Photoluminescence spectrum study of defects of potassium dihydrogen phosphate crystals irradiated by different laser fluences

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 <inline-formula><tex-math id="Z-20200908091635-1">\begin{document}$ \rm H_2PO_4^{-} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200482_Z-20200908091635-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200482_Z-20200908091635-1.png"/></alternatives></inline-formula> 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/cm<sup>2</sup>, the new band at 231.55 nm emerges in the spectra of KDP crystal irradiated by 9.0 J/cm<sup>2</sup>. 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/cm<sup>2</sup>, and the long chain structure is mainly in the crystal irradiated by 11.5 J/cm<sup>2</sup>. 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 3<i>s</i> orbitals and oxygen 2<i>p</i> 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/cm<sup>2</sup>. It suggests that the structure of the retired component and the structure of KDP crystal irradiated by 9.0 J/cm<sup>2</sup> 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.