Self-standing 3D magnetoplasmonic nanostructures with enhanced Faraday effect and outstanding sensing performance

Engineered magnetoplasmonic nanostructures offer an efficient route to enhance the magneto-optical (MO) activity of materials, greatly improving their practical utility. Especially, three-dimensional (3D) magnetoplasmonic nanostructures offer richer MO properties through multi-mode excitations, providing new paradigms for miniaturized non-reciprocal photonic devices. However, the complex geometrical configurations of 3D nanostructures impose significant fabrication challenges compared to conventional planar nanostructures, which creates substantial obstacles for practical implementation. Here, we present free-standing metallic hole-vertical nanoplate (MH-VNP) nanostructures fabricated by high-precision focused ion beam (FIB) technology on Au/Co/Au trilayers. Under y-polarized excitation, MH-VNPs exhibit an unusual Fano resonance arising from in-phase conductive coupling between the nanoplate and metallic hole, thereby forming 3D currents. This resonance induces strong magnetic field localization, causing a larger Faraday rotation (FR) and enabling FR sign inversion. Systematic modulation of geometric parameters allows precise control of FR magnitude and resonance positions, offering versatile tunability. The self-supported architecture maximizes exposure to the surrounding medium, achieving an exceptional refractive index sensitivity of up to 1500 nm/RIU (RIU = refractive index unit). Overall, this work demonstrates an environmentally friendly fabrication route for 3D magnetoplasmonic structures, offering a versatile approach for tailoring MO responses in nonreciprocal photonic and sensing applications.