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1 micrometer high-efficient radiation resistant laser crystal: Nd:YSAG
Journal of Luminescence 214 (2019) 116596
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
1 micrometer high-efficient radiation resistant laser crystal: Nd:YSAG a,*
b
Shoujun Ding , Haoyu Wang , Jianqiao Luo
c,**
c
d
, Wenpeng Liu , Yufei Ma , Qingli Zhang
T c
a
School of Mathematics and Physics, Anhui University of Technology, Maanshan, Anhui, 243002, PR China Science and Technology on Applied Physical Chemistry Laboratory, Shaanxi Applied Physics and Chemistry Research Institute, Xi′an, 710061, PR China Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, Anhui, 230031, PR China d National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin, 150001, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Nd:YSAG Radiation resistant Laser crystal Czochralski method Gamma-ray
With the increased requirements of solid state lasers from some special areas, such as space craft, reactor inspection and high level radioactive waste detection, developing laser crystals with radiation resistant ability become more and more important. In this study, high quality Nd-doped Y3Sc2Al3O12 (Nd:YSAG) crystal was grown successfully by Czochralski method and its radiation resistant ability was evaluated. XRD and Raman spectrum characterization results indicate that the structure of the as-grown crystal remain unchanged after the irradiation of 20 Mrad gamma-ray. Besides, the highest decrease of the transmittance was only 5% for the crystal after the irradiation of 10 Mrad gamma-ray suggesting the excellent radiation resistant ability of the crystal. Importantly, the almost unchanged absorption at 808 nm and fluorescence properties for the crystal after the irradiation of 20 Mrad gamma-ray confirmed the excellent radiation resistant ability of Nd:YSAG crystal again. High efficient LD pumped 1.06 μm laser was realized in Nd:YSAG crystal and the slightly decreases of the laser efficiency after the irradiation indicate the crystal can be considered as a promising high efficient radiation resistant laser material for the application in harsh radiation environment.
1. Introduction Since the first laser was realized in ruby crystal by T.H. Maiman in 1960 and the success in generating continuous-wave (CW) laser in Nd:YAG crystal in 1964 [1,2], solid state lasers have undergone fast development for almost 60 years. In recent years, solid state lasers, especially for 1 μm lasers, have been widely used in medical treatment [3], metal processing and environmental instrumentation measurement [4,5], and have demonstrated potential applications for future nuclearfusion [6,7]. However, new challenges have been aroused for solid state lasers due to the requirements from some special areas, such as space craft [8], reactor inspection and high level radioactive waste detection [9,10]. Generally, the working materials in the laser system will produce various significant changes in their structural, chemical, physical as well as optical properties after the exposure of high-energy radiation (X-ray, gamma-ray and neutrons) [11]. Therefore, developing laser crystals with radiation resistant ability are necessary and have attracted considerable attentions from all over the world. Recently, great progresses have been achieved in the studies of radiation effect on the luminescent properties of laser crystals [12–15]. One fascinating conclusion is that scandium (Sc) contained garnet
*
crystals generally exhibit excellent radiation resistant properties in comparison with the mostly used crystals [12,13,16], such as yttrium aluminum garnet (YAG) crystal [17], phosphate crystal [9], gadolinium gallium garnet (GGG) crystal and fluoride crystal (LiYF4) [18]. For examples: (1) Two additional absorption bands were observed in the spectra of Nd:GGG crystal after the gamma-ray irradiation while only one weak additional absorption band was appeared for Nd3+ doped gadolinium scandium gallium garnet (GSGG) crystal [12]. (2) The spectroscopic properties of Yb,Er:GSGG crystal was almost undeteriorate after the irradiation of 50 Mrad gamma-ray [13]. (3) The laser output performance of Er3+ doped gadolinium yttrium scandium gallium garnet (GYSGG) crystal was only affected slightly by the 100 Mrad gamma-ray irradiation [16]. Unfortunately, all these radiation resistant crystals mentioned above contains easily volatile element Gallium (Ga) indicating that it is difficult to keep the compositions consistent and grow single crystals with large size. Recently, gadolinium scandium aluminum garnet (GSAG) crystal, which colligates the advantages of GSGG and YAG crystals, was designed successfully with good radiation resistant ability and without volatile element Ga [11]. Yttrium aluminum scandium garnet (YSAG) is the other crystal that colligates the advantages of GSGG and YAG
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Ding), [email protected] (J. Luo).
**
https://doi.org/10.1016/j.jlumin.2019.116596 Received 11 March 2019; Received in revised form 28 June 2019; Accepted 2 July 2019 Available online 02 July 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
Journal of Luminescence 214 (2019) 116596
S. Ding, et al.
crystal. To the best of our knowledge, the influence of the gamma-ray irradiation on the optical and laser properties of Nd:YSAG crystal has not been reported elsewhere. In this study, a Nd:YSAG crystal was grown by Czochralski (Cz) method. The structure, Raman, absorption and fluorescence spectra of the as-grown crystal were investigated before and after irradiation with different doses gamma-ray. Importantly, high efficient 1 μm CW laser was realized successfully in Nd:YSAG crystal before and after irradiation with different doses gamma-ray. The results indicate that Nd:YSAG crystal is a promising high efficient 1 μm radiation resistant laser crystal.
950 spectrometer with a spectral interval of 1 nm. An Edinburgh FLSP920 spectrometer was employed to record the emission spectra and fluorescence decay curves of the crystal. The excitation source used to generate the emission spectra and fluorescence decay curves were Xenon-lamp and optical parametric oscillation (OPO) laser (Opolette 355 I), respectively. A Horiba-Jobin Yvon LABRAM-HR Raman spectrometer system equipped with a confocal microscope was employed to record the Raman spectra of the crystal at room temperature. The excitation source used to generate the Raman spectra was a frequency doubled diode pumped Nd:YAG laser at 532 nm.
2. Experimental details
2.4. Laser experiment
2.1. Crystal growth
The schematic diagram of laser diode (LD) pumped 1.06 μm continuous-wave laser experiments is shown in Fig. 2, where a plano to plano resonator is used. A fiber-coupled LD is employed as the pumping source with the central wavelength at around 808 nm and maximum output power of 30 W. The core diameter and numerical aperture (NA) of the fiber are 400 μm and 0.22, respectively. M1 is the input mirror with 808 nm anti-reflection coated on the pumping side, and 808 nm high-transmission and 1.06 μm high-reflection coated on the other side. M2 is the output couple mirror with the transmittance of 5% at 1.06 μm. The cavity length (the distance between M1 and M2) is fixed as 18 mm. The as-grown Nd:YSAG crystal samples, cut with the dimension of 2 × 2 × 4 mm3 and two 2 × 2 mm2 faces parallel to (111) crystalline orientation and polished carefully, are used as the working materials. In order to remove the heat generated in the crystal timely during the laser experiment, the crystal is wrapped with indium foil and placed into water-cooled copper heat sink with microchannel structure. The temperature of the cooling water is maintained at 19 °C. The output laser power is measured by an OPHIR 30A-BB-18 power meter.
Nd:YSAG crystal was grown by the Cz method using a JGD-60 furnace (CETC 26th, China) with an automatics diameter controlled (ADC) growth system. The oxide compounds Nd2O3 (99.999%), Y2O3 (99.999%), Sc2O3 (99.999%), Al2O3 (99.999%) were used as raw materials and weighed according to the designed composition of Nd0.03Y2.97Sc2Al3O12. After being mixed adequately, the raw materials were pressed into disks and dried with muffle furnace. Then the disks were loaded into iridium crucible and grown to crystal in nitrogen atmosphere. After the growth process finished, the crystal was cooled down to room temperature at a rate of 30–50 °C/h and subsequently annealed in air at 1100 °C for 72 h to eliminate the strain inside the crystal. The used seed crystal was a YAG crystal with (111) orientation. The used pulling speed and rotation rate were 0.5–1.5 mm/h and 5–15 rpm, respectively. The photograph of the as-grown crystal boule is shown in Fig. 1 (a). 2.2. Gamma-ray irradiation
3. Results and discussion The samples cut from the post-annealed crystal were irradiated by a60Co gamma-ray source with dose rate of 82 Gy/min, radiation times of 20 h and 40 h, corresponding to the doses of 10 Mrad and 20 Mrad, respectively. As shown in Fig. 1 (b), the color of the crystal becomes darker with increasing the irradiation doses.
3.1. Crystal structure The XRC of the (111) crystalline surface of Nd:YSAG crystal manifests as one single symmetric peak and without splitting as shown in Fig. 3 (a). The full width at half maximum (FWHM) of this peak is fitted to be 0.09°, suggesting the high crystalline quality of the as-grown crystal. The XRD patterns of the post-annealed Nd:YSAG and pure YSAG from card ICSD#67055 are shown in Fig. 3 (b). As it can be seen, all the measured diffraction peaks are sharp and the peaks position well coincident with that from card ICSD#67055, indicating that the crystal is well-crystallized and the crystal structure unchanged by the doping ions. Thus, the as-grown crystal belongs to the cubic phase with space group of Ia-3d. Using the structural parameters from the card ICSD#67055 as the initial values, the unit cell parameters of Nd:YSAG
2.3. Structure and spectroscopic measurements The crystal structure was identified by powder X-ray diffraction (XRD) using a Philips X'pert Pro X-ray powder diffractometer equipped with Cu Kα radiation. A scan step of 0.033° was applied to record the patterns in the 2θ range of 10°–90°. A high resolution X'pert Pro MPD diffractometer equipped with hybrid Kα1 monochromator was employed to record the X-ray rocking curve (XRC). The absorption spectra of the samples were recorded by a PerkinElmer UV-VIS-NIR lambda-
Fig. 1. (a) Photograph of the as-grown Nd:YSAG crystal boule. (b) Polished Nd:YSAG crystal slices after irradiation with 0, 10 and 20 Mrad doses gamma-ray, respectively. 2
Journal of Luminescence 214 (2019) 116596
S. Ding, et al.
Fig. 2. Schematic diagram of the LD pumped 1.06 μm continuous-wave laser experiment.
are obtained with Rietveld refinement method [19,20]. The lattice parameters are refined to be a = b = c = 12.258 Å and α = β = γ = 90°, respectively, by general structure analysis software (GSAS). The refined results are shown in Table 1 and Fig. 3 (c) with the refined residuals Rp and Rwp are both less than 10%, indicating that the refined results are reliable. The XRD patterns of Nd:YSAG crystal before and after irradiation with different doses gamma-ray are shown in Fig. 3 (d). It is obvious that the position and intensity of the diffraction peaks are identically before and after the irradiation, which indicates the crystal structure do not be changed by the irradiation. Thus, the color change as appeared in Fig. 1 (b) could be attributed to the formation of color center in the crystal after the irradiation.
Table 1 Rietveld refinement results of Nd:YSAG crystal. Atoms
X
Nd 0.0000 Y 0.0000 Sc 0.0000 Al 0.0000 O 0.0302 Lattice Parameters R factors
Y
Z
Occupancy
0.2500 0.1250 0.015 0.2500 0.1250 0.985 0.0000 0.0000 1.000 0.2500 0.3750 1.000 0.0512 0.6564 1.000 a = b = c = 12.258 Å α = β = γ = 90° Rp = 7.50%, Rwp = 9.86%
Uiso 0.0051 0.0051 0.0074 0.0041 0.0067
3.2. Spectroscopic properties The Raman spectra of Nd:YSAG crystal before and after irradiation with different doses gamma-ray are shown in Fig. 4 (a). The Raman
Fig. 3. (a) X-ray rocking curve of (111) crystalline face of Nd:YSAG crystal. (b) Measured XRD patterns of Nd:YSAG crystal and XRD patterns from ICSD#67055. (c) Rietveld refinement results of Nd:YSAG crystal (Obs.: the observed data, Calc.:the calculated data, Bkg.:the background, Diff.: the difference between the observed and calculated data). (d) The XRD comparison for Nd:YSAG crystal before and after irradiation with different doses gamma-ray. 3
Journal of Luminescence 214 (2019) 116596
S. Ding, et al.
Fig. 4. (a) The Raman spectra of Nd:YSAG crystal before and after irradiation with different doses gamma-ray. (b) The transmission spectra of Nd:YSAG crystal before and after irradiation with different doses gamma-ray. Inset: The transmission spectra at around 808 nm. (c) The transmission spectra of non-irradiated and 20 Mrad irradiated samples between 290 and 450 nm. (d) The absorption coefficient of Nd:YSAG crystal. Inset: The comparison of absorption between Nd:YAG and Nd:YSAG crystal at around 808 nm.
contained garnet crystals, such as Nd:YSAG and Nd:GSGG, show excellent radiation resistant ability and can be expected using in harsh radiation environment. The transmission spectra of non-irradiated and 20 Mrad irradiated samples between 290 and 450 nm are shown in Fig. 4 (c). As it can be seen, two additional peaks with central wavelength at 307 and 310 nm appear in the 20 Mrad irradiated sample in comparison with non-irradiated sample. According to the reported reference [24], the additional absorption bands at around 300 nm after irradiation could be assigned to F+-centers, namely, the color centers generated in the crystal after irradiation are belonging to F+-centers. Fig. 4 (d) illustrated the absorption coefficient of Nd:YSAG crystal before and after irradiation with different doses gamma-ray. Nine absorption bands appeared in the figure are corresponding to the transitions of Nd3+ from ground state 4I9/2 to different excited states. The absorption cross section σabs of Nd:YSAG crystal can be calculated using the formula σabs = α(λ)/ Nc , where α(λ) is the absorption coefficient and Nc is the concentration of Nd3+ ions in the crystal. The calculated σabs at 808.9 nm is 11.5 × 10−20 cm2, suggesting that the Nd:YSAG crystal can be pumped efficiently by the commercialized 808 nm LD. In addition, the broader absorption band at around 808 nm for Nd:YSAG (as shown in inset of Fig. 4 (d)), in comparison with the Nd:YAG crystal, is beneficial to decrease the temperature dependence of the LD pumping source and improve the laser efficiency. Fig. 5 (a) illustrated the emission spectra of Nd:YSAG crystal before
peaks position and intensity are almost unchanged after the irradiation, which means the internal vibrational mode frequencies in the crystal are unaffected by the irradiation [21]. Therefore, the crystal structure is unchanged by the irradiation, which is consistent with the XRD characterization result. According to the reported Raman spectra of some other garnet crystals [22], the Raman active modes are assigned and denoted in Fig. 4 (a). Additionally, the maximum phonon energy of Nd:YSAG is determined to be 763 cm−1 from the Raman spectra, which is lower than that of Nd:YAG [23] (776 cm−1). The lower maximum phonon energy of Nd:YSAG is advantageous for achieving population inversion. The transmission spectra of Nd:YSAG crystal before and after irradiation with different doses gamma-ray are illustrated in Fig. 4 (b). As can be seen, despite the significant decrease of the transmittance after the gamma-ray irradiation in the wavelength below 730 nm, the transmittance affected by the irradiation is quite slightly in the wavelength above 730 nm, especially at 808 nm (as shown in inset of Fig. 4 (b)). That is to say, the crystal after the gamma-ray irradiation can still match well with the commercialized 808 nm diode laser pumping source. The crystal transmittance decrease ratio Dra caused by the irradiation can be estimate by the formula Dra =
Tλb − Tλa Tλb
, where Tλb and Tλa
represents the crystal transmittance at wavelength λ before and after irradiation, respectively. According to this formula, the calculated Dra at wavelength 400 nm for Nd:YSAG crystal and some other Nd3+ doped radiation resistant crystals are listed in Table 2. As can be seen, Sc Table 2 The calculated Dra for different Nd3+ doped radiation resistant crystals. Crystals
Nd:YSAG
Nd:GSGG
Nd:GGG
Nd:YAG
Nd:YLF
Nd: phosphate
Irradiation doses Dra References
10 Mrad 5% This work
100 Mrad 23% [12]
100 Mrad 40% [12]
10 Mrad 18% [17]
20 Mrad 85% [18]
1 Mrad 46% [9]
4
Journal of Luminescence 214 (2019) 116596
S. Ding, et al.
Fig. 5. (a) The emission spectra of Nd:YSAG crystal before and after irradiation with different doses gamma-ray. (b) The fluorescence decay curve of non-irradiated Nd:YSAG crystal. (c) The fluorescence decay curve of Nd:YSAG crystal after irradiation with 10 Mrad gamma-ray. (d) The fluorescence decay curve of Nd:YSAG crystal after irradiation with 20 Mrad gamma-ray.
and after irradiation with different doses gamma-ray. It can be noted that the fluorescence intensities are almost uninfluenced by the irradiation, suggesting the excellent radiation resistant ability of Nd:YSAG crystal. Three emission bands observed in the measured range could be assigned to the transitions of Nd3+ from 4F3/2 to 4I9/2, 4I11/2 and 4I13/2, respectively, as denoting in Fig. 5 (a). The fluorescence decay curves of 4 F3/2 → 4I11/2 transition for Nd:YSAG crystal before and after irradiation with different doses gamma-ray are shown in Fig. 5 (b), (c) and (d). Using the single exponential function to fit the curves, the fluorescence lifetimes are fitted to be 240, 243 and 253 μs for the crystal irradiated with doses of 0, 10 and 20 Mrad, respectively. On the one hand, the slightly fluctuation of the lifetime after the irradiation indicates the excellent radiation resistant ability of Nd:YASG crystal again. On the other hand, the longer lifetime of Nd:YSAG crystal in comparison with that of Nd:YAG crystal suggesting the better energy storage capacity of Nd:YSAG crystal, which is advantageous for realizing laser output. According to the Fuchtbauer-Ladenburg (F-L) formula [25]:
σem (λ ) =
λ5I (λ ) ∫ λI (λ ) dλ
Fig. 6. The 1.06 μm laser output power of Nd:YSAG crystal before and after irradiated with different doses gamma-ray versus the absorbed incident power.
8πn2cτm
where I(λ) is the fluorescence intensity at the wavelength of λ, n is the reflective index, c is the velocity of light, τm is the measured lifetime. The stimulated emission cross section of Nd:YSAG crystal at 943 nm and 1060 nm are calculated to be 2.2 and 10.1 × 10−20 cm2, respectively. The relative large emission cross section at ~940 nm indicates that Nd:YSAG crystal not only can achieve 1 μm laser output, but also is promising for generating ~940 nm laser, which wavelength has important applications in water vapor detection by differential absorption lidar (DIAL) technique [26,27].
40.8% and 38.6% for Nd:YSAG with irradiation doses of 0, 10 and 20 Mrad, respectively. The decrease of the laser efficiency with increasing the irradiation dose could be explained by Matkovski's. proposal [28,29]: the formed color center in the crystal after the irradiation can absorb pumping light as well as reabsorb laser radiation. Importantly, such a small decrease of the laser efficiency after the irradiation, suggesting that the Nd:YSAG crystal is an excellent high efficient radiation resistant crystal for application in harsh radiation environment.
3.3. Laser performance 4. Conclusions
Fig. 6 illustrated the realized laser output power versus the absorbed incident power using the Nd:YSAG with irradiation doses of 0, 10 and 20 Mrad, respectively. The slope efficiency are fitted to be 43.2%,
High quality Nd:YSAG single crystal was grown successfully by Cz 5
Journal of Luminescence 214 (2019) 116596
S. Ding, et al.
method. The post-annealed crystal was irradiated with different doses gamma-ray to study its radiation resistant ability. XRD and Raman characterization results indicate that the crystal structure remain unchanged after the irradiation, namely, the color changes of the crystal after irradiation could be attributed to the formation of color centers in the crystal. The excellent radiation resistant ability of Nd:YSAG crystal was proved by the slightly fluctuation of the absorption and fluorescence spectra after the gamma-ray irradiation. High efficient LD pumped 1.06 μm laser was realized in the as-grown crystal. Importantly, the laser efficiency was slightly influenced by the gammaray irradiation, suggesting that the crystal can be considered as a promising high efficient radiation resistant laser material for the application in harsh radiation environment.
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Acknowledgements [18] [19] [20]
The Startup Foundation for Advanced Talents of Anhui University of Technology, National Natural Science Foundation of China (Grants No. 51502292), National Key Research and Development Program of China (Grants No. 2016YFB0402101) supported this study.
[21] [22] [23]
Appendix A. Supplementary data
[24]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jlumin.2019.116596.
[25] [26]
References [27] [28]
[1] J. Geusic, H. Marcos, L. Van Uitert, Appl. Phys. Lett. 4 (1964) 182. [2] T.H. Maiman, Nature 187 (1960) 493. [3] R. Moncorgé, B. Chambon, J. Rivoire, N. Garnier, E. Descroix, P. Laporte, H. Guillet, S. Roy, J. Mareschal, D. Pelenc, Opt. Mater. 8 (1997) 109. [4] A. Ikesue, Y.L. Aung, Nat. Photon. 2 (2008) 721.
[29]
6
A.K. Misra, S.K. Sharma, P.G. Lucey, Appl. Spectrosc. 60 (2006) 223. M.H. Key, Nature 412 (2001) 775. S. Nakai, K. Mima, Rep. Prog. Phys. 67 (2004) 321. M. Toyoshima, Y. Takayama, H. Kunimori, T. Jono, S. Yamakawa, Opt. Eng. 49 (2010) 083604. V. Rai, B.R. Sekhar, S. Kher, S. Deb, J. Lumin. 130 (2010) 582. S.J. Zinkle, L.L. Snead, Annu. Rev. Mater. Res. 44 (2014) 241. Y. Chen, Q. Zhang, F. Peng, W. Liu, D. Sun, R. Dou, H. Zhang, Y. He, S. Han, S. Yin, J. Lumin. 205 (2019) 109. D. Sun, J. Luo, Q. Zhang, J. Xiao, J. Xu, H. Jiang, S. Yin, J. Lumin. 128 (2008) 1886. D. Sun, J. Luo, Q. Zhang, J. Xiao, W. Liu, S. Wang, H. Jiang, S. Yin, J. Cryst. Growth 318 (2011) 669. J. Luo, D. Sun, H. Zhang, Q. Guo, Z. Fang, X. Zhao, M. Cheng, Q. Zhang, S. Yin, Opt. Lett. 40 (2015) 4194. D. Sugak, A. Matkovskii, A. Durygin, A. Suchocki, I. Solskii, S. Ubizskii, K. Kopczynski, Z. Mierczyk, P. Potera, J. Lumin. 82 (1999) 9. J. Chen, D. Sun, J. Luo, J. Xiao, R. Dou, Q. Zhang, Optic Commun. 301 (2013) 84. T. Rose, R. Fields, R. Hutcheson, Radiation Hardening of Nd: YAG by Transition Metal Ion Codopants, Advanced Solid State Lasers, Optical Society of America, 1994, p. NS4. T. Rose, M. Hopkins, R. Fields, IEEE J. Quantum Electron. 31 (1995) 1593. M.t. Sakata, M. Cooper, J. Appl. Crystallogr. 12 (1979) 554. S. Ding, Q. Zhang, F. Peng, W. Liu, J. Luo, R. Dou, G. Sun, X. Wang, D. Sun, J. Alloy. Comp. 698 (2017) 159. L.G. Cançado, A. Jorio, E.M. Ferreira, F. Stavale, C. Achete, R. Capaz, M. Moutinho, A. Lombardo, T. Kulmala, A.C. Ferrari, Nano Lett. 11 (2011) 3190. J. Koningstein, O.S. Mortensen, J. Mol. Spectrosc. 27 (1968) 343. Z. Fang, D. Sun, J. Luo, H. Zhang, X. Zhao, C. Quan, L. Hu, M. Cheng, Q. Zhang, S. Yin, Opt. Express. 25 (2017) 21349–21357. N. Mironova-Ulmane, I. Sildos, E. Vasil'chenko, G. Chikvaidze, V. Skvortsova, A. Kareiva, J. Muñoz-Santiuste, R. Pareja, E. Elsts, A. Popov, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 435 (2018) 306. S. Ding, Q. Zhang, D. Sun, F. Peng, W. Liu, J. Luo, G. Sun, J. Lumin. 201 (2018) 65. G.J. Koch, B.W. Barnes, M. Petros, J.Y. Beyon, F. Amzajerdian, J. Yu, R.E. Davis, S. Ismail, S. Vay, M.J. Kavaya, Appl. Opt. 43 (2004) 5092. E. Browell, S. Ismail, W. Grant, Appl. Phys. B 67 (1998) 399. A. Matkovskii, D. Sugar, S. Ubizskii, I. Kityk, Radiat. Eff. Defect Solid 133 (1995) 153. A. Matkovskii, P. Potera, D. Sugak, L. Grigorjeva, D. Millers, V. Pankratov, A. Suchocki, Cryst. Res. Technol.: J. Exp. Ind. Crystallogr. 39 (2004) 788.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
1 micrometer high-efficient radiation resistant laser crystal: Nd:YSAG a,*
b
Shoujun Ding , Haoyu Wang , Jianqiao Luo
c,**
c
d
, Wenpeng Liu , Yufei Ma , Qingli Zhang
T c
a
School of Mathematics and Physics, Anhui University of Technology, Maanshan, Anhui, 243002, PR China Science and Technology on Applied Physical Chemistry Laboratory, Shaanxi Applied Physics and Chemistry Research Institute, Xi′an, 710061, PR China Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, Anhui, 230031, PR China d National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin, 150001, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Nd:YSAG Radiation resistant Laser crystal Czochralski method Gamma-ray
With the increased requirements of solid state lasers from some special areas, such as space craft, reactor inspection and high level radioactive waste detection, developing laser crystals with radiation resistant ability become more and more important. In this study, high quality Nd-doped Y3Sc2Al3O12 (Nd:YSAG) crystal was grown successfully by Czochralski method and its radiation resistant ability was evaluated. XRD and Raman spectrum characterization results indicate that the structure of the as-grown crystal remain unchanged after the irradiation of 20 Mrad gamma-ray. Besides, the highest decrease of the transmittance was only 5% for the crystal after the irradiation of 10 Mrad gamma-ray suggesting the excellent radiation resistant ability of the crystal. Importantly, the almost unchanged absorption at 808 nm and fluorescence properties for the crystal after the irradiation of 20 Mrad gamma-ray confirmed the excellent radiation resistant ability of Nd:YSAG crystal again. High efficient LD pumped 1.06 μm laser was realized in Nd:YSAG crystal and the slightly decreases of the laser efficiency after the irradiation indicate the crystal can be considered as a promising high efficient radiation resistant laser material for the application in harsh radiation environment.
1. Introduction Since the first laser was realized in ruby crystal by T.H. Maiman in 1960 and the success in generating continuous-wave (CW) laser in Nd:YAG crystal in 1964 [1,2], solid state lasers have undergone fast development for almost 60 years. In recent years, solid state lasers, especially for 1 μm lasers, have been widely used in medical treatment [3], metal processing and environmental instrumentation measurement [4,5], and have demonstrated potential applications for future nuclearfusion [6,7]. However, new challenges have been aroused for solid state lasers due to the requirements from some special areas, such as space craft [8], reactor inspection and high level radioactive waste detection [9,10]. Generally, the working materials in the laser system will produce various significant changes in their structural, chemical, physical as well as optical properties after the exposure of high-energy radiation (X-ray, gamma-ray and neutrons) [11]. Therefore, developing laser crystals with radiation resistant ability are necessary and have attracted considerable attentions from all over the world. Recently, great progresses have been achieved in the studies of radiation effect on the luminescent properties of laser crystals [12–15]. One fascinating conclusion is that scandium (Sc) contained garnet
*
crystals generally exhibit excellent radiation resistant properties in comparison with the mostly used crystals [12,13,16], such as yttrium aluminum garnet (YAG) crystal [17], phosphate crystal [9], gadolinium gallium garnet (GGG) crystal and fluoride crystal (LiYF4) [18]. For examples: (1) Two additional absorption bands were observed in the spectra of Nd:GGG crystal after the gamma-ray irradiation while only one weak additional absorption band was appeared for Nd3+ doped gadolinium scandium gallium garnet (GSGG) crystal [12]. (2) The spectroscopic properties of Yb,Er:GSGG crystal was almost undeteriorate after the irradiation of 50 Mrad gamma-ray [13]. (3) The laser output performance of Er3+ doped gadolinium yttrium scandium gallium garnet (GYSGG) crystal was only affected slightly by the 100 Mrad gamma-ray irradiation [16]. Unfortunately, all these radiation resistant crystals mentioned above contains easily volatile element Gallium (Ga) indicating that it is difficult to keep the compositions consistent and grow single crystals with large size. Recently, gadolinium scandium aluminum garnet (GSAG) crystal, which colligates the advantages of GSGG and YAG crystals, was designed successfully with good radiation resistant ability and without volatile element Ga [11]. Yttrium aluminum scandium garnet (YSAG) is the other crystal that colligates the advantages of GSGG and YAG
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Ding), [email protected] (J. Luo).
**
https://doi.org/10.1016/j.jlumin.2019.116596 Received 11 March 2019; Received in revised form 28 June 2019; Accepted 2 July 2019 Available online 02 July 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
Journal of Luminescence 214 (2019) 116596
S. Ding, et al.
crystal. To the best of our knowledge, the influence of the gamma-ray irradiation on the optical and laser properties of Nd:YSAG crystal has not been reported elsewhere. In this study, a Nd:YSAG crystal was grown by Czochralski (Cz) method. The structure, Raman, absorption and fluorescence spectra of the as-grown crystal were investigated before and after irradiation with different doses gamma-ray. Importantly, high efficient 1 μm CW laser was realized successfully in Nd:YSAG crystal before and after irradiation with different doses gamma-ray. The results indicate that Nd:YSAG crystal is a promising high efficient 1 μm radiation resistant laser crystal.
950 spectrometer with a spectral interval of 1 nm. An Edinburgh FLSP920 spectrometer was employed to record the emission spectra and fluorescence decay curves of the crystal. The excitation source used to generate the emission spectra and fluorescence decay curves were Xenon-lamp and optical parametric oscillation (OPO) laser (Opolette 355 I), respectively. A Horiba-Jobin Yvon LABRAM-HR Raman spectrometer system equipped with a confocal microscope was employed to record the Raman spectra of the crystal at room temperature. The excitation source used to generate the Raman spectra was a frequency doubled diode pumped Nd:YAG laser at 532 nm.
2. Experimental details
2.4. Laser experiment
2.1. Crystal growth
The schematic diagram of laser diode (LD) pumped 1.06 μm continuous-wave laser experiments is shown in Fig. 2, where a plano to plano resonator is used. A fiber-coupled LD is employed as the pumping source with the central wavelength at around 808 nm and maximum output power of 30 W. The core diameter and numerical aperture (NA) of the fiber are 400 μm and 0.22, respectively. M1 is the input mirror with 808 nm anti-reflection coated on the pumping side, and 808 nm high-transmission and 1.06 μm high-reflection coated on the other side. M2 is the output couple mirror with the transmittance of 5% at 1.06 μm. The cavity length (the distance between M1 and M2) is fixed as 18 mm. The as-grown Nd:YSAG crystal samples, cut with the dimension of 2 × 2 × 4 mm3 and two 2 × 2 mm2 faces parallel to (111) crystalline orientation and polished carefully, are used as the working materials. In order to remove the heat generated in the crystal timely during the laser experiment, the crystal is wrapped with indium foil and placed into water-cooled copper heat sink with microchannel structure. The temperature of the cooling water is maintained at 19 °C. The output laser power is measured by an OPHIR 30A-BB-18 power meter.
Nd:YSAG crystal was grown by the Cz method using a JGD-60 furnace (CETC 26th, China) with an automatics diameter controlled (ADC) growth system. The oxide compounds Nd2O3 (99.999%), Y2O3 (99.999%), Sc2O3 (99.999%), Al2O3 (99.999%) were used as raw materials and weighed according to the designed composition of Nd0.03Y2.97Sc2Al3O12. After being mixed adequately, the raw materials were pressed into disks and dried with muffle furnace. Then the disks were loaded into iridium crucible and grown to crystal in nitrogen atmosphere. After the growth process finished, the crystal was cooled down to room temperature at a rate of 30–50 °C/h and subsequently annealed in air at 1100 °C for 72 h to eliminate the strain inside the crystal. The used seed crystal was a YAG crystal with (111) orientation. The used pulling speed and rotation rate were 0.5–1.5 mm/h and 5–15 rpm, respectively. The photograph of the as-grown crystal boule is shown in Fig. 1 (a). 2.2. Gamma-ray irradiation
3. Results and discussion The samples cut from the post-annealed crystal were irradiated by a60Co gamma-ray source with dose rate of 82 Gy/min, radiation times of 20 h and 40 h, corresponding to the doses of 10 Mrad and 20 Mrad, respectively. As shown in Fig. 1 (b), the color of the crystal becomes darker with increasing the irradiation doses.
3.1. Crystal structure The XRC of the (111) crystalline surface of Nd:YSAG crystal manifests as one single symmetric peak and without splitting as shown in Fig. 3 (a). The full width at half maximum (FWHM) of this peak is fitted to be 0.09°, suggesting the high crystalline quality of the as-grown crystal. The XRD patterns of the post-annealed Nd:YSAG and pure YSAG from card ICSD#67055 are shown in Fig. 3 (b). As it can be seen, all the measured diffraction peaks are sharp and the peaks position well coincident with that from card ICSD#67055, indicating that the crystal is well-crystallized and the crystal structure unchanged by the doping ions. Thus, the as-grown crystal belongs to the cubic phase with space group of Ia-3d. Using the structural parameters from the card ICSD#67055 as the initial values, the unit cell parameters of Nd:YSAG
2.3. Structure and spectroscopic measurements The crystal structure was identified by powder X-ray diffraction (XRD) using a Philips X'pert Pro X-ray powder diffractometer equipped with Cu Kα radiation. A scan step of 0.033° was applied to record the patterns in the 2θ range of 10°–90°. A high resolution X'pert Pro MPD diffractometer equipped with hybrid Kα1 monochromator was employed to record the X-ray rocking curve (XRC). The absorption spectra of the samples were recorded by a PerkinElmer UV-VIS-NIR lambda-
Fig. 1. (a) Photograph of the as-grown Nd:YSAG crystal boule. (b) Polished Nd:YSAG crystal slices after irradiation with 0, 10 and 20 Mrad doses gamma-ray, respectively. 2
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Fig. 2. Schematic diagram of the LD pumped 1.06 μm continuous-wave laser experiment.
are obtained with Rietveld refinement method [19,20]. The lattice parameters are refined to be a = b = c = 12.258 Å and α = β = γ = 90°, respectively, by general structure analysis software (GSAS). The refined results are shown in Table 1 and Fig. 3 (c) with the refined residuals Rp and Rwp are both less than 10%, indicating that the refined results are reliable. The XRD patterns of Nd:YSAG crystal before and after irradiation with different doses gamma-ray are shown in Fig. 3 (d). It is obvious that the position and intensity of the diffraction peaks are identically before and after the irradiation, which indicates the crystal structure do not be changed by the irradiation. Thus, the color change as appeared in Fig. 1 (b) could be attributed to the formation of color center in the crystal after the irradiation.
Table 1 Rietveld refinement results of Nd:YSAG crystal. Atoms
X
Nd 0.0000 Y 0.0000 Sc 0.0000 Al 0.0000 O 0.0302 Lattice Parameters R factors
Y
Z
Occupancy
0.2500 0.1250 0.015 0.2500 0.1250 0.985 0.0000 0.0000 1.000 0.2500 0.3750 1.000 0.0512 0.6564 1.000 a = b = c = 12.258 Å α = β = γ = 90° Rp = 7.50%, Rwp = 9.86%
Uiso 0.0051 0.0051 0.0074 0.0041 0.0067
3.2. Spectroscopic properties The Raman spectra of Nd:YSAG crystal before and after irradiation with different doses gamma-ray are shown in Fig. 4 (a). The Raman
Fig. 3. (a) X-ray rocking curve of (111) crystalline face of Nd:YSAG crystal. (b) Measured XRD patterns of Nd:YSAG crystal and XRD patterns from ICSD#67055. (c) Rietveld refinement results of Nd:YSAG crystal (Obs.: the observed data, Calc.:the calculated data, Bkg.:the background, Diff.: the difference between the observed and calculated data). (d) The XRD comparison for Nd:YSAG crystal before and after irradiation with different doses gamma-ray. 3
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Fig. 4. (a) The Raman spectra of Nd:YSAG crystal before and after irradiation with different doses gamma-ray. (b) The transmission spectra of Nd:YSAG crystal before and after irradiation with different doses gamma-ray. Inset: The transmission spectra at around 808 nm. (c) The transmission spectra of non-irradiated and 20 Mrad irradiated samples between 290 and 450 nm. (d) The absorption coefficient of Nd:YSAG crystal. Inset: The comparison of absorption between Nd:YAG and Nd:YSAG crystal at around 808 nm.
contained garnet crystals, such as Nd:YSAG and Nd:GSGG, show excellent radiation resistant ability and can be expected using in harsh radiation environment. The transmission spectra of non-irradiated and 20 Mrad irradiated samples between 290 and 450 nm are shown in Fig. 4 (c). As it can be seen, two additional peaks with central wavelength at 307 and 310 nm appear in the 20 Mrad irradiated sample in comparison with non-irradiated sample. According to the reported reference [24], the additional absorption bands at around 300 nm after irradiation could be assigned to F+-centers, namely, the color centers generated in the crystal after irradiation are belonging to F+-centers. Fig. 4 (d) illustrated the absorption coefficient of Nd:YSAG crystal before and after irradiation with different doses gamma-ray. Nine absorption bands appeared in the figure are corresponding to the transitions of Nd3+ from ground state 4I9/2 to different excited states. The absorption cross section σabs of Nd:YSAG crystal can be calculated using the formula σabs = α(λ)/ Nc , where α(λ) is the absorption coefficient and Nc is the concentration of Nd3+ ions in the crystal. The calculated σabs at 808.9 nm is 11.5 × 10−20 cm2, suggesting that the Nd:YSAG crystal can be pumped efficiently by the commercialized 808 nm LD. In addition, the broader absorption band at around 808 nm for Nd:YSAG (as shown in inset of Fig. 4 (d)), in comparison with the Nd:YAG crystal, is beneficial to decrease the temperature dependence of the LD pumping source and improve the laser efficiency. Fig. 5 (a) illustrated the emission spectra of Nd:YSAG crystal before
peaks position and intensity are almost unchanged after the irradiation, which means the internal vibrational mode frequencies in the crystal are unaffected by the irradiation [21]. Therefore, the crystal structure is unchanged by the irradiation, which is consistent with the XRD characterization result. According to the reported Raman spectra of some other garnet crystals [22], the Raman active modes are assigned and denoted in Fig. 4 (a). Additionally, the maximum phonon energy of Nd:YSAG is determined to be 763 cm−1 from the Raman spectra, which is lower than that of Nd:YAG [23] (776 cm−1). The lower maximum phonon energy of Nd:YSAG is advantageous for achieving population inversion. The transmission spectra of Nd:YSAG crystal before and after irradiation with different doses gamma-ray are illustrated in Fig. 4 (b). As can be seen, despite the significant decrease of the transmittance after the gamma-ray irradiation in the wavelength below 730 nm, the transmittance affected by the irradiation is quite slightly in the wavelength above 730 nm, especially at 808 nm (as shown in inset of Fig. 4 (b)). That is to say, the crystal after the gamma-ray irradiation can still match well with the commercialized 808 nm diode laser pumping source. The crystal transmittance decrease ratio Dra caused by the irradiation can be estimate by the formula Dra =
Tλb − Tλa Tλb
, where Tλb and Tλa
represents the crystal transmittance at wavelength λ before and after irradiation, respectively. According to this formula, the calculated Dra at wavelength 400 nm for Nd:YSAG crystal and some other Nd3+ doped radiation resistant crystals are listed in Table 2. As can be seen, Sc Table 2 The calculated Dra for different Nd3+ doped radiation resistant crystals. Crystals
Nd:YSAG
Nd:GSGG
Nd:GGG
Nd:YAG
Nd:YLF
Nd: phosphate
Irradiation doses Dra References
10 Mrad 5% This work
100 Mrad 23% [12]
100 Mrad 40% [12]
10 Mrad 18% [17]
20 Mrad 85% [18]
1 Mrad 46% [9]
4
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Fig. 5. (a) The emission spectra of Nd:YSAG crystal before and after irradiation with different doses gamma-ray. (b) The fluorescence decay curve of non-irradiated Nd:YSAG crystal. (c) The fluorescence decay curve of Nd:YSAG crystal after irradiation with 10 Mrad gamma-ray. (d) The fluorescence decay curve of Nd:YSAG crystal after irradiation with 20 Mrad gamma-ray.
and after irradiation with different doses gamma-ray. It can be noted that the fluorescence intensities are almost uninfluenced by the irradiation, suggesting the excellent radiation resistant ability of Nd:YSAG crystal. Three emission bands observed in the measured range could be assigned to the transitions of Nd3+ from 4F3/2 to 4I9/2, 4I11/2 and 4I13/2, respectively, as denoting in Fig. 5 (a). The fluorescence decay curves of 4 F3/2 → 4I11/2 transition for Nd:YSAG crystal before and after irradiation with different doses gamma-ray are shown in Fig. 5 (b), (c) and (d). Using the single exponential function to fit the curves, the fluorescence lifetimes are fitted to be 240, 243 and 253 μs for the crystal irradiated with doses of 0, 10 and 20 Mrad, respectively. On the one hand, the slightly fluctuation of the lifetime after the irradiation indicates the excellent radiation resistant ability of Nd:YASG crystal again. On the other hand, the longer lifetime of Nd:YSAG crystal in comparison with that of Nd:YAG crystal suggesting the better energy storage capacity of Nd:YSAG crystal, which is advantageous for realizing laser output. According to the Fuchtbauer-Ladenburg (F-L) formula [25]:
σem (λ ) =
λ5I (λ ) ∫ λI (λ ) dλ
Fig. 6. The 1.06 μm laser output power of Nd:YSAG crystal before and after irradiated with different doses gamma-ray versus the absorbed incident power.
8πn2cτm
where I(λ) is the fluorescence intensity at the wavelength of λ, n is the reflective index, c is the velocity of light, τm is the measured lifetime. The stimulated emission cross section of Nd:YSAG crystal at 943 nm and 1060 nm are calculated to be 2.2 and 10.1 × 10−20 cm2, respectively. The relative large emission cross section at ~940 nm indicates that Nd:YSAG crystal not only can achieve 1 μm laser output, but also is promising for generating ~940 nm laser, which wavelength has important applications in water vapor detection by differential absorption lidar (DIAL) technique [26,27].
40.8% and 38.6% for Nd:YSAG with irradiation doses of 0, 10 and 20 Mrad, respectively. The decrease of the laser efficiency with increasing the irradiation dose could be explained by Matkovski's. proposal [28,29]: the formed color center in the crystal after the irradiation can absorb pumping light as well as reabsorb laser radiation. Importantly, such a small decrease of the laser efficiency after the irradiation, suggesting that the Nd:YSAG crystal is an excellent high efficient radiation resistant crystal for application in harsh radiation environment.
3.3. Laser performance 4. Conclusions
Fig. 6 illustrated the realized laser output power versus the absorbed incident power using the Nd:YSAG with irradiation doses of 0, 10 and 20 Mrad, respectively. The slope efficiency are fitted to be 43.2%,
High quality Nd:YSAG single crystal was grown successfully by Cz 5
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method. The post-annealed crystal was irradiated with different doses gamma-ray to study its radiation resistant ability. XRD and Raman characterization results indicate that the crystal structure remain unchanged after the irradiation, namely, the color changes of the crystal after irradiation could be attributed to the formation of color centers in the crystal. The excellent radiation resistant ability of Nd:YSAG crystal was proved by the slightly fluctuation of the absorption and fluorescence spectra after the gamma-ray irradiation. High efficient LD pumped 1.06 μm laser was realized in the as-grown crystal. Importantly, the laser efficiency was slightly influenced by the gammaray irradiation, suggesting that the crystal can be considered as a promising high efficient radiation resistant laser material for the application in harsh radiation environment.
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Acknowledgements [18] [19] [20]
The Startup Foundation for Advanced Talents of Anhui University of Technology, National Natural Science Foundation of China (Grants No. 51502292), National Key Research and Development Program of China (Grants No. 2016YFB0402101) supported this study.
[21] [22] [23]
Appendix A. Supplementary data
[24]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jlumin.2019.116596.
[25] [26]
References [27] [28]
[1] J. Geusic, H. Marcos, L. Van Uitert, Appl. Phys. Lett. 4 (1964) 182. [2] T.H. Maiman, Nature 187 (1960) 493. [3] R. Moncorgé, B. Chambon, J. Rivoire, N. Garnier, E. Descroix, P. Laporte, H. Guillet, S. Roy, J. Mareschal, D. Pelenc, Opt. Mater. 8 (1997) 109. [4] A. Ikesue, Y.L. Aung, Nat. Photon. 2 (2008) 721.
[29]
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A.K. Misra, S.K. Sharma, P.G. Lucey, Appl. Spectrosc. 60 (2006) 223. M.H. Key, Nature 412 (2001) 775. S. Nakai, K. Mima, Rep. Prog. Phys. 67 (2004) 321. M. Toyoshima, Y. Takayama, H. Kunimori, T. Jono, S. Yamakawa, Opt. Eng. 49 (2010) 083604. V. Rai, B.R. Sekhar, S. Kher, S. Deb, J. Lumin. 130 (2010) 582. S.J. Zinkle, L.L. Snead, Annu. Rev. Mater. Res. 44 (2014) 241. Y. Chen, Q. Zhang, F. Peng, W. Liu, D. Sun, R. Dou, H. Zhang, Y. He, S. Han, S. Yin, J. Lumin. 205 (2019) 109. D. Sun, J. Luo, Q. Zhang, J. Xiao, J. Xu, H. Jiang, S. Yin, J. Lumin. 128 (2008) 1886. D. Sun, J. Luo, Q. Zhang, J. Xiao, W. Liu, S. Wang, H. Jiang, S. Yin, J. Cryst. Growth 318 (2011) 669. J. Luo, D. Sun, H. Zhang, Q. Guo, Z. Fang, X. Zhao, M. Cheng, Q. Zhang, S. Yin, Opt. Lett. 40 (2015) 4194. D. Sugak, A. Matkovskii, A. Durygin, A. Suchocki, I. Solskii, S. Ubizskii, K. Kopczynski, Z. Mierczyk, P. Potera, J. Lumin. 82 (1999) 9. J. Chen, D. Sun, J. Luo, J. Xiao, R. Dou, Q. Zhang, Optic Commun. 301 (2013) 84. T. Rose, R. Fields, R. Hutcheson, Radiation Hardening of Nd: YAG by Transition Metal Ion Codopants, Advanced Solid State Lasers, Optical Society of America, 1994, p. NS4. T. Rose, M. Hopkins, R. Fields, IEEE J. Quantum Electron. 31 (1995) 1593. M.t. Sakata, M. Cooper, J. Appl. Crystallogr. 12 (1979) 554. S. Ding, Q. Zhang, F. Peng, W. Liu, J. Luo, R. Dou, G. Sun, X. Wang, D. Sun, J. Alloy. Comp. 698 (2017) 159. L.G. Cançado, A. Jorio, E.M. Ferreira, F. Stavale, C. Achete, R. Capaz, M. Moutinho, A. Lombardo, T. Kulmala, A.C. Ferrari, Nano Lett. 11 (2011) 3190. J. Koningstein, O.S. Mortensen, J. Mol. Spectrosc. 27 (1968) 343. Z. Fang, D. Sun, J. Luo, H. Zhang, X. Zhao, C. Quan, L. Hu, M. Cheng, Q. Zhang, S. Yin, Opt. Express. 25 (2017) 21349–21357. N. Mironova-Ulmane, I. Sildos, E. Vasil'chenko, G. Chikvaidze, V. Skvortsova, A. Kareiva, J. Muñoz-Santiuste, R. Pareja, E. Elsts, A. Popov, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 435 (2018) 306. S. Ding, Q. Zhang, D. Sun, F. Peng, W. Liu, J. Luo, G. Sun, J. Lumin. 201 (2018) 65. G.J. Koch, B.W. Barnes, M. Petros, J.Y. Beyon, F. Amzajerdian, J. Yu, R.E. Davis, S. Ismail, S. Vay, M.J. Kavaya, Appl. Opt. 43 (2004) 5092. E. Browell, S. Ismail, W. Grant, Appl. Phys. B 67 (1998) 399. A. Matkovskii, D. Sugar, S. Ubizskii, I. Kityk, Radiat. Eff. Defect Solid 133 (1995) 153. A. Matkovskii, P. Potera, D. Sugak, L. Grigorjeva, D. Millers, V. Pankratov, A. Suchocki, Cryst. Res. Technol.: J. Exp. Ind. Crystallogr. 39 (2004) 788.