Raman scattering from surface molecules excited by guided optical waves and surface plasmons

Abstract

Raman scattering measures vibrational frequencies in materials, and is used to identify and characterize species and structures. Generally, scattering of laser radiation from a surface upon which molecules are deposited leads only to very weak Raman signals because the number of molecules which are able to interact with the beam is very small. We have recently used integrated optics, i.e., in the form of light propagation in dielectric waveguides, and plasmon surface polaritons (surface electromagnetic waves) on metals, to increase the number of interacting molecules, in the first case, and to greatly increase the strength of the optical field, in the second. The details of the techniques will be described and illustrated with spectra from interfacial molecules. First, the optical properties of a film are determined from measurements of the angular dependence of the reflectivity or transmission of light. Analysis of these results can give the film thickness and refractive index. Second, the scattered light is focused on the slit of a double monochromator and the Raman spectrum recorded as a spectral shift from the exciting light. Frequency shifts, intensity changes and polarization variations are observed in the spectra of surface molecules compared to bulk molecules. Scattering from the substrate and from defects in the films are also observed. Measurements of this type aid in the characterization of surfaces and our ability to engineer its properties for specific uses. © 1983 SPIE.