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We report the investigation of Brillouin scattering from thermal surface acoustic waves on Agcoated holographic gratings. The usual wave-vector conservation condition for Brillouin scattering is modi6ed due to the added periodicity in the direction of phonon propagation, but this effect is observed only when surface plasmons act as an intermediate state in the scattering process. The involvement of surface plasmons results in an'enhancement of the Brillouin scattering cross section which depends on the grating amplitude.Brillouin scattering from surface phonons is the dominant phonon-scattering process in materials of high absorption in which the depth of interaction of light is restricted to a small region near the sample surface. ' This process is governed by the conservation of wave vectors parallel to the surface, q~= k, sin8, +k, sin8, ,where 8; and 8, are the angles that the wave vectors k; and k, of the incident and scattered light make with the surface normal. The frequency shift~of the scattered light is then given by co= viqi,where U is the velocity of the scattered phonon. If an additional periodicity, such as a surface corrugation, is added in the direction of phonon propagation, the parallelwave-vector conservation condition should, in principle, only be obeyed modulo k, where kg is the wave vector of the added periodicity. We show here that under conditions in which an intermediate excitation, surface plasmons (SP's) in this case, mediates the scattering process, a relaxation of wave-vector conservation is clearly observable. Moreover, the large surface electromagnetic fields associated with SP's lead to a sizable enhancement of the Brillouin signal in a similar fashion to that observed for a variety of other nonlinear optical phenomena such as second harmonic generation, Raman scattering from adsorbates on gratings, ' coherent anti-Stokes Raman scattering (CARS), and surface-acoustic-wave (SAW) Brillouin scattering in an attenuated-total-reAection prism configuration.Holographic gratings with sinusoidal surface profiles were produced by exposing thin films of photoresist (Shipley 1470) deposited on glass slides to two interfering expanded beams from an argon-ion laser (A. =457.9 nm, I=3.5 mW cm ). After development, the photoresist was coated with 300 nm of Ag by magnetron sputtering.Gratings with different amplitudes were produced by varying the exposure time on the photoresist. The grating amphtude was estimated using a technique described by Pockrand'o and the grating spacing (=720 nm) determined by measuring the first-order diffraction angle and using the grating equation. Srillouin-scattering measurements were performed using a tandem Fabry-Perot interferometer in a (5+2)-pass configuration. "SP's are a collective oscillation of the electrons at the boundary of a metal-insulator interface. For a Hat metal-vacuum interface the SP dispersion relation is given by' 41S3

We have determined the index of refraction of AlxGa−xAs over the wavelength range 0.76–1.15 μm, and the composition range 0≤x<0.33, using a grating to couple light into waveguides. We find the mode indices of multimode slab waveguides from the coupling angle and grating period, then calculate the bulk indices of the core and cladding materials by a root searching technique using the analytical formula for the effective index of a guided mode. The method gives the core index within ±0.001, and the cladding index within ±0.01. We are in agreement with high precision index values for GaAs in the literature, confirming the method. We are in substantial agreement with literature values for AlGaAs but find a significant systematic difference in the composition dependence. An analysis of measurement uncertainties shows that the determination of composition is the dominant variable.

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Analysis of infrared images of spatial beats between two discrete modes in a single waveguide is shown to provide a useful consistency check on the thicknesses and refractive indices of the dielectric layers.