RADIATION SCATTERING IN INTENSE-FIELD HARMONIC-GENERATION EXPERIMENTS

Multiphoton ionization (MPI) of a noble gas produces an electron plasm a, acting as a defocusing lens for the laser beam. In this paper we sh ow 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 exa mining the far-held intensity distribution of the laser fundamental. I n the limit of small electron density, describing the scattering withi n the framework of the Born approximation, an analytic expression of t he scattered field has been obtained. For the other cases, we have dev eloped a numerical approach based on the expansion of the field in a s eries of Gaussian beams. On the other hand, the aberration of the fund amental beam interferes notably with the process of harmonic generatio n in the gas medium. Our theoretical findings agree well with experime ntal data relative to the fundamental and the third harmonic of a Nd:Y AG laser radiation (where YAG denotes yttrium aluminum garnet) produce d in the nonressonant 14-photon ionisation of Ar. This agreement confi rms that the distributions reported in literature describe quite well the plasma formed in our 30-ps experiments.