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基于四面体基元响应机理的深紫外非线性光学材料设计研究
雷兵华
Subtype博士
Thesis Advisor杨志华
2018-05-24
Degree Grantor中国科学院大学
Place of Conferral北京
Degree Discipline物理电子学
Keyword第一性原理 非线性光学 光学各向异性 四面体基元
Abstract

紫外/深紫外非线性光学材料是用于产生短波长激光光源的核心材料。因其能实现短波长、高能量输出等优点,在激光光刻、精密加工、激光医疗及激光武器等领域具有突破性应用。为加快新材料的研发步伐、缩短材料研发周期,基于微观基元对宏观性质响应机理的设计研究是实现激光技术发展急需的性能优异紫外/深紫外非线性光学材料设计制备的有效策略。在设计过程中,具有显著结构各向异性的平面基元(BO3,B3O6等)被认为是获得紫外/深紫外非线性光学材料的优势基元,然而其结构上的各向异性可能会导致晶体生长的层状习性。四面体基元(BO4,PO4)易于构成三维网络结构有利于晶体生长有效避免层状习性,但相较于三角平面基元及化合物的研究,还存在尚未明确的关键科学问题:i)四面体在化合物中的光学性能响应并不明确。传统理论认为,四面体具有弱的光学各向异性,但仍然存在大倍频的四面体化合物。现存的非中心对称结构的四面体化合物都具有较小的光学各向异性,但在中心化合物中却存在双折射率较大的材料。ii)缺乏针对四面体化合物的非线性光学材料的设计策略。在三角平面基元的化合物中,同向排列可以保证化合物的倍频响应,共面排列可以保证化合物的各向异性。但在四面体化合物中,并没有相应的设计策略来探索新型的深紫外/紫外非线性光学材料。因此,以四面体基元为基础,设计能同时满足大带隙、适中的倍频、合适的双折射率要求的深紫外非线性光学材料是一个巨大的挑战。本文以含四面体基元紫外/深紫外非线性光学材料合理设计为目标,揭示了微观结构特别是四面体基元对紫外/深紫外非线性光学材料的核心性能即带隙(紫外截止边)、双折射率(相位匹配波长)、非线性光学效应的影响机理,确定了系列材料影响或决定核心性能的相关结构及电子态,并在此基础之上进行功能模块设计及组装,设计出含四面体基元的深紫外非线性光学材料。本文主要从以下四个方面阐述:1、稀土阳离子对非线性光学性能的影响机制研究通过研究 LaBeB3O7, LaBGeO5, La9Na3B8O27, La2Na3B3O9, La2CaB10O19,Na4La2(NO3)10?2H2O和K2La(NO3)5?2H2O等一系列含稀土元素的化合物,发现具有一定共价相互作用的稀土阳离子对四面体的倍频响应、材料的带隙及光学各向异性都有着重要的影响。稀土阳离子的d轨道与O-2p轨道容易形成(d-p)π键,而它的强弱与相应阴离子基团的分子轨道有关,并决定着材料的带隙与倍频响应。另外,稀土元素周围的配位环境影响着材料的光学各向异性,不均一的配位环境有利于产生大的双折射率。因此,此项研究结果有利于对稀土元素对光学性质及带隙影响的全面了解,为紫外/深紫外非线性材料设计提供了有效信息。2、四面体倍频响应研究倍频效应是非线性光学材料的最重要部分,直接影响激光转换效率。前期研究发现大多数由四面体组成的材料具有较弱的倍频响应,这使得四面体基元常被排除在能增强倍频效应功能基团之外。然后,在近几年的探索中,科学家们也发现了倍频效应较大的四面体化合物。本文选择了光学性能表征较全面的SrBPO5与BaBPO5作为研究对象,探索四面体的倍频响应机制。SrBPO5与BaBPO5带隙分别为6.44及 6.14 eV, 倍频效应分别为KH2PO4 (KDP)晶体的0.57 及 0.43倍.我们通过对四面体相关电子态响应研究发现四面体的基元的倍频响应不仅与四面体基元本身的电子结构有关,阳离子与分子轨道相互作用对四面体基元倍频响应的影响也是不可忽略的。3、光学各向异性研究双折射率是决定紫外/深紫外非线性光学材料角度相位匹配的关键参数。众所周知,具有显著结构各向异性的平面基元有利于获得大的双折射率,而含四面体基元材却不具有明显结构各向异性。针对该问题,我们系统研究了系列含四面体基元材料的双折射率,在研究过程中发现其具有大能隙并且能有效阻止层状习性的四面体基元具有普遍较小的双折率,因此严重影响其在紫外/深紫外的应用。调研中发现YPO4及ScPO4等化合物具有较大的光学各向异性,我们通过数值计算与分析,提出了基于键价理论的评估晶体材料双折射率模型(Response Electronic Distribution Anisotropy, REDA模型)。计算结果表明YPO4等材料的光学各向异性主要来自具有一定角度偏离的PO4四面体基元。该模型不仅可用于四面体化合物,而且也适用于大多数线性与非线性光学材料。更重要的是基于该模型,可以定量分析原子及基元对光学各向异性即双折射率的贡献,这为探索更多更优秀的线性与非线性光学材料提供了理论依据。4、基于四面体基元的光学响应研究设计新型深紫外非线性光学晶体在四面体基元对带隙、双折射率、倍频效应的影响机理研究基础上,我们通过调控阳离子、PO4四面体功能模块组装进行深紫外非线性光学材料设计,设计出当前磷酸盐相位匹配波长唯一可达到深紫外非线性光学材料(α-YSc(PO4)2)。该研究提出基于四面体影响机理的功能模块设计和组装设计策略,设计出深紫外非线性光学材料,对功能导向性功能材料研发提供设计思路和借鉴,为紫外/深紫外非线性光学领域提供备选材料。

Other Abstract

The solid-state laser has many significant applications in a broad area, such as laser micromaching, semiconductor photolithography, photochemical synthesis, laser communication and so on. Owing to the ability to shorten the wavelength of light based on the process of second-harmonic generation (SHG), the nonlinear optical (NLO) crystals become the key material used in the frequency conversion device. With the development of society and technology, the requirement of NLO material is increasing. In exploring next generation NLO materials, the research of structure-property relationship becomes very important. The planar triangle groups are good modules to design ultraviolet (UV) or deep-UV NLO materials but it also induces the layer habit and influences large size crystal growth. However, the tetrahedral modules can form three-dimension crystal structure so that the layer habit can be avoided, but there are two scientific questions which should be addressed: i) The optical response of tetrahedra in the crystal is indistinct. Traditionally, the SHG response of tetrahedra is very weak but in experiment, some compounds consisting of tetrahedra have large SHG effect. For birefringence, in the existed NLO materials only containing tetrahedral groups, the birefringence is very small but in centrosymmetrical systems only containing tetrahedra, there are several crystals which have large birefringence. ii) The strategy for design UV or deep-UV NLO materials is lacking. For planar triangle groups, the parallel arrangement of the groups will guarantee the SHG response and optical anisotropy. However, for the tetrahedral groups, there is no any strategy for design UV or deep-UV NLO materials. Therefore, design UV or deep-UV NLO materials with tetrahedral groups is significant and a big challenge.In this dissertation, based on the existing materials, through studying the structure-property relationship and analyzing the results, we reveal response mechanisms such as SHG response, birefringence, and band gap of tetrahedral compounds. In addition, based on the results, we have designed new NLO materials with high-performance. The main contents are listed below:1. Influence of metal cations (rare-earth element, etc.) on optical propertiesA new crystal, Na4La2(NO3)10?2H2O (NLN) containing these two units NO3 and d0 transition metals,was obtained for the first time by slow evaporation method at room temperature with dimensions of 3 × 2 × 0.75 mm3. It processes an anomalous high UV cutoff edge totally different from alkali/alkaline-earth metals nitrates and trivalent d0 transition metal borates. Meanwhile, K2La(NO3)5?2H2O (KLN) whose structure similar to NLN has a noticable improvement in linear optical property compared to NLN and a large SHG effect.To clarify the inner mechanism or synergy of NO3 and d0 transition metals on influencing electronic structure and optical properties, the trivalent d0 transition metal nitrates such as Na4La2(NO3)10?2H2O and K2La(NO3)5?2H2O have been studied systematically by the first-principles in comparison with related borates. It reveals that the unique (d-p)π interaction generating from trivalent d0 transition metal and oxygen of NO3 hinders the large band gap. In particular, the hierarchical band gap is related to the degree of interaction that depends on the delocalization of O-2p orbitals in the groups, which offers a way to design new functional materials with special characters by controlling cations or anionic groups.2. SHG response of tetrahedra groupsSrBPO5 and BaBPO5 display large experimental gaps (6.44 and 6.14 eV) and moderate SHG coefficients (0.57 and 0.43 times of KH2PO4 (KDP)). Based on the further insight into the relationship between NLO properties and crystal structure within density functional theory (DFT), we discover that BO4 groups and metal cations take essential roles in SHG effect of MBPO5 (M=Sr, Ba) but not PO4 groups as expected. Moreover, PO4 groups play a counteractive role in BaBPO5 because of indirect influence from metal cations.3. Optical anisotropy of compounds with tetrahedral groupsAfter investigating the relationship between properties response and structure feature, the response electronic distrubtion approximation (REDA) model based on bond valence theory is proposed to analyze the optical anisotropy of tetrahedral chromophores compounds. Thus, the study of bonding electron in anionic groups will facilitate the design and synthesizing compounds with desirable properties based on the characters of groups. Further, the optical anisotropy of tetrahedral chromophore compounds with common small birefringence and more excellent nonlinear optical properties can be enhanced and obtained by optimizing the distribution.4. Module design and assembling strategy for deep-UV NLO materialsPhosphates are promising nonlinear optical candidates since these crystals have deep-UV cutoff edges (≤200 nm) and have three-dimensional network structure avoiding the layer habit to obtain large single crystals easily. However, so far, few NLO phosphates can be applied in deep-UV region due to the universal plight of small birefringence. We studied phosphates systematically and characterized the birefringence of crystals by our REDA model. And the functional module controlling birefringence in phosphates was discovered. By assembling with functional modules, a series of NCS phosphates with larger birefringence than existing deep-UV phosphates were designed, which breakthrough the small birefringence wall in phosphates. The large birefringence and appreciate SHG effect render them promising deep-UV NLO materials. Of them, α-YSc(PO4)2, is the sole phosphate meeting the stringent deep-UV NLO criteria up to now.

Pages120
Document Type学位论文
Identifierhttp://ir.xjipc.cas.cn/handle/365002/5460
Collection材料物理与化学研究室
Recommended Citation
GB/T 7714
雷兵华. 基于四面体基元响应机理的深紫外非线性光学材料设计研究[D]. 北京. 中国科学院大学,2018.
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