Lithium niobate (LN) crystal is already one of the most widely used optical materials , due to its superior electro-optical properties which can efficiently convert electronic signals into optical signals. With the widespread use and maturity of semiconductor manufacturing processes on commercial optical-grade lithium niobate films, the material’s mechanical, thermal, and acoustic properties have begun to be significant. With its excellent nonlinear optics (electro-optical, nonlinear frequency conversion, photorefractive, etc.), the performance of thin film lithium niobate will gradually find a series of applications in the field of integrated photonic information in the future. People are beginning to look forward to the arrival of the ultra-bandwidth "optical valley" era that is not affected by the semiconductor carrier migration rate.

However, lithium niobate (LN) crystal material has a low optical damage threshold (0.12 GW/cm2) and opacity in the deep ultraviolet band (LN, 350nm-5000nm), which limits its applications in high power devices and deep ultraviolet bands. In contrast, lithium tantalate (LT) crystal material has a wider and lower transparent band (280nm-5500nm) and a higher damage threshold (0.5 GW/cm2), which makes it capable of operating in the ultraviolet band and high power functional devices.

We fabricated an on-chip LTOI microdisk with a high damage threshold and high Q factor (105 ) at the telecommunication band based on LTOI by using the FIB milling method. Using the device, we have achieved efficient SHG and cascading THG. The LTOI microdisk can be loaded with more than 500 mW input power without damaging it, making the SH power to reach the microwatt level (2 µW). The order of magnitude of the laser damage threshold of lithium tantalate film was predicted to be about 100 KW/cm2. What is more, we observed about an order of magnitude Q-factor improvement on some modes after high input power “annealing.”

Fig. 1.  Left : LTOI sample and fabrication processing flow of an LTOI microdisk. Right: Transmission spectrum of a 50 um diameter z-cut LTOI microdisk in the telecommunication band. Lorentzian fitting of a measured mode around 1544.24 nm indicate by a black arrow in (a), exhibiting a Q factor of 2.67 × 〖10〗^5.

Fig. 2.  (a) SH signal generated around 773.68 nm. Inset: the optical image of scattered SH wave and simulated mode profile of the FW, SH waves. (b) Quadratic relationship of the SH power on the input FW power. (c) TH signal generated around 515.91 nm. Inset: the optical image of scattered SH and TH waves and the simulated mode profile of TH waves. (d) cubic relationship of the TH power on the input FW power. White arrow indicates the polarization, horizontal arrow: TE and Vertical arrow: TM.

Fig. 3. Quadratic simulated relationship of the SH power on the high input FW power from 70 mW to 503 mW. Inset: the optical image of scattered SH and TH waves with high input power.

This research was published in “Xiongshuo Yan, Yi’an Liu, Licheng Ge, Bing Zhu, Jiangwei Wu, Yuping Chen*, and Xianfeng Chen, High optical damage threshold on-chip lithium tantalate microdisk resonator，Optics Letters, 45(15), 4100 (2020)”.

Link: https://doi.org/10.1364/OL.394171