Research On Growth And Nonlinear Optical Properties Of BaMgF₄ Crystal And Microcrystal

In the past decades,studies on all-solid-state lasers working in vacuum ultraviolet?VUV?region are becoming hot areas for the demands of the scientific research and production.Nowadays,lasers working in UV/VUV region are mainly excimer lasers,such as KrF?248nm?laser,ArF?193 nm?laser and F₂?157 nm?laser.But their disadvantages such as easy decomposition,toxicity,poor beam quality and large working wavelength restrict their wide applications.In contrast,VUV all-solid-state lasers have many advantages,such as small size,narrow linewidth,good beam quality,tunability and stability.They can be widely used in VUV lithography,microfabrication,photoelectron spectroscopy,optical clock,etc.Nonlinear optical crystal,as one of the key points to realize VUV coherent laser output,is naturally becoming the focus of scientific research.Up to now,among the few nonlinear optical crystals which can meet the above requirements,BaMgF₄ crystal has been considered as one of the most promising candidates for coherent output of VUV lasers due to its extremely short UV absorption edges?130 nm?and it can realize full-band quasi-phase matching.In this thesis,the growth of BaMgF₄ crystal by temperature gradient technique?TGT?and Bridgman method were studied.For TGT,the BaMgF₄ single crystal with diameter of 75mm and length of 50 mm was successfully obtained.During crystal growth,the synthesis of raw materials,the construction of thermal field and the growth technology were studied carefully.The BaMgF₄ crystal sample has been characterized by X-ray diffraction spectroscopy,UV transmission spectroscopy,X-ray energy spectrum,differential thermal analysis,Raman spectroscopy and so on.By Bridgman method,the largest and complete BaMgF₄ single crystal with diameter of 80 mm and length of 55 mm has been grown by optimizing the temperature gradient and descent rate.A symmetrical metal-cladding waveguide?SMCW?was fabricated by vacuum evaporation method.The electro-optic effect of the BaMgF₄ crystal was studied with the help of the high sensitivity of the structure.In the experiment,the different external incident angles of the same order guided mode in the waveguide under different applied electric fields was measured.Afterwards,the electro-optic coefficient of BaMgF₄ crystal was determined to be?_{1 3}?28?-36.2 pm/V by linear fitting.With same experimental condition,the value of?_{1 3} of LiNbO₃ crystal was measured to be 9.1 pm/V,which is in good agreement with the previous reported results.The|?₁₃|of the BaMgF₄ crystal is even bigger than the maximum electro-optic coefficient of LiNbO₃ crystal,which is?₅₁?28?32.6 pm/V.In addition,BaMgF₄crystal has excellent VUV cut-off wavelength and it is expected to be used in the manufacture of VUV electro-optic modulators and optical switches in the future.The principal refractive index ellipsoid of BaMgF₄ crystal was calculated and the relation between the optical axis of the crystal and the wavelength was obtained.The principle of the Maker fringe method for measuring second harmonic generation coefficients of the materials was introduced and the second harmonic generation coefficients of BaMgF₄crystal was determined.The obtained second harmonic generation coefficients is about one order of magnitude larger than that reported in earlier paper.However,it is in good agreement with the rencent theoretical calculation results.In addition,the phase matching angle,effective second harmonic generation coefficients under birefringent phase matching condition and grating period parameters under quasi-phase matching condition of BaMgF₄crystal were calculated.Afterwards,the experiment of generating sum frequency light with355 nm in BaMgF₄ crystal was carried out by using 1064 nm and 532 nm fundamental waves.The growth of BaMgF₄ microcrystal was studied by hydrothermal method.High purity Mg?AC?₂·4H₂O,BaCO₃ and CF₃COOH were selected as raw materials and the BaMgF₄microcrystal was grown under high temperature and pressure.The X-ray diffraction spectrum of BaMgF₄ microcrystal was measured and it fits well with the standard data.Also,the morphology of the BaMgF₄ microcrystal was characterized by SEM.The shape of the microcrystal was cuboidal and the size was about 5-10?m.The size of some larger ones was about 100?m.BaMgF₄ microcrystal powders were deposited on an ITO glass by chemical electrophoresis,and the structure of BaMgF₄ microcrystal film with 100?m thickness was prepared.The 532 nm second harmonic light was observed when the film was radiated by1064 nm laser.When the fundamental wavefront was modulated by spatial light modulator with the optimization of genetic algorithm,the second harmonic focusing signal with 532 nm wavelength was realized in BaMgF₄ microcrystal film.With the similar theoretical model,the conversion efficiency of second harmonic generation in weakly scattering BaMgF₄ crystal under birefringent phase matching and phase mismatching condition was both recoverd.