Magnetoplasmonic Response of Nonmagnetic Ag Films at Surface Plasmon Resonance

Surface plasmon resonance (SPR) is highly sensitive to perturbations in the dielectric properties of metal films, enabling precise detection of external stimuli. While magnetoplasmonic studies typically rely on ferromagnetic layers to induce magneto-optical activity, the influence of magnetic fields on pristine nonmagnetic metals under SPR conditions has remained challenging and underexplored. Here, we demonstrate that a uniform static magnetic field induces measurable and direction-dependent shifts in the SPR angle of a pristine Ag thin film without incorporating any magnetic material. Experiments performed in two geometries, magnetic field parallel and orthogonal to the plane of incidence, reveal a distinct, polarity-dependent, and nonlinear angular response. To interpret these observations, we model the magnetic-field-induced modifications to the dielectric tensor using the Drude magnetoplasma framework and map the resulting perturbations to shifts in the SPR resonance. The excellent agreement between theoretical predictions and experimental data confirms that resonance-enhanced plasmonic orbital currents generate an anisotropic magneto-optical response even in nonmagnetic noble metals. These findings establish SPR as a highly sensitive platform for probing weak magnetoplasma effects under an applied magnetic field, opening new pathways toward magnetic-field-tunable plasmonic devices based on simple metallic films.