Scattering Defects And Mechanism Of TeO₂ Crystals

TeO₂crystal has excellent acousto-optic properties and is the preferred acousto-optic dielectric material in the visible and near-infrared spectral regions,and is widely used in the production of various acousto-optic devices.In recent years,with the continuous expansion of applications of TeO₂crystals and their devices in many fields,the demand and quality requirements for TeO₂crystals have also become higher.Light scattering defects are the most common defects in TeO₂crystals,which easily cause light loss and lower the light damage threshold.Serious light scattering defects can also affect the yield of TeO₂single crystals.Therefore,eliminating light scattering defects in TeO₂crystals is one of the main tasks to improve the quality of TeO₂crystals.We used the crucible descent method to grow the<110>,<001>orientation of TeO₂single crystals.We observed and analyzed defects such as cracks,bubbles,inclusions,dislocations,and scattering.The high optical quality TeO₂single crystal has been improved,and the yield of TeO₂single crystal has been improved.（1）TeO₂single crystals of different sizes were grown by the crucible descending method,and the growth process parameters were optimized:the TeO₂polycrystalline material was pressed into a block,and the excess gas was discharged as much as possible;the crystal growth direction was selected as<110>and<001>,To reduce the crystal cracking caused by thermal stress;the furnace temperature is controlled at about 790～820℃;the crucible descending speed is between 0.3 and 0.6 mm/h;the cooling rate is between 20 and 40℃/h.By optimizing the process and selecting raw materials with smaller particle size and concentrated particle size distribution,the crystals obtained have high optical quality and good integrity.The obtained TeO₂crystal（110）plane and（001）plane double rocking curves show that its half-peak width（FWHM）Are as low as 16″,with good left-right symmetry,and the transmittance of the crystal is 74%-76%in the wavelength range of 335 nm to 800nm.（2）Macroscopic defects such as cracks,bubbles and inclusions were observed in TeO₂single crystals,and the formation mechanism of these macroscopic defects was analyzed:TeO₂single crystals have weak cleavage planes,and improper temperature gradient control will cause the crystals to follow the cleavage planes.<100>and<010>direction cracking;the formation of bubbles is related to the removal of impurities,raw material briquetting and long-term heat preservation can basically eliminate bubbles;EDS energy spectrum analysis shows that the black inclusions are mainly crucible Pt residues.Through process optimization,these macroscopic defects can be eliminated,but light scattering defects are difficult to completely eliminate.（3）The corrosion morphology of[110]and[001]orientations of TeO₂crystals are studied.The dislocation corrosion morphology of the TeO₂wafer on the front and back sides of the[001]orientation is a square pyramid with four times symmetry,and the corrosion morphology conforms to the crystallographic tetragonal symmetry;the TeO₂wafer is on the front and back sides of the[110]orientation.The dislocation corrosion morphology is a rectangular cone with quadratic symmetry,and the corrosion morphology conforms to crystallographic quadratic symmetry.（4）By optimizing the process and selecting raw materials with smaller particle size and concentrated particle size distribution,the number of scattering defects can be reduced,and higher-quality TeO₂crystals can be obtained,but different growth methods cannot completely eliminate scattering.When the TeO₂crystal is irradiated with a laser with a wavelength of 532nm,the light scattering defect appears as a diffuse light column.The sample was refined by X-ray,no new phase was found,no grain boundary was observed under the optical microscope,and no abnormal peak position was found in the Raman spectrum.We believe that the light scattering defect may be a very thin-walled domain,which is an inherent defect of TeO₂ crystal.