Coherent VUV radiation by harmonic conversion of mixed fields in gases

Harmonic generation in gases has been investigated by mixing of parallel-polarized fundamental (1064 nm) and second harmonic (532 nm) of a 100-mJ 35-ps Nd-YAG laser. The wave-mixing scheme generates even and odd harmonics whose conversion efficiency is enhanced by at least two orders of magnitudes compared with single-frequency excitation. We have investigated the harmonic spectrum in Ar and N-2, up to the 14th harmonics, versus the gas pressure and laser intensities for two different interaction lengths. Harmonic generation is barely affected by phase mismatch in a large range of gas density variation, and for both configurations of active medium. Conversion efficiency of each harmonic versus laser intensities decreases steeply near the saturation threshold of ionization of selected gas.