Lithium Niobate Nonlinear Nanophotonics

Abstract

Lithium niobate (LiNbO3) is by far the most widely used second order nonlinear optical material due to its high χ(2) coefficient, wide transparency window (400 nm – 5 µm) and relatively high refractive index (~ 2.2). In nonlinear wavelength conversion, periodically poled lithium niobate is a standard platform for generating classical and quantum light sources, as well as frequency combs. In optical communications, lithium niobate is the material of choice for electro-optic modulators that require large data bandwidth, high signal integrity and low insertion loss. Conventional lithium niobate devices, however, achieve waveguiding by local perturbation of the crystal (e.g. ion diffusion) with low refractive index contrast, large mode size, and reduced nonlinear interaction strength. As a result, these devices are large, expensive, and require lots of power to operate, posing increasing challenges to modern optics applications that demand scalable solutions with low cost and low power consumption. This thesis describes the realization of integrated lithium niobate photonics by combining the superior material properties of lithium niobate and top-down nanofabrication capability. By utilizing lithium niobate thin films (300 ~ 700 nm thick) bonded on top of silicon dioxide substrate, wavelength scale optical waveguides and cavities with excellent light confinement and strong nonlinear interactions can be realized. In Chapter 2, we first describe the optimization of nanofabrication approaches, in particular the dry etching process, that are capable of delivering on-chip photonic resonators with high quality factors (> 100,000). Next, we show that the optimized thin-film lithium niobate platform can be used for versatile on-chip nonlinear optics applications. In Chapter 3, we propose and experimentally realize two distinct phase matching schemes for second harmonic generation in lithium niobate nanowaveguides, i.e. modal phase matching in uniform waveguides and quasi-phase matching in periodically grooved waveguides. In Chapter 4, we utilize a hybrid system consisting of amorphous silicon phase gradient metasurfaces and lithium niobate waveguides to control the light wave propagation properties inside the waveguides. Based on this platform, we show both efficient linear TE-TM mode conversion and phase-matching-free second harmonic generation. In Chapter 5, we show that by shrinking down the waveguide dimensions, lithium niobate electro-optic modulators can be brought into a new design paradigm which allows for much higher data bandwidth and lower driving voltage than their bulk counterparts. In Chapter 6, we present the design of photonic crystal nanogroove cavities with high quality factor and small modal volume that are promising for ultra-compact on-chip switches and nonlinear wavelength converters.