| With good monochromaticity,directionality and coherence,lasers have attracted a lot of attention since their introduction.Polarization devices can be used to effectively regulate the direction and intensity of laser to make it more effective for production and life.With the rapid development of laser technology,various types of polarizing devices have appeared one after another,mainly including polarizers,polarizing prisms,and various delayers.Polarizing crystals are the cornerstone for the preparation of polarizing devices.Currently,there are commercial polarizing crystal materials such as α-BBO,calcite and YVO4 in the UV-Vis-NIR region,but the research on polarizing crystals in the mid-and far-infrared band is still in its infancy.Therefore,it is urgent to explore polarized crystals with excellent performance for this band.Mercurous halide crystals with wide transmission range,high birefringence,low sound velocity and high acousto-optic figure of merit are a class of polarized crystals with great potential for mid-and far-infrared wavelength applications.The polarization and acoustooptical devices prepared by mercury halide crystals are expected to serve in the defense industry such as fiber optic communication and electronic countermeasures,so foreign research units such as the United States and the Czech Republic have invested a lot of research into this type of crystals as early as last century,and the results are quite fruitful.However,similar to the situation of "necking" in our chip industry,foreign countries have implemented technology blockade and embargo policy for mercurous halide crystals,which has led to the difficult situation of our research based on Hg2X2 crystals.Coupled with the late start of our exploration for Hg2X2 crystals,the research on Hg2Cl2 crystals in China is still blank.Therefore,this thesis takes the lead in the growth,property study and device preparation of Hg2Cl2 crystals based on the current status of domestic research on Hg2X2 crystals.The main contents and conclusions are described below.Ⅰ.Feedstocks purification and crystal growth of Hg2Cl2 crystalThe Hg2Cl2 raw materials were purified by sublimation method,and the dense Hg2Cl2 ingots suitable for crystal growth were obtained by continuous optimization of the process.XRD tests showed that the black impurity was Hg5Cl2O4,which appeared with high frequency during the purification.The yellow smoke was presumed to be nitrogen dioxide,a phenomenon that generally occurs only during the first purification process.The transparent PVT furnaces for Hg2Cl2 crystal growth have been developed and used to grow Φ 35 mm and Φ 50 mm Hg2Cl2 crystals.The first-generation transparent PVT furnace was mainly used for the growth of Φ 35 mm Hg2Cl2 crystals.By exploring the effects of temperature field,crucible drop rate and cooling process on the nucleation,growth and crystalline of crystals,we successfully obtained Hg2Cl2 crystals with Φ 35×40 mm3 with high transparency and smooth surface,which achieved a breakthrough in this key technology in China.Based on this,we performed the growth technology of larger size Φ 50 mm Hg2Cl2 crystals.Compared with the first-generation transparent PVT furnace,the refinements were described below:the structure of control system and execution unit were separated;the singletube PVT furnace was increased to double-tube;the insulation facilities were further enhanced;the ampoule rotation device was added to the mechanical drive unit,etc.The process improvement resulted in the successful growth of high quality,smooth and transparent Hg2Cl2 crystals of 50 mm in diameter,which filled the research gap in this field in China.Ⅱ.Generation and suppression of crystallinity and cleavage striations in Hg2Cl2 crystals(1)Using the formula given by Darwin*,the full width at half maximum(FWHM)of ideal Hg2X2 crystals were calculated:the FWHMs of ideal Hg2Cl2,Hg2Br2,and Hg2I2 single crystals in the(110)plane are 36.264",41.906",and 44.184",respectively;the FWHMs of ideal Hg2Cl2,Hg2Br2,and Hg2l2 single crystals in the(004)plane are 27.388",29.992",and 30.009",respectively.The large intrinsic FWHMs of Hg2X2 are mainly due to the large structural amplitudes of the heavy mercury and halogen atoms and the large scattering of x-rays.(2)The micro-striations in(001)surface of polished Hg2Cl2 crystals was observed for the first time.This striation’s structure was regularly arranged,with a bright striati on width of about 30 μm,a striation spacing of about 20 μm,and a minimum repeating unit of 50 μm.And a reasonable explanation for this phenomenon was provided:the mechanical stress introduced by the late cutting and polishing caused the Hg2Cl2 crystal to cleave along the<110>direction.(3)After a series of exploration of Hg2Cl2 crystal processing process,the relationship between crystallinity-striations-processing was finally obtained:the improvement of processing process can weaken the striations depth of crystals and reduce the FWHM of crystals,thus further improving the crystallinity of crystals.Ⅲ.Development and performance characterization of Hg2Cl2 crystal Gran PrismThe Hg2Cl2 polarizer with a structure angle of 27.76°(the Brewster angle at 10.6 μm)was designed and prepared for the first time in China,and the extinction ratio of the Hg2Cl2 prism was tested:the extinction ratio was 18,000:1@1064 nm,indicating that the Hg2Cl2 polarizing prism have reached the international level,and also confirming the superiority of Hg2Cl2 crystal in making polarized prisms. |
