The excellent electro-optic (EO) and nonlinear optical properties of lithium niobate ( or LN) have long established it as a prevailing photonic material for the long-haul telecom modulator and wavelength-converter markets. Indeed, the first nonlinear experiment in any waveguide platform was a demonstration of Cherenkov radiation from titanium-diffused LN.1 Conventional LN waveguides are most commonly formed by in-diffusion of titanium (Ti) dopants2 or by the proton exchange process.3 However, these conventional lithium niobate optical waveguides have a low index-contrast, hence are bulky compared with modern integrated platforms, such as silicon photonics. The bulkiness impedes photonic circuit implementations and imposes high optical power requirements for nonlinear applications.
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