Title: Plasmon-enhanced Brillouin Light Scattering spectroscopy for magnetic systems. I. Theoretical Model

Abstract: Brillouin light scattering (BLS) spectroscopy is an effective method for detecting spin waves in magnetic thin films and nanostructures. While it provides extensive insight into the properties of spin waves, BLS spectroscopy is impeded in many practical cases by the limited range of detectable spin wave wavenumbers and its low sensitivity. Here, we present a generalized theoretical model describing plasmon-enhanced BLS spectroscopy. Three types of plasmonic nanoparticles in the shape of an ellipsoid of rotation are considered: a single plasmon resonator, a sandwiched plasmonic structure in which two nanoparticles are separated by a dielectric spacer, and an ensemble of metallic nanoparticles on the surface of a magnetic film. The effective susceptibilities for the plasmonic systems at the surface of the magnetic film are calculated using the electrodynamic Green functions method, and the enhancement coefficient is defined. It is analytically shown that the ratio of the plasmon resonator height to its radius plays the key role in the development of plasmon-enhanced BLS spectroscopy. The developed model serves as a basis for numerical engineering of the optimized plasmon nanoparticle morphology for BLS enhancement.