Abstract
The analysis of two-photon resonant third-harmonic generation is generalised to include, in particular, the effects of imperfect index matching and two-photon tuning. Furthermore, whereas previously the authors considered only the temporal properties of the generated harmonic signal, here is spectral characteristics are also considered. The authors comment on a discrepancy between the predictions of semiclassical and quantised field formalisms. In the semiclassical analysis, it is shown that for large energy input pulses the harmonic spectrum is double peaked even near exact resonance and that this structure can be interpreted in terms of transitions between sets of doublet states of the Autler-Townes (1955) type. In a quantised field formalism, however, the picture is radically altered near exact resonance where a three-peaked spectrum is predicted.