Integrated optics components based on second-order nonlinearities are provided by structures presenting departure from centrosymmetry, both at microscopic and at macroscopic levels.
In a non-exhaustive review, we consider optical methods for the evaluation of molecular first hyperpolarizabilities, presenting Hyper-Rayleigh Scattering (HRS) as a more direct method than inference from the standard Electric Field-Induced Second Harmonic Generation (EFISH). From a macro perspective on films, also alternatively to EFISH, we consider determination of second-harmonic susceptibilities from the Maker fringe technique, as well as of electro-optic (EO) coefficients by several methods, using a dc/modulated electric field and light in either transmission or reflection, external or internal.
As a case study from our laboratory, with the standard p-nitroaniline as reference, we illustrate application of HRS to the newer 2-(2’-hydroxy-4’-aminophenyl)-6-nitrobenzoxazole (HBO-BO6) molecule. The guest-host systems formed by its incorporation, as well as of commercial chromophores, into silicate sol-gel film matrices, were subjected to Corona poling. Application of the Poling Optical Polarimetry (POP) method to these systems during poling has revealed significant birefringence. For the resulting oriented structures with high-hyperpolarizability ingredients, such as the HBO-BO6 molecule, POP results have indicated the importance of EO characterization methods with explicit consideration of anisotropy, associated with very high EO coefficient materials.