Measurement of nonlinear coefficients by two-beam second-harmonic generation

Second-order nonlinear optical processes depend sensitively on light polarization and material symmetry. Second-order techniques are therefore powerful tools in studying effects associated with the structure of materials, e.g., the orientation of molecules on surfaces. In such techniques, it is important to make the distinction between the parameters that are directly accessible in a nonlinear optical experiment and more fundamental properties of the material, such as the nonlinear susceptibility tensor. The accuracy of the result depends on the validity of the theoretical model and the precision of the experimental parameters. Here, we show that the precision of the experimental parameters can be significantly improved by using two beams at the fundamental frequency instead of one as is commonly done.