Taichiro Fukui, Kei Sumita, Hiroki Miyano, Go Soma, Warakorn Yanwachirakul, Eisaku Kato, Ryota Tanomura, Jiahao Liu, Toshiki Yamada, Akira Otomo, Kasidit Toprasertpong, Mitsuru Takenaka, Shinichi Takagi, Yoshiaki Nakano, Takuo Tanemura
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 ( Q ) 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- Q 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.