Dispersion and localization of guided acoustic modes in a Langmuir-Blodgett film studied by surface-plasmon-polariton-enhanced Brillouin scattering

We present the results of our surface-Brillouin-scattering measurements on a film of 18 monolayers of cadmium arachidate deposited on a Ag film by the Langmuir-Blodgett (LB) dipping technique. By using the Kretschmann attenuated-total-reflection geometry, the resonant-Brillouin-scattering measurements on the prism side are almost exclusively sensitive to guided acoustic modes that ripple the active interface between the Ag film and LB film. This is in contrast to conventional Brillouin measurements taken with light impinging directly onto the LB film, for which there are strong interference effects between the ripple and elasto-optic scattering mechanisms. The resonant Brillouin scattering is therefore used to investigate the localization of guided acoustic modes at the CdA/Ag interface. The hybridization of the dispersion relations between the guided modes of the LB film and the Rayleigh-Sezawa modes of the supported Ag film is investigated both experimentally and theoretically. We also study for the first time the angular dependence of the intensity of the surface-plasmon-polariton-enhanced Brillouin scattering for a dielectric-metal-prism system. The enhancement factor defined as the ratio of the Brillouin signal obtained at optimum coupling to surface plasmons to the signal away from the resonance is found to be greater than 15.