LiNbO3 has various excellent optical properties with broad transmission window, thus multiple optical applications can be realized in this wide transmission range. In addition, LiNbO3 as a birefringent negative uniaxial crystal, has a high refractive index, which can strongly restrict the light. The whispering-gallery-mode (WGM) microresonator with small mode volumes and low optical loss can effectively confine photons in the cavity for a long time, greatly boosting the interaction between light and matter. Since the nonlinear process strongly depends on the energy density of the light field, thin film lithium niobate (TFLN) microresonators are undoubtedly an ideal platform for studying nonlinear optics. With the aid of a strong pump, SFG can effectively convert low-frequency weak light into high-frequency light with low noise. So cost effective visible light detectors can effectively monitor infrared, far-infrared signals, and even terahertz photons through the SFG process. The frequency and pulse shape of single photons can also be manipulated through the SFG process without destroying their quantum properties.

Though careful simulation and design, we report on the observation of an effectively phase-matched SHG and SFG in an on-chip x-cut TFLN microdisk resonator via cyclic quasi-phase matching (CQPM).

Fig. 1. (a) Schematic of the experimental setup. (b) Transmission spectrum of the TFLN microdisk. (c) The Lorentzian fitting of the resonances at 1553.2 nm.

In the experiment, we prepare an X-LONI microdisk by combining photolithography and chemo-mechanical polishing, measure its transmission spectrum in the wavelength range of 1520 nm-1560 nm, and determine the free spectrum range (FSR). The results are shown in Figure 1, FSR is 6.1 nm, and quality factor Q is 5.8×10^6.

Fig. 2. (a) Spectrum of the generated SFG and SHG. Right inset: 774.3 nm SFG signal scattering from the microdisk; left inset: 767.5 nm SFG signal. (b) The 774.3 nm SFG signal varies with the input Pump 2 power while Pump 1 power is fixed at 5 mW. (c)-(d) Lorentzian fitting (red curve) of the resonances at the wavelengths of 1541.2 nm and 1555.8 nm, respectively.

SFG signal varing with the input Pump is observed by changing the power of one pump light, while the power and wavelength of the other pump light remain unchanged. By linear fitting the experimental data, the SFG normalization efficiency is calculated to be 2.52×10^-4/mW.

Fig. 3. (a) Calculated mode profiles of TE(1,252) and TE(1,244) of Pump1 and Pump2, and TE(3,509) of SFG. (b)-(c) Effective nonlinear coefficient deff and effective refractive index neff varies with the azimuthal angle. (d) Phase mismatch varies with azimuthal angle.

According to the size and mode distribution of the microdisk, the inverted lattice vector of the 13th level grating can be calculated, which just falls on the wave vector mismatch, with four intersections in the range of 0 to 2pi, which means that the pump light and the SFG light can fully meet the phase matching in four positions during the process of propagating in the microdisk for one week. The SFG conversion efficiency is improved by an order of magnitude. This work shows a high efficiency frequency converter, which is expected to be applied in integrated optics on chip in the future.

This research is published in “Jiefu Zhu, Xuerui Sun, Tingting Ding, Yongzhi Tang, Shijie Liu, Yuanlin Zheng, and Xianfeng Chen, Sum-frequency generation in high-quality thin film lithium niobate microdisk via cyclic quasi-phase matching, Journal of the Optical Society of America B, 40(5), D44-49 (2023)”.

Link: https://doi.org/10.1364/JOSAB.482270