Classical and quantum light sources in thin-film lithium niobate waveguides
Keywords: Humanities and Sciences.
Applied Physics.

Thin-film lithium niobate (TFLN) is a rapidly emerging nanophotonic platform for linear and nonlinear optics. An impressive range of integrated optical components with unprecedented performance have been developed on this platform within the past half-decade. These developments include all aspects of a multi-functional photonic circuit, including various distinct approaches for generation, manipulation, as well as detection of light. It is expected that a seamless integration of these multitude of components will enable numerous novel applications, both classical and quantum. In this dissertation, we focus on one particular approach for optical source development in the TFLN platform: Chi-2 nonlinear optics. We discuss the importance of two engineering techniques, namely, quasi-phasematching and dispersion engineering, which can be obtained jointly only in a nonlinear nanophotonic platform such as TFLN. We design and demonstrate some hitherto inaccessible and highly promising sources in TFLN based on a combination of these techniques. These sources can be categorized based on their potential classical or quantum applications. The classical source involves generation of mid-infrared light via a difference-frequency process of two near-infrared input sources, on a novel TFLN-on-sapphire platform that is transparent up to approximately 5 micron. We demonstrate that such a source can have at least an order of magnitude higher normalized conversion efficiency compared to conventional nonlinear-optic sources. We also show that this source, when dispersion engineered, can have a significantly large bandwidth in the mid-infrared (~700-nm full-width-half-max bandwidth around 3.4 micron generation wavelength) without compromising the conversion efficiency. Such a source is expected to be very useful for spectroscopic applications in the 3-5 micron mid-infrared spectral region. The quantum source involves a first demonstration of using lithium niobate for generation of separable biphotons in the telecommunications band. While unattainable through lithium niobate's material dispersion alone, this source could be achieved via dispersion-engineering and Gaussian-apodized periodic poling of TFLN-on-silica waveguides. Such a source is crucial for indistinguishable heralded single photon generation, which is essential for several photonic quantum computing protocols. We demonstrate a heralded state purity of 86%, as well as a spectral purity upper-bound of ~95%. Other than demonstrating these sources, we also discuss in some detail the development of periodic poling of TFLN for quasi-phasematching in this thesis. We expect that the classical and quantum sources shown in this dissertation will open up new avenues for the TFLN platform's unique capabilities. Furthermore, we hope that in future, the combination of quasi-phasematching and dispersion engineering can also be used for designing such novel sources in other nanophotonic platforms in development, which will possess unique capabilities contingent on those materials' linear and nonlinear optical properties