Abstract

In a recent publication, Chen et al.¹ describe their observation of phase matched third-harmonic generation (THG) in a highly ionized gas under laser intensities sufficiently large (2 × 10¹⁷ W/cm²,) that relativistic effects are expected to be important. The harmonic radiation is found to be emitted in a narrow cone (of radius 5.6 degrees) in the near forward direction, with a reasonably large (2 × 10⁻⁵) conversion efficiency. Perhaps most surprisingly they found that the intensity of the THG was comparable for linear and circular polarization of their incident laser beam. In traditional nonlinear optics the process of THG is known to vanish identically for an isotropic medium for a circularly polarized fundamental laser beam.² Nonlinear optical interactions are often described in terms of a nonlinear optical susceptibility, which for the case of third-harmonic generation relates the polarization of the medium to the third power of the electric field amplitude of the incident laser beam.

© 2001 Optical Society of America