Polarization-resolved measurement of forward volume spin waves by micro-focused Brillouin light scattering

We show how the micro-focused Brillouin light scattering (BLS) signal of forward volume spin waves is formed, and why it remains observable despite symmetry-based “suppression” expectations. A reciprocity-theorem based model with vectorial diffraction-limited focusing identifies the nonnegligible longitudinal focal-field component as the key element responsible for BLS sensitivity in the forward volume geometry. We further demonstrate that full polarization analysis, implemented through polarizer–analyzer maps of coherently excited spin waves, provides information beyond the conventional crossed polarizer–analyzer readout. In a bismuth-doped yttrium iron garnet thin film, the measured maps exhibit Stokes/anti-Stokes polarization asymmetries and nontrivial patterns that stem from quadratic magneto-optical coupling terms. Fitting the data with a model including Voigt and Cotton–Mouton contributions yields an effective Cotton–Mouton constant and shows that the quadratic response is comparable to the linear Voigt contribution.