Toward Magnetically Tunable Metasurfaces

Metasurfaces have emerged as a versatile platform for manipulating electromagnetic waves with unprecedented spatial resolution; however, realizing dynamic tunability remains a critical challenge for practical photonic applications. This thesis investigates the design, fabrication, and characterization of magnetically tunable metasurfaces, exploiting Gires-Tournois (GT) resonant architectures to significantly enhance magneto-optical (MO) effects in the visible and near-infrared spectral regions. The research explores two distinct material platforms: metallic magnetoplasmonic systems based on Cobalt (Co) and all-dielectric systems utilizing Cerium-substituted Bismuth Iron Garnet (Ce:BIG). The study employs a combination of theoretical modelling, numerical simulations (COMSOL Multiphysics®), and experimental validation. For the metallic systems, Co-based GT metasurfaces were designed to couple localized surface plasmon resonances (LSPRs) and surface lattice resonances (SLRs) with cavity modes. These structures were fabricated using physical vapor deposition and electron beam lithography (EBL) and characterized via reflectance spectroscopy and magneto-optical Kerr effect (MOKE) measurements. Experimental results demonstrate that the nanostructured GT metasurfaces achieve a remarkable enhancement of the polar Kerr response, up to two orders of magnitude in the near-infrared and one order of magnitude in the visible spectrum, compared to bulk cobalt films. Furthermore, a Co-based intensity modulator exhibiting a reflectance contrast of approximately 25% was designed. In the dielectric regime, the thesis addresses the need for low-loss tunable devices in the telecommunication band. Numerical simulations of Ce:BIG-based GT metasurfaces operating at 1550 nm predict high-efficiency modulation capability. A key achievement is the design of a magnetically tunable bifocal metalens capable of switching its focal length from 1.5 mm to 4.5 mm upon magnetization reversal, providing a proof-of-concept for non-mechanical active optical systems. Overall, this work establishes that synergistically combining magnetic materials with resonant GT optical cavities is a scalable and effective strategy for developing compact, non-reciprocal, and actively tunable photonic devices, paving the way for advancements in optical modulation and sensing technologies.