Raman enhancement methods for molecular structure characterization of optical thin films

Abstract

Raman scattering from thin dielectric films and multilayers carries information about the chemical and physical state of the film and its interfaces. Because of the low Raman scattering probability and the short optical paths in films and interfaces, several techniques to enhance Raman signals have been developed. Among these are interference enhancement in multilayer structures, path length extension by waveguide methods, coherent non-linear Raman spectroscopies and resonant Raman enhancement by the use of short wavelength excitation. We shall discuss here the advantages and limitations of interference and resonance enhancement methods. A common theme of both methods is the measurement of a Raman excitation profile spectrum by tuning the probe laser wavelength across an optical interference passband or across the electronic band gap of the film material. While interference is an optical effect which enhances the Raman spectrum uniformly, under resonance Raman conditions selective enhancement is observed for vibrational modes which are strongly coupled to the allowed electronic transitions in the dielectric material. The principal limitation of resonance Raman probes of solid films is the optical damage threshold which is easily exceeded by typical UV pulsed probe power levels near the electronic band edge. Examples and techniques to achieve resonance Raman excitation profiles in dielectric films will be discussed.