Ultrabroadband, Subvolt Thin‐Film Lithium Niobate Modulators via Codesigned Optical and RF Slow‐Wave Structures

Low‐voltage, high‐bandwidth electro‐optic modulators are essential for next‐generation optical interconnects and CMOS‐compatible photonic systems. Here, we demonstrate thin‐film lithium niobate (TFLN) Mach–Zehnder modulators that combine slow‐light optical waveguides with inductively loaded slow‐wave RF electrodes to simultaneously reduce the half‐wave voltage ( V π ) and extend the modulation bandwidth. The optical section employs a dispersion‐engineered 1D photonic crystal waveguide with an enhanced group index, achieving an ultralow voltage–length product of ≈0.3 V cm across all designs. To maintain broadband operation, the RF transmission lines are engineered to increase the RF effective index and achieve close RF‐optical index matching. Using two distinct electrode configurations, the resulting modulators achieve 3‐dB electro‐optic bandwidths of 75–203 GHz, V π values of 0.25–1.0 V, and interaction lengths of 2.7–12.8 mm. These findings demonstrate that codesigned optical and RF slow‐wave structures offer an effective strategy for realizing compact, low‐drive‐voltage, and ultrabroadband integrated modulators.