Nonlinear Optics of Oriented Chiral Systems

Abstract

Circular dichroic (CD) ratios often exceeding 100% have been routinely reported in surface second harmonic generation (SHG) measurements of chiral surface films, 1-6 offering promise for the development of unique characterization methods to study biologically interesting surface systems. To date, theoretical treatments designed to explain the anomalous sensitivity of SHG to chirality have focused almost exclusively on the chirality of the chromophore.5,7,8 In this work, we present an alternative possibility, in which surface structure directs circular dichroism in SHG.9 Specifically, considering only electric-dipole contributions in a two-state chromophore we demonstrate that (1) all chiral elements of the molecular nonlinear optical tensor can be made to disappear by redefining the internal coordinates (i.e., all chiral information about the chromophore is lost in a two-state system), (2) surface packing and orientation rather than molecular chirality can still lead to significant values of the chiral elements of the tensor describing the surface nonlinear response, (3) surface optics play a critical role in predicting the measured response, and (4) SHG-CD is predicted to be possible even in uniaxial films of achiral chromophores. Preliminary experiments have been performed supporting many key prediction of the model.