Abstract:The laser-induced damage to potassium dihydrogen phosphate (KDP) crystal restricts the development of high power laser systems and attract the attention of researchers. The defects are essential for the understanding of the laser-induced damage to KDP crystals. The defects in KDP crystals are commonly related to $ \rm H_2PO_4^{-} $ groups. The defects of KDP crystal have been studied extensively, however the changes of defects of KDP crystal with low fluence and high fluence have not been investigated sufficiently. The synchrotron radiation technology is a sensitive method of detecting the defects. The vacuum ultraviolet photoluminescence (PL) emission spectra can provide microscopic structural changes in KDP crystals. In this work, we investigate the defects of KDP crystals irradiated with different fluences by vacuum ultraviolet PL emission spectra. The vacuum ultraviolet spectra are obtained at the 4B8 beam line in Beijing synchrotron radiation facilities. Each KDP crystal spectrum is measured from 200 to 400 nm and 400 to 800 nm. The emission spectra of KDP crystal irradiated with different fluences are fitted for illustration. Each Gaussian curve represents a kind of defect. Comparing the retired components with KDP crystal irradiated by 11.5 J/cm2, the new band at 231.55 nm emerges in the spectra of KDP crystal irradiated by 9.0 J/cm2. The intrinsic luminescence band is assigned to the radiative annihilation of self-trapped excitons. According to our previous work, the short chain structures mainly exist in the crystal irradiated by 9.0 J/cm2, and the long chain structure is mainly in the crystal irradiated by 11.5 J/cm2. The retired components have the short, medium and long chain. The length of P—O bond in the short chain is shorter than that in the long chain structure. The overlap between phosphorus 3s orbitals and oxygen 2p increases, and the radiative annihilation of STEs becomes stronger. So the band at 231.55 nm emerges in the spectrum of KDP crystal irradiated by 9.0 J/cm2. It suggests that the structure of the retired component and the structure of KDP crystal irradiated by 9.0 J/cm2 are different. The results provide an insight into the defects in KDP crystals. It is meaningful to study the mechanism of laser-induced damage to KDP crystal. Keywords:potassium dihydrogen phosphate crystal/ defect/ photoluminescence
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--> --> --> 1.引 言在高功率系统中如国家点火装置, 磷酸二氢钾(potassium dihydrogen phosphate, KDP)晶体一般用于电光转换和频率转换[1-3]. 但是KDP晶体的激光诱导损伤限制了激光系统的光强, 它吸引了大批研究者的关注[4,5]. 激光诱导损伤源于晶体中一些杂质或者缺陷的光吸收[1,6-10]. 晶体中的杂质和缺陷一般来自于晶体的生长及加工过程[11-14]. Pommiès等[15]用光热探测器和荧光光谱研究了损伤前驱体, 他们发现金属杂质在激光诱导损伤过程中起着重要作用. 聚集的金属包裹体具有较高的吸收系数和热导率, 它们在激光诱导损伤过程中起着损伤前驱的作用[16-18]. KDP晶体中的缺陷一般和H2PO4–基团相关, 很多****研究了KDP晶体中A和B基, 它们主要是由晶体中的氢键引起的. Duchateau等[19]的研究显示出在飞秒激光辐照下KDP和DKDP晶体在800 nm处出现B基和自捕获激子缺陷, 在266 nm处则显示出A和B基[20]. 虽然很多****研究了KDP晶体中的缺陷, 但是对高低通量下KDP晶体中缺陷变化的研究还不够. 同步辐射是一种具有高灵敏度的检测技术, 用真空紫外光谱(vacuum ultraviolet, VUV)可以得到KDP晶体微观结构变化. 本文用VUV光谱研究了不同通量下KDP晶体的缺陷, 对比了退役元件与高通量辐照下KDP晶体荧光光谱特征. 文章的第二部分是样品及实验信息, 第三部分是实验的结果及分析讨论, 第四部分是总结. 这项研究显示了退役元件与高通量辐照下晶体中的缺陷是不同的. 2.实验参数一部分样品是退役元件, 通过慢生长法生长, 原晶体尺寸为300 mm × 300 mm × 10 mm, 实验所用样品是从原晶体中切下的三块20 mm × 10 mm × 1 mm的薄片. 退役元件经过了400个脉冲辐照, 其中激光波长为355 nm, 半高宽为1 ns. 激光通量最大值为1.8 J/cm2, 平均通量为1.2 J/cm2. 另一部分样品用慢生长法生长, 通过单点金刚石飞切技术加工为10 mm × 10 mm × 1 mm. 用不同激光通量辐照KDP晶体形成损伤点, 激光波长为355 nm, 半高宽和频率分别为6.8 ns和1 Hz. 激光在样品表面的光斑大小为0.22 mm2, 详细的样品及激光参数可参考文献[21]. 在北京同步辐射4 B8线站进行VUV实验. 同步加速器的能量为2.5 GeV, 电流范围为60—150 mA, 光斑为1 mm × 2 mm, 1 m的Seya单色仪(1200 g/mm, 120—450 nm, 1 nm)和Acton SP-308单色仪(600 g/mm, 330—900 nm)用于测试发射谱, 激光激发波长为155 nm. 样品腔中的真空度为1 × 10–5 mbar. 每个KDP样品分别测试了200—400 nm和400—800 nm范围的发射谱, 步长为1 nm, 时间间隔为3 s, 用Hamamatsu H8259-01光子计数探测信号.
3.结果与讨论图1显示了不同通量辐照下KDP晶体的光致发光(photoluminescence, PL)发射谱. 在400—800 nm范围内有一个宽带, 这个波段的中心位置是477 nm. 高能量边的范围为200—400 nm, 9.0 J/cm2辐照样品的中心位置是232 nm, 而退役元件与11.5 J/cm2辐照样品的中心位置为269 nm. 从图1中可以看出, 9.0 J/cm2辐照下KDP晶体的光谱中有一个强峰. 不同通量辐照下KDP晶体的发射谱拟合如图2和图3所示, 每个高斯峰代表了一种缺陷. 图 1 不同通量辐照下KDP晶体的PL发射谱, 测量能量范围为 (a) 200—400 nm; (b) 400—800 nm Figure1. PL emission spectra of KDP crystal with different flux irradiations measured from (a) 200 to 400 nm and (b) 400 to 800 nm.
图 2 400—800 nm范围内不同通量辐照下KDP晶体的PL发射谱 (a) 退役元件; (b) 9.0 J/cm2; (c) 11.5 J/cm2. 黑色线是实验光谱, 红线是拟合叠加谱, 蓝线为高斯拟合曲线 Figure2. PL emission spectra of KDP crystals with different flux irradiations measured from 400 to 800 nm: (a) Retired; (b) 9.0 J/cm2; (c) 11.5 J/cm2. The black solid lines represent the experiment spectra, the red dotted lines represent the simulated spectra, and the blue lines represent the Gaussian fitting curve.
图 3 200—400 nm范围内不同通量辐照下KDP晶体的PL发射谱 (a) 退役原件; (b) 9.0 J/m2; (c) 11.5 J/m2. 图中黑色线是实验光谱, 红线是拟合叠加谱, 蓝线为高斯拟合曲线 Figure3. PL emission spectra of KDP crystals with different flux irradiations were measured from 200 to 400 nm: (a) Retired; (b) 9.0 J/m2; (c) 11.5 J/m2. The black solid lines represent the experiment spectra, the red dotted lines represent the simulated spectra and blue lines represent the Gaussian fitting curve.
表1400—800 nm范围内不同通量辐照下KDP晶体PL发射谱的高斯拟合参数 Table1.Parameters of peaks with Gaussian fitting for samples irradiated by different flux irradiations measured from 400 to 800 nm.
表2200—400 nm范围内不同通量辐照下KDP晶体PL发射谱的高斯拟合参数 Table2.Parameters of peaks with Gaussian fitting for samples irradiated by different flux irradiations measured from 200 to 400 nm.
图 1 不同通量辐照下KDP晶体的PL发射谱, 测量能量范围为 (a) 200—400 nm; (b) 400—800 nm
图 2 400—800 nm范围内不同通量辐照下KDP晶体的PL发射谱 (a) 退役元件; (b) 9.0 J/cm2; (c) 11.5 J/cm2. 黑色线是实验光谱, 红线是拟合叠加谱, 蓝线为高斯拟合曲线
图 3 200—400 nm范围内不同通量辐照下KDP晶体的PL发射谱 (a) 退役原件; (b) 9.0 J/m2; (c) 11.5 J/m2. 图中黑色线是实验光谱, 红线是拟合叠加谱, 蓝线为高斯拟合曲线
