New opportunities for ferroelectric deep-UV nonlinear optical crystals

Deep-ultraviolet(deep-UV)light with the wavelength below200 nm is one of the most crucial elements of both laser-driven industrial and scientific applications in recent years[1].DeepUV light has been produced using a variety of techniques,such as gas discharge lamps,synchrotron radiation,excimer lasers,free electron lasers,and nonlinear frequency conversion.

Clamping effect driven design and fabrication of new infrared birefringent materials with large optical anisotropy

Infrared(IR)birefringent materials with large optical anisotropy and wide transparency range are important for efficient light manipulation in various IR optical devices.Herein,two new IR birefringent materials AMgGeSe_(3)(A=Li,Na)with large optical anisotropy were rationally designed by a rigid octahedron and flexible dimer combined strategy and fabricated in experiment.The introduction of rigid[LiSe_(6)]/[NaSe_(6)]and[MgSe_(6)]octahedra effectively regulates the geometry and arrangement of the flexible[Ge2Se6]dimers,resulting in the birefringence as large as 0.334@1,064 nm in LiMgGeSe_(3) and 0.445@1,064 nm(the largest one in the reported[Ge_(2)Se_(6)]dimer-contained selenides)in NaMgGeSe_(3).Density functional theory(DFT)calculations and statistical analyses highlight the influence of polarizability anisotropy,density,arrangement of units,as well as layer distance on birefringence.The results indicate that AMgGeSe_(3)(A=Li,Na)crystals are the promising IR birefringent materials and it gives an insight into the exploration of new IR birefringent materials with large birefringence based on the clamping effect from rigid groups.

Recent progress on sulfide infrared nonlinear optical materials with large SHG response and wide band gap

As the core device of all-solid-state laser,infrared(IR)nonlinear optical(NLO)material plays a critical role in exploring new coherent lights by frequency conversion technologies,which are widely used in noninvasive medical diagnostics,spectroscopy,environmental monitoring and long-distance laser communication.To satisfy the growing demand of current laser technology in IR regions,high performance NLO materials with large second-harmonic generation(SHG)response,high laser-induced damage threshold(LIDT)and wide transparency range are highly expected.Recently,numerous efforts and advances have been achieved in sulfide system.This review highlights the progress of high-performance sulfide IR NLO materials with a good balance between strong SHG response(≥1.0×AgGaS_(2))and wide band gap(≥3.0 eV)since 2018.The chemical synthesis,crystal structure,optical properties and structure-property relationship of 13 high performance metal sulfide IR NLO materials are summarized and reviewed in detail.Finally,the potential opportunities and challenges in IR NLO materials are discussed.

Fluorooxoborates:A precious treasure of deep-ultraviolet nonlinear optical materials

As the key components of all-solid-state deep-ultraviolet(DUV,λ<200 nm)laser sources,exploring DUV nonlinear optical(NLO)materials has been a research hotspot for a long time.However,as the only material that has been practically used to generate DUV coherent light through the direct second harmonic generation(SHG)process,KBe_(2)BO_(3)F_(2)(KBBF)still meets bottlenecks,namely,severe layered crystal growth tendency,for extensive application.In the search for the next generation of DUV NLO materials,fluorooxoborates are highly expected due to their excellent NLO performances,flexible structures,and rich structural chemistry.So far,a large number of materials with prominent properties have emerged in the fluorooxoborate system.By investigating the differences in structures and properties among fluorooxoborates and borates and borate fluorides,the excellent NLO per-formances of fluorooxoborates are attributed to functionalized B-O/F chromophores.In this review,by analyzing the published fluorooxoborates,the fluorooxoborate system is a precious treasure for exploring DUV NLO ma-terials.The synthesis,structures,and properties of some representative fluorooxoborates in the research history are introduced and summarized.Finally,we put forward the opportunities and challenges of future research in fluorooxoborate DUV NLO crystals.

A Cation-Driven Approach toward Deep-Ultraviolet Nonlinear Optical Materials

The design of new materials with special performances is still a great challenge,especially for the deep-ultraviolet nonlinear optical materials in which it is difficult to balance large bandgaps and strong second harmonic generation responses due to their inverse relationship.Cation variation not only influences the whole structure frameworks but also directly participates in the formation of electronic structures,both of which could lead to the uncontrollability of the properties of the designed materials.Here,a novel approach,aiming at purposeful and foreseeable material designs,is proposed to characterize the role of cations.By the verification of several series of borates,the influences of cation variation on property changes are explored systematically.Accordingly,a feasible strategy of designing deep-ultraviolet nonlinear optical materials by substituting barium for lead has been concluded,which could obviously blue-shift the ultraviolet cutoff edge and maintain the relatively strong second harmonic generation response(more than 2 times of KH2PO4),achieving the property optimization,and especially works efficiently in fluorooxoborates.The property optimization design strategy and the cation characterization method are not only helpful in exploring nonlinear optical materials but also enlightening in material design and selection.

共价键合氟优化氟化硼酸盐的深紫外非线性光学性能

Nonlinear optical (NLO) crystals are pivotal in laser application technology, which can convert the laser frequency to generate coherent light in the deep-ultraviolet (deep-UV, wavelengthλ≤200 nm) to far-infrared spectral range, and widely used in solid-state system detection, tissue imaging, quantum information,and more [1]. To expand the diversity of optical functional units and then discover novel NLO materials.

以LiB_(3)O_(5)为模板设计深紫外零级波片材料

以著名的LiB_(3)O_(5)为模板,通过增强其结构中B_(2)O_(5)的夹角使双折射更小,在封闭体系中通过高温溶液法获得了两种新的用于制造零级波片的双折射晶体Li_(2)MLaB_(18)O_(30)(M=Rb,Cs).有趣的是,相邻的B_(2)O_(5)接近垂直排列,使得它们表现出比商用石英晶体(Δn~0.0092在可见光区域)小得多的双折射率(分别为0.0032@532 nm和0.0027@532 nm).此外,Li_(2)MLaB_(18)O_(30)(M=Rb,Cs)晶体的紫外截止边均低于200 nm,表明它们可用作深紫外区域的零级波片材料.

LiAlS_(2):一种具有超宽带隙和高激光损伤阈值的潜在红外频率转换材料

具有宽带隙、高激光损伤阈值和无双光子吸收的红外非线性光学晶体材料在民用和军事领域具有许多重要应用.本工作通过系统的实验表征和全面的第一性原理研究了LiAlS_(2)晶体的综合光学性质.结果表明,LiAlS_(2)是一致熔融的晶体材料,在2.09μm光源下能够实现Ⅰ类相位匹配,倍频效应(SHG)和有效SHG系数均为AgGaS_(2)的0.2倍;LiAlS_(2)具有目前红外非线性光学晶体材料中最宽的光学带隙,为5.13 eV,激光诱导损伤阈值约为AgGaS_(2)的28倍.此外,对其折射率色散关系的研究发现,LiAlS_(2)为正双轴晶体,最短相位匹配波长为1.98μm;结构-性能关系分析表明[AlS_(4)]四面体决定了其光学带隙,同时是材料SHG响应的主要贡献来源.以上结果表明,LiAlS_(2)是一种潜在的能够在高能激光系统中应用的红外非线性光学材料.

电子离域诱导性能提升的深紫外非线性光学材料模板结构设计研究

探索具有深紫外相位匹配能力的新型非线性光学材料是当前研究热点.本文从硼氧阴离子框架出发,发现由π共轭BO_(3)基团组成一维(BO_(2))∞链的电子离域导致了极化特性增强,可实现非线性光学性能的优化.据此,本文预测了以(BO_(2))∞链为阴离子框架的CaBO_(2)F体系.重点研究了三种能量最低的结构CaBO_(2)F-Ⅰ,CaBO_(2)F-Ⅱ和CaBO_(2)F-Ⅲ,设计了热力学上可进行的反应路径.第一原理计算表明,CaBO_(2)F-Ⅰ,CaBO_(2)-F-Ⅱ和CaBO_(2)F-Ⅲ具有较宽的带隙,合适的双折射(0.076–0.108@1064 nm)和较大的倍频系数(2.1–2.7×KDP).其中,计算得到CaBO_(2)F-Ⅰ的最短相位匹配波长在约为195 nm(基于HSE06带隙修正剪刀算符),证明了含有定向电子离域的(BO_(2))∞链是深紫外非线性光学材料的潜在阴离子框架.本文通过将机理研究与验证性的结构预测相结合实现了自上而下设计深紫外非线性光学结构模板,该研究为探索新型非线性光学材料提供了新的策略.

非线性光学数据库:理论预测助力材料的快速发现

现代激光技术迫切需要能够通过二次谐波产生相干光的非线性光学材料.然而,只有很少一部分非中心对称晶体材料的非线性光学性质被实验或者理论研究,针对高性能非线性光学晶体材料的探索仍非常有限.本工作建立了迄今为止最大的计算非线性光学晶体材料数据库,其中包含2354个非中心对称晶体结构,以及它们的倍频系数和带隙.本文提供了理论数据的计算细节和其与实验数据的对比验证,以及数据库的详细描述.该数据库的重要特点之一是它包含大量通过进化算法搜索发现的新的热力学稳定和亚稳定结构,这为寻找具有更好性能的新型非线性光学材料提供了可能性.

Ba_(2)B_(9)O_(13)F_(4)·BF_(4):first fluorooxoborate with unprecedented infinite [B_(18)O_(26)F_(8)] tubes and deep-ultraviolet cutoff edge

Fluorooxoborate has emerged as a promising system for exploring optical materials due to their rich structural chemistry and physicochemical properties,yet so far not fully studied.Herein,we report a new fluorooxoborate,Ba_(2)B_(9)O_(13)F_(4)·BF_(4),which contains the greatest variety of structural basic units,[BO_(3)],[BO_(2)F_(2)],[BO_(3)F],and[BF_(4)],and exhibits unprecedented 1D infinite[B_(18)O_(26)F_(8)]tubes.The unique structure avoids the dangling bonds caused by terminal oxygens,which is beneficial to the large bandgap and deep-ultraviolet cutoff edge.This article expands the crystal structure of fluorooxoborates and contributes to the exploration of deep-ultraviolet optical materials in fluorooxoborate systems.

An excellent deep-ultraviolet birefringent material based on[BO2]∞ infinite chains

Birefringent materials play indispensable roles in modulating the polarization of light and are vital in the laser science and technology.Currently,the design of birefringent materials operating in the deep-ultraviolet region(DUV,λ≤200 nm)is still a great challenge.In this work,we developed a new DUV birefringent crystal LiBO_(2) based on[BO2]∞infinite chains in the Li-B-O system,which simultaneously achieves the shortest UV cutoff edge(164 nm)and the largest birefringence(≥0.168 at 266 nm)among all the reported borate-based DUV birefringent materials.Single crystals of LiBO_(2) with dimensions up to Ø55×34 mm^(3) were grown by the Czochralski method,providing access to large-sized single crystal with low cost.Moreover,it has a high laser damage threshold and stable physicochemical properties.These outstanding characters unambiguously support that LiBO_(2) can be an excellent birefringent material for DUV application.