Femtosecond laser fabrication of ultrahigh Q lithium niobate microresonators (Conference Presentation)

Abstract

The realization of micro-disk resonators (MDRs) of high quality (Q) factors using lithium niobate on insulator (LNOI) as the substrate has spurred great interest in developing on-chip nanophotonic structures which hold the promise for efficient nonlinear wavelength conversion, fast electrooptic light modulation, and high density photonic integration. Here, we report on fabrication of crystalline lithium niobate microresonators with quality factors above 10^7 as measured around 770 nm wavelength, which is almost one order of magnitude higher than the state-of-the-art Q factors around the visible and near-infrared wavelengths reported so far. Our fabrication process includes four steps. First, a thin layer of chromium (Cr) was deposited on the surface of the LNOI by thermal evaporation coating. Subsequently, the Cr film on the LNOI sample was patterned into a circular disk using space-selective femtosecond laser direct writing. Next, the chemo-mechanical (CM) polishing process was performed to fabricate LN MDRs by a wafer polishing machine, the surface smoothness is greatly improved by the CM polishing process, leading to a significant increase of the Q factor. Finally, the fabricated structure was first immersed in Cr etching solution, and then underwent a chemical wet etching in a buffered hydrofluoric acid (HF) solution to partially remove the SiO_2 layer beneath the LN microdisk to produce the freestanding LN MDRs. We have also demonstrated nonlinear processes including second harmonic generation and Raman scattering in our LN MDRs.