Electric-field induced harmonic generation as a probe of the focal region of a laser beam.

The amount of optical second harmonic generated in a centrosymmetric medium in the presence of a dc electric field depends on a convolution of the optical and dc field distributions in space. Thus, measurements of the harmonic generation associated with a known dc field distribution gives information on the optical field distribution. This method has been used to investigate the focal region of a laser beam. It avoids difficulties with resolution and probe damage that arise when conventional methods are used near the focus. The detailed data are in excellent agreement with results predicted for a pure TEM(oo) laser beam with the notable exception that the effective confocal parameter is about a factor of 2 smaller than that indicated by conventional measurements. We believe this effect is associated with the truncation of the laser beam by the intracavity mode-selecting aperture, and indicates that dc induced second-harmonic .generation constitutes a sensitive probe of experimentally interesting mode distortios.