Lithium niobate on insulator (LNOI) has excellent electro-optical and nonlinear optical properties，which has been considered as the most promising basic material for the integrated photonics. Whispering gallery mode (WGM) optical microresonators are important building blocks in photonic integrated circuits. They are also ideal platforms for studying various optical phenomena. LNOI microdisk resonators therefore have great potential application prospects. There are rich and diverse modes in LNOI microdisk resonators. Operation of such cavities on specific lower- or higher-order transverse modes has much interest in application perspectives.

We demonstrate active mode selection through introducing nano defects in LNOI microdisks by focused ion beam (FIB) method. FIB method has the advantage of easy processing and high resolution. Either fundamental or high-order transverse WGMs can be retained by properly designing the size and location of the defects. There is no severe degradation in quality factor of microdisks with defects by characterizing the transmission spectra. Figure 1 shows the simulated electric field intensity distribution of different radial order TE(q, m) modes in a LNOI microdisk with a radius of 25 μm and a thickness of 300 nm. The area enclosed by the white rectangle is the microdisk, and the surrounding area is the air. As the radial mode number q increases from 1 to 6, the maximum of the electric field profile moves toward the center of the microdisk. The main idea of the proposed method is to introduce extra defects on the surface of the microdisk to disturb WGMs by increased scattering loss while leaving the designated one least impacted. The nano slit acts as a strong scattering center, so as to suppress the excitation of the specific transverse mode and finally to achieve the mode selection. Figure 2 shows spectral simplification in microdisk with defects of different sizes and locations. For sample S2, the modes of TE(q=2), TE(q=3) and TE(q=4) are cleared, while the TE(q=1), TE(q=5) and TE(q=6) modes are retained. For sample S3, only TE(q=5) and TE(q=6) modes remain. For sample S4, The spectrum is sparser than that of S3, with the 5th- and 6th-order radial modes retained.

This study achieves active mode selection, which fills the gap in the mode control of the lithium niobate material microresonator, and further promotes the development of lithium niobate-based photonic integrated circuits.

Fig. 1. Calculated mode profiles for different radial order TE (q, m) modes.

Fig. 2. (a)-(d) Transmission spectra corresponding to the S1, S2, S3 and S4 LNOI microdisks in the communication C-band. (e)-(h) Optical microscope images of the respective samples. (i) The enlarged view of the orange region of the transmission spectrum.

This research was published in “Shubin Su, Xiaona Ye, Shijie Liu, Yuanlin Zheng, Xianfeng Chen, Active mode selection by defects in lithium niobate on insulator microdisks, Optics Express, 29(8), 11885-11891 (2021) ”

Link：https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-8-11885&id=449779