17-GHz lossless InP-membrane active metasurface

High-speed active metasurfaces enable spatiotemporal light control within an ultrathin device layer, offering previously unexplored possibilities for optical communication, sensing, and computing. However, a trade-off between electrical conductivity and optical loss has hindered the realization of a high-speed, low-loss device. Here, we experimentally demonstrate an active metasurface operating at 1.5-μm wavelength range that achieves a record-high 17.5-GHz modulation bandwidth while maintaining a high quality (<i>Q</i>) factor of 102 and an ultralow optical loss of 0.56 dB. This is enabled by the indium phosphide (InP) membrane platform, where n-InP offers high electron mobility and low free-carrier absorption simultaneously. A high-<i>Q</i> Friedrich-Wintgen quasi-bound-state-in-the-continuum mode within the InP-membrane high-contrast grating (InP HCG) traps light in the organic electro-optic material for efficient modulation. The InP HCG also functions as an ultralow-resistance interdigitated electrode, enabling 50-fold faster modulation than silicon-based counterparts. Our work paves the way toward active metasurfaces for high-speed spatiotemporal light control beyond the GHz regime.