8 - Nonlinear integrated photonics in thin-film lithium niobate

Abstract

Lithium niobate (LN) has excellent electrooptic and nonlinear-optical properties and is a prevailing photonic material for long-haul telecom modulators and nonlinear wavelength converters. However, conventional LN optical waveguides are low index-contrast and hence bulky compared to modern integrated platforms such as silicon photonics. The bulkiness impedes photonic circuit implementations and imposes high optical power requirements for nonlinear applications. Thin-film lithium niobate (TFLN) photonics is a promising integrated platform to address these shortcomings and has been an emerging discipline in the last decade. TFLN and its applications in second-order nonlinear integrated photonics are the main topic of this chapter. It begins with a broad introduction to nonlinear integrated optics and tailors the phenomenological models for ultracompact waveguides with an emphasis on quasi-phase matching techniques such as periodic poling. It is followed by a review of applications of LN in nonlinear optics and the need for thin-film solutions, and the early integrated platform developments and milestone achievements towards efficient nonlinear devices on TFLN. The chapter then gives an overview of broader applications of TFLN but focuses on progress in fundamental second-order nonlinear processes on TFLN in the last few years based on periodically-poled straight waveguide and microring structures. Finally, prospects for further developments in terms of performance, heterogeneous integration with other materials, and circuit-level applications are presented.