KDP、DKDP晶体拉曼散射特性研究

发布时间：2018-05-03 13:57

本文选题：KDP(DKDP)晶体 + 晶格振动　；参考：《山东大学》2015年博士论文

【摘要】：磷酸二氢钾(KH2PO4,简称KDP)晶体和磷酸二氘钾(K(H1-xDx)2PO4,简称DKDP)晶体是一种性能优良的的非线性光学晶体材料,其生长研究已有80多年的历史。KDP和DKDP晶体具有较大的非线性系数、较高的激光损伤阈值等性能,被广泛作为激光倍频材料；另外,KDP和DKDP晶体还是良好的压电、电光材料。近年来惯性约束核聚变(ICF)工程得到了越来越多国家的关注,ICF是一种可控制的热核爆炸,为解决人类未来能源短缺问题提供了新的路径。到目前为止,能满足ICF研究所需要的高光学性能和大尺寸晶体仅有KDP、DKDP晶体,其在该系统中主要作为变频材料和电光开关材料。在ICF系统中,高通量激光通过大尺寸KDP晶体组成的倍频系统时,自发拉曼散射光与泵浦光场耦合使自发拉曼散射受激放大,受激拉曼散射在横向获得较大增益形成横向受激拉曼散射(TSRS)。大尺寸KDP晶体中的TSRS不但会造成泵浦光能量损失,而且其强度可能超过激光的损伤阈值,造成晶体永久损伤。KDP晶体中的TSRS效应严重制约了激光系统能量的提升,国内外采取了一系列措施降低大尺寸晶体中的TSRS效应。众所周知利用DKDP晶体取代KDP晶体可有效降低其TSRS增益系数,但DKDP晶体氘含量和其TSRS增益系数的定量关系尚未明确,难以得出降低TSRS效应的最佳氘含量；另外,在更高激光功率条件下DKDP晶体中的TSRS效应也将显著,且DKDP晶体生长工艺困难,成本高。因此,继续开展KDP、DKDP晶体TSRS效应相关研究有重要意义。受激拉曼散射起源于晶体的自发拉曼散射,且其增益系数可根据其自发拉曼光谱评估,故受激拉曼散射与自发拉曼散射密切相关。本文系统地研究了KDP、DKDP晶体的拉曼散射特性,我们首先根据群论对KDP、DKDP晶体晶格振动模进行了详细归类并对拉曼峰进行了指认；然后开展了KDP晶体的受激拉曼散射实验,分析了三倍频过程中的TSRS效应；并研究了晶体生长方法、杂质离子、氘含量和温度等因素对KDP、DKDP晶体拉曼散射的影响。论文主要内容如下：1.首先利用因子群分析法得到了KDP晶体的基本晶格振动模：T=4A1+5A2+6B1+7B2+13E,然后利用位置群分析法对振动模进行了详细分类。验证了背向拉曼散射时A1振动模的角度特性,A1振动模角度特性表明实际晶体的对称性低于D2d。另外,退火和未退火晶体的拉曼光谱无明显差别,故A1振动模的角度特性与晶体的内应力无关,是由KDP晶体内部结构决定的。我们测量了KDP和DKDP晶体不同散射配置下的拉曼光谱,并对KDP和DKDP晶体的拉曼峰进行了指认。结果表明,虽然所测光谱中存在和D2d拉曼选择定则不符的情况,但是光谱整体上符合D2d拉曼选择定则。2.根据受激拉曼散射理论推导了KDP晶体受激拉曼散射增益系数和激发波长和散射配置的关系。采用532 nm的皮秒激光脉冲泵浦在腔外单次通过方式下实现了KDP晶体的受激拉曼散射(SRS),实验上只观察到了增益系数最大的v1振动模的三阶stokes光(559.43 nm、589.74 nm、623.5 nm)。另外,随着SRS阶次增高Stokes光的强度减弱,泵浦光的阈值功率呈非线性增长规律。因此,高功率激光系统中大尺寸KDP晶体中的TSRS效应主要考虑v1振动模的一阶受激拉曼散射。受激拉曼散射增益系数和激发波长和散射配置有关,在三倍频过程中KDP晶体中将会出现几种不同类型的TSRS效应。本文定量比较了Ⅰ类和Ⅱ类KDP晶体组成的三倍频系统中的几种不同类型的TSRS效应,结果表明在高的倍频效率和激光通量下,三倍频晶体中的TSRS效应较为显著,尤其是三倍频光激发的TSRS效应。根据本文结果在倍频过程中若得到各阶段的倍频效率即可预测各种TSRS效应的相对强度,并采取措施有效降低晶体坯片在使用过程中的激光损伤几率。3.研究了生长方法(传统法和快速法)、热退火和杂质离子(Cr3+和Ni2+)对KDP晶体拉曼光谱的影响,并分析了这些因素对KDP晶体v1振动模的受激拉曼散射的影响。结果表明这些因素对晶体的光学性能(透过率和散射颗粒)均有不同程度的影响,但对晶体拉曼光谱及v1振动模的受激拉曼散射增益系数的影响不明显。另外,受激拉曼散射强度与还其作用范围内晶体的光学性能(透过率、散射颗粒等)有关,并且相互作用长度越大受激拉曼散射传输过程中受晶体光学质量的影响越显著,故大尺寸KDP晶体中的TSRS效应受生长方法、热退火和杂质离子等因素影响,这需要进一步的在线实验验证。4.测量了不同氘含量DKDP晶体X(ZZ)-X和X(YY)-X散射配置下v1振动模的自发拉曼光谱。随着氘含量的增加v1振动模发生红移,且X(ZZ)-X散射配置下的频移大于X(YY)-X散射配置下的频移,v1振动模半峰宽先增加后减小,而峰值强度随氘含量的变化和半峰宽的变化趋势相反。根据振动模耦合理论拟合了DKDP晶体的拉曼光谱,分析得出DKDP晶体v1振动模的红移主要是由于v1振动模和δ(OD)振动模耦合引起的,而v1振动模和v3振动模耦合对v1振动模频移也有较小贡献。另外,基于DKDP晶体掺氘后晶体结构的变化建立了DKDP晶体拉曼散射的强度叠加模型,根据这一模型计算了不同氘含量DKDP晶体的拉曼光谱,结果与实验测得的光谱符合很好,根据这一模型解释了氘含量对DKDP晶体半峰宽和峰值强度的影响,并且发现峰值强度随氘含量的变化是由半峰宽的变化引起。采用和水的自发拉曼光谱对比法得出了KDP晶体和不同氘含量DKDP晶体的TSRS增益系数,测量误差为15%。结果发现KDP晶体的TSRS增益系数为0.3cm·GW-1,且随着氘含量的增加DKDP晶体的TSRS增益系数先减小至KDP晶体的40.1%,后增大至KDP晶体的68.9%。建立了DKDP晶体TSRS增益系数和其v1振动模拉曼频移的关系,利用此关系可以实现DKDP晶体TSRS增益系数及其均匀性在线无损测量。通过分析与T SRS增益系数有关的参数,我们认为氘含量引起的半峰宽的变化是引起其TSRS增益系数变化的主要原因,故寻求其它增大拉曼散射半峰宽的方法可达到抑制TSRS效应的目的。此外,结合第三章中KDP晶体TSRS效应的分析,我们给出了DKDP晶体作为三倍频材料时3ω光激发TSRS效应阂值强度和氘含量的关系。5.采用背向拉曼散射装置测量了KDP晶体和不同氘含量DKDP晶体v1振动模在285.4 K～345.2 K范围内的拉曼光谱,并分析了温度对其v1振动模拉曼频移、半峰宽、散射强度和SRS增益系数的影响。随着温度的升高,KDP晶体中的v1振动模频率逐渐减小,半峰宽逐渐增大。而温度对DKDP晶体拉曼频移和半峰宽的影响和其氘含量有关,对于低氘和高氘的DKDP晶体,温度对其拉曼散射的影响和KDP情况类似；对于其他氘含量的DKDP晶体,拉曼频移和半峰宽随温度的变化规律表现出较大波动,我们认为这主要是因为温度同样引起了O-D键的变化。另外,随着温度的升高KDP、DKDP晶体拉曼散射强度没有明显的变化趋势,但其稳态受激拉曼散射增益系数有所降低,对于KDP晶体与285.4 K相比,345.2 K时的增益系数降低了约12%。因此,在高功率激光系统中大尺寸KDP、DKDP晶体在较高温度下应用有助于降低TSRS效应。
[Abstract]:KH2PO4 (KH2PO4, KDP) crystals and phosphoric acid two (H1-xDx) 2PO4 (H1-xDx) 2PO4 (abbreviated as DKDP) crystals are a kind of nonlinear optical crystal materials with excellent properties. Their growth studies have been studied for more than 80 years in the history of.KDP and DKDP crystals with larger nonlinear coefficients and higher laser damage thresholds, and are widely used as laser doubling materials. In addition, KDP and DKDP crystals are also good piezoelectric, electro-optic materials. In recent years, the inertial confinement fusion (ICF) engineering has attracted more and more attention. ICF is a controllable thermonuclear explosion, which provides a new way to solve the problem of human energy shortage in the future. So far, the high optical properties of ICF research are met. The energy and large size crystals are only KDP, DKDP crystals, which are mainly used as frequency conversion materials and electro-optical switching materials in this system. In ICF systems, when high throughput lasers are composed of large size KDP crystals, spontaneous Raman scattering light is coupled with the pump light field to induce spontaneous Raman scattering to be stimulated and stimulated, and stimulated Raman scattering is obtained in the transverse direction. The large gain form transverse stimulated Raman scattering (TSRS). The TSRS in large size KDP crystal not only causes the energy loss of the pump light, but also the intensity may exceed the laser damage threshold, resulting in the TSRS effect in the crystal permanent damage to the crystal, which seriously restricts the energy improvement of the laser system. A series of measures have been taken to reduce the large scale at home and abroad. TSRS effect in an inch crystal. It is known that using the DKDP crystal to replace the KDP crystal can effectively reduce the gain coefficient of the TSRS, but the quantitative relation between the deuterium content and the TSRS gain coefficient of the DKDP crystal is not clear, and it is difficult to obtain the best deuterium content to reduce the TSRS effect. In addition, the TSRS effect in the DKDP crystal will also be displayed under the higher laser power condition. In addition, the growth of DKDP crystal is difficult and the cost is high. Therefore, it is important to continue to develop the KDP, the related research on the TSRS effect in the DKDP crystal is of great significance. The stimulated Raman scattering originated from the spontaneous Raman scattering of the crystal, and its gain coefficient can be evaluated according to its spontaneous Raman spectrum. The Raman scattering characteristics of KDP, DKDP crystal are studied. We first classify the lattice vibration modes of KDP and DKDP crystals in detail according to the group theory and identify the Raman peaks. Then the stimulated Raman scattering experiments of KDP crystals are carried out, the TSRS effect in the three frequency doubling process is analyzed, and the crystal growth method, impurity ions, deuterium are also studied. The influence of the content and temperature on the Raman scattering of KDP and DKDP crystals. The main contents of the paper are as follows: 1. first, the basic lattice vibration mode of the KDP crystal is obtained by the factor group analysis method, T=4A1+5A2+6B1+7B2+13E, and then the vibration modes are classified in detail by the position group analysis method. The A1 vibration mode of the backscattering Raman scattering is verified. The angle characteristic of the A1 vibration mode shows that the symmetry of the actual crystal is lower than that of D2d., and the Raman spectra of the annealed and unannealed crystals have no obvious difference. Therefore, the angle characteristic of the A1 vibration mode is independent of the internal stress of the crystal. It is determined by the internal structure of the KDP crystal. We measured the Raman spectra under the different scattering configuration of the KDP and DKDP crystals. The Raman peaks of KDP and DKDP crystals are identified. The results show that, although the spectra are not consistent with the D2d Raman selection rule, the spectrum is consistent with the D2d Raman selection rule, and.2. derives the relationship between the stimulated Raman scattering gain coefficient and the excitation wavelength and the scattering configuration of the KDP crystal based on the stimulated Raman scattering theory. The stimulated Raman scattering (SRS) of the KDP crystal is realized by a 532 nm picosecond laser pulse pump in a single pass through cavity. The three order Stokes light (559.43 nm, 589.74 nm, 623.5 nm) of the V1 vibration mode with the maximum gain coefficient is observed experimentally. In addition, the threshold power of the pump light decreases with the increase of the SRS order of Stokes light. Therefore, the TSRS effect in the large size KDP crystal in high power laser system mainly considers the first order stimulated Raman scattering of the V1 vibration mode. The stimulated Raman scattering gain coefficient is related to the excitation wavelength and the scattering configuration. In the three frequency doubling process, there will be several different types of TSRS effects in the KDP crystal. This paper quantified Several different types of TSRS effects in the three frequency doubling system composed of class I and class II KDP crystals are compared. The results show that the TSRS effect in the three frequency doubling crystals is more significant at high frequency doubling efficiency and laser flux, especially the TSRS effect of three frequency doubling light excitation. The relative intensity of various TSRS effects can be predicted, and measures are taken to effectively reduce the laser damage probability in the use process of the crystal blank.3. to study the growth method (traditional method and fast method), the effect of thermal annealing and impurity ions (Cr3+ and Ni2+) on the Raman spectra of the KDP crystal, and the analysis of these factors on the V1 vibration modes of the KDP crystal. The effect of stimulated Raman scattering shows that these factors have different effects on the optical properties (transmittance and scattering particles) of the crystal, but the influence of the Raman scattering and V1 vibration modes on the stimulated Raman scattering gain coefficient is not obvious. The higher the interaction length is, the greater the interaction length is, the more significant the optical mass of the crystal is affected by the stimulated Raman scattering. So the TSRS effect in the large size KDP crystal is influenced by the growth method, the thermal annealing and the impurity ions. This requires an online experiment to verify that.4. has measured the DKDP crystal with different deuterium content. The spontaneous Raman spectra of the V1 vibration modes under the configuration of the bulk X (ZZ) -X and X (YY) -X scattering. With the increase of the deuterium content, the V1 vibration mode occurs red shift, and the frequency shift of the X (ZZ) -X scattering is larger than the frequency shift of the X. The half peak width of the vibration mode increases first and then decreases, while the peak intensity is opposite to the variation of deuterium content and the half peak width. According to the vibration mode coupling theory, the Raman spectra of the DKDP crystal are fitted. It is found that the red shift of the V1 vibration mode of the DKDP crystal is mainly caused by the coupling of the V1 vibration mode and the Delta (OD) vibration mode, while the V1 vibration mode and the V3 vibration mode coupling have little contribution to the V1 vibration mode frequency shift. In addition, the crystal structure changes based on the deuterium doping of the DKDP crystal have been established. The intensity superposition model of DKDP crystal Raman scattering is used to calculate the Raman spectra of DKDP crystals with different deuterium content. The results are in good agreement with the experimental spectra. According to this model, the effect of deuterium content on the half peak width and peak intensity of the DKDP crystal is explained, and the change of the peak intensity with deuterium content is from the half peak width. The TSRS gain coefficient of KDP crystal and DKDP crystal with different deuterium content was obtained by the comparison with the spontaneous Raman spectroscopy of water. The measurement error was 15%. and the TSRS gain coefficient of KDP crystal was 0.3cm. GW-1, and with the increase of deuterium content the TSRS gain of DKDP crystal decreased first to 40.1% of KDP crystal and then increased to KDP. The relationship between the gain coefficient of the DKDP crystal TSRS and the Raman shift of its V1 vibration mode is established by the 68.9%. crystal. Using this relationship, the gain coefficient and the uniformity of the DKDP crystal TSRS gain coefficient and its on-line nondestructive measurement can be realized. By analyzing the parameters related to the gain coefficient of T SRS, we believe that the variation of the half peak width caused by the deuterium content is the cause of the TSRS gain system. The main reason for the change is to find other methods to increase the half peak width of Raman scattering. In addition, in combination with the analysis of the TSRS effect of the KDP crystal in the third chapter, we give the relationship between the 3 Omega light excitation TSRS effect and the deuterium content when the DKDP crystal is a three frequency doubling material, and.5. uses the back Raman scattering. The Raman spectra of KDP crystal and V1 vibration modes of different deuterium content DKDP crystals in the range of 285.4 K to 345.2 K were measured, and the effects of temperature on the Raman shift, half peak width, scattering intensity and SRS gain coefficient of the V1 vibration modes were analyzed. With the increase of temperature, the frequency of V1 vibratory mode in KDP crystal gradually decreased and the half peak width gradually increased. The influence of degree on the Raman shift and the half peak width of DKDP crystal is related to its deuterium content. For low deuterium and high deuterium DKDP crystal, the effect of temperature on its Raman scattering is similar to that of KDP. For other deuterium content DKDP crystals, the Raman shift and the half peak width vary with the temperature. We think this is mainly due to the temperature. Degree also causes the change of O-D bond. In addition, with the increase of temperature KDP, the Raman scattering intensity of DKDP crystal has no obvious change trend, but the gain coefficient of the stable stimulated Raman scattering is reduced. The gain coefficient of the KDP crystal is reduced by about 12%. when compared with 285.4 K, so the large size KDP and DK in the high power laser system are in DK. The application of DP crystals at higher temperatures helps to reduce the TSRS effect.

【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O734;O614.113

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