Ultra-high-<i>Q</i> photonic crystal nanobeam cavity for etchless lithium niobate on insulator (LNOI) platform

The expansive spectral coverage and superior optical properties of lithium niobate (LN) offer a comprehensive suite of tools for exploring novel functionalities. Achieving high-quality (<italic>Q</italic>) photonic resonator cavities is crucial for enhancing light-matter interactions. However, this task is challenging as the device performance is heavily dependent on the fabrication quality of the LN. In this paper, we present experimental validation of an etchless approach to fabricating high-<italic>Q</italic> photonic crystal nanobeam cavities (PCNBCs). We successfully fabricate PCNBCs with <italic>Q</italic> factors exceeding 10⁵ while maintaining high transmittance by capitalizing on the low waveguide loss and high fabrication tolerance of TE-polarized mode. Remarkably, the <italic>Q</italic> factor achieved here exceeds previous reports on etchless LN PCNBCs by over an order of magnitude. Benefiting from this advancement, we further explore a variety of optical functions, including thermo-optic tuning, optically induced bistability, and Fano line shapes generation. These findings present promising prospects for a versatile platform technique, facilitating the development of high-performance electro-optic or acousto-optic modulators, optical logic devices, and quantum photonics, highlighting its significant impact in the field of photonic integration.