RADIATION SCATTERING IN INTENSE-FIELD HARMONIC-GENERATION EXPERIMENTS

Multiphoton ionization (MPI) of a noble gas produces an electron plasma, acting as a defocusing lens for the laser beam. In this paper we show that some parameters ofthe MPI process, e.g., order of nonlinearity p, and of the plasma, e.g., size and density, can be estimated by examining the far-field intensity distribution of the laser fundamental. In the limit of small electron density, describing the scattering within the framework of the Born approximation, an analytic expression of the scattered field has been obtained. For the other cases, we have developed a numerical approach based on the expansion of the field in a series of Gaussian beams. On the other hand, the aberration of the fundamental beam interferes notably with the process of harmonic generation in the gas medium. Our theoretical findings agree well with experimental data relative to the fundamental and the third harmonic of a Nd:YAG laser radiation (where YAG denotes yttrium aluminum garnet) produced in the nonressonant 14-photon ionisation of Ar. This agreement confirms that the distributions reported in literature describe quite well the plasma formed in our 30-ps experiments.