Thin-film lithium niobate electro-optic modulator based on slow-light waveguides and slow-wave electrodes

High-speed optical interconnects demand electro-optic modulators that simultaneously offer low drive voltage, ultrabroad bandwidth, and a compact footprint. However, thin-film lithium niobate (TFLN) traveling-wave modulators typically face an intrinsic trade-off between modulation efficiency and electro-optic bandwidth. Here, we propose a co-designed TFLN Mach–Zehnder modulator that combines slow-light waveguides with slow-wave electrodes. A Bragg-grating slow-light section is integrated into the modulation arm to increase the optical group index and enhance phase-modulation sensitivity, whereas a T-shaped slow-wave traveling-wave electrode is employed to increase the effective microwave index and maintain broadband impedance matching. This co-design extends the optical–microwave velocity-matching window and suppresses high-frequency roll-off. Numerical modeling and full-wave simulations indicate that, with an ultrashort modulation length of 124.8 μm, the device can achieve VπL=0.143 V·cm together with an electro-optic bandwidth of 188 GHz. When the group index is increased to ng=20 and 24.5, the modulation efficiency is further enhanced, whereas the electro-optic bandwidth decreases to 110 and 72 GHz, respectively, revealing a predictable efficiency–bandwidth trade-off. This co-design strategy provides a transferable pathway toward compact, low-voltage, ultrabroadband TFLN-integrated modulators.