Spectral-temporal effects in resonant harmonic generation in gases

A theoretical investigation is made of unsteady quasiresonant optical third-harmonic generation in a gaseous nonlinear medium pumped by amplitude-modulated pulses of constant collimated radiation. It is shown that two effective unsteadiness mechanisms exist: "wave" unsteadiness, associated with the difference between the propagation velocities of the pump and harmonic waves, and "local" unsteadiness caused by the finite relaxation time of the appropriate nonlinear component of the polarization of the medium. In spectral terms these correspond to the resonant dispersion of the phase matching and the dispersion of the nonlinear polarizability of the medium, respectively. The output pulse profile is determined and the influence of the main parameters on its profile is analyzed. The stimulated emission spectrum is also studied for conditions when the pump field is a steady-state random Gaussian process.