Titanium Oxide-coated Lithium Niobate

Background

Microring resonators are particularly suitable as integrated photonic components given their flexibility of device engineering and potential for large-scale integration, but their susceptibility to temperature fluctuation has become a major challenge for their implementation in a practical environment. The realization of lithium niobate (LN) microring resonators insensitive to temperature fluctuation is thus crucial for a variety of applications in communication, sensing, and nonlinear/quantum photonics. LN possesses a wide transparent window, strong electro-optics effect, and large second-/third-order nonlinearities, but exhibits a strong photorefractive effect that induces optical damage when the optical intensity becomes significant inside the material. To mitigate this issue, the LN crystal has either to be doped with certain ions (such as Mg) via a sophisticated doping process to increase the threshold of optical damage, or to be processed via a complicated day-long “optical cleaning” procedure at elevated temperatures above 180°C.

Technology Overview

Researchers propose a novel approach to eliminate the photorefractive effect in thin-film lithium niobate by coating it with a thin layer of titanium oxide. Titanium oxide cladding has been used to engineer the thermo-optic properties of silicon and silicon nitride photonic waveguides. However, it has never been applied to address the photorefractive effect.

Benefits

For a bare LN microring resonator without the titanium oxide cladding, the transmission spectrum is dramatically distorted and the resonance wavelength is significantly shifted when the optical power increases inside the resonator—a combined effect of thermo-optic nonlinear effect and photorefractive effect. In strong contrast, for the LN microring resonator with a titanium oxide cladding, the transmission spectrum remains nearly intact and the wavelength position of the cavity resonance remains fixed even when the optical power increases to about 6.1 W inside the cavity, indicating that both the thermo-optic nonlinear effect and the photorefractive effect are eliminated in the resonator. Additionally, cladding greatly suppresses the temperature sensitivity while keeping the optical quality factor nearly intact.