| Laser is one of the greatest inventions in the twentieth century.It is widely used in military defense,modern industry and social life owing to the good monochromaticity,directionality and coherence.The direction,intensity and frequency of the laser can be effectively regulated by the acousto-optic interaction,thereby further broadening the application scope of the laser.Acousto-optic crystals are a class of acousto-optic interaction media with excellent performance.At present,the commercialized acousto-optic crystals such as TeO2,LiNbO3 and PbMoO4 have been widely applied in the ultraviolet-visible and near-infrared regions.However,acousto-optic crystals suitable for the mid and far infrared region are rare.Therefore,the exploitation of commercialized acousto-optic crystals with excellent performance in the mid and far infrared region is an urgent problem to be solved.Hg2X2(X=Cl,Br,I)series crystals are a class of potential acousto-optic materials with excellent performance,which have a wide transmission range(0.4～30 μm),large figure of merit,large birefringence,low acoustic velocity and attenuation coefficient.Some countries such as the Czech Republic,the United States and Russia have carried out relevant researches on Hg2X2 series crystals as early as the last century,and have achieved outstanding results.However,in view of the significant application of Hg2X2 series crystals in national defense and military,foreign countries have implemented technical blockade and embargo policies on our country.Our country started late in the research of Hg2X2 series crystals,and has not mastered the growth technology of large-size Hg2X2 series crystals with high-quality.Especially,no researches have been carried out on Hg2Br2 crystals.The growth,performance characterization,defect and device fabrication of Hg2Br2 crystals based on the great demand for far infrared acousto-optic crystals in the field of military defense and social life were investigated in this thesis for the first time in China.The main research contents and conclusions of this thesis are as follows:Ⅰ.Feedstocks purification and crystal growth of Hg2Br2 crystalThe purification of Hg2Br2 feedstocks is carried out by sublimation-condensation method.The effects of the quartz ampoule structure,the temperature field,the holding time,the purification times and the purification directions on the purification efficiency of feedstocks were explored.It is clear that the best purification temperature field of Hg2Br2 feedstocks is 265℃ at high temperature and 225℃ at low temperature.The impurities such as Hg5Br2O4 and HgO,can be effectively removed and Hg2Br2 feedstocks with high-purity can be obtained by horizontal purification for 5 times under the optimal purification process,followed by vertical sublimation purification,which lays a solid foundation for the growth of high-quality crystals.A home-designed transparent physical vapor transport furnace is used for the growth of Hg2Br2 crystals,and the effects of ampoule structure,temperature field,ampoule descent speed and cooling time on crystal nucleation,interface evolution and quality are explored.Under the temperature field of 380/350℃ in the high/low temperature region,the crystal growth was carried out with the ampoule descending rate of 0.2 mm/h and the ampoule rotating rates of 5 r/min,and the Φ 35×40 mm3 Hg2Br2 crystals with smooth surface were successfully obtained,which achieved a zero breakthrough of key technology in this field in China.High-resolution X-ray diffraction,ZYGO optical interferometer and X-ray Laue diffraction reflect that the as-grown Hg2Br2 crystals are of high quality.Ⅱ.Properties characterization and acousto-optic tunable filter fabrication of Hg2Br2 single crystalThe density,mechanical,optical,thermal,acoustic and electrical properties of the crystals were systematically characterized on the basis of obtaining large size Hg2Br2 crystals with high-quality,which provided basic data for the fabrication of acousto-optic devices.The results show that Hg2Br2 crystals exhibit high transmittance in a broad region(0.4～35 μm)with the maximum transmittance is around 80%.The Hg2Br2 crystal has a large birefringence,which is as high as 0.97 at 1 μm.The shear wave acoustic velocity and attenuation coefficient of Hg2Br2 crystal along the[001]direction are 1061.41 m/s and 0.005 dB/cm,respectively.Hg2Br2 crystal has low acoustic velocity and attenuation coefficient,which is beneficial to the fabrication of acousto-optic devices with high diffraction efficiency.The hardness,Young’s modulus,thermal expansion and thermal conductivity of Hg2Br2 crystal are quite anisotropic,and[001]oriented crystal has better mechanical and thermal properties.The large anisotropy could be explained by the first-principles calculation,which is attributed to the special one-dimensional chain structure of the Hg2Br2 crystal.At the same time,Hg2Br2 crystal still shows excellent comprehensive properties at low temperature,and the thermal conductivity of the crystal at-140℃ is more than twice that at 150℃,which ensures the performance stability of acousto-optic devices in low temperature packaging environment in practical applications.In addition,an acousto-optic tunable filter based on Hg2Br2 crystal was prepared,and the diffraction efficiency of AOTF at different incident angles was measured with the driving frequency.Ⅲ.The crystal processing,defects characterization and annealing modification of Hg2Br2 crystalThe orientation of Hg2Br2 crystal is accurately determined by the X-ray orientation instrument combined with multiple(1 10)natural planes.The effects of cutting speed,rotating speed of the cutting machine,grinding pad,grinding liquid,grinding time,polishing liquid,polishing pad and time on the surface damage,roughness and scratches of Hg2Br2 crystal were systematically investigated,and the optimal processing technology was preliminarily determined.Typical defects such as cracking,growth striations and dislocations in Hg2Br2 crystals are comprehensively identified and analyzed,and effective measures to suppress defect are proposed.The effects of annealing temperature on the crystallinity and stress of Hg2Br2 crystal were investigated.It was confirmed that after annealing at 150℃ for 60 h,the crystallinity of Hg2Br2 crystal was significantly improved,the stress was reduced,and the crystal quality was significantly improved.Ⅳ.Environmental interactivity of Hg2Br2 crystalsThe photochromic properties of Hg2Br2 crystal are explored.The Hg2Br2 crystal darkens under the irradiation of UV light shorter than 450 nm,and the darkened crystals can be restored after annealing at 160℃ for 72 h or irradiation of high-power infrared light.The mechanism of photo-darkening and annealing recovery of Hg2Br2 crystals was determined by X-ray photoelectron spectroscopy.Hg2Br2 decomposed into HgBr2 and Hg under the irradiation of UV light,and Hg and HgBr2 recombined after annealing.The results of transmission spectrum,UV-visible diffuse reflectance spectrum,refractive index and high-resolution XRD indicated that the optical properties and crystalline quality of Hg2Br2 crystals decreased after the irradiation of UV light,while these properties could be restored after annealing.In addition,the reason of Hg2Br2 crystal darkening induced by methanol and absolute ethanol under external action such as ultrasonic or polishing was studied.The results of Raman spectrum,transmission spectrum and microscopic observation indicated that the corrosion of Hg2Br2 crystal by methanol and absolute ethanol under ultrasonic condition is the main reason for the darkening of crystal color. |
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