Helicity-Preserving Metasurfaces for Magneto-Optical Enhancement in Ferromagnetic [Pt/Co]_N Films

All-optical control and detection of magnetic states for high-density recording necessitate nanophotonic approaches to amplify local light intensity below the diffraction limit. Sculpting the near-field phase and polarization can additionally strengthen magneto-optical effects that rely on circularly polarized pulses, such as all-optical helicity-dependent switching, imaging, and spin-wave excitation. Here, high-refractive-index dielectric nanoantennas illuminated with circularly polarized light resonantly enhance local electric field rotation by more than sixfold within [Pt/Co]_N thin films. Sub-wavelength arrays of amorphous Si nanodisks, or metasurfaces, patterned on perpendicularly magnetized films support Mie-type resonances that modulate reflection and transmission dissymmetry by >±2% in experiments. Spatial and spectral interference between dipolar modes, proximity effects, and gain are evaluated by varying disk aspect ratio, metasurface–metal separation, and magnetic film thickness, respectively. Simulated enhancements in magnetic circular birefringence and differential absorption are correlated with amplified local field rotation at electric dipolar modes. Greater achievable amplifications are shown via simulations with single-crystalline Si metasurfaces exhibiting lower losses, including a 12-fold strengthened electric field rotation within ferromagnetic layers. The metasurface design rules established here could enable nanoscale localization of all-optical magnetic switching with lowered laser fluence thresholds, as well as enhanced magneto-optical responses for light-assisted reading in spintronic devices.