High-performance Hybrid Lithium Niobate Electro-optic Modulators Integrated with Low-loss Silicon Nitride Waveguides on a Wafer-scale Silicon Photonics Platform

TL;DR

This work demonstrates high-performance electro-optic modulators by heterogeneously integrating thin-film lithium niobate with low-loss silicon-nitride waveguides on a wafer-scale silicon photonics platform. The hybrid-mode design distributes optical power between LN and SiN, achieving an on-chip insertion loss around $3$–$6$ dB, a low-frequency $V_ ext{π}L$ near $3.8\,\text{V·cm}$, and a 3-dB EO bandwidth exceeding $110\ \text{GHz}$ across three device designs, with the best performance from Design 3. The results, supported by a traveling-wave electro-optic model and robust fabrication on standard $200\ \text{mm}$ silicon wafers, establish a scalable, cost-effective route to large-scale PICs that integrate high-performance LN EOMs with passive SiN photonics for applications in communications, waveform generation, sensing, and quantum photonics.

Abstract

Heterogeneously-integrated electro-optic modulators (EOM) are demonstrated using the hybrid-mode concept, incorporating thin-film lithium niobate (LN) by bonding with silicon nitride (SiN) passive photonics. At wavelengths near 1550 nm, these EOMs demonstrated greater than 30 dB extinction ratio, 3.8 dB on-chip insertion loss, a low-frequency half-wave voltage-length product ($V_πL$) of 3.8 $V.{}cm$, and a 3-dB EO modulation bandwidth exceeding 110 GHz. This work demonstrates the combination of multi-layer low-loss SiN waveguides with high-performance LN EOMs made in a scalable fabrication process using conventional low-resistivity silicon (Si) wafers.

Figures (7)

• Figure 1: (a) Photograph of the fabricated wafer (100 mm diameter). (b) Magnified photograph of the electro-optic modulator (EOM) devices discussed in this report. (c) Schematic drawing of the wafer cross-section. Abbreviations: S and G: signal and ground electrodes, respectively; TFLN: thin-film lithium niobate; SiN: silicon nitride; Si: silicon; Al: aluminum. (d)-(f) Simulations of the optical mode in the feeder, transition, and hybrid sections, which are indicated in panel (b). The optical effective index ($n_\text{eff}$), effective area ($A_\text{eff}$) and mode confinement fraction in LN ($\Gamma_\text{LN}$) are tabulated.
• Figure 2: Process flow diagram showing the main steps, (a) - (f), in the fabrication of the hybrid TFLN-SiN modulators. (Schematic diagrams, not to scale.)
• Figure 3: (a) Schematic for the measurement of the half-wave voltage, $V_\pi$. (b) Example of the (overdriven) optical response (red line) to the trapezoidal electrical drive signal (blue line). The vertical axis for the optical response is on an arbitrary linear scale determined by the photodetector. (c) Example of the normalized optical power measured as a function of the applied voltage at frequencies of 1 kHz (red) and 1 MHz (blue). The solid line is a fit to the 1 MHz data using the squared-cosine functional form, which yields $V_\pi$ as shown. (d) The same data as in panel (c) for 1 kHz with a logarithmic vertical scale to quantify the extinction ratio, $\mathrm{ER} > 34\ \mathrm{dB}$.
• Figure 4: (a) Schematic for the measurement of the electro-optic response (EOR). (b) EOR measurement data between 1 GHz and 50 GHz for the three MZM designs on one test chip. (c) EOR measurement data between 1 GHz and 111 GHz for a Design 3 EOM device, normalized to the corresponding EOR value at 1 GHz. The solid black line is a fit to the data using a theoretical model of the EOM. (d) The calculated RF $V_\pi$ as a function of driving frequency using Eq. \ref{['eqn_Vpif']}.
• Figure S1: (a) Cross-sectional scanning-electron microscope (SEM) image showing the trenches etched into the Si substrate to lower the propagation loss of the traveling-wave microwave mode. (b) Higher-resolution SEM image of a test chip showing the SiN2 waveguide layer and the TFLN bonded region.