With the progress of optical communication technology, optoelectronic devices are developing toward low power consumption, large data bandwidth, and high integration. The electro-optic modulator (EOM), a key optoelectronic device, plays a vital role in connecting the electric and optical fields, where the on-chip integration, high efficiency, low power consumption, and large bandwidth are always the crucial development directions of EOMs. Up to now, lithium niobate (LN) is still one of the most ideal materials for electro-optic modulation due to its excellent properties of wide transparent windows, strong Pockels effect, as well as stable physical and chemical features. However, the currently used EOMs are based on the bulk LN material, and the key modulation waveguides are formed by titanium diffusion or proton exchange on the bulk LN. Therefore, the formed waveguides have a low refractive index contrast (?n≈0.02), which leads to a large waveguide size required to well confine the optical mode, and the EOM footprint is also relatively large inevitably. Recently, the thin-film lithium niobate (TFLN) wafer has been fabricated by the smart cutting process and made available by several commercial companies. The TFLN wafer not only inherits some excellent material properties of LN but also has a high refractive index contrast (?n≈0.8), a feature considerably beneficial for shrinking the device footprint and making the on-chip compact integration available. In general, the TFLN-based EOMs can be divided into two types. One performs etching on the TFLN wafer to form the required waveguide, and the other deposits other high refractive index materials atop or below the TFLN wafer to form the waveguide, where the TFLN wafer does not need to be etched. By comparison, the etching-free TFLN scheme can reduce the fabrication difficulty. Therefore, we focus on the etching-free TFLN structure and propose a heterogeneously integrated EOM using embedded filling layers.

the structural design of the modulation waveguide, the electrode structure, as well as the coupling structure between the modulation region and the input/output waveguides. The silicon nitride (SiN_x) modulation waveguide is under the TFLN, and a layer of BCB is filled between them to reduce the half-wave-voltage length product (V_πL) and optical loss. On this basis, we employ such structure as the interference arms in a Mach-Zehnder interferometer (MZI) waveguide structure, where the modulation electrodes are arranged as a ground-signal-ground (G-S-G) configuration. The modulation electrodes are deposited on the TFLN, and a SiO₂ layer is sandwiched in between as an isolating layer to further reduce the optical loss, microwave loss, and the refractive index of the effective mode. Additionally, we propose an inverted stepped TFLN structure to achieve efficient coupling between input/output waveguide and modulation waveguide. Finally, we simulate and analyze the proposed structure using tools of COMSOL Multiphysics and FDTD Solutions to demonstrate its high-speed modulation performance.

The BCB layer is filled between bottom SiN_x modulation waveguide and TFLN. We simulate the influence of different thicknesses of the BCB layer and the SiO₂ layer on V_πL and the optical loss of the device. Results show that the proposed structure can effectively reduce V_πL and optical loss (Fig. 2). At the same time, we optimize the electrode gap, and the optimum V_πL of the device is 1.77 V·cm (Fig. 4). Further, we fill the SiO₂ layer between modulation electrode and TFLN layer. The filled SiO₂ layer can not only further reduce the optical loss (Fig. 3) and microwave loss (Fig. 5) of the device but also contribute to the index matching (Fig. 7). The high-speed analysis shows that the 3 dB modulation bandwidth of our proposed modulator is 140 GHz (Fig. 8). Finally, we design an inverted stepped thin-film structure, which can reduce the refractive index mismatch of the effective mode between SiN_x waveguide region and SiN_x-LN hybrid region. The simulation results show that the single-ended coupling loss of this structure is 0.73 dB (Fig. 9).

In this paper, we propose a heterogeneously integrated EOM based on TFLN. The modulation waveguide is formed by the bottom SiN_x and top TFLN that are sandwiched by a BCB layer. The modulation electrodes are deposited on the TFLN, and a SiO₂ layer is sandwiched in between as an isolating layer, which contributes to the index matching and the reduction in optical loss and microwave loss. Further, we construct an MZI-based EOM, where an inverted stepped thin-film structure is proposed to achieve the efficient coupling between input/output waveguide and modulation waveguide. After the high-speed matching design and optimization of the proposed electro-optic modulator, we obtain a V_πL of 1.76 V·cm and a 3 dB bandwidth of 140 GHz in a modulation length of only 5 mm, and the single-ended coupling loss is reduced from 1.23 dB to 0.73 dB. Given these characteristics, we believe the proposed device structure could be applied in the large-bandwidth design of the TFLN-based EOM and would boost the development of TFLN-based photonic integrated devices.