新型深紫外非线性光学晶体“氟驱动”材料设计:研究进展及挑战

Fluorine-driven materials design of novel deep-ultraviolet nonlinear optical crystal materials:Research progress and challenges

深紫外全固态激光(波长短于200 nm)因波长短、光子能量高等特点,在工业、科研等领域有着重要的应用价值.当前,利用可见/近红外全固态激光器为基频光源,通过深紫外非线性光学晶体的非线性谐波技术多级变频,是发展深紫外全固态激光器的有效途径.本文概述了近年来深紫外非线性光学晶体设计与制备方面的重要进展;分析了潜在深紫外非线性光学晶体材料的体系分布情况;重点探讨了新型深紫外非线性光学晶体材料氟化驱动设计策略;介绍了氟化硼酸盐等典型非线性光学材料实例,研究表明,氟与非金属键合的“氟化”对氟化硼酸盐的结构和性质均起到调控作用.最后,展望了深紫外非线性光学晶体面临的挑战,并对新材料设计、晶体生长、性质极限、“无人区”新应用等方面的发展方向进行了讨论.

Deep-ultraviolet(UV)all-solid-state lasers(with a phase-matching wavelength shorter than 200 nm)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 63 years of the development of nonlinear optical crystals,borates have been the main system for exploring deepultraviolet(UV)nonlinear optical crystals,whose microstructural motifs are mainly[BO3].The[BO3]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 KBe2BO3F2(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.