Novel Functional Devices on Thin-Film Lithium Niobate Platform

Abstract

Modern data centers and long-distance communication systems increasingly utilize optical interconnects to handle larger data volumes while reducing energy consumption. Photonic Integrated Circuits (PICs) are emerging as a key platform, allowing for the integration of numerous active and passive photonic components on a small chip. Despite advances with materials like silicon, indium phosphide, and silicon nitride, they still see critical limitations in simultaneously satisfying the requirements of low transmission loss, high integration density, and efficient high-speed modulation, hindering the progress of integrated photonics.

Lithium niobate (LiNbO₃, LN), stand out due to its low absorption loss, high refractive index, broad optical transparency window, and superior electro-optic and nonlinear coefficients. These properties make LN an ideal material for large-scale PICs. Recent advancements in wafer-scale lithium niobate on insulator (LNOI) thin films and fabrication techniques have enabled the creation of highly integrated, low-loss linear and nonlinear devices. These include ultra-high-Q microresonators, high-performance electro-optic modulators, narrow-linewidth on-chip lasers, high-gain waveguide amplifiers, broadband optical frequency combs, among others.

This thesis builds on top of these advancements and further expands the functional and application scopes of the thin-film lithium niobate (TFLN) platform, which investigates and develops various devices aimed at diverse applications. These include the design and experimental demonstrations of efficient continuous-wave THz generation based on TFLN THz-photonic chips that support strong broadband THz-optical wave interactions; a broadband adiabatic polarization rotator and splitter that manipulates polarization states over a wide spectral range, crucial for advanced high-speed communication; as well as ultra-compact and high-performance photonic devices using inverse design methods for high-density photonic integration.

The outline of this thesis is as follows:
Chapter 1 introduces LN material properties and the recent developments of TFLN photonic devices and systems.

Chapter 2 focuses on the theoretical model of THz generation and introduces an efficient continuous wave THz generation method using hybrid LNOI-silicon platforms.

Chapter 3 describes a monolithic integrated photonic chip for efficient THz generation and manipulation at frequencies up to 500 GHz.

Chapter 4 introduces the design and characterization of broadband adiabatic polarization rotators and splitters on TFLN platform.

Chapter 5 shows the results of inverse design methods for TFLN devices, including three devices of mode multiplexer, waveguide crossing and waveguide bend.

Chapter 6 provides a summary of research findings in this thesis and prospects of TFLN technologies in photonic applications.

Date of Award	7 Nov 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Cheng WANG (Supervisor) & S. Pang (Co-supervisor)