Waveguide Raman and Brillouin scattering

Raman and Brillouin spectroscopy in planar waveguides can be performed by coupling the laser beam to the waveguide with a prism and by collecting the scattered light from the from surface. Brillouin spectra, which are different for excitation in the different modes of the waveguides, can be accounted for by a model that considers the space distribution of the exciting field. This allows measuring the sound velocity in the film. This technique has been applied to a silica-titania planar waveguide, obtained by r.f. sputtering. The same experimental setup is used for collecting the Raman spectra of the glassy waveguides. In graded index waveguides, as those produced by ion-exchange in glass, different guided modes propagate in layers with different thickness. By comparison of the Raman spectra, taken by waveguide excitation in different modes, it is possible to characterize the guide at different depths. The method has been applied to waveguides obtained by ion-exchange of silver in a soda-lime substrate. Raman scattering from the optical vibrations of the glass and from the acoustic vibrations of silver nanoclusters depends on the silver concentration, providing different spectra for excitation in different modes of the waveguide