Generation of the third harmonic of light pulses in gases with a spatial self-interaction

An investigation is reported of the influence of the pump pulse profile on the efficiency of third-harmonic generation (THG) in gases. Generation of the third harmonic from parabolic pulses with different parabola orders is investigated analytically on the basis of a Kerr-free model. Gases with a nonlinear phase mismatch and with self-focusing or self-defocusing are studied by numerical simulation. It is shown that the efficiency of THG in a medium with pump self-focusing can decrease significantly (by an order of magnitude) if the pump pulses are not rectangular. In a medium with pump self-defocusing the reduction in the efficiency is roughly the same (approximately by a factor of two) as that deduced on the basis of the Kerr-free model. Control of the pump pulse profile and/or a search for gas mixtures with self-defocusing are recommended for increasing the THG efficiency.