Nonlinear optical properties of chiral media.

Abstract

This thesis concerns nonlinear optical phenomena in liquids and at interfaces. Optical processes are considered in chiral liquids and at chiral surfaces to second order in the applied electric fields, and to third-order in fluids. Time-dependent perturbation theory is used to describe the response of molecules to electromagnetic fields in terms of molecular electric-dipole polarizabilities. The inclusion of phenomenological damping in the sum-overstates expressions for the nonlinear polarizabilities is discussed, both at optical frequencies and in the limit that one or more applied electromagnetic fields are static. The reality condition for the polarization and the applied field is shown to define the sign of the damping terms. Electro-optical effects, linear in the electrostatic field, are considered in chiral liquids. Symmetry arguments are presented to show that the Pockels effect is forbidden in any non-conducting isotropic medium, and it is shown that this also follows from the statistical average of its sum-over-states expression when damping is correctly incorporated. A linear effect of an electrostatic field on the intensity of sum- and difference-frequency generation in a chiral liquid is predicted. The effect changes sign with the enantiomer and on reversing the direction of the electrostatic field. The sum-frequency polarizability and the electric field-induced sum-frequency polarizability interfere and their contributions to the scattering power can be distinguished. Experiments are described where, symmetry permitting, the third-harmonic is generated simultaneously in a second and in a third-order nonlinear optical mixing process. The third-order nonlinear optical properties of some simple liquids are characterized by all-electric phase-matched four-wave mixing. However, no three-wave mixing was observed in chiral liquids. Finally, a theory of surface nonlinear optical activity, applicable to various surface symmetries, is developed for sum-frequency generation. The potential of the three-wave mixing processes as probes of molecular chirality is examined. An optically active conformation for water in a static electric field parallel to the surface is proposed.