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

TL;DR

This paper investigates how micro-focused Brillouin light scattering detects forward volume spin waves, revealing the importance of longitudinal focal fields and polarization analysis for understanding magneto-optical effects in BiYIG films.

Contribution

It introduces a reciprocity-theorem based model explaining BLS sensitivity and demonstrates advanced polarization analysis revealing quadratic magneto-optical effects.

Findings

Longitudinal focal-field component enables BLS detection in forward volume geometry.

Polarization asymmetries indicate quadratic magneto-optical coupling.

Quadratic response is comparable to linear contributions in BiYIG films.

Abstract

We show how the micro-focused 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 BiYIG 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.