Enhanced-Spectral-Range Brillouin Light Spectroscopy by sub-diffraction confinement of light

Microfocus Brillouin Light Spectroscopy (BLS) is a standard optical technique which allows for the direct observation of quasiparticles such as phonons or magnons in magnetic films. A major limitation of the technique is that due to the small wavevector of light, the technique is only sensitive to the dynamical modes near the center of the Brillouin zone, making a large part of the spectrum optically inaccessible. We show that by using a nanoscale optical mask on top of the magnetic film, the BLS signal from short-wavelength spin waves can be enhanced by up to an order of magnitude. The enhancement is consistent with the broadened distribution of wavevectors caused by the spatial confinement of light, analogous to the uncertainty principle. Additionally, by engineering the geometry of the optical mask, we selectively enhance the sensitivity to specific spin wave modes, allowing us to reconstruct the spin wave dispersion.