Multi-resonant THz modulation with thin-film lithium niobate

Thin-film lithium niobate (TFLN) has emerged as a leading platform for integrated photonics owing to its outstanding electro-optic and nonlinear optical properties. Despite its widespread success in optical devices, the potential for active terahertz (THz) modulation remains largely unexplored because of the intrinsically weak interaction between free-space THz waves and subwavelength-thick lithium niobate (LN) films. Here, we demonstrate a TFLN-integrated THz metasurface supporting multiple quasi-bound states in the continuum (QBICs) for resonance-enhanced all-optical THz modulation. The metasurface is engineered via band folding and controlled symmetry breaking of a double-split-ring-resonator supercell, enabling three high-quality-factor QBIC resonances under normal incidence. By integrating a 600-nm-thick x-cut LN film with the metasurface, weak pump-induced dielectric perturbations in LN are efficiently converted into measurable transmission modulation through the strong field confinement and steep spectral dispersion of the QBIC resonances. Under continuous-wave optical pumping at 532 nm, distinct modulation responses are experimentally observed at three resonance frequencies, with a maximum modulation depth of ∼0.8%. In contrast, the modulation remains negligible when the QBIC resonances are not excited, highlighting the importance of resonance-enhanced light–matter interaction. Our results establish an effective route for translating TFLN to active THz devices and provide a versatile platform for future tunable THz modulators, programmable spectral components, and integrated THz photonic systems.