Open Access
Add to BibSonomyAbstract
Metal inclusions play critical roles in laser-induced damage in large potassium dihydrogen phosphate optics. In this paper, the distribution and variation of copper, zinc and iron were studied by synchrotron X-ray fluorescence spectrometry. The results showed that these metals were reduced quickly with the expansion of damage sites, which indicated that copper, zinc and iron played key roles in the damage procedure. And the distribution of potassium was used to describe the size of each crater which proved that an observable damage site was formed by some smaller pinpoints. The residual metal inclusions would be concentrated at the edge of pinpoints.
© 2015 Optical Society of America
Introduction
Potassium dihydrogen phosphate (KDP) is one of the significant materials used for transmitting the high power lasers such as the Laser Megajoule (LMJ) and National Ignition Facility (NIF) due to its combination of nonlinear optical and electro-optical properties [1–3]. At present, it is the most suitable nonlinear material to make frequency converters and Pockel cells. However, the low laser induced damage threshold (LIDT) of KDP-based components becomes a tremendous limitation to the useful output of high power laser [4,5]. And this is an urgent issue to be solved.
Laser-induced damage in KDP usually initiated at nano-absorbing centers which are so called precursors [1,6,7]. Some of these precursors are the concentrated metal inclusions which imported during the growth of KDP [8–10]. And in a virgin one, the distribution of these metal inclusions were random. These metal inclusions act as origin of the damage progress for their high absorption coefficient and high thermal conductivity [11–13]. As we know that, under the action of high power laser, the metal inclusions absorbed energy and conduct to the surrounding base material then leading to the band gap decline. After that, the surrounding base material were ionized through absorbing energy, then these base material became the new absorption origin. By this process, the metal inclusions completed the energy deposition and finally these deposed energy released through explosion and plasma shock wave. At the end formed the damage craters [[14–17]]. Pommiès et al. investigated some precursors (cerium and iron) in KDP crystals by the conventional optical techniques such as photo-thermal deflection and fluorescence spectroscopy, and concluded that metal inclusions might have an impact on the laser-damage resistance [18]. For the purpose of knowing the relationship between metal inclusions and damage sites, here the synchrotron X-ray fluorescence (SXRF) has been used.
XRF is an effective method to identify elements. SXRF microprobe with higher resolution is more suitable to map the metals distribution than XRF. After using the synchrotron sources to produce the exciting micro-beam, with the advantages of high intensity, natural collimation and high degree of linear of polarization, high elemental and chemical sensitivity and spatial resolution can be achieved. This paper reported a microanalysis of metal inclusions using SXRF in KDP induced by high fluence UV 3ω (355 nm) laser pulses. We attempt to establish the possible relationship between metal inclusions and damage craters. And in this paper, we use the fluorescence signals to stand for these elements concentration degree. The count of the signal larger the elements more concentrated.
1. Samples and experimental procedure
The KDP samples we used in this study is a retired component of a Q-switched Nd-YAG laser system which was grown at State Key Laboratory of Crystal Materials of Shandong University (by traditional growth method [19]) and fabricate by single point diamond face flycutting technique. The size of the sample was 300mm × 300mm × 10mm, and nine 10 × 10 × 10 mm pieces from the original one were cut for experiments. During its service duration, it undergone 400 pulses. The wavelength was 355nm and pulse length (full width at high maximum, FWHM) was 1ns. The energy of each pulse was measured by a calorimeter. The average laser fluence of 400 pulses was 1.2J/cm2 and the peak flunce at 1.8J/cm2.
SXRF microanalysis was carried out at the BL15U1 beam line of SSRF which is a third generation synchrotron radiation facility with 3.5 Gev energy of electron beam in the storage ring. The beam come from the storage ring then focused by a pair of Kirpatrick-Baez-mirrors (KB mirrors) and monochromatized with a double-crystal Si (1, 1, 1) monochromator. The light spot could be focus from 2 to 100 μm in diameter. Samples were placed on a three dimentional movable table and each axis had a precision of 0.1μm. The X-ray fluorescence signals needed for normalization, detector dead time and ring current were detected with a nitrogen cooled energy dispersive Si drift detector. The synchrotron beam and the detector was arranged with a 90° geometry. As the synchrotron beam was highly polarized, the background caused by the scattering of X-rays would be reduced greatly. In order to excite the maximum signal, the excitation energy was set to 14 kev. Under this circumstances, we can collect the signals of potassium, copper, zinc, and iron. With the help of sample microscopy and beam-focusing microscopy, we can easily control the beam spot and get any position on the sample we desired. To get an accurate results and consider other conditions, the beam spot with a diameter of 5 μm was chosen. And the acquisition time was set at 2 seconds per point when we collect the area scan data. To get a continual scanning result, the step size of the motor was set up at 5μm. The laser confocal scanning microscope (LCMS) was used to select the scanning area according to the size of the craters.
2. Results
We measured 20 different damage sites and the statistical data was shown in Table 1.
Table 1. Statistical result of damage crater’s size.
Figure 1(d) shows a typical damage crater and Fig. 1(a)-1(c) shows the distribution of copper, zinc, and iron for the crater displayed in Fig. 1-2. It should be noted that the scale color bar on the right of each elemental distribution map shows counts normalized to the compton scattering which can only be used for comparing purposes between the same elements. As Fig. 1(a)-1(c) displayed the elements concentrate in somewhere of the damage crater, but we cannot find an accurate relationship between the damage crater and metal inclusions. Thus, we scanned a line instead of mapping an area for getting a more precisely results. When we used a line to instead the mapping area, the acquisition time of each point can be set at 20 seconds while the motor step size do not change which can made the fluorescence signals more clearly. And we need to explain one thing that is the lines we choose to instead the area are two which arranged 90° in geometry and this method is appropriate for the result is statistic.
We have scanned 20 damage sites and Fig. 2 shows a representative result we have found. As shown in Fig. 2, copper, iron and zinc have an obvious concentration in the damage sites and Table 2 shows the mean value and standard deviation of normalized counts of each element. Thus, the elements’ distribution has some relationship with the size of the damage craters.
Table 2. The mean value and square deviation of each element in discussion.
3. Discussion
3.1 The variation of metal inclusion
In this paper, the comparison among different metals has not been discussed for the variable efficiency of fluorescence production and detection based on the atomic number and fluorescence energy. Therefore, the comparison is confined in the same elements. Figures 3(a)-3(c) depict a significant variation between damage crater size and the metal inclusions decrement. In Fig. 3(d), we calculate the element density (it is the result of normalized counts integration divide the corresponding length) of each damage crater. Actually, when the KDP was irradiated by high laser fluence, it would undergone metal inclusions disperse or subsequent ejection of bulk KDP. If the laser fluence is high enough, the temperature of the metal inclusion can easily exceed 10,000 K. Then high pressure (larger than 70 Gpa) could be produced on the surface. Thus, the bulk will breakdown immediately. At the damage sites, the concentration of metal inclusions act as the precursor of the process, after pulses of laser irradiated, they reduced with the ejection of bulk KDP [16].
3.2 The distribution of potassium at different size of damage craters
P. DeMange et al, found that the average pinpoint size was about 13.5μm × 23.6μm, and the damage sites were larger than it. They postulated that the second pulse deposited a large amount of energy on the plasma created at damage sites from the first pulse giving rise to more severe damage as a result of shock wave propagation and melting [20]. But we cannot distinguish whether the damage sites were formed independent or base on the pinpoints (which are very small damage sites) from the shape of it. Here, we tried a new way to study this question.
As we know that, under the extremely high temperature and high tensile pressure, there must have experienced some phase transitions and degradations. Actually, R.A. Negres et al. have studied this question by Raman spectroscopy in early 2005 [21]. For the potassium was distributed evenly in the nonirradiated KDP crystal, when the distribution of potassium was changed, the KDP must have experienced phase transitions or degradations. So we believe that the variation of potassium can stand for the damage sites. Thus, the distribution of potassium at different damage sites was studied. The results are shown in Fig. 4. It depicts the distribution of potassium at different size of damage craters and the corresponding fitted curve which is matched by Gaussian Function. These damage craters covered all the size range we have measured. Table 3 shows the FWHM of each peak in the fitted curve. From Table 3, we can conclude that each damage sites is composed by some pinpoints and the average radius is about 20μm. In large damage craters, such as L = 230μm, there appears some large radius (about 50μm) pinpoints, this may because there are two or three small pinpoints mixed in that area. Therefore, we can conclude that a damage site is consist of many pinpoints. And we need to point out one thing that is the valley and straight line of potassium curve stand for good area or slightly effected.
Fig. 4 The distribution of potassium and iron at different damage sites and there Gaussian fitted curves.
Table 3. The FWHM of each peak of every fitted curve at different damage sites.
3.3 The position of residual metal inclusions and potassium
From Fig. 3 we can know that iron, copper and zinc are almost concentrated at the same position, so here we just compare the position of iron and potassium. In Fig. 4, we compare the position of iron and potassium to study their relationship and we can concluded that the peaks of iron are corresponding to the valley of potassium. And as the valley stand for good area, which means iron are concentrated at the edge of damage craters or good area. We can explain this result from the damage process. When the KDP was unbroken, the metal inclusions were dispersed randomly, while absorbed laser energy, they became the center of explosion or the source of shock waves. By these process, the metal inclusions were transported from the center to the edge of damage craters and finally formed the results of Fig. 4 showing. For the big damage craters, as its complicated circumstance, some pinpoints overlapped, so some iron concentrated at the valley of the damage crater, however, the valley is the edge of the other adjacent pinpoint. And the diffused distance are equal or larger than the radius of the adjacent pinpoints.
3.4 The comparison with other method
The classical method to measure the damage craters is scanning electron microscope, which can observe the damage sites more clearly, however it can’t depict the formation process of damage sites. While use the variation of potassium to depict the damage sites we can easily know that a damage crater is composed by some pinpoints.
By means of photothermal deflection, it shows that the absorption of 3ω is very well related to the iron concentration [18]. But this method can’t show an accurate relationship between iron and damage sites. And by the SXRF, we have known that after laser irradiated, metal inclusions were transported to the edge of damage craters.
Conclusion
The distribution and variation of metal inclusions in KDP irradiated by 355 nm laser pulses were studied by the SXRF microanalysis. Mapping results show that copper, zinc and iron are very well correlated with the damage procedure. With the damage crater expansion, the metal inclusions will splash or defused from the original position. This conclusion will be useful to the engineers who are working at manufacture optics for large laser facilities. We use the distribution of potassium to depict the size of the craters can also be a helpful way to study the mechanism of laser induced damage in KDP. Finally, we compared the concentrated position of metal inclusions and concluded that they are gathered at the edge of the pinpoints, but most are still in the damage area, which can be useful in the optical components reprocessing.
Acknowledgments
We acknowledge Dr. Lili Zhang and Dr. Hui Jiang at the Shanghai Synchrotron Radiation Facility for their beam line support during data collection and Dr. Heliang Sui for his advices during the formation of this paper.
References and links
1. C. W. Carr, H. B. Radousky, and S. G. Demos, “Wavelength dependence of laser-induced damage: determining the damage initiation mechanisms,” Phys. Rev. Lett. 91(12), 127402 (2003). [CrossRef] [PubMed]
2. K. Boopathi, P. Rajesh, P. Ramasamy, and P. Manyum, “Comparative studies of glycine added potassium dihydrogen phosphate single crystals grown by conventional and Sankaranaryanan–Ramasamy methods,” J. Opt. Matt. 35(5), 954–961 (2013). [CrossRef]
3. J. J. De Yoreo, A. K. Burnham, and P. K. Whitman, “Developing KH2PO4 and KD2PO4 crystals for the world ’ s most powerful laser,” Int. Mater. Rev. 47(3), 113–152 (2002). [CrossRef]
4. R. A. Negres, N. P. Zaitseva, P. Demange, and S. G. Demos, “A new expedited approach to evaluate the importance of different crystal growth parameters on laser damage performance in KDP and DKDP,” Proc. SPIE 6403, 225780 (2006). [CrossRef]
5. P. Demange, R. A. Negres, C. W. Carr, and S. G. Demos, “A new damage testing system for detailed evaluation of damage behavior of bulk KDP and DKDP,” Proc. SPIE 5647, 208281 (2005). [CrossRef]
6. P. Demange, R. A. Negres, N. P. Zaitseva, H. B. Radousky, and S. G. Demos, “Correlation of laser-induced damage performance to crystal growth conditions in KDP and DKDP crystals,” in Proceedings of IEEE Conference on Lasers & Electro-Optics/Quantum Electronics and Laser Science Conference(IEEE, 2006), 4628122 (2006). [CrossRef]
7. G. H. Hu, Y. A. Zhao, S. Sun, D. W. Li, X. F. Liu, and X. Sun, “Growth Characteristics and Mechanism of Surface and Bulk Damage in KDP and DKDP Crystals,” Chin. Phys. Lett. 26, 8 (2005).
8. D. C. Guo, X. D. Jiang, J. Huang, F. R. Wang, H. J. Liu, X. Xiang, G. X. Yang, W. G. Zheng, and X. T. Zu, “Effects of γ-ray irradiation on optical absorption and laser damage performance of KDP crystals containing arsenic impurities,” Opt. Express 22(23), 29020–29030 (2014). [CrossRef] [PubMed]
9. N. Y. Garces, K. T. Stevens, L. E. Halliburton, M. Yan, N. P. Zaitseva, and J. J. DeYoreo, “Optical absorption and electron paramagnetic resonance of Fe ions in KDP crystals,” J. Cryst. Growth 225(2–4), 435–439 (2001). [CrossRef]
10. C. M. R. Rem, W. Paraguassu, G. D. Saraiva, D. P. Pereira, P. C. de Oliveira, and P. T. C. Freire, “Temperature-dependent Raman scattering of KDP : Mn (0.9% weight of Mn) crystal,” J. Raman Spectrosc. 41(5), 1318–1322 (2010).
11. P. DeMange, C. W. Carr, R. A. Negres, H. B. Radousky, and S. G. Demos, “Laser annealing characteristics of multiple bulk defect populations within DKDP crystals,” J. Appl. Phys. 104(10), 103103 (2008). [CrossRef]
12. K. Wang, C. Fang, J. Zhang, X. Sun, S. Wang, Q. Gu, X. Zhao, and B. Wang, “Laser-induced damage mechanisms and improvement of optical qualities of bulk potassium dihydrogen phosphate crystals,” J. Cryst. Growth 287(2), 478–482 (2006). [CrossRef]
13. P. Demange, R. A. Negres, C. W. Carr, H. B. Radousky, and S. G. Demos, “Laser-induced defect reactions governing damage initiation in DKDP crystals,” Opt. Express 14(12), 5313–5328 (2006). [CrossRef] [PubMed]
14. M. F. Koldunov, “Theory of laser-induced inclusion-initiated damage in optical materials,” Opt. Eng. 51, 121811 (2002).
15. A. A. Manenkov, “Fundamental mechanisms of laser-induced damage in optical materials: today’s state of understanding and problems,” Opt. Eng. 53(1), 010901 (2014). [CrossRef]
16. C. W. Carr, H. B. Radousky, A. M. Rubenchik, M. D. Feit, and S. G. Demos, “Localized dynamics during laser-induced damage in optical materials,” Phys. Rev. Lett. 92(8), 087401 (2004). [CrossRef] [PubMed]
17. B. Ma, Y. Zhang, H. Ma, H. Jiao, X. Cheng, and Z. Wang, “Influence of incidence angle and polarization state on the damage site characteristics of fused silica,” Appl. Opt. 53(4), A96–A102 (2014). [CrossRef] [PubMed]
18. M. Pommiès, D. Damiani, X. Le Borgne, C. Dujardin, A. Surmin, J. C. Birolleau, F. Pilon, B. Bertussi, and H. Piombini, “Impurities detection by optical techniques in KH2PO4 crystals,” Opt. Commun. 275(2), 372–378 (2007). [CrossRef]
19. B. A. Liu, G. H. Hu, Y. A. Zhao, M. X. Xu, S. H. Ji, L. L. Zhu, L. Zhang, X. Sun, Z. Wang, and X. Xu, “Laser induced damage of DKDP crystals with different deuterated degrees,” Opt. Laser Technol. 45, 469–472 (2013). [CrossRef]
20. P. DeMange, C. W. Carr, H. B. Radousky, and S. G. Demos, “System for evaluation of laser-induced damage performance of optical materials for large aperture lasers,” Rev. Sci. Instrum. 75(10), 3298–3301 (2004). [CrossRef]
21. R. A. Negres, S. O. Kucheyev, P. DeMange, C. Bostedt, T. van Buuren, A. J. Nelson, and S. G. Demos, “Decomposition of KH2PO4 crystals during laser-induced breakdown,” Appl. Phys. Lett. 86(17), 171107 (2005). [CrossRef]
