Surface enhancement of coherent anti-Stokes Raman scattering by colloidal spheres

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Abstract
Coherent anti-Stokes Raman scattering signals may be strongly enhanced when the active molecules are located near the surface of a small silver particle. The theoretical analysis is similar to the electrodynamic mechanism for surface-enhanced Raman scattering, except that there are four instead of two electric fields that stimulate collective electron oscillations within the particle. The general analysis is presented for a sphere of arbitrary size, for arbitrary angle between pump and probe beams, and for arbitrary polarization between pump and probe beams. This is then specialized to the small-particle limit for numerical computation. The peak enhancement for a monolayer of benzene on a silver particle (excitation wavelength 404 nm, Raman shift 992 cm−1) is 1012 when both incident beams are polarized perpendicular to the incident plane and 1021 when these beams are cross polarized. These values are averaged over scattering angle. While the coherent anti-Stokes Raman spectroscopy amplitudes depend on scattering angle, only the enhancement factor for one of the cross-polarized components depends on scattering angle. Enhanced signals from a silver organosol (silver dispersed in neat benzene) should be measurable.