*Article* **An Ultra-High-Q Lithium Niobate Microresonator Integrated with a Silicon Nitride Waveguide in the Vertical Configuration for Evanescent Light Coupling**

**Jianhao Zhang 1,2, Rongbo Wu 1,2, Min Wang 3,4,\*, Youting Liang 3,4, Junxia Zhou 3,4, Miao Wu 3,4, Zhiwei Fang 3,4 , Wei Chu 4,\* and Ya Cheng 1,3,4,5,6,7,8,\***


**Abstract:** We demonstrate the hybrid integration of a lithium niobate microring resonator with a silicon nitride waveguide in the vertical configuration to achieve efficient light coupling. The microring resonator is fabricated on a lithium niobate on insulator (LNOI) substrate using photolithography assisted chemo-mechanical etching (PLACE). A fused silica cladding layer is deposited on the LNOI ring resonator. The silicon nitride waveguide is further produced on the fused silica cladding layer by first fabricating a trench in the fused silica while using focused ion beam (FIB) etching for facilitating the evanescent coupling, followed by the formation of the silicon nitride waveguide on the bottom of the trench. The FIB etching ensures the required high positioning accuracy between the waveguide and ring resonator. We achieve Q-factors as high as 1.4 <sup>×</sup> <sup>10</sup><sup>7</sup> with the vertically integrated device.

**Keywords:** lithium niobate microring resonator; silicon nitride waveguide; photolithography assisted chemo-mechanical etching
