Lithium niobate (LiNbO₃ or LN) is a well-known multifunctional crystal that has been widely applied in various areas of photonics, electronics, and optoelectronics. In the past decade, “ion-cut” has become the key technique to produce wafer-size, high-quality, sub micrometer-thickness crystalline LiNbO₃ thin films, i.e., lithium-niobate-on-insulator (LNOI). With the rapid development of LNOI technology and the tremendous progress of associated surface structuring and engineering techniques over the last few years, many novel chip-integrated LiNbO₃-based devices and applications with reduced cost, complexity, power, and size, are demonstrated, boosting the resurgence of integrated photonics based on this material. The remarkable achievements are largely facilitated by the most recent technological progress in photonic integration and performance optimization of LNOI on-chip devices, such as high-quality surface domain engineering, advanced heterogeneous integration technology, powerful dispersion engineering, fine polishing lithography, and wafer-scale fabrication. Accordingly, batch-compatible chip-integrated platforms for more complex photonic integrated circuits, such as quantum optical circuits, are within reach. This article provides a timely review of the key advances in LNOI technology and a reasonable perspective on the near-future directions for both integrated photonics and applied physics communities.

The layered two-dimensional (2D) materials, e.g., the well-known graphene, transition metal dichalcogenides, and topological insulators, have attracted great interest of researchers from fundamental research as well as industries owing to their intriguing properties in a number of aspects. In practical applications, the efficient modification of the features of 2D materials is compelling and essential to achieve desired functionalities. Ion implantation has been successfully applied to synthesize graphene. The most exotic advantage of ion beam technology is that it offers distinct options of energetic beams, which has recently shown the unique capability to modify and tailor the properties of versatile 2D materials. To name a few, the energetic ion beams could implement the surface morphology or layer-to-layer structural engineering of 2D materials. At the microcosmic level, the introduction of ion beam induced defects and intentional doping of specific ions are the basis of tailoring properties of 2D materials. By manipulating the parameters of ion beams (energies, species, fluences, incident angles, etc.), the modified 2D materials may possess novel properties, which are unprecedented in pristine ones. Promising applications based on these 2D materials with ion beam tailored features have been realized in a broad range of fields. In this work, we systematically review the latest research progress on the deployed techniques and property modifications of graphene and other 2D materials under the treatment of various ion beams. A few selected applications are presented to indicate the practical potentials of ion beam modified 2D materials in distinct areas. Perspectives on future developments are also provided by focusing on several promising topics.

Optical waveguides are far more than mere connecting elements in integrated optical systems and circuits. Benefiting from their high optical confinement and miniaturized footprints, waveguide structures established based on crystalline materials, particularly, are opening exciting possibilities and opportunities in photonic chips by facilitating their on-chip integration with different functionalities and highly compact photonic circuits. Femtosecond-laser-direct writing (FsLDW), as a true three-dimensional (3D) micromachining and microfabrication technology, allows rapid prototyping of on-demand waveguide geometries inside transparent materials via localized material modification. The success of FsLDW lies not only in its unsurpassed aptitude for realizing 3D devices but also in its remarkable material-independence that enables cross-platform solutions. This review emphasizes FsLDW fabrication of waveguide structures with 3D layouts in dielectric crystals. Their functionalities as passive and active photonic devices are also demonstrated and discussed.

Optical dielectric crystals play crucial roles in a large number of optical and photonic applications.^{1 – 3} For example, crystalline quartz is an ideal material for waveplates to modify the polarization state of light. Nonlinear optical crystals, such as ${\mathrm{KTiOPO}}_4$,⁴ $\mathrm{\beta }\text{-}\mathrm{BBO}$,⁵ or ${\mathrm{LiNbO}}_3$,⁶ are used to realize selected wavelength conversion in broadband covering visible to the mid-infrared (MIR). Laser crystals serve as gain media that are capable of emitting coherent laser radiation in various solid-state systems.⁷ As an emerging direction of intriguing applications of dielectric crystals, quantum photonic devices are also achievable based on specific crystal materials.^{8 – 15} Regardless of the attractive features of crystals, there is a strong desire to construct on-chip or three-dimensional (3D) devices to implement multifunctions in compact systems.^{16 – 19} In photonics, such devices are based on structures of optical waveguides. The light propagation can be confined in waveguides in a diffraction-free way due to the total internal reflection, in which the optical intensity is able to reach a high level.^20,21 With these advantages, the waveguide-based devices therefore have a compact size, robust functionality, high efficiency, and high-cost effectiveness.²² Optical waveguides based on crystals are attractive for a variety of applications, owing to the combination of advantages of both waveguiding structures and crystal bulk features.^23,24 Toward this purpose, several techniques have been developed to fabricate waveguides in crystal bulks, including metal–ion indiffusion,²⁵ ion exchange,²⁶ optical induction,²⁷ ion implantation,²⁸ and femtosecond laser inscription/writing.^{29 – 31} However, due to the diversity of the crystal structures and physical, chemical, or optical properties, some techniques are only applicable to specific materials. Among these solutions, femtosecond laser writing has been recognized as an efficient tool to fabricate waveguides in versatile crystals owing to the powerful 3D engineering capability and microscale localized refractive index modification.^23,32 The first scenario of dielectric waveguides was realized in glasses through femtosecond laser writing by Davis et al. in 1996.³³ Since then, quite extensive studies have been performed on this topic, and the materials used have also been extended to single crystals, ceramics, and organic materials.^{34 – 38} Various fabrication techniques of laser-written waveguides and related photonic applications have taken a considerable leap forward in the past two decades. For example, using structured light writing, the production of complex waveguides can be implemented in single-way scanning.^39,40 The beam shaping of femtosecond laser pulses can form tracks along selected orientations inside the dielectrics.^41,42 In addition to well-developed waveguide lasers⁷ and wavelength converters,⁴³ emerging applications in quantum photonics have been realized in laser-written waveguides, such as quantum chips^{44 – 49} and memories.^{50 – 53} There are several reviews demonstrating the research progress of the topic from different perspectives;^{9,23,24,31,34,35,37,54} however, since this is a hot topic with rapid development of technical solutions and applications, a timely concise review of the new results obtained in recent years on selected branches is desired, particularly on femtosecond laser-written crystal waveguides. In this work, we concentrate on the crystalline waveguides on the newly developed femtosecond laser inscription techniques, the diverse waveguide geometries, the engineering of waveguide modes, and selected applications by presenting an overview of the recent advances in this topic.