Study On Growth,annealing And Defects Of Infrared Nonlinear Optical Crystal LiInSe₂

1-3 ?m(near-infrared),3-5 ?m(middle-infrared),8-12 ?m(long wave infrared)bands of light waves have a good propagation in the atmosphere,so the applications of the corresponding three bands of laser source in military and civil are very important.The nonlinear frequency transformation through nonlinear optical(NLO)crystals is one of the main ways to generate infrared(IR)lasers.At present,there are representative crystals with excellent performance in the near-infrared(NIR)and mid-infrared(MIR)regions,while there is still a lack of representative crystals with excellent comprehensive performance in the long wave infrared regions.LiInSe2 has important application potential in long wave infrared laser due to its wide transmission range(0.45-14 ?m),large laser damage threshold(40 MW/cm2@10 ns,1064 nm),small two-photon absorption at 1 ?m,and relatively easy crystal growth.Therefore,on the basis of the previous work of our research group,we continued to carry out a series of work,including:solidifying LiInSe2 polycrystalline synthesis and single crystal growth processes,exploring the appropriate annealing conditions to improve the quality of the crystals,and studying the defects of LiInSe2 crystals.The main research contents and conclusions of this thesis are as follows:(1)The research status of IR NLO crystals and the research progress of LiInSe2 crystalsThe research background was introduced in this thesis,including the application of infrared laser in military and civil fields,and the generation modes of infrared laser,the principle of nonlinear frequency transformation,the research status of infrared nonlinear optical crystal was introduced in detail.In addition,the principle and characteristics of crystal growth by vertical bridgman method were systematically introduced,and the present research status of LiInSe2 crystal was reviewed in detail.(2)Polycrystalline synthesis and single crystal growth of LiInSe2The polycrystalline synthesis process was optimized,so that the yield of polycrystalline synthesis reached to be 100%.The high-purity polycrystalline material of about 160 g can be synthesized for each time with two tubes in one furnace.High quality and large size LiInSe2 single crystals were grown by the vertical Bridgman method using oriented seed crystal.To the best of our knowledge,the largest size of LiInSe2 single crystals with a diameter of 40 mm were successfully grown.The transmittance of as grown LiInSe2 was measured to be 73%(crystal wafer thickness d=2.6 mm),which is close to the theoretical transmittance.And the full width at half maximum(FWHM)of the rocking curve is 48 for the as grown LiInSe2 crystal,suggesting the high crystallization quality of as grown crystal.The optical homogeneity root mean square of 1.58×10-5,indicats that the crystal has high optical uniformity.Therefore,the crystals that we grow were proved to be high quality LiInSe2 crystals.(3)Study on annealing of LiInSe2 crystalUnder the appropriate condition,LiInSe2 poly crystalline powder and singer crystal in the same environment,the volatile elements of polycrystalline powder serve as annealing source.It will give rise to obtain the stoichiometric crystals.In addition,the annealing treatment can reduce the dislocation in crystal to further improve the quality of crystal.Therefore,we obtained the opimimal annealing parameters of 730?,LiInSe2 atmosphere for 150 h.The quality of LiInSe2 crystals can be significantly improved by the annealing treatments.The grown and annealed LiInSe2 crystals were characterized by transmittance spectra,laser damage threshold,XPS,temperature dependence resistance and photo response.We conclude that the annealing process improves the transmittance and laser damage threshold of LiInSe2 crystal,and reduces the internal defects density of LiInSe2 crystal;(4)Study on Crystal Defects of LiInSe2 crystalNaOH solution was used to corrode LiInSe2 crystal for the first time,and obvious dislocation corrosion pits was obtained.We found the corrosion pit morphology and corrosion rate of different crystal face(a,b,and c-face)is obviously different.It was caused by the different atomic packing density of three crystal faces.a-face was easily preserved in the process of corrosion due to the largest atomic packing density.At this time,the dislocation priority to be corroded duo to the higher energy.Therefore,it is more likely to appear dislocation corrosion pits.However,the dislocation corrosion pits are not easy to appear on c-face due to the lowest atomic packing density,when the concentration of etch is high,the whole surface is corroded before the dislocation pit appears.The corrosion pits are of regular shape,and the pit wall corresponds to the crystal face where the atoms are packed closely.We analyzed the shapes of corrosion pits and the corresponding crystal faces of pit walls on different crystal faces.Point defects in LiInSe2 crystals were investigated by PL specta,and VSe,VIn,InLi,LiIn point defects were found in LilnSe2 crystals.