Circular-difference effects in second-harmonic generation from thin films

Abstract

Chiral molecules have received much attention in nonlinear optics because they are inherently noncentrosymmetric. It has been shown that second-harmonic generation (SHG) from chiral, isotropic surfaces and thin films is sensitive to the handedness of circularly-polarized fundamental light. These circular-difference (CD)-effects in SHG are used to probe the chirality of a sample. However, an achiral, anisotropic surface can also give rise to CD-effects in SHG. This can be explained by the fact that for some orientations of the sample, the total geometry becomes chiral. We investigated a chiral, anisotropic Langmuir–Blodgett film. For such a sample both chirality and anisotropy give rise to CD-effects in SHG. Our goal was to extract the optical activity effects due to chirality. Our technique is based on the fact that CD-effects originating from anisotropy will critically depend on the azimuthal rotation angle of the sample in the set-up whereas the CD-effects coming from chirality will remain constant.