Surface-plasmon-enhanced Brillouin scattering on silver films: Double-resonance effect

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Abstract
We report a continuation of our investigation of surface-plasmon-induced enhancement of Brillouin scattering of thermal surface acoustic waves of thin silver films. The experiments were performed in a Kretschmann attenuated-total-reflection configuration. In the backscattering mode, the surface plasmons are resonantly involved both in the incident and scattered radiation. The scattered radiation appears on the surface of a cone defined by the resonant interaction angle and contains Brillouin couplets, which are analyzed by a five-pass Fabry-Perot interferometer. The enhancement factor is determined from the ratio of the peak radiation on this cone to the Brillouin scattered radiation in an external reflection configuration not involving surface plasmons. The theory and experiment agree in that the double resonance gives a much larger enhancement than does the singly resonant process, which was observed in our earlier investigation for the forward-scattering configuration. The enhancement factor observed, 750 (±25%), is very dependent on the imaginary part of the dielectric constant of the Ag film, which is determined experimentally for our film as -9.0+i0.57. Finally, we contrast the results of our Brillouin scattering experiment with those of Raman scattering from chemical adsorbates.