Fig. 1. (a) Conventional TFLN waveguide with wide-gap electrodes. (b) LN-silica hybrid waveguide with narrow-gap electrodes. (c) Optical absorption loss of optical waveguides with/without the silica buffer layer. RF modes in (d) TFLN waveguide with wide-gap electrodes and (e) LN-silica hybrid waveguide with narrow-gap electrodes.

Fig. 2. (a) Minimum allowable electrode spacing and electro-optic overlap factor for different silica buffer layer thicknesses. (b) Refractive index variation of the hybrid waveguide with 1 V drive voltage. A 100-nm-thick silica layer is chosen to obtain the maximum refractive index change.

Fig. 3. (a) Top view of the CL-TWEs. (b) Duty cycle of T-rails for different T-rail gaps under capacitance matching condition. (c) Variation of microwave loss with the width of unloaded signal electrode under inductance matching condition. The star in (b) and (c) indicates the designed value.

Fig. 4. (a) Demonstrated modulator 3D schematic. The unloaded electrodes have 50 µm signal electrode width, 15 µm electrode spacing, and bent tapers to match with the microwave probes. The inset shows the SEM image of the 3-µm-spaced T-rails with 50 µm period and 90% duty cycle. (b) Insertion loss of 5-mm-long modulators with different electrode gaps. (c) Normalized optical transmission as a function of modulation voltage. (d) Microwave transmission S₂₁ and reflection S₁₁ of the traveling-wave electrodes as well as the electro-optic response of the TFLN modulator up to 67 GHz. (e) Extracted microwave refractive index, which shows excellent matching with the group index of the optical mode (n_g ∼ 2.25).

Fig. 5. Extended electrical bandwidth measurement to 110 GHz.