Broadband Electro-Thermal Full-Space Modulation in Freeform VO2 Metagrating in the Telecom Band

Electrically tunable metasurfaces for free-space light modulation have been widely studied, with VO2-based phase-change metasurfaces standing out for their large, broadband material tunability and reliable electrical modulation. However, simultaneously modulating both transmitted and reflected beams across a broad bandwidth is a multi-objective problem that is difficult to satisfy with intuitive metasurface designs built on one or a few eigenmodes, calling for complex multimode structures accessible only through inverse design. Here, we demonstrate broadband electro-thermal modulation of both reflected and transmitted beams in an adjoint-optimized freeform Au-VO2 metasurface on sapphire, operating as an amplitude modulator across the telecom bands (1200-1600 nm), with the Au layer serving as both electrode and plasmonic field concentrator. Measurements reproduce the simulated complementary switching of the two channels, yielding band-averaged modulation depths of ∼0.4 (transmittance) and ∼0.7 (reflectance) and optical efficiencies of ∼0.18 (transmittance) and ∼0.15 (reflectance) for y-polarized incidence. A coupled Bloch mode analysis then uncovers how the optimizer meets these competing objectives, the origins of the polarization selectivity, and the broadband nature of the switching. This work establishes adjoint optimization combined with multi-mode analysis as a framework for VO2-based active metasurfaces, opening a route toward compact full-space light modulators for optical communications, interconnects, and free-space optical computing in the telecom bands.