Abstract

Recently third harmonic generation in Hg vapour has been demonstrated¹ by making use of four photon resonance between the ground state 6s ¹S₀ and the excited 6d¹D₂ and 6d³D₂ states. Efficient third harmonic generation was observed on the red side of the four photon resonance. The resonant ionization signal due to these four photon resonances was found to be strongly dependent on the amount of third harmonic generated in that both the position and the width of the ionization signal concided with those of the generated third harmonic signal. We have developed a theory for four photon resonant third harmonic generation in atomic vapour. Third harmonic generation in this system is possible via the fifth order susceptibility χ⁵(3ω; ω, ω, ω, ω,-ω). Note that at high density of atomic vapour (>10^-3 Torr) a two photon resonant process between the same resonant levels starts to compete with the process involving the four photons of the fundamental wave. For this two photon process one photon comes from the generated third harmonic wave and the other from the fundamental wave.

© 1986 Optical Society of America