Athermal lithium niobate microresonator.

Lithium niobate (LN), possessing wide transparent window, strong electro-optic effect, and large optical nonlinearity, is an ideal material platform for integrated photonics application. Microring resonators are particularly suitable as integrated photonic components, given their flexibility of device engineering and their potential for large-scale integration. However, the susceptibility to temperature fluctuation has become a major challenge for their implementation in a practical environment. Here, we demonstrate an athermal LN microring resonator. By cladding an x-cut LN microring resonator with a thin layer of titanium oxide, we are able to completely eliminate the first-order thermo-optic coefficient (TOC) of cavity resonance right at room temperature (20C), leaving only a small residual quadratic temperature dependence with a second-order TOC of only 0.37 pm/K. It corresponds to a temperature-induced resonance wavelength shift within 0.33 nm over a large operating temperature range of (-10 - 50)C that is one order of magnitude smaller than a bare LN microring resonator. Moreover, the TiO-cladded LN microring resonator is able to preserve high optical quality, with an intrinsic optical Q of 5.8 × 10 that is only about 11% smaller than that of a bare LN resonator. The flexibility of thermo-optic engineering, high optical quality, and device fabrication compatibility show great promise of athermal LN/TiO hybrid devices for practical applications, elevating the potential importance of LN photonic integrated circuits for future communication, sensing, nonlinear and quantum photonics.