VUV generation by frequency tripling the third harmonic of a picosecond kHz Nd:YLF laser in xenon and mercury vapour

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^vac=351.165 nm) of a ps 1 kHz Nd:YLF laser system is frequency tripled in xenon and mercury vapour. About 4×10⁴ photons per pulse, i.e. 4×10⁷ photons/s, are generated in xenon yielding a conversion efficiency of η=3×10^-10. The unusual frequency tripling in xenon takes place in a positive dispersive wavelength region. It is shown that Kerr-induced dispersion in the atomic system and a fifth-order process rather than a third-order process can explain the frequency tripling. For comparison a four-wave mixing process is investigated in negative dispersive mercury vapour. Due to absorption of the generated VUV radiation in the autoionization region of mercury the observed effective efficiency is, in our experimental arrangement, even lower than in xenon. An analysis of the VUV generation with respect to absorption is given.