resonance absorption. In the presence of the self-focusing of the laser beam, on the other hand, other mechanisms have been found to be dominant. It was tentatively concluded that they are the unstable motion of the critical surface of the plasma or the self-phase modulation of the laser pulse