This special issue includes 11 invited and 5 contributed papers by the active groups on LiNbO3-based photonic devices. The fabrication of new LNOI wafers is the base of high-performance functional devices. Hu et al. reported on the fabrication of new hybrid Si/LiNbO3 thin film platforms and investigated their properties. Zhao et al. studied the ridge waveguide fabrication in LiNbO3 by oxygen ion implantation and precise diamond dicing, which is helpful to understand the ion–lattice interaction process (also being a fundamental of LNOI wafer fabrication). The experimental techniques for characterization of LNOI-based devices are crucial for their applications. Luo et al. demonstrated the polarization diversity of a two-dimensional grating coupler on x-cut LNOI.

One of the major applications of LNOI-based photonic devices is to realize nonlinear frequency conversion, for example, second harmonic generation (SHG). In this special issue, nonlinear LNOI devices have been presented as follows. Lu et al. reviewed the advances of SHG based on the thin film LiNbO3 platform, summarizing the up-to-date progress of nonlinear LNOI devices for integrated photonic applications. Xie et al. studied the effect of dimension variation for LNOI waveguides towards SHG, indicating that the SHG profile and efficiency can be greatly affected by the waveguide cross-section dimension variations. Li et al. theoretically proposed a Si-covered LNOI waveguide structure for highly efficient SHG, which may be useful to design new types of LNOI wafers. Hu et al. reported on local periodically poled LNOI ridge waveguides for SHG, and up to normalized conversion efficiency of 435.5%W−1·cm−2 was obtained. In addition to the typical nonlinear optical effects, Sheng et al. investigated nonlinear Talbot self-healing in periodically poled LiNbO3 crystals, opening up new possibilities for defect-tolerant optical lithography and printing. Li et al. reported on the direct generation of vortex beams in the second harmonic by a spirally structured fundamental wave.

Modulators are typical photonic devices. The configurations of the LNOI-based modulators are multiple. In this special issue, Cai et al. reported on an integrated thin film LiNbO3 modulator based on Fabry–Perot geometry, and Liu et al. realized a wideband thin film LiNbO3 modulator with low half-wave-voltage length product of only 1.7 V·cm based on the Mach–Zehnder configuration. Moreover, Fang et al. produced high-quality-factor optical racetrack micro-resonators (intrinsic Q factor ∼2.8×106) based on LNOIs with an electro-optical tuning range spanning over one free spectral range of 86 pm. Wang et al. demonstrated LiNbO3 microring resonators with intrinsic Q factors over 105 using standard lift-off metallic masks and dry etching, which was considered as a fully compatible process with wafer-scale production.

In addition to the above devices, Bo et al. reported on the fabrication of on-chip erbium-doped LNOI waveguide amplifiers, and a net internal gain of ∼30dB/cm in the telecommunication band was achieved. Jia et al. proposed LNOI-based dielectric metasurfaces to realize surface lattice resonances for enhanced light–matter interaction, offering valuable information for the design and optimization of high-quality-factor optical LiNbO3 metasurfaces. Ge et al. theoretically constructed a broadband and lossless valley photonic crystal waveguide robust against sharp bend and chirality in LiNbO3 valley photonic crystals. This work can provide guidance on the design of the high-performance topological waveguides with a lossless edge state for low refractive index materials.

It is noted that the commercialization of the LNOI wafer has sparked significant on-chip photonic integration requirements in the past five years. To provide a common platform compatible with Si photonics, Hu et al. explored and fabricated the heterogeneous integration of Si and LiNbO3 thin films of 3 inch wafers, which combines the advantages of excellent electric properties and mature semiconductor processing technology of Si, as well as the remarkable photonic, acousto-optic, electro-optic, and piezoelectric nature of LiNbO3. The Si-LNOI wafer will drive new promising devices and potential applications for PICs.

This special issue is expected to provide a glance to the rapidly growing topics in LiNbO3-based photonics, covering selected branches of recent theoretical and experimental research related to LiNbO3-based photonic structures and devices (particularly on thin film LiNbO3 platforms). We strongly encourage our colleagues working in this area to pay attention to this special issue and submit their excellent works to Chinese Optics Letters in the future.

Guest Editors:

Prof. Feng Chen, Shandong University

Email: drfchen@sdu.edu.cn

Prof. Yuping Chen, Shanghai Jiao Tong University

Email: ypchen@sjtu.edu.cn