The Measurement of Several Optical Nonlinearities Using Focused Gaussian Laser Beams

摘要

The twofold purpose of this study was: (1) To analyze optical second-harmonic generation (SHG) in the focus of the lowest order transverse mode of a cw gas laser beam; (2) to utilize the power enhancement available from focusing to measure smaller nonlinearities on a cw basis than previously had been done. The results of the analysis give the dependence of SHG in index-matching crystals upon the crystal length, the crystal double-refraction angle, and the laser beam focal spot size w0. The dependence of SHG upon w0 was measured using varying degrees of focusing. The absolute power levels were checked by measuring the absolute value of the nonlinearity in ADP. The excellent agreement between analysis and experiment means that focused beams can be used to make accurate measurements of crystal nonlinearities. The second section of this study describes measurements of several optical nonlinearities in calcite made under conditions of optimum focusing using cw gas lasers. Since calcite has a center of inversion, the nonlinearities measured are of higher order than normal dipolar SHG; the resulting effects are much weaker, requiring the use of focused beams to detect them. Measurements were made of quadrupole-type SHG, electric-field-induced SHG, and the Quadratic electro-optic effect. (Author)