Third harmonic generation in gases under self-focusing and self-defocusing conditions

A theory of third harmonic generation (THG) is developed. It allows for the spatial self-interaction of radiation and for the nonlinear phase mismatch, subject to their inherent unity. A study is made of the THG efficiency and of the mode composition of the third harmonic under self-focusing and self-defocusing conditions. Expressions are obtained for estimating the conversion efficiency and the optimal phase mismatch of each mode. A comparison is made with the results of numerical calculations. It is shown that the THG efficiency can be increased considerably in media capable of self-focusing and self-defocusing the pump radiation.