Ultralow-loss lithium niobate photonic integrated circuits for nonlinear and electro-optic applications

Doctoral thesis, 2025

Lithium niobate (LN) has emerged as a promising integrated photonic platform due to its unique combination of electro-optic, nonlinear, and acousto-optic properties, combined with broad optical transparency, high refractive index contrast, and ultra-low optical losses. These characteristics enable diverse applications including high-speed modulators, frequency combs, quantum light sources, and nonlinear wavelength converters.
A critical challenge in LN photonics lies in fabricating tightly confined waveguides through dry etching of this chemically and physically stable material. Although the 2017 Harvard breakthrough demonstrated ultralow-loss waveguides using pure physical etching, most subsequent implementations employed partially etched structures with compromised light confinement due to etching selectivity limitations.
To further increase the light confinement, in this thesis, a fully etched LN waveguide with an etching depth of 600 nm was demonstrated. The fully etched waveguides showed significantly improved light confinement (4-fold improvement compared to the partially etched waveguides), while maintaining an ultralow propagation loss of 5.8 dB/m. Relying on the fully etched waveguide platform, we demonstrated a few nonlinear applications, such as high repetition rate Kerr microcombs (500 GHz), octave-spanning supercontinuum combs, and stimulated Brillouin scattering. To extend the functionalities of our LN waveguide platform, we also studied efficient high-speed modulators and sought the possibility of co-integration with nonlinear devices.
The developed platform significantly enhances the light-matter interaction while maintaining fabrication compatibility, opening new possibilities for complex photonic systems.