Fourier transform waveguide Raman spectroscopy of laminate films

Abstract

The investigation of the chemistry and physics of thin films has had a considerable impact on the fields of optics, microelectronics, coating technology, and imaging. To better understand the nature of thin films and interfaces, several methods of characterization have been developed recently. Of these waveguide Raman spectroscopy (WRS) has been demonstrated to have a large potential for investigating a wide array of surface and interfacial molecular properties. Near-infrared excitation provides a method to eliminate the fluorescence that is commonly associated with polymers, making the measurement of Raman spectra much easier. The combination of FT-Raman spectroscopy with integrated optical techniques has made the characterization of structure, orientation, and morphology of polymeric films with Raman spectroscopy more tractable.1,2 The spectra measured in the waveguide Raman experiment are intimately related to the characteristics of the film as a planar waveguide. More specifically, the intensity of the Raman signal is directly related to the intensity of the light that is present in each layer of the waveguide. Thus for multilayer films, the measured Raman spectrum will be a superposition of the spectrum of each layer weighted by the proportion of the radiant flux in each layer. It is useful to model the distribution of the optical field intensity within the waveguide in order to be able to interpret the measured relative intensities. Of the experimental work reported in the literature, films of one or two layers have been investigated. Modeling the waveguide properties of these films is straight forward and easily done. Of further interest are polymer laminates of several layers as well as polymer films with concentration gradients. As part of a larger overall effort to study multilayer film structures, computational models of optical waveguides of as many as 100 layers have been developed. With these models, it is possible to observe the changes in the distribution of the optical field intensity (OFI) within the waveguide, and thus the resulting Raman intensity, by varying the number of layers, the refractive indices and thicknesses of each layer, and the wavelength. This has led to some interesting insights into the design of planar optical waveguides for spectroscopy.