Accurate cw Measurements of Optical Second-Harmonic Generation in Ammonium Dihydrogen Phosphate and Calcite

Optimally focused cw gas-laser beams were used to make an accurate absolute measurement of optical second-harmonic generation (SHG) in ammonium dihydrogen phosphate (ADP) and accurate relative measurements of the higher order quadrupole-type SHG and electric-field-induced SHG in calcite. The result of the first set of experiments was d₃₆ (ADP) =1. 3810^-9 esu16%, in excellent agreement with the value determined by Francois using unfocused beams. This agreement demonstrates that focused laser beams can be used to make accurate measurements of crystal nonlinearities, and it provides further evidence that the value of d₃₆ in potassium dihydrogen phosphate (KDP), normally taken as the standard nonlinearity, is considerably smaller than the value usually quoted. The nonlinearity describing quadrupole-type SHG in calcite was found to be 2. 810^-4d₃₆ (ADP) 14%, and that for electric-field-induced SHG was 2. 110^-5d₃₆ (ADP) 24%. The second-harmonic power from calcite could not have been measured without the enhancements available from focusing. A displacement of the parabolic curve of second-harmonic power as a function of the applied electric field was noted, and it was shown that this shift was not caused by inhomogeneous applied electric fields. The very small induced birefringence in calcite which depends quadratically upon the applied electric field was also measured.