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系列硼酸盐紫外非线性光学晶体的结构性能及材料设计研究
张兵兵
学位类型博士
导师杨志华
2016-05-26
学位授予单位中国科学院大学
学位授予地点北京
学位专业物理电子学
关键词非线性光学材料 硼酸盐 氟硼酸盐 密度泛函 原子切割
摘要

紫外/深紫外激光在激光微加工,激光光刻,高分辨率光发射光谱光源等领域有着广泛的应用。非线性光学晶体材料是拓展激光波段至紫外/深紫外的核心材料。从激光发展的近几十年间非线性光学晶体研究取得了重要的进展,目前已产业化的晶体包括β-BaB2O4(BBO), LiB3O5(LBO), CsB3O5(CBO), CsLiB6O10(CLBO)并已成功应用于可见-紫外波段。KBe2BO3F2 (KBBF)是目前唯一可通过直接倍频输出深紫外波段 (<200 nm)的非线性光学晶体,但是KBBF因其含有剧毒的Be元素同时难以克服层状生长习性,限制了它的应用。随着激光技术发展,目前已有的晶体无法满足需求,新的性能更优异的可用在紫外波段非线性光学晶体材料和可以替代KBBF的深紫外非线性光学材料亟待开发。一种合格的紫外/深紫外非线性光学晶体必须满足以下基本条件:透过范围达到深紫外、非线性光学效应大、双折射率适中、光学损伤阈值高、易于生长大尺寸单晶,化学性质稳定。如何进行有效设计制备满足指标功能的材料成为目前该领域的关键难题。但是传统的通过试错实验寻找新材料的方法成功率低、研发周期长,阻碍了新材料的研发。我们从材料的结构-组成-性质之间的关系出发,基于第一性原理方法,开发材料性能分析程序,计算和研究了一系列材料的非线性光学性质,揭示了非线性光学效应微观机理,标定了与深紫外非线性光学材料基本性能要求相关的结构特征。基于此,我们通过调控结构,设计合成了性能更优异的紫外非线性光学材料。(1)系统研究碱金属/碱土金属硼酸盐体系结构-光学效应微观机制,揭示(2p-π*)“电荷转移激发”是该材料体系非线性光学效应的关键机理。碱金属硼酸盐是一类很有优势的深紫外非线性光学材料候选体系,在复合碱金属硼酸化合物体系中的我们组发现了一类深紫外二阶非线性光学材料Li3Cs2B5O10 (L3CBO), Li4Cs3B7O14 (L4CBO)和Li6Rb5B11O22 (L6RBO)。它们可以用一个化学通式LinMn-1B2n-1O4n-2 (M=Cs/Rb, n=3, 4, 6)概括。这三个晶体都拥有短的紫外截止边(低于190 nm)和类似的BO基团拓扑结构,并通过LiOn (n= 4, 5), Rb/CsOn (n=8, 9, 10)多面连接。我们通过能带结构的能带解析 (band-resolved) 和倍频密度 (SHG-density) 方法研究了这类晶体的非线性光学机理。研究结果显示,氧原子的占据态未成键2p轨道到BO基团中BO3平面的未占据π*和 2p间的“电荷转移激发”是该材料体系非线性光学效应的关键机理。通过系统分析晶体结构和非线性光学效果之间的关系,我们发现BO基团的的排列方式是影响这一系列材料的倍频效应的主要因素,因此我们以倍频效应最大的Li4Cs3B7O14为模板材料,设计并用固相反应合成出了一个新的晶体Li4Rb3B7O14,它与Li4Cs3B7O14同构,拥有短的紫外截止边(低于190 nm)和2/3×KDP的倍频效应。该研究为非线性光学材料的发现提供了一种新的理论指导实验合成的依据。(2)研究紫外/深紫外非线性光学材料光学各项异性及相匹配波长调控方案。从之前的研究我们可以看出,紫外/深紫外非线性光学材料,除了应具有较宽的带隙、较大的倍频效应之外,还需满足相位匹配要求的折射率色散和双折射率指标。氯硼酸钾(K3B6O10Cl, KBOC)是我们组发现的性能优异的非线性光学晶体,其相位匹配波长(倍频波长)可达到272 nm,但无法满足 266 nm或更低波长应用。我们通过原子切割分析方法发现KBOC和KBOB (K3B6O10Br) 的B6O10基团是材料光学各项异性的主要来源。鉴于此,我们提出了利用外界压力将类平面的B6O10更加平面化从而增加双折射的调控方案,并在密度泛函理论(DFT)基础上得到验证。研究发现加压引起的光学异性增加可以显著拓展KBOC在紫外区的相匹配范围,致使倍频波长紫移30-50 nm。通过模拟加压的研究,为实现非线性光学晶体更短相干光输出提供了一种压力工程方案。(3)通过引入F离子至BO基团平衡双折射率和截止边的矛盾关系,进行深紫外用非线性光学材料设计研究。基于硼酸盐体系结构-非线性光学性能、光学各项异性研究,发现硼酸盐很难平衡深紫外非线性光学晶体的各项标准,我们认为若要满足深紫外非线性晶体的基本条件,需对BO基团进行离子引入调控来消除端氧的悬挂键使吸收边紫移,同时增大晶体的光学各项异性。因此,我们希望找到一种潜在的可以突破传统硼酸盐限制的深紫外非线性光学材料体系。分子水平的第一性原理计算表明,(BO3F)4?, (BO2F2)3?和 (BOF3)2?四面体基团有较大的固有偶极矩,拥有比BO4更大的极化率各项异性,大于BO3基团的带隙,且拥有非常大的超极化率。通过分析和筛选,我们发现[BOF]基团是非常优异的、可平衡深紫外非线性光学材料各项条件的功能基团。计算表明已发现的氟硼酸盐中,Li2B6O9F2是一种非常有前景的深紫外非线性光学晶体,其结构特点有效平衡了双折射和带隙这两个基本要求,同时拥有较大的非线性系数。我们在密封的石英管中用高温固相法合成了Li2B6O9F2纯相,粉末倍频实验测试表明该材料可是实现266 nm倍频输出并满足相匹配,效应达到1/3的BBO。进一度的理论分析显示,正是[BOF]基团平衡了相互矛盾的材料指标,使得该材料可以突破传统材料的限制。该工作表明,氟硼酸盐是一类潜在的紫外/深紫外非线性光学晶体材料,为深紫外非线性光学的合成指出了一个非常有前途的体系。(4)基于电荷密度分布的原子贡献分析软件研发。为加速材料研发,实现材料设计,关键技术问题就是如何正确鉴定基团贡献。原子实空间波函数切割是一种可以确定材料中特定原子或原子基团对材料某些性能贡献的重要方法,作为目前重要且唯一的量化工具在对材料光学性质的分析方面有不可替代作用。这种将原子作为球体进行切割的缺点是需人为确定切割半径,这种方法不能实现全空间的切割,破坏了共价键的自然属性。为此我们发展了一种基于材料电荷密度分布,在原子之间电荷密度最小的位置将原子分开的切割策略——AIM原子切割方法。这种方法避免人为设定切割半径,实现了全空间不留间隙切割。该方法研发可有效划分或鉴定功能基元,为建立功能基元数据库、材料设计提供支持。不仅可以用于线性和非线性光学性质的计算,而且可以推广到更多材料性质的结构性能分析,对设计特定功能材料具有重要价值。

其他摘要

Ultraviolet/deep-ultraviolet (UV/DUV) laser has numerous applications such as superhigh-resolution photoemission spectroscopy, laser micromaching and photolithography. Nonlinear optical (NLO) crystals as the key materials to extent the work range of laser wavelength have made an important progress. Many borates have been widely used in UV/DUV and visible regions, such as β-BaB2O4 (BBO), LiB3O5 (LBO), CsB3O5 (CBO), CsLiB6O10 (CLBO). KBe2BO3F2 (KBBF) is the only practically usable DUV material to date that can generates coherent light of wavelength below 200 nm by the direct second harmonic generation (SHG). Unfortunately, KBBF contains the highly poisonous element beryllium and features a strong layer growth habit which hinders its applications. Thus, it is of great requirement to discover other new DUV NLO materials. It is a great challenge to discover new DUV NLO crystals because of rigorous prerequisites. A good DUV NLO crystal must simultaneously meet the following basic criteria that include a wide transparency window down to the DUV spectral region, a large SHG effect and a sufficient birefringence to achieve phase matchability, the resistance of the crystal to high laser intensity, ease of growth, chemical stability. In this work, the plane wave pseudopotential density functional theory method was used to investigate the structure-compersion-properites relationship of a series of borates NLO materials. The key mechanism of NLO properties of these materials was revealed. The structure that related features of the high SHG response, birefringence, and band gap were highlighted and studied. Accordingly, the new UV/DUV NLO crystals could be designed and synthezied.(1) Alkali metal borates are promising DUV second-order NLO materials. Here we study the mechanism of NLO properties of this class of crystals, Li3Cs2B5O10 (L3CBO), Li4Cs3B7O14 (L4CBO), and Li6Rb5B11O22 (L6RBO). The results reveal that the “charge-transfer excitation” from the non-bonding 2p occupied states of O atoms to the π* and 2p unoccupied states of BO3 substructure is the key mechanism of NLO properties of this material family. Through systematic analyses on the relationship between crystal structure and NLO effects, a new crystal, Li4Rb3B7O14 was designed and subsequently synthesized through solid state reaction, which is isomorphic with Li4Cs3B7O14, exhibits a short UV cutoff edge (below 190 nm) and a SHG response of 2/3 × KDP. Unfortunately, this series materials exhibit a very small birefringence that restrict the phase-matching in DUV range. (2) Small birefringence restricts various crystals from achieving DUV laser output although they exhibit short UV cutoff edges and high SHG intensities. An access to achieve deeper coherent light output through external pressure on NLO crystal, K3B6O10Cl is proposed and demonstrated through computer experiment based on the first principles theory. It is found that the quasi-planar (B6O10)2? group is the dominant contributing unit to optical anisotropy. The pressure-induced increase of polarizability anisotropy of (B6O10)2? group can notably enlarge birefringence which extends the shortest achievable wavelength of K3B6O10Cl frequency conversion. The results show that pressure engineering may be a promising scheme to overcome the drawback of small birefringence of some NLO crystals. The electronic structure show that the p orbitals of Cl and Br in these two crystals located at Fermi level that result in a red shif of band gap. Meanwihile, their birefringences are too small to achieve phase-matching in 266 nm.(3) It is a huge challenge to discover other DUV LNO materials in borates which simultaneously satisfy the criteria mentioned above due to the limitation of BO groups’ natural character of structure. It is a great challenge to discover new DUV NLO crystals because of rigorous prerequisites. Here, a material system is proposed as promising candidates that could break the limitation of traditional borates as new DUV NLO crystal materials. Li2B6O9F2 is found to be an attractive candidate for the next generation of DUV NLO material according to the prediction on existent materials. UV-Vis-NIR diffuse reflectance analysis indicates that the short-wavelength absorption edge of Li2B6O9F2 is below 200 nm. Powder second-harmonic generation (SHG) tests show that Li2B6O9F2 exhibits a large SHG signal of approximately 1/3 times that of β-BaB2O4 (BBO) under doubled-frequency light at 266 nm, and it is also phase-matchable at the above wavelengths. Detail analysis shows that the fundamental building blocks (FBBs) in this material are the key to balance the inter-constraint requires.(4) The crucial question is how to determine the contribution of specific atom or atom groups in molecular or crystals materials to speed up the development period. Atom cutting method is one of tools to identify the contribution of each atom or atom groups. For this purposes, we develop a Bader atom cutting analysis program according to Bader AIM theory. This method overcomes the disadvantage of the present spherical atom cutting method. It could divide space into regions by surfaces that run through minima in the charge density. This method could cut atoms without remnant spaces that spherical cutting method cannot avoid. Furthermore, it does not need to set a cutting radius. Test shows the atom-cutting method could more precisely analyze the contribution of specific atom or atom groups in condensed material or molecular to optical properties or other properties that calculated from wave function. This tool could find out the property-active structure feature and further permit to find the corresponding criteria and to guide the design of functional materials.

文献类型学位论文
条目标识符http://ir.xjipc.cas.cn/handle/365002/4589
专题材料物理与化学研究室
作者单位中国科学院新疆理化技术研究所
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张兵兵. 系列硼酸盐紫外非线性光学晶体的结构性能及材料设计研究[D]. 北京. 中国科学院大学,2016.
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