【作者】 冯志书；

【导师】 高锦岳； 苏雪梅；

【作者基本信息】 吉林大学 ， 光学， 2014， 博士

【摘要】 硒化镓及其掺杂晶体光学、倍频及飞秒激光损伤特性的研究非线性光学晶体材料是光电子技术,尤其是激光技术的重要物质基础，被广泛用于激光频率转换和信号存储等，在现代高新技术和军事上都起到了非常重要的作用。GaSe晶体是可以用于参量频率转换装置中的最佳非线性晶体材料之一，它具有较宽的透明范围（0.62～20μm），较大的二阶非线性系数（54pm/V）以及较高的损伤阈值（接近被称为“中红外标准晶体”的ZnGeP2）。然而，GaSe晶体的层状结构也导致了其较差的机械强度：莫氏硬度约为零且容易劈裂，这也严重限制了其实际应用。研究发现，在GaSe晶体中掺杂杂质元素可以明显改善其机械性能，例如S、In、Al、Te等，从而使其更适宜用于参量频率转换装置中。本文对几种不同的掺杂GaSe晶体的光学特性、倍频特性以及飞秒激光损伤特性进行了研究。1. GaSe：Er晶体的光学和倍频特性使用改进的布里奇曼法合成了厘米级尺寸的GaSe:Er (0.025，0.1，0.5，1，2at.%)晶体，从光谱短波和长波吸收限的变化、喇曼光谱和EDX图像可以确定Er原子确实进入了GaSe晶体中，但在合成的晶体中Er的含量要比装料时低很多，真正的Er含量分别为0.009，0.019，0.033，0.042和0.048at.%。使用飞秒OPG和CO2激光对晶体进行作用，实验发现GaSe：Er晶体的相位匹配角与纯GaSe晶体相比没有明显变化，这也说明Er的真实掺杂含量很小。仅靠吸收光谱不能确定最佳的Er掺杂含量，根据倍频实验结果可以确定在我们所使用的晶体中，最佳的Er掺杂含量为0.033at.%，该晶体将二阶有效非线性系数提高了20%，研究发现晶体的光学质量提高是倍频转换效率提高的原因。2. GaSe：Te晶体的光学和倍频特性对掺杂浓度为0.05~10mass.%的GaSe：Te晶体进行了研究，发现GaSe：Te (≤5mass.%)晶体和ε型GaSe晶体一样具有六角形结构，其中GaSe：Te (≤0.5mass.%)单晶具有最好的光学质量。测量了飞秒OPG和CO2激光泵浦条件下GaSe：Te晶体的相位匹配角，实验发现Te掺杂晶体的相位匹配角与纯GaSe晶体相比几乎没有变化。Te掺杂晶体较高的光学质量和较大的损伤阈值使其频率转换效率较纯GaSe晶体有所提高。根据以往的实验和本实验的结果可以推测，对于较重的掺杂元素，例如In、Te和Er等，最佳的掺杂浓度应该在0.5mass.%以下。3. GaSe：InS晶体的光学特性用熔融的GaSe和Ins（1，5，20mass.%）合成了四元化合物Ga1-yInySe1-xSx，研究表明该晶体为ε型结构。通过对化学成分、晶格结构以及光学特性的分析表明该晶体可以用于参量频率转换装置中。比GaSe1-xSx低的光学损耗以及比纯GaSe和Ga1-xInxSe晶体高的损伤阈值是Ga1-yInySe1-xSx晶体的优势所在。同时，我们认为S和In掺杂浓度不同的Ga1-yInySe1-xSx晶体在非线性应用中可能更具有吸引力。4. GaSe：S和GaSe：In晶体的飞秒激光损伤特性研究了飞秒激光脉冲与GaSe:S (0.5，1，2，3，7，10mass.%)晶体和GaSe:In(0.5，1.32，2，2.32mass.%)晶体的相互作用情况，并与纯GaSe晶体进行了对比。研究发现，在用波长为0.8μm的飞秒激光作用时，使用肉眼观察作为判断标准得到的晶体损伤阈值与实验结果并不符合，晶体表面出现黑点并没有使晶体透过率和频率转换效率产生明显下降。选择一个可接受的透过率值以后，泵浦强度的限制可以由透过率曲线的变化情况来确定。透过率曲线在下降到初始值的10%以前都是可逆的，这种情况下不影响晶体在飞秒激光系统中的应用。最佳的In掺杂晶体GaSe：In（2mass.%）可以使最大可泵浦强度提高40~50%，而最佳的S掺杂晶体GaSe：S（3mass.%）可以使相应值达到纯GaSe晶体的4.5倍。研究发现限制飞秒激光脉冲泵浦强度的关键因素是非线性多光子吸收。更多还原

【Abstract】 Nonlinear optical crystal is the important material foundation of photoelectrictechnology, especially laser technology. It can be used for laser frequencyconversion and signal storage and plays an important role in both modern high-techand military.The key physical properties that render the layered GaSe as one of the bestnonlinear crystals for parametric frequency conversion (PFC) are the broadtransparency range from0.62to20μm, the large second-order nonlinearity of54pm/V and the high damage threshold that can be similar to that of the so-called“mid-IR standard crystal” ZnGeP2. However, the layered structure of GaSe leads toextremely weak mechanical properties: almost zero hardness by Mohs scale andeasy cleaving that limit large size crystals application.It is found that the mechanical properties of GaSe can be improved significantlyby doping with S, In, Te, Al and so on, which makes it more suitable to be used inPFC systems.The optical, second harmonic and damage properties of several kinds of dopedGaSe crystals are investigated in this thesis.1. The optical and second harmonic properties of GaSe:Er crystalsCentimeter-sized Er-doped single crystals were grown from the meltsGaSe:Er(0.025,0.1,0.5,1and2at.%) by modified Bridgman technology.Incorporation of Er atoms into GaSe structure is confirmed by fixing specifictransformation features in the short-and long-wave absorption edges, Raman spectra and EDX patterns. It is found that Er content in all grown ingots issurprisingly lower to that in the melt compositions. Real Er content is ascertained as0.009,0.019,0.033,0.042and0.048at.%, respectively. The phase matching anglesunder fs OPG and CO2laser pump are measured. No visible differences can befound between phase matching angles for GaSe:Er crystals and that for pure GaSe,which also confirms the low concentration of Er. The optimal doping of Er in GaSecannot be determined by absorption spectroscopy. Optimal doping of0.033at.%ofEr is established from SHG experiment that results in about20%increased efficientnonlinearity. It is found that improved optical quality is a reason for increased SHGefficiency.2. The optical and second harmonic properties of GaSe:Te crystalsGaSe crystals with0.05~10mass.%Te-doping are studied. GaSe:Te (≤5mass.%) crystals show the hexagonal structure like ε-GaSe. It is found that thecentimetre-sized single crystal of GaSe:Te (≤0.5mass.%) possesses high opticalquality. The phase matching angles under fs OPG and CO2laser pumping aremeasured. No visible differences can be found between phase matching angles forGaSe:Te crystals and that for pure GaSe. Nevertheless, the improved opticalquality and damage threshold lead to increased frequency conversion efficiency.From the available and obtained data, it is proposed that the optimal doping levelof such heavier elements as In, Te, Er in GaSe is below0.5mass.%for nonlinearoptical applications.3. The optical properties of GaSe:InS crystalsSingle crystals are grown from the melt of GaSe and InS (1,5,20mol.%) thatare identified as ε-polytype of quaternary solid solution crystals Ga1-yInySe1-xSx.Chemical composition, lattice structure and optical properties have beeninvestigated that shown its usefulness as parametric frequency converters that forthe first time were confirmed experimentally. Lower nonlinear optical losses to that for solid solution crystals GaSe1-xSxand higher damage threshold to that forpure GaSe and solid solution crystals Ga1-xInxSe are their advantages. It isproposed that Ga1-yInySe1-xSxcrystals with independce of S and In concentrationmay be more attractive in nonlinear applications.4. The fs laser damage properties of GaSe:S and GaSe:In crystalsThe impact of fs pulses on doped crystals GaSe:S (0.5,1,2,3,7,10mass.%)and GaSe:In (0.5,1.32,2,2.32mass.%) has been studied in comparison with thaton pure GaSe crystal. It is established that visual criterion in the determination ofthe crystal damage threshold under0.8μm fs pulse pumping is not consistentbecause observation of the black matter damage spots on the crystal surfaces donot decrease noticeably both the transparency and frequency conversion efficiency.The pump intensity limit can be readily determined from the transparency decaycurve measurements and that is still reversible after a decline down to themagnitude of0.1choosing an acceptable transparency level. High advantages inthe limit pump intensity as up to40~50%for optimally In-doped crystal GaSe:In(2mass.%) and up to4.5times for optimally S-doped crystal GaSe:S (3mass.%)crystals have been demonstrated under expose to fs Ti:Sapphire laser systempulses. The nonlinear multiphoton absorption has been identified as key factorlimiting the fs Ti:Sapphire laser system pulse pump intensities.更多还原