  A novel thin-film LiNbO3 (TFLN) electro-optic modulator is proposed and
demonstrated. LiNbO3-silica hybrid waveguide is adopted to maintain low optical
loss for an electrode spacing as narrow as 3 {\mu}m, resulting in a record low
half-wave-voltage length product of only 1.7 V*cm. Capacitively loaded
traveling-wave electrodes (CL-TWEs) are employed to reduce the microwave loss,
while quartz substrate is used in place of silicon substrate to achieve
velocity matching. The fabricated TFLN modulator with a 5-mm-long modulation
region exhibits a half-wave-voltage of 3.4 V and merely 1.3 dB roll-off in
electro-optic response up to 67 GHz, and a 3-dB modulation bandwidth over 110
GHz is predicted.
