Raman scattering of light by polaritons in thin films; surface polaritons and size effects

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Abstract
In this paper, we present a general theory of Raman scattering of light by polaritons in thin films. We consider both forward and back scattering of light from a film laid on a transparent substrate. With the assumption that the light couples with polaritons in the film via the Raman tensor and electro-optic coupling within the film, we relate the Raman cross section to the spectral density of the electic field fluctuations in the film. The spectral density functions are constructed from electromagnetic Green's functions derived previously. Our expressions for the Raman cross section describe scattering by volume LO phonons, by volume TO polaritons, and by surface polaritons associated with the substrate-film-vacuum structure. We apply the general formulas to simple special cases, and examine the form of the cross section for both forward and back scattering from surface polaritons, along with the nature of size-dependent corrections to the cross sections for scattering from volume excitations. We present numerical calculations of the shape of the cross section for both forward and back scattering from the film. We also discuss a number of qualitative features of the scattering process, such as the origin of the large forward-backward asymmetry in the surface-polariton cross.