探索新型非线性光学功能基团，通过协调基团间的相互作用来优化晶体的性能，设计新颖的功能材料，是当今非线性光学领域的研究热点之一。本论文通过第一性原理计算方法，研究了三类非线性光学材料的晶体结构-电子结构-光学性质间的构效关系，为后期材料设计合成提供了指导性建议，部分理论预测已经得到了实验证实。具体包括以下三方面内容：

一，单氟磷酸盐深紫外非线性光学晶体的理论预测与实验验证。首次提出可用于深紫外区域的新型功能基团-[PO₃F]^2-：与各向同性的[PO₄]^3-相比，[PO₃F]^2-拥有略窄的能隙(9.4 eV)和更大的极化各向异性和超极化率。前线轨道分析指出不对称的[PO₃F]^2-的电子分布是光学各向异性增强的主要原因。为了判断含[PO₃F]^2-的单氟磷酸盐是否具有成为新型深紫外非线性光学材料的潜力，对100多个已知单氟磷酸盐进行深入调查和相关电子结构及光学性质计算分析。理论研究指出(NH₄)₂PO₃F，(C(NH₂)₃)₂PO₃F和NaNH₄PO₃F·H₂O的带隙值均大于6.5 eV，二阶倍频响应强度是商用材料KH₂PO₄(KDP)的0.9?1.3倍，双折射率是0.029?0.053@532 nm。通过介电函数分析以及光轴-晶轴-磷氟键的夹角关系分析，首次发现结构中一致排列的极性磷氟键有效地增强了单氟磷酸盐的光学各向异性，使其最短相位匹配波长可进入深紫外区域。随后，合作者实验证明了以上理论预测。本章工作为高性能深紫外非线性光学材料的设计和探索开辟了新的途径。

二：提出“π共轭限域”新策略，并将上述理论成功地用于碳酸磷酸盐深紫外非线性光学晶体的理论预测。为了打破单基团的本征分子构型和杂化轨道特性造成的带隙-双折射率困境，我们首次提出了“π共轭限域”策略：利用非π共轭基团分离π共轭基团间的耦合作用来抬高晶体导带底部能量，从而提升带隙；并保持π共轭基团密度和共面性来产生大的光学各向异性和双折射率。在该策略指导下，碳酸磷酸盐被理论预测为新的深紫外非线性光学体系。我们计算了Sr₃Y(PO₄)(CO₃)₃(SYPC₃)和Na₃X(PO₄)(CO₃)(X = Ba, Sr, Ca, Mg, NXPC)的深紫外激光输出能力，计算结果表明碳酸磷酸盐的双折射率是同阳离子的磷酸盐的3–24倍，且带隙比同阳离子碳酸盐增大0.2–1.7 eV，其中SYPC₃的最短相匹配波长181 nm突破了“200 nm”的堡垒。

三：Li_xAg_1-xGaQ₂(Q = S, Se)系列中远红外非线性光学晶体的理论研究。I42d-AgGaS₂和Pna2₁-LiGaS₂的构效关系，理论设计了属于I42d型和Pna2₁型的Li_xAg_1-xGaQ₂晶体。形成能和声子谱计算结果指出，Li_xAg_1-xGaQ₂ (Q = S, Se)能保持I42d结构的x最大值分别为0.65和0.76。接着评估了稳定结构下的Li_xAg_1-xGaQ₂ (Q = S, Se)的中远红外激光输出能力：在带隙方面，Li_xAg_1-xGaQ₂的计算值随x单调递增，原因是Li⁺的掺杂能够削弱Li_xAg_1-xGaQ₂价带顶部Ag的d轨道贡献，并减小导带底部Ga-Q杂化的色散；在倍频强度方面，I42d-Li_0.5Ag_0.5GaS₂和I42d-Li_0.76Ag_0.24GaSe₂分别是AgGaS₂和AgGaSe₂的1.1倍和0.75倍，原因是Li⁺的掺杂通过降低结构中原子半径的差异性降低S原子的畸变程度，进而增强[GaS₄]四面体的协同作用；又因Se的半径较大，Li⁺对[GaSe₄]的调控作用不明显。我们预测Li_0.65Ag_0.35GaS₂和Li_0.76Ag_0.24GaSe₂可以实现高激光损伤阈值和大的倍频效应的平衡，并获得部分实验证实。

Exploring the new nonlinear optical (NLO) fundamental modules, achieveing the good performance balance in one system and designing novel functional materials is a hot spot in NLO field. Herein, we major studied the crsytal structure-electronic structure-optical property relationship in three new NLO systems by first principles calculations, which provides helpful guide for experimental synthesis and exploration. More importantly, some theoretical predictions have been confirmed by recent experiments. The main work is listed as follows:

(1) Theoretical study and experimental verification of monofluorophosphates NLO crystals for DUV applications. We first identified the [PO3F]2- group as a new DUV NLO functional module. Compared with the isotropic [PO4] 3- unit, [PO3F]2- posseses a slightly narrowed energy gap of 9.4 eV, larger polarizability anisotropy and hyperpolarizability. Frontier molecular orbital distributions indicated that the fluorine induced anisotropic electronic distribution in [PO3F]2- is responsible for the enhancement of optical anisotropy. Subsequently, we conducted a thorough screen of all available compounds (approximately 100) and theoretically evaluated whether the monofluorophosphates met the rigorous prerequisites for being DUV NLO materials. (NH4)2PO3F, (C(NH2)3)2PO3F and NaNH4PO3F·H2O were identified as having promising DUV NLO properties: large bandgap (Eg > 6.50 eV), impressive second harmonic generation (SHG) intensities (dij = 0.9– 1.3 × KDP) and moderate birefringence (?n = 0.028–0.053 at 532 nm). Combined dielectric function analysis with optical axis-crystal axis relationship, we revealed that the best uniform polar P-F bond orientation of monofluorophosphates generates a remarkable enhancement anisotropy, which make the shortest SHG phase-matching (PM) wavelength (λPM) entering the DUV region possible. Then, the experimental reports have confirmed our prediction. This work opens a new way for the design and exploration of high-performance DUV NLO materials. 

(2) The proposed π-conjugated confinement strategy and theortical study and rational design in deep ultraviolet carbonophosphates NLO crystals. The extremely difficult dilemma caused by the contradiction between band gap and birefringence which are the inherent qualities of the mono structure building units. To break this delimma, herein, we propose a general π-conjugated confinement strategy, mechanistically, the π-conjugated confinement decouples partially the interunit π-conjugated interactions with the separation of non-π-conjugated group so as to increase the band gap; and meanwhile to keep a suitable density of the π-conjugated group to maintain a large optical anisotropy. Predicted by this principle, the carbonophosphate, was discovered for the first time as a promising DUV NLO candidate system. Sr3Y[PO4][CO3]3 (SYPC3) and Na3X[PO4][CO3] (NXPC, X = Ba, Sr, Ca, Mg) (some are known as mineral), exhibit not only a greatly enhanced Δn that is 3–24 times larger than that of phosphate, but also an enlarged Eg that is 0.2–1.7 eV wider than that of the carbonate. More importantly, SYPC3 exhibits a λPM = 181 nm, being the shortest SHG output wavelength among phosphates to date.

(3) Theoretical study on LixAg1-xGaQ2 (Q = S, Se) mid-infrared NLO crystals. Based on the known structure-relationship between I42d-AgGaQ2 and LiGaQ2, We first theoretically designed LixAg1-xGaQ2 based on I42d and Pna21 space groups, the formation energy and phonon spectrum calculation determined that the maximum of x of I42d-LixAg1-xGaQ2 (Q = S, Se) was 0.65 and 0.76, respectively. Then we calculated the electronic structure and optical properties of LixAg1- xGaQ2 (Q = S, Se) in stable structure. With the increase of x, Li+ can weaken the contribution of Ag d orbital at the VBM and reduce the dispersion caused by Ga-Q hybridization at the CBM, consequently the Eg of LixAg1-xGaQ2 was enlarged. In terms of SHG intensity, the d36 of Li0.5Ag0.5GaS2 and Li0.75Ag0.25GaSe2 was 1.1 and 0.75 times that of AGS and AGSe, respectively. The SHG enhancement in Li0.5Ag0.5GaS2 was caused by the dislocation reduction in the lattice and cosequently a more perfect geometric superposition of [GaS4], while because of the larger radii of Se, the Li+ adjusement was less effective. We proposed that I42d-Li0.5Ag0.5GaS2 and Li0.75Ag0.25GaSe2 will achieve the good balance between wide bandgap and strong SHG effect. Then, several experimental reports have confirmed our prediction.