Sub-100 Femtosecond All-Optical Modulation Beyond Electron-Phonon Limits.

Ultrafast all-optical modulators are central to the advancement of next-generation photonic computing and signal-processing systems. However, the intrinsic electron-phonon relaxation bottleneck in plasmonic materials has long constrained modulation speeds to the picosecond regime, hindering the realization of sub-100 fs modulation. Here, we report a metastructured silver-single-crystal silicon nanodisk antenna that delivers experimentally resolved sub-100 fs all-optical modulation. Distinct from conventional planar metal-semiconductor junctions, the nanodisk architecture spatially co-localizes plasmonic energy deposition with the metal-semiconductor transfer boundary within a nanoscale-confined volume. This configuration markedly shortens hot-carrier transport pathways and preferentially activates interfacial carrier extraction during the earliest relaxation stage, thereby establishing an interface-dominated modulation pathway that precedes electron-phonon thermalization. By enabling modulation on timescales comparable to intrinsic electronic response limits, this work establishes a physical foundation for ultrafast photonic modulation, including femtosecond free-space photonic computing architectures, temporal optical gating, and other ultrafast systems constrained by carrier or cavity lifetimes.