Linear and Nonlinear Light Scattering from the Surfaces of Liquids

Abstract

The linear optical properties of matter, e.g. the refractive index, the absorption coefficient and the reflectivity, can be interpreted in terms of the oscillating charge and current densities induced by the optical field. For most purposes, it is sufficient to consider the oscillating dipole moment 11= OI.E = OI.E(O) exp(-ioot+ik. r), where 01. is the mean polarisability of a molecule, or of a unit cell, in a plane wave propagating with wave vector k, angular frequency 00 and amplitude E(O). A static electric field F polarises the material and may affect the reflected light linearly in F. If F is in the plane of incidence there is a change of phase of the reflected light, and if F is perpendicular to the plane of incidence there is a change in polarisation and intensity. If the intensity of the optical field is high, nonlinear scattering is observed from the surface of a liquid. The surface breaks the symmetry of the fluid, leading to partial molecular orientation and hence to a non·vanishing first hyperpolarisability P that gives the induced dipole at the frequency 200 proportional to the square of E. With intense laser sources a molecular monolayer or sub-monolayer can be detected. Surface selectivity can be achieved by tuning an infrared laser of frequency oolR to a vibrational frequency of the monolayer and detecting a coherent beam at the sum frequency oo+ooIR, when the surface is simultaneously subjected to pulses at 00 and oolR.