Brillouin scattering in planar waveguides. II. Experiments

Abstract

Silica-titania planar waveguides of different thicknesses and compositions have been produced by radio-frequency sputtering and dip coating on silica substrates. Waveguides were also produced by silver exchange on a soda-lime silicate glass substrate. Brillouin scattering of the samples has been studied by coupling the exciting laser beam with a prism to different transverse-electric ͑TE͒ modes of the waveguides, and collecting the scattered light from the front surface. In multimode waveguides, the spectra depend on the m mode of excitation. For waveguides with a step index profile, two main peaks due to longitudinal phonons are present, apart from the case of the TE 0 excitation, where a single peak is observed. The energy separation between the two peaks increases with the mode index. In graded-index waveguides, mϪ1 peaks of comparable intensities are observed. The spectra are reproduced very well by a model which considers the space distribution of the exciting field in the mode, a simple space dependence of the elasto-optic coefficients, through the value of the refraction index, and neglects the refraction of phonons. A single-fit parameter, i.e., the longitudinal sound velocity, is used to calculate as many spectra as is the number of modes in the waveguide.

Diagnostic techniques for photonic materials based on Raman and Brillouin spectroscopies

“…** E-mail:mattarel@science.unitn.it RS with waveguided excitation in a mode of a planar waveguide (WG) has the advantage of obtaining information only on the region of interest, avoiding the contribution of the substrate. Moreover, in a graded index WG, different guided modes propagate at different depths in the guiding film leading to a depth-selective spectroscopic characterization of a WG by RS and BS [10,13] .…”

“…RS with waveguided excitation in a mode of a planar waveguide (WG) has the advantage of obtaining information only on the region of interest, avoiding the contribution of the substrate. Moreover, in a graded index WG, different guided modes propagate at different depths in the guiding film leading to a depth-selective spectroscopic characterization of a WG by RS and BS [10,13] .In a light scattering experiment, the spectral density of the scattered light is given bywhere q and are the exchanged wavevector and energy, respectively and is the fluctuation of the dielectric constant. The inelastic light scattering is due to time dependent dielectric constant fluctuations [2,12] .…”