Authors: Zhihua Yang | Shilie Pan
Source: Chinese Science Bulletin (Chinese Version) (article, 2024-07-01)
Field: Materials Science — Electronic, Optical and Magnetic Materials

Fluorine-driven materials design of novel deep-ultraviolet nonlinear optical crystal materials: Research progress and challenges <p indent="0mm">Deep-ultraviolet (UV) all-solid-state lasers (with a phase-matching wavelength shorter than <sc>200 nm)</sc> have important application value in scientific research, industry, and other fields due to their short wavelength and high photon energy. Currently, the most effective way to achieve practical deep-UV laser output is through deep-UV all-solid-state laser technology based on the frequency conversion technique by using deep-UV nonlinear optical crystals. The exploration of deep-UV nonlinear optical crystals is still urgent since it constrains the development and application of all-solid-state deep-UV lasers. Deep-UV nonlinear optical crystals must satisfy the basic properties of “large bandgap, large second harmonic generation effect, and large birefringence” to achieve the high-efficiency output of deep-UV lasers via second harmonic generation. However, achieving a balance among these properties is a key scientific problem to be solved in the design of deep-UV NLO materials, as the mechanisms underlying these key properties are complex and mutually constrained. In the past <sc>63 years</sc> of the development of nonlinear optical crystals, borates have been the main system for exploring deep-ultraviolet (UV) nonlinear optical crystals, whose microstructural motifs are mainly [BO<sub>3</sub>]. The [BO<sub>3</sub>] planar unit exhibits a large microscopic hyperpolarizability and polarizability anisotropy, which is conducive to achieving a large second harmonic generation (SHG) effect and high birefringence. In the 1990s, beryllium-containing borate fluorides, represented by KBe<sub>2</sub>BO<sub>3</sub>F<sub>2</sub> (KBBF), were discovered, driving the development of deep-UV laser technology. However, due to its layer growth habit, it is difficult to grow KBBF along the c-axis, which hinders its wide application based on prism coupling technology. Since then, the exploration of deep-UV crystals has been ongoing. In this paper, we review the potential deep-UV nonlinear optical crystals that have been initially evaluated (or calculated) to have the shortest phase-matched wavelengths extending into the deep-UV region. Based on their compositions, these crystals include borate fluorides, borates (without fluorine), carbonate fluorides, borophosphates, fluorooxoborates, fluorophosphates, and others. In 2017, it was proved for the first time that the introduction of fluorine into covalent tetrahedral groups can serve as deep-UV functional units to modulate optical properties, and a series of new deep-UV nonlinear optical crystals of fluorooxoborates were designed. This opens a new system for the design of deep-UV nonlinear optical materials. For fluorooxoborates, the non-planar fluorooxoborate units are crucial for achieving the balance of “large band gap-large SHG effect-sufficient birefringence”. Since the advent of deep-UV nonlinear optical materials, significant achievements have been made in the development of theoretical systems, the identification of deep-UV nonlinear optical functional motifs, the expansion of deep-UV design strategies, and the innovation of deep-UV material systems. Facing the development needs of laser technology, research on deep-UV nonlinear optical crystals still has a long way to go, and new original innovations and breakthroughs are still needed in material design, crystal growth technology, overcoming performance limitations, and novel applications in unexplored areas. Specifically, how to effectively leverage AI technology in traditional areas like crystal growth and accelerate the discovery of new crystals remains one of the challenges. Computer science Nonlinear Optical Materials Research ; Crystal Structures and Properties ; Nonlinear Optical Materials Studies