Application of microscopic models of chirality to second harmonic reflection

Classical models of chirality are extended to calculate the second-order nonlinear optical response of an isotropic layer of chiral molecules. The one-electron approach and the excitonic coupling model give strikingly different results about the origin of the chiral component. Furthermore, it is possible to discriminate between local (electric dipolar) and nonlocal (magnetic dipolar and electric quadrupolar) contributions to the nonlinear optical activity when playing with the polarizations of the fundamental and second harmonic beams. Surface second harmonic generation experiments are performed to validate these calculations, with a femtosecond titanium-sapphire laser, in a two-photon resonant configuration. We study a stilbene with an asymmetric carbon which corresponds to a one-electron chirality, and a Tröger base exhibiting an excitonic coupling. Both experiments show a very good agreement with the theory, and emphasize the interest of second harmonic generation to access information about the microscopic origin of optical activity in chiral molecules.