120 GHz Sub-2 V Thin-Film Lithium Niobate Modulators on Silicon Substrate Using Thick Capacitively Loaded Slow Wave Electrodes

Electro-optic modulators with large bandwidth and low voltage are crucial for the high-baud-rate digital communication and high-carrier-frequency analog links. Recently, thin-film lithium niobate (TFLN) modulators with sub-1 V voltage and 140 GHz bandwidth have been emerged on the low permittivity substrates, such as quartz. However, on the high permittivity environment, such as silicon substrates, the bandwidth dramatically reduced to below 100 GHz even if the voltage was raised to around 3 V. We break the voltage-bandwidth trade-off limit in TFLN modulators on silicon substrates using low inductance thick metal traveling wave electrodes, which reduce RF phase index and microwave losses while preserving high EO modulation efficiency. We demonstrate a TFLN EO modulator on silicon substrate with 3 dB EO bandwidth > 120 GHz and half-wave voltage (V_\pi) < 2 V. Bandwidth/V_\pi reach 60, which is significantly larger than traditional TFLN modulators. The proposed thin-film lithium niobate modulators offers a practical solution for the hybrid integration of silicon and lithium niobate.