Lithium niobate materials have attracted extensive attention because of their wide transparent window, high second-order nonlinear coefficient and linear electro-optical coefficient, and stable chemical properties. Lithium niobate crystal has excellent thermoelectric effect, piezoelectric effect, inverse piezoelectric effect and photorefractive effect, and has become one of the preferred materials for advanced photonic components.

F-P cavity, microring cavity, microdisk cavity and microsphere cavity are important components of photonic integrated chips, and there is still room for further compression of the volume of the photon region. The photonic crystal cavity can localize the light at sub-wavelength size and exhibit flexible and adjustable dispersion curve, which has certain advantages over the traditional whispering gallery mode resonator. Lithium niobate sensors, modulators and opto-mechanical couplers have been gradually realized on photonic crystal platforms. Traditional review articles on lithium niobate photonics often focus on the fabrication process and performance parameters of lithium niobate waveguides, whispering gallery mode resonator and periodically poled lithium niobate. The fabrication process, characterization and performance parameters of thin film lithium niobate photonic crystals have not been reported. Due to the material redeposition effect caused by stable chemical properties, the preparation process of thin film lithium niobate photonic crystals is often different from that of silicon photonic crystals. In addition, the performance parameters of each component of thin film lithium niobate photonic crystal should also be summarized.

In order to fill the relevant gaps, the team of Professor Chen Yuping from Shanghai Jiao Tong University reviewed the common preparation, characterization and main applications of thin film lithium niobate photonic crystals. The relevant results were published in Chinese Optics Letters, Volume 22, Issue 3, 2024 Ge Rui, Wu Jiangwei, Liu Xiangmin, Chen Yuping, and Chen Xianfeng, Recent progress in thin-film lithium niobate photonic crystal [J], Chinese Optics Letters, 2024, 22(3): 033602.

In this review, several fabrication techniques of thin film lithium niobate photonic crystals are summarized, such as focused ion beam etching, electron beam lithography combined with argon ion etching, ion beam enhanced etching and femtosecond laser direct writing. The processing parameters, auxiliary materials and advantages and disadvantages of each method are summarized. For each fabrication process, the hole-etching effect corresponding to different processing parameters and the quality factor of the corresponding microcavity are shown.

Several common characterization methods of thin film lithium niobate photonic crystals are summarized, such as fiber coupling, end-face coupling, spatial light coupling, and grating coupling. The application scenarios of different coupling methods are compared.

In the third part, the main applications of thin film lithium niobate photonic crystals are summarized, such as parametric up conversion, parametric down conversion, sensors, modulators and opto-mechanical couplers. The performance parameters of each component are shown.

The main challenges and possible solutions for the preparation of thin film lithium niobate photonic crystals are reviewed. The main development directions of thin film lithium niobate photonic crystals in the future are pointed out. In the future, various optical components based on thin-film lithium niobate photonic crystals can be integrated into the same chip, as shown in Figure 1. In the future, the team will focus on thin film lithium niobate photonic crystal active devices such as lasers to further broaden the application range of lithium niobate photonic components.

Schematic of integrated LNPhC devices, including wavelength converter, sensor, modulator, opto-mechanical cavity, and superprism.