Overcoming the spectroscopic limitations of inelastic light scattering by sub-diffraction light confinement*

The small momentum of light strongly limits inelastic light scattering techniques. For instance, only long-wavelength spin waves are accessible to Brillouin light spectroscopy (BLS) widely utilized in studies of magnetic materials and nanostructures [1,2]. We overcome this limitation by utilizing a nanoscale metallic antenna on yttrium iron garnet (YIG) film. The antenna facilitates sub-diffraction confinement of light, generating momentum significantly larger than that of free-space light, and simultaneously enhances the local field due to a combination of geometric phase matching and optical reflection. We also present evidence for the plasmonic effects further enhancing the sensitivity and the spectral range. Our approach can be extended to other types of excitations and light scattering techniques. The demonstrated momentum enhancement can also facilitate light absorption in indirect-gap semiconductors, improving the efficiency of solar cells and optical detectors.

[1] S.O. Demokritov and V.E. Demidov, IEEE Trans. Magn. 44, 6 (2008).
[2] V.E. Demidov et al. Phys. Rep. 673, 1 (2017).