Thin Film Lithium Niobate Photonics

Abstract

Lithium niobate (LN) has emerged as an outstanding material in photonics due to its exceptional optical and physical properties. Its high Pockels' coefficient, broad transparency window from visible to IR, low optical loss, and excellent thermo-optic and acousto-optic characteristics make it uniquely well suited for high-speed modulation, frequency conversion, and nonlinear optical applications. Compared to III-V materials and silicon-based devices that rely on plasma dispersion effects, lithium niobate enables higher modulation speeds with lower loss. However, traditional bulk lithium niobate devices are constrained by large optical mode sizes and low refractive index contrast, resulting in large bending radii, low modulation efficiency, and limited integration density. The advent of thin-film lithium niobate (TFLN) has addressed these limitations. Recent advancements in smart-cut technology have facilitated the realization of crystal ion-sliced thin-film lithium niobate on insulator (TFLNOI). This development enables more highly confined optical modes, thus reducing the electrode gap separation, which enhances modulation efficiency, increases bandwidth, and reduces the footprint of the device.

Recent developments in the TFLNOI material platform have employed diverse waveguide platforms, including ridge-etched and strip-loaded with materials such as silicon (Si), silicon nitride (SiN), and indium phosphide (InP). These approaches have effectively reduced waveguide loss and optical mode size compared to traditional bulk lithium niobate. The versatility of TFLN has led to the development of various optical devices on TFLNOI platforms, such as Mach-Zehnder modulators (MZMs), phase modulators, electro-optic (EO) and thermo-optic (TO) optical switches, optical filters, and optical frequency comb generators. Compared to their bulk lithium niobate counterparts, these devices exhibit significant improvements in optical waveguide loss, drive voltage, and 3-dB bandwidth. However, with the increasing demand for lower power consumption and higher operational bandwidth, significant challenges remain in further advancing TFLNOI devices.

The work presented in this dissertation focuses on the design, simulation, fabrication, and characterization of TFLNOI photonics to realize state-of-the-art modulators featuring low waveguide loss, high 3-dB bandwidth, and low power consumption. A TiO$_2$-on-TFLN etchless hybrid waveguide has been developed to achieve high mode confinement and low propagation loss of 1.2 dB/cm. To address the trade-off between V$_\pi$ and 3-dB bandwidth, capacitive loaded traveling wave electrodes (CLTWEs) on quartz were utilized to develop low waveguide optical propagation loss of 0.25 dB/cm, low-loss radio-frequency (RF) electrode of 0.21 dB/(cm $\cdot$ GHz$^{1/2}$) and broadband index matching. The device demonstrates a V$_\pi$ of 1 V, a 3-dB bandwidth of approximately 100 GHz, in a dual output configuration, and a high extinction ratio (more than 40 dB). Based on the optimized electro-optic modulator design, a multifold structure was designed and fabricated, resulting in a phase modulator with an ultra-low V$_\pi$ of 0.52 V.