Study On The Design Of Nonlinear Optical Crystal And Birefringent Crystal In Fluorine-Contain Alkali Metal Borates

With the rapid development of laser science and technology,there is an urgent demand for deep-ultraviolet（deep-UV）optical functional crystals.Especially,nonlinear optical（NLO）crystals,as a key laser device by the frequency-conversion technology to switch laser sources to new spectral regions,have driven the development of advanced scientific and technical areas.So far,NLO technology is the most mature method to change or expand the limited wavelength range that can be directly accessed by actual laser light sources.At the same time,birefringent crystals can be inserted into optical devices as polarizers or optical isolators,which is also of great significance in advanced laser technology.In recent years,computer-aided material discovery strategy has become an effective way to find new materials with target properties.Borate is an optimal system for exploring UV or deep-UV linear optical or NLO materials.[BOF]groups with excellent microscopic properties are introduced into borates to form fluorooxoborates,which not only enrich the structural types,but also facilitate the acquisition of deep-UV linear optical or NLO materials with equilibrium properties.Based on this,this dissertation takes the alkali metal fluorooxoborates system as the research object.Firstly,the electronic structures and microscopic properties of three different six-membered rings are analyzed by means of first-principles calculation.A new fluorooxoborate structure is designed reasonably based on the microscopic groups with excellent microscopic properties.Secondly,in order to solve the problem that it is difficult to obtain high optical materials in experiments,this paper proposes an optical material discovery paradigm based on interconnection prediction-experimental strategy.through this strategy,the fluorooxoborate system was studied and the directional preparation of novel fluorooxoborate was realized.The main contents are as follows:1.Design of deep-ultraviolet nonlinear optical materials based on fundamental building blocks with excellent microscopic propertiesNLO materials are designed based on the fundamental building blocks（FBBs）with excellent microscopic properties,which is beneficial to obtain high performance materials.In this paper,the[B₃O₇]and[B₃O₆F]groups are taken as the research object.By calculating the electronic structure of these two groups,it is found that when[BO₃F]replaces[BO₄]in[B₃O₇]group to form[B₃O₆F]group,The HOMO-LUMO（highest occupied molecular orbital-lowest unoccupied molecular orbital）gap of[B₃O₆F]group increases,it is found that when the F atom replaces the O atom at the end,the BD*orbital of BO moves upward.The HOMO of the above functional groups is roughly in the same energy level,while the LUMO moves to a higher energy level.The microcosmic properties such as polarizability anisotropy and hyperpolarizability of[B₃O₇],[B₃O₆F]and[B₃O₆F+BO₃]groups were calculated,and it is found that the symmetry of[B₃O₆F]group is lower than that of[B₃O₇]and[B₃O₆]group.In addition,the reason for the symmetry destruction of the[B₃O₆F]group relative to the[B₃O₇]group is analyzed by using the localized orbital indicator function.In the process of exploration,we find that the[B₃O₆F]group can better balance the relationship among HOMO-LUMO gap,polarization anisotropy and hyperpolarizability than[B₃O₇]group.On this basis,we designed a new type of intraocular crystal Li₂B₃O₅F.Comparatively speaking,the medium SHG coefficient(0.3×KH₂PO₄（KDP）,(d₃₆=0.39 pm/V),deep-UV transmission（<200nm E_g>6.2 e V）,and the appropriate birefringence（0.10@1064 nm）.Assembling functional modules can accelerate the discovery of potential UV or deep-UV NLO materials.2.An interlinked prediction-experiment paradigm discovering deep-ultraviolet fluorooxoborates with desired optical nonlinearity and birefringenceIn order to solve the difficulty in obtaining materials with high optical properties experimentally,we propose a new optical material discovery paradigm based on the interlinked prediction-experiment strategy.The high-throughput prediction and object-oriented synthesis of the fluorine-based system Na B₄O₆F（NBF）were implemented.Novel fluorooxoborates crystals in the NBF system are predicted successfully,and all the structures with enthalpy difference less than 100 me V compared with ground state structure are obtained.These thermodynamically s Tab.or metas Tab.NBF structures show rich structural types and enrich the structural chemistry of fluoroborates.At the same time,the comprehensive performance of these structures is excellent,realizes a balance between deep-ultraviolet transparency and large birefringence,or deep-UV phase-matching wavelengths and strong nonlinear optical response.This is very important for the application of deep ultraviolet birefringence and nonlinear materials.Concurrently,the experimentally oriented preparation of a new crystal NBF-C2/c and the known compound NBF-C2 is achieved,confirming the validity of the paradigm.Further results indicate that NBF-C2/c is a new-generation birefringent crystal with large birefringence（0.219@193 nm）and deep-ultraviolet transparency.And,the phase-matching curves of NBF-C2 show the shortest phase-matching wavelength for type I and type II being 166 and 200 nm.This research has enabled the customization of functional materials with high performances via the interlinked discovery paradigm.