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A promising high-efficient radiation resistant laser crystal Nd:GSAG
Infrared Physics and Technology 102 (2019) 103005
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Regular article
A promising high-efficient radiation resistant laser crystal Nd:GSAG a,⁎
b
c
c
c,d
T c
Shoujun Ding , Yuanzhi Chen , Wenpeng Liu , Jianqiao Luo , Yingying Chen , Xiuli Li , Qingli Zhangc a
School of Mathematics and Physics, Anhui University of Technology, Maanshan, Anhui 243002, PR China School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, PR China Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, Anhui 230031, PR China d University of Science and Technology of China, Hefei, Anhui 230026, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Gamma-ray Nd:GSAG Spectral properties F+-center Continuous-wave laser
In this study, the effect of gamma-ray irradiation on the spectral and laser properties of Nd:GSAG crystal is investigated. With 20 Mrad gamma-ray irradiation, the crystal’s absorption at around 808 nm and emission at 1060 nm are only affected slightly. In addition, the lifetime of the 4F3/2 → 4I11/2 transition becomes longer when increasing the irradiation doses. All these optical properties changes after the irradiation could be attributed to the formation of F+-centers in the crystal. Using LD pumping, high-efficient 1.06 μm continuous-wave laser operation was realized in the crystal. It is worthy to note that the laser efficiency is influenced slightly by the gamma-ray irradiation, which suggest that Nd:GSAG crystal can be considered as a potential efficient laser gain medium for being used in radiant environment.
1. Introduction Thanks to the big breakthrough in laser diode (LD) technology, LD pumped all solid-state lasers (DPSSLs) are increasingly becoming the most attractive field in laser research owing to its high efficiency, high stability and compactness [1–3]. Among all of the materials used in DPSSLs, neodymium (Nd) doped crystal is the mostly investigated category and successfully applied in many fields, such as medical treatment, optical communication, environment exploration, etc [4–8]. It may not be an overstatement to say that DPSSLs based on Nd-doped materials will gradually become ubiquitous in the field of laser research. Recently, new challenges have been aroused for solid state lasers due to the requirements from some special areas, such as space craft, high level radioactive waste detection, reactor inspection, etc [9,10]. It is well known that materials working in these areas (radiant environment) will generate significant changes on their structural, chemical and optical properties due to the ionization and displacement in the materials caused by high-energy radiation [9]. Accordingly, searching for laser materials with radiation resistant ability is of great significance. Previous studies have shown that the garnet crystals with scandium (Sc) contained always exhibit excellent radiation resistant ability. For examples: (1) The laser performance of Nd,Cr:GSGG is influenced very slightly after the gamma-ray irradiation [11]. Besides,
⁎
singly doped GSGG crystals, such as Nd:GSGG and Cr:GSGG, are also verified with excellent radiation resistant abilities [12,13]. (2) The spectral and laser properties of Er:GYSGG crystal are almost undeteriorate after the irradiation with 100 Mrad gamma-ray [14]. Unfortunately, both GSGG and GYSGG crystals contain easily volatile element Ga, indicating that growing them with high-quality and largesize are very difficult. Nowadays, another Sc contained but without Ga contained garnet crystal GSAG has attracted much more attentions. Chen reported that Er:GSAG and Er,Pr:GSAG crystals both can be considered as promising 2.6–2.9 μm laser materials with radiation resistant ability [15,16]. However, up to now, laser generations in Er:GSAG and Er,Pr:GSAG have not been realized. Although, Nd,Cr:GSAG was reported as 1 μm efficient radiation resistant laser crystal [17], it is also difficult to grow high-quality Nd,Cr:GSAG crystal due to the mismatch of the ionic radius between Cr3+ and Gd3+. Therefore, it is very necessary to investigate the radiation resistant properties of singly Nd3+ doped GSAG crystal. In this study, Nd:GSAG crystal was irradiated by 60Co gamma-ray with different doses to investigate the irradiation effect on its spectral and laser properties. Using LD pumping, high-efficient 1.06 μm continuous-wave laser output was realized in Nd:GSAG crystal. Remarkably, its laser performance and spectral properties are almost unaffected by the irradiation, indicating that the crystal is of excellent radiation ability.
Corresponding author. E-mail address: [email protected] (S. Ding).
https://doi.org/10.1016/j.infrared.2019.103005 Received 2 July 2019; Received in revised form 4 August 2019; Accepted 5 August 2019 Available online 05 August 2019 1350-4495/ © 2019 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 102 (2019) 103005
S. Ding, et al.
where α(λ) is the absorption coefficient and Nc is the concentration of Nd3+ ions in the crystal. According to this formula, the absorption cross section at 808 nm is calculated to be 5. 21 × 10−20 cm2, which is comparable to that of Nd:YAG [18]. In addition, the full width at half of maximum (FWHM) of the absorption peak at 808 nm is fitted to be 2.9 nm (as denoted in the inset of Fig. 2(a)), which is almost twice wider than that of Nd:YAG crystal. The large absorption cross section and wide absorption band of Nd:GSAG crystal at 808 nm indicate that the crystal is very suitable for pumping by commercialized 808 nm LD and benefit to decrease the temperature dependence of the LD. The comparison of the absorption before and after irradiation with different doses of gamma-ray is shown in Fig. 2(b). As it can be seen, the absorption at around 808 nm is only influenced slightly by the gamma-ray irradiation. That is to say, the crystal can still absorb energy from the 808 nm commercialized LD pumping source efficiently after the gamma-ray irradiation. Additional absorption band with central wavelength at around 390 nm is observed when using the 20 Mrad irradiated sample to subtract the absorption data of non-irradiated sample (as shown in Fig. 3). According to the reported reference [19], this additional absorption could be assigned to the absorption of F+-centers.
2. Experiments 2.1. Gamma-ray irradiation experiment A 60Co gamma-ray source was employed to perform the irradiation experiment. An irradiation dose rate of 26 Gy/min was used for all the irradiation experiments. In this work, three samples were irradiated by the source with irradiation times of 12.5, 63 and 125 h, corresponding to the irradiation doses of 2, 10 and 20 Mrad, respectively. 2.2. Spectroscopic characterizations The absorption spectra of the samples were recorded by a PerkinElmer UV-VIS-NIR lambda-950 spectrometer with a spectral interval of 1 nm. The emission spectra and fluorescence decay curves of the samples were recorded by an Edinburgh FLSP-920 spectrometer. The excitation source used to generate the emission spectra and fluorescence decay curves were Xenon flash lamp and OPO laser (Opolette 355 I, OPOTEK, Inc, USA), respectively. The interval of the emission spectra was 0.5 nm. All the experiments were performed on the room temperature. 2.3. Laser experiment
3.2. Emission spectra
The experimental setup used to perform the LD pumped 1.06 μm continuous-wave (CW) laser experiment is shown in Fig. 1. An 808 nm fiber coupled LD with maximum output power of 30 W was employed as the pumping source. The numerical aperture (N.A.) and core diameter of the fiber are 0.22 and 400 μm, respectively. The M1 and M2 shown in Fig. 1 represents input mirror and output coupler, respectively. The cavity length (distance between M1 and M2) was about 20 mm in the experiment. The 808 nm anti-reflection (AR) film was coated on the pumping side of M1, whereas the 808 nm high-transmission (HT) and 1060 nm high-reflection (HR) films were coated on the other side. The transmittance of M2 is 5% at the wavelength of 1060 nm. The dimension of the Nd:GSAG crystal samples used in the laser experiment was 2 × 2 × 4 mm3, in which two 2 × 2 mm2 faces were cut perpendicular to (1 1 1)-orientation and polished carefully. During the laser experiment, the crystal was wrapped by indium foil and placed into copper sink with water cooled to remove the generated heat in the crystal timely. The cooling water temperature was controlled at 20 °C. An OPHIR 30A-BB-18 power meter was employed to measure the output laser power.
The Emission spectrum of Nd:GSAG crystal is shown in Fig. 4(a). Three emission bands with central wavelengths of 940, 1060, and 1340 nm, respectively, are observed in the measured range, which are corresponding to the transitions from 4F3/2 state of Nd3+ to states 4I9/2, 11/2, 13/2, respectively. The stimulated emission cross section σem could be estimated from the fluorescence spectrum by Fuchtbauer-Ladenburg (F-L) formula [20]:
σem (λ ) =
λ5I (λ ) ∫ λI (λ ) dλ
8πn2cτr
where I(λ) is the fluorescence intensity, n is the reflective index which was reported in the reference [21], c is the velocity of light, τr is the radiative lifetime which is determined to be 263 μs from Judd-Ofelt calculation [22,23]. Using the above formula, the stimulated emission cross section of Nd:GSAG crystal at 1060 nm is calculated as high as 6.18 × 10−20 cm2. The large absorption cross section at 808 nm and emission cross section at 1060 nm suggest that Nd:GSAG crystal is very promising for realizing high-efficiency and low-threshold 1060 nm laser. In addition, the emission cross section at 940 nm is calculated to be 2.68 × 10−20 cm2. The relative large emission cross section at 940 nm indicates that Nd:GSAG crystal is also potential for obtaining 940 nm lasers, which is an important wavelength for application in water vapor detection by differential absorption lidar (DIAL) technique [17,24–26]. Fig. 4(b) shows the fluorescence spectra of Nd:GSAG crystal before and after irradiation with different doses gamma-ray. It is inspiring that the fluorescence intensity is almost unaffected by the gamma-ray irradiation, suggesting the excellent radiation resistant ability of Nd:GSAG crystal.
3. Results and discussion 3.1. Absorption spectra The absorption spectrum of Nd:GSAG crystal is shown in Fig. 2(a). Nine absorption bands are observed in the measured range, which are corresponding to the transitions from the ground state (4I9/2) of Nd3+ to different excited states, as denoted in the Fig. 2(a). The absorption cross section σabs can be calculated with the formula σabs = α (λ )/Nc ,
Fig. 1. Schematic of the LD pumped Nd:GSAG crystal laser experiment. 2
Infrared Physics and Technology 102 (2019) 103005
S. Ding, et al.
Fig. 2. . (a) The room temperature absorption spectrum of Nd:GSAG crystal. (b) The absorption spectra of Nd:GSAG crystal before and after irradiation with different doses gamma-ray.
irradiations. Yadav reported that the defects in the host trap electrons and thus potentially resulting in delayed of the lifetime [27]. A simplified schematic showing the trap mechanism is presented in Fig. 5(b). Therefore, the higher density of the defects (F+-centers), the longer of the lifetimes. In addition, the lifetime only increased 13% ( 281 − 248 ) with 248 the irradiation dose of 20 Mrad, which indicates the excellent radiation resistant ability of Nd:GSAG crystal again.
3.4. Laser performances Fig. 6 illustrated the realized laser output power versus the absorbed incident power using the Nd:GSAG with irradiation doses of 0, 2, 10 and 20 Mrad, respectively. As it can be seen, the output power increases linearly with the absorbed pump power. The laser thresholds for Nd:GSAG crystal before and after irradiation with 2, 10 and 20 doses gamma-ray are 2.00, 2.12, 2.25 and 2.30 W, respectively. The slope efficiency for Nd:GSAG crystal before and after irradiation with 2, 10 and 20 doses gamma-ray are fitted to be 47.15%, 45.89%, 41.12% and 39.5%, respectively. The laser efficiency is decreased 16.2% ( 47.15 % − 39.5 % ) after irradiation with 20 Mrad gamma-ray suggesting the 47.15 % good radiation resistant ability of Nd:GSAG crystal again. In addition, the laser threshold is increased very slightly with the increasing of the irradiation doses, which is because the defects caused by the irradiation absorbed small partial of the pumping energy. Accordingly, Nd:GSAG crystal is a potential high-efficiency 1 μm radiation resistant crystal that can be used for outer-space laser application
Fig. 3. Additional absorption between non-irradiated and 20 Mrad irradiated samples.
3.3. Fluorescence decay curves The fluorescence decay curves of 4F3/2 → 4I11/2 transition for Nd:GSAG crystal before and after irradiation with different doses gamma-ray are shown in Fig. 5(a). With single exponential function, the lifetimes before and after irradiation with 2, 10, 20 Mrad gamma-ray are fitted to be 248, 253, 268 and 281 μs, respectively. As mentioned above, F+-centers will be generated in the crystal after irradiation and the density of the F+-centers increase with the increasing of the
Fig. 4. . (a) The room temperature emission spectrum of Nd:GSAG crystal. (b) The emission spectra of Nd:GSAG crystal before and after irradiation with different doses gamma-ray. 3
Infrared Physics and Technology 102 (2019) 103005
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Fig. 5. . (a) The fluorescence decay curves of 4F3/2 → 4I11/2 transition for Nd:GSAG crystal before and after irradiation with different doses gamma-ray. (b) A simplified schematic showing the trap mechanism.
of China (Grants No. 51502292), National Key Research and Development Program of China (Grants No. 2016YFB0402101) supported this study. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.infrared.2019.103005. References [1] X. Duan, L. Li, L. Zheng, B. Yao, Z. Zou, D. Jiang, L. Su, Infrared Phys. Technol. 94 (2018) 7. [2] R. Yan, C. Zhao, X. Li, X. Yu, Z. Liu, X. Wen, W. Yao, J. Gao, F. Peng, Q. Zhang, Infrared Phys. Technol. 94 (2018) 32. [3] S. Ding, F. Peng, Q. Zhang, J. Luo, W. Liu, D. Sun, R. Dou, J. Gao, G. Sun, M. Cheng, J. Alloy. Compd. 693 (2017) 339. [4] H. Yu, J. Liu, H. Zhang, A.A. Kaminskii, Z. Wang, J. Wang, Laser Photonics Rev. 8 (2014) 847. [5] L. Su, J. Xu, H. Li, L. Wen, Y. Zhu, Z. Zhao, Y. Dong, G. Zhou, J. Si, Chem. Phys. Lett. 406 (2005) 254. [6] J. Li, Y. Wu, Y. Pan, H. Kou, Y. Shi, J. Guo, Ceram. Int. 34 (2008) 1675. [7] Y. Ma, H. Sun, Z. Peng, S. Ding, J. Peng, Q. Zhang, X. Yu, Infrared Phys. Technol. 97 (2019) 371. [8] Y. Ma, Z. Peng, S. Ding, F. Peng, Q. Zhang, X. Yu, Infrared Phys. Technol. 92 (2018) 295. [9] V. Rai, B.R. Sekhar, S. Kher, S. Deb, J. Lumin. 130 (2010) 582. [10] S.J. Zinkle, L.L. Snead, Annu. Rev. Mater. Res. 44 (2014) 241. [11] E. Zharikov, I. Kuratev, V. Laptev, S. Nasel'skii, A. Ryabov, G. Toropkin, A. Shestakov, I. Shcherbakov, Bull. Acad. Set. USSR, Phys. Ser. 48 (1984) 103. [12] A.K. Misra, S.K. Sharma, P.G. Lucey, Appl. Spectrosc. 60 (2006) 223. [13] D. Sun, J. Luo, Q. Zhang, J. Xiao, J. Xu, H. Jiang, S. Yin, J. Lumin. 128 (2008) 1886. [14] J. Chen, D. Sun, J. Luo, J. Xiao, R. Dou, Q. Zhang, Opt. Commun. 301 (2013) 84. [15] Y. Chen, Q. Zhang, F. Peng, W. Liu, Y. He, R. Dou, H. Zhang, J. Luo, D. Sun, Opt. Mater. 84 (2018) 172. [16] 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. [17] S. Ding, H. Wang, W. Liu, J. Luo, Y. Ma, Q. Zhang, J. Lumin. 213 (2019) 249. [18] F. Peng, H. Yang, Q. Zhang, J. Luo, W. Liu, D. Sun, R. Dou, G. Sun, Appl. Phys. B 118 (2015) 549. [19] A. Matkovskii, P. Potera, D. Sugak, L. Grigorjeva, D. Millers, V. Pankratov, A. Suchocki, Cryst. Res. Technol. 39 (2004) 788. [20] S. Ding, Q. Zhang, F. Peng, W. Liu, J. Luo, R. Dou, G. Sun, X. Wang, D. Sun, J. Alloy. Compd. 698 (2017) 159. [21] J. Su, B. Liu, L.H. Xu, Q.L. Zhang, S.T. Yin, J. Alloys Compd. 512 (2012) 230. [22] B.R. Judd, Phys. Rev. 127 (1962) 750. [23] G. Ofelt, J. Chem. Phys. 37 (1962) 511. [24] S. Strohmaier, H. Eichler, C. Czeranowsky, B. Ileri, K. Petermann, G. Huber, Opt. Commun. 275 (2007) 170. [25] A. Karapuzikov, A. Malov, I. Sherstov, Infrared Phys. Technol. 41 (2000) 77. [26] A. Karapuzikov, I. Ptashnik, I. Sherstov, O. Romanovskii, G. Matvienko, Y.N. Ponomarev, Infrared Phys. Technol. 41 (2000) 87. [27] S.K. Yadav, B.P. Uberuaga, M. Nikl, C. Jiang, C.R. Stanek, Phys. Rev. Appl 4 (2015) 054012.
Fig. 6. The 1.06 μm laser output power of Nd:GSAG crystal before and after irradiated with different doses gamma-ray versus the absorbed pump power.
4. Conclusion In this work, Nd:GSAG crystal was irradiated by 60Co gamma-ray with different doses to investigate the irradiation effect on its spectral and laser properties. After exposure with the irradiation dose as high as 20 Mrad, the absorption at around 808 nm and emission at 1060 nm of the crystal are almost unaffected. With the increasing of the irradiation doses, the lifetime of the 4F3/2 → 4I11/2 transition only increases slightly, which can be explained by the generation of defects in the crystal after irradiation. Using LD pumping, 1.06 μm continuous-wave laser output was realized in Nd:GSAG crystal with high efficiency. Importantly, the laser performance is only decreased slightly after 20 Mrad gamma-ray irradiation. All the results indicate that Nd:GSAG crystal is of great potential for application in harsh radiant environment. Declaration of Competing Interest The authors declare that there are no conflict of interest. Acknowledgements The University Natural Science Research Project of Anhui Province, China (KJ2019ZD06), Startup Foundation for Advanced Talents of Anhui University of Technology, National Natural Science Foundation
4
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Regular article
A promising high-efficient radiation resistant laser crystal Nd:GSAG a,⁎
b
c
c
c,d
T c
Shoujun Ding , Yuanzhi Chen , Wenpeng Liu , Jianqiao Luo , Yingying Chen , Xiuli Li , Qingli Zhangc a
School of Mathematics and Physics, Anhui University of Technology, Maanshan, Anhui 243002, PR China School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, PR China Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, Anhui 230031, PR China d University of Science and Technology of China, Hefei, Anhui 230026, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Gamma-ray Nd:GSAG Spectral properties F+-center Continuous-wave laser
In this study, the effect of gamma-ray irradiation on the spectral and laser properties of Nd:GSAG crystal is investigated. With 20 Mrad gamma-ray irradiation, the crystal’s absorption at around 808 nm and emission at 1060 nm are only affected slightly. In addition, the lifetime of the 4F3/2 → 4I11/2 transition becomes longer when increasing the irradiation doses. All these optical properties changes after the irradiation could be attributed to the formation of F+-centers in the crystal. Using LD pumping, high-efficient 1.06 μm continuous-wave laser operation was realized in the crystal. It is worthy to note that the laser efficiency is influenced slightly by the gamma-ray irradiation, which suggest that Nd:GSAG crystal can be considered as a potential efficient laser gain medium for being used in radiant environment.
1. Introduction Thanks to the big breakthrough in laser diode (LD) technology, LD pumped all solid-state lasers (DPSSLs) are increasingly becoming the most attractive field in laser research owing to its high efficiency, high stability and compactness [1–3]. Among all of the materials used in DPSSLs, neodymium (Nd) doped crystal is the mostly investigated category and successfully applied in many fields, such as medical treatment, optical communication, environment exploration, etc [4–8]. It may not be an overstatement to say that DPSSLs based on Nd-doped materials will gradually become ubiquitous in the field of laser research. Recently, new challenges have been aroused for solid state lasers due to the requirements from some special areas, such as space craft, high level radioactive waste detection, reactor inspection, etc [9,10]. It is well known that materials working in these areas (radiant environment) will generate significant changes on their structural, chemical and optical properties due to the ionization and displacement in the materials caused by high-energy radiation [9]. Accordingly, searching for laser materials with radiation resistant ability is of great significance. Previous studies have shown that the garnet crystals with scandium (Sc) contained always exhibit excellent radiation resistant ability. For examples: (1) The laser performance of Nd,Cr:GSGG is influenced very slightly after the gamma-ray irradiation [11]. Besides,
⁎
singly doped GSGG crystals, such as Nd:GSGG and Cr:GSGG, are also verified with excellent radiation resistant abilities [12,13]. (2) The spectral and laser properties of Er:GYSGG crystal are almost undeteriorate after the irradiation with 100 Mrad gamma-ray [14]. Unfortunately, both GSGG and GYSGG crystals contain easily volatile element Ga, indicating that growing them with high-quality and largesize are very difficult. Nowadays, another Sc contained but without Ga contained garnet crystal GSAG has attracted much more attentions. Chen reported that Er:GSAG and Er,Pr:GSAG crystals both can be considered as promising 2.6–2.9 μm laser materials with radiation resistant ability [15,16]. However, up to now, laser generations in Er:GSAG and Er,Pr:GSAG have not been realized. Although, Nd,Cr:GSAG was reported as 1 μm efficient radiation resistant laser crystal [17], it is also difficult to grow high-quality Nd,Cr:GSAG crystal due to the mismatch of the ionic radius between Cr3+ and Gd3+. Therefore, it is very necessary to investigate the radiation resistant properties of singly Nd3+ doped GSAG crystal. In this study, Nd:GSAG crystal was irradiated by 60Co gamma-ray with different doses to investigate the irradiation effect on its spectral and laser properties. Using LD pumping, high-efficient 1.06 μm continuous-wave laser output was realized in Nd:GSAG crystal. Remarkably, its laser performance and spectral properties are almost unaffected by the irradiation, indicating that the crystal is of excellent radiation ability.
Corresponding author. E-mail address: [email protected] (S. Ding).
https://doi.org/10.1016/j.infrared.2019.103005 Received 2 July 2019; Received in revised form 4 August 2019; Accepted 5 August 2019 Available online 05 August 2019 1350-4495/ © 2019 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 102 (2019) 103005
S. Ding, et al.
where α(λ) is the absorption coefficient and Nc is the concentration of Nd3+ ions in the crystal. According to this formula, the absorption cross section at 808 nm is calculated to be 5. 21 × 10−20 cm2, which is comparable to that of Nd:YAG [18]. In addition, the full width at half of maximum (FWHM) of the absorption peak at 808 nm is fitted to be 2.9 nm (as denoted in the inset of Fig. 2(a)), which is almost twice wider than that of Nd:YAG crystal. The large absorption cross section and wide absorption band of Nd:GSAG crystal at 808 nm indicate that the crystal is very suitable for pumping by commercialized 808 nm LD and benefit to decrease the temperature dependence of the LD. The comparison of the absorption before and after irradiation with different doses of gamma-ray is shown in Fig. 2(b). As it can be seen, the absorption at around 808 nm is only influenced slightly by the gamma-ray irradiation. That is to say, the crystal can still absorb energy from the 808 nm commercialized LD pumping source efficiently after the gamma-ray irradiation. Additional absorption band with central wavelength at around 390 nm is observed when using the 20 Mrad irradiated sample to subtract the absorption data of non-irradiated sample (as shown in Fig. 3). According to the reported reference [19], this additional absorption could be assigned to the absorption of F+-centers.
2. Experiments 2.1. Gamma-ray irradiation experiment A 60Co gamma-ray source was employed to perform the irradiation experiment. An irradiation dose rate of 26 Gy/min was used for all the irradiation experiments. In this work, three samples were irradiated by the source with irradiation times of 12.5, 63 and 125 h, corresponding to the irradiation doses of 2, 10 and 20 Mrad, respectively. 2.2. Spectroscopic characterizations The absorption spectra of the samples were recorded by a PerkinElmer UV-VIS-NIR lambda-950 spectrometer with a spectral interval of 1 nm. The emission spectra and fluorescence decay curves of the samples were recorded by an Edinburgh FLSP-920 spectrometer. The excitation source used to generate the emission spectra and fluorescence decay curves were Xenon flash lamp and OPO laser (Opolette 355 I, OPOTEK, Inc, USA), respectively. The interval of the emission spectra was 0.5 nm. All the experiments were performed on the room temperature. 2.3. Laser experiment
3.2. Emission spectra
The experimental setup used to perform the LD pumped 1.06 μm continuous-wave (CW) laser experiment is shown in Fig. 1. An 808 nm fiber coupled LD with maximum output power of 30 W was employed as the pumping source. The numerical aperture (N.A.) and core diameter of the fiber are 0.22 and 400 μm, respectively. The M1 and M2 shown in Fig. 1 represents input mirror and output coupler, respectively. The cavity length (distance between M1 and M2) was about 20 mm in the experiment. The 808 nm anti-reflection (AR) film was coated on the pumping side of M1, whereas the 808 nm high-transmission (HT) and 1060 nm high-reflection (HR) films were coated on the other side. The transmittance of M2 is 5% at the wavelength of 1060 nm. The dimension of the Nd:GSAG crystal samples used in the laser experiment was 2 × 2 × 4 mm3, in which two 2 × 2 mm2 faces were cut perpendicular to (1 1 1)-orientation and polished carefully. During the laser experiment, the crystal was wrapped by indium foil and placed into copper sink with water cooled to remove the generated heat in the crystal timely. The cooling water temperature was controlled at 20 °C. An OPHIR 30A-BB-18 power meter was employed to measure the output laser power.
The Emission spectrum of Nd:GSAG crystal is shown in Fig. 4(a). Three emission bands with central wavelengths of 940, 1060, and 1340 nm, respectively, are observed in the measured range, which are corresponding to the transitions from 4F3/2 state of Nd3+ to states 4I9/2, 11/2, 13/2, respectively. The stimulated emission cross section σem could be estimated from the fluorescence spectrum by Fuchtbauer-Ladenburg (F-L) formula [20]:
σem (λ ) =
λ5I (λ ) ∫ λI (λ ) dλ
8πn2cτr
where I(λ) is the fluorescence intensity, n is the reflective index which was reported in the reference [21], c is the velocity of light, τr is the radiative lifetime which is determined to be 263 μs from Judd-Ofelt calculation [22,23]. Using the above formula, the stimulated emission cross section of Nd:GSAG crystal at 1060 nm is calculated as high as 6.18 × 10−20 cm2. The large absorption cross section at 808 nm and emission cross section at 1060 nm suggest that Nd:GSAG crystal is very promising for realizing high-efficiency and low-threshold 1060 nm laser. In addition, the emission cross section at 940 nm is calculated to be 2.68 × 10−20 cm2. The relative large emission cross section at 940 nm indicates that Nd:GSAG crystal is also potential for obtaining 940 nm lasers, which is an important wavelength for application in water vapor detection by differential absorption lidar (DIAL) technique [17,24–26]. Fig. 4(b) shows the fluorescence spectra of Nd:GSAG crystal before and after irradiation with different doses gamma-ray. It is inspiring that the fluorescence intensity is almost unaffected by the gamma-ray irradiation, suggesting the excellent radiation resistant ability of Nd:GSAG crystal.
3. Results and discussion 3.1. Absorption spectra The absorption spectrum of Nd:GSAG crystal is shown in Fig. 2(a). Nine absorption bands are observed in the measured range, which are corresponding to the transitions from the ground state (4I9/2) of Nd3+ to different excited states, as denoted in the Fig. 2(a). The absorption cross section σabs can be calculated with the formula σabs = α (λ )/Nc ,
Fig. 1. Schematic of the LD pumped Nd:GSAG crystal laser experiment. 2
Infrared Physics and Technology 102 (2019) 103005
S. Ding, et al.
Fig. 2. . (a) The room temperature absorption spectrum of Nd:GSAG crystal. (b) The absorption spectra of Nd:GSAG crystal before and after irradiation with different doses gamma-ray.
irradiations. Yadav reported that the defects in the host trap electrons and thus potentially resulting in delayed of the lifetime [27]. A simplified schematic showing the trap mechanism is presented in Fig. 5(b). Therefore, the higher density of the defects (F+-centers), the longer of the lifetimes. In addition, the lifetime only increased 13% ( 281 − 248 ) with 248 the irradiation dose of 20 Mrad, which indicates the excellent radiation resistant ability of Nd:GSAG crystal again.
3.4. Laser performances Fig. 6 illustrated the realized laser output power versus the absorbed incident power using the Nd:GSAG with irradiation doses of 0, 2, 10 and 20 Mrad, respectively. As it can be seen, the output power increases linearly with the absorbed pump power. The laser thresholds for Nd:GSAG crystal before and after irradiation with 2, 10 and 20 doses gamma-ray are 2.00, 2.12, 2.25 and 2.30 W, respectively. The slope efficiency for Nd:GSAG crystal before and after irradiation with 2, 10 and 20 doses gamma-ray are fitted to be 47.15%, 45.89%, 41.12% and 39.5%, respectively. The laser efficiency is decreased 16.2% ( 47.15 % − 39.5 % ) after irradiation with 20 Mrad gamma-ray suggesting the 47.15 % good radiation resistant ability of Nd:GSAG crystal again. In addition, the laser threshold is increased very slightly with the increasing of the irradiation doses, which is because the defects caused by the irradiation absorbed small partial of the pumping energy. Accordingly, Nd:GSAG crystal is a potential high-efficiency 1 μm radiation resistant crystal that can be used for outer-space laser application
Fig. 3. Additional absorption between non-irradiated and 20 Mrad irradiated samples.
3.3. Fluorescence decay curves The fluorescence decay curves of 4F3/2 → 4I11/2 transition for Nd:GSAG crystal before and after irradiation with different doses gamma-ray are shown in Fig. 5(a). With single exponential function, the lifetimes before and after irradiation with 2, 10, 20 Mrad gamma-ray are fitted to be 248, 253, 268 and 281 μs, respectively. As mentioned above, F+-centers will be generated in the crystal after irradiation and the density of the F+-centers increase with the increasing of the
Fig. 4. . (a) The room temperature emission spectrum of Nd:GSAG crystal. (b) The emission spectra of Nd:GSAG crystal before and after irradiation with different doses gamma-ray. 3
Infrared Physics and Technology 102 (2019) 103005
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Fig. 5. . (a) The fluorescence decay curves of 4F3/2 → 4I11/2 transition for Nd:GSAG crystal before and after irradiation with different doses gamma-ray. (b) A simplified schematic showing the trap mechanism.
of China (Grants No. 51502292), National Key Research and Development Program of China (Grants No. 2016YFB0402101) supported this study. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.infrared.2019.103005. References [1] X. Duan, L. Li, L. Zheng, B. Yao, Z. Zou, D. Jiang, L. Su, Infrared Phys. Technol. 94 (2018) 7. [2] R. Yan, C. Zhao, X. Li, X. Yu, Z. Liu, X. Wen, W. Yao, J. Gao, F. Peng, Q. Zhang, Infrared Phys. Technol. 94 (2018) 32. [3] S. Ding, F. Peng, Q. Zhang, J. Luo, W. Liu, D. Sun, R. Dou, J. Gao, G. Sun, M. Cheng, J. Alloy. Compd. 693 (2017) 339. [4] H. Yu, J. Liu, H. Zhang, A.A. Kaminskii, Z. Wang, J. Wang, Laser Photonics Rev. 8 (2014) 847. [5] L. Su, J. Xu, H. Li, L. Wen, Y. Zhu, Z. Zhao, Y. Dong, G. Zhou, J. Si, Chem. Phys. Lett. 406 (2005) 254. [6] J. Li, Y. Wu, Y. Pan, H. Kou, Y. Shi, J. Guo, Ceram. Int. 34 (2008) 1675. [7] Y. Ma, H. Sun, Z. Peng, S. Ding, J. Peng, Q. Zhang, X. Yu, Infrared Phys. Technol. 97 (2019) 371. [8] Y. Ma, Z. Peng, S. Ding, F. Peng, Q. Zhang, X. Yu, Infrared Phys. Technol. 92 (2018) 295. [9] V. Rai, B.R. Sekhar, S. Kher, S. Deb, J. Lumin. 130 (2010) 582. [10] S.J. Zinkle, L.L. Snead, Annu. Rev. Mater. Res. 44 (2014) 241. [11] E. Zharikov, I. Kuratev, V. Laptev, S. Nasel'skii, A. Ryabov, G. Toropkin, A. Shestakov, I. Shcherbakov, Bull. Acad. Set. USSR, Phys. Ser. 48 (1984) 103. [12] A.K. Misra, S.K. Sharma, P.G. Lucey, Appl. Spectrosc. 60 (2006) 223. [13] D. Sun, J. Luo, Q. Zhang, J. Xiao, J. Xu, H. Jiang, S. Yin, J. Lumin. 128 (2008) 1886. [14] J. Chen, D. Sun, J. Luo, J. Xiao, R. Dou, Q. Zhang, Opt. Commun. 301 (2013) 84. [15] Y. Chen, Q. Zhang, F. Peng, W. Liu, Y. He, R. Dou, H. Zhang, J. Luo, D. Sun, Opt. Mater. 84 (2018) 172. [16] 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. [17] S. Ding, H. Wang, W. Liu, J. Luo, Y. Ma, Q. Zhang, J. Lumin. 213 (2019) 249. [18] F. Peng, H. Yang, Q. Zhang, J. Luo, W. Liu, D. Sun, R. Dou, G. Sun, Appl. Phys. B 118 (2015) 549. [19] A. Matkovskii, P. Potera, D. Sugak, L. Grigorjeva, D. Millers, V. Pankratov, A. Suchocki, Cryst. Res. Technol. 39 (2004) 788. [20] S. Ding, Q. Zhang, F. Peng, W. Liu, J. Luo, R. Dou, G. Sun, X. Wang, D. Sun, J. Alloy. Compd. 698 (2017) 159. [21] J. Su, B. Liu, L.H. Xu, Q.L. Zhang, S.T. Yin, J. Alloys Compd. 512 (2012) 230. [22] B.R. Judd, Phys. Rev. 127 (1962) 750. [23] G. Ofelt, J. Chem. Phys. 37 (1962) 511. [24] S. Strohmaier, H. Eichler, C. Czeranowsky, B. Ileri, K. Petermann, G. Huber, Opt. Commun. 275 (2007) 170. [25] A. Karapuzikov, A. Malov, I. Sherstov, Infrared Phys. Technol. 41 (2000) 77. [26] A. Karapuzikov, I. Ptashnik, I. Sherstov, O. Romanovskii, G. Matvienko, Y.N. Ponomarev, Infrared Phys. Technol. 41 (2000) 87. [27] S.K. Yadav, B.P. Uberuaga, M. Nikl, C. Jiang, C.R. Stanek, Phys. Rev. Appl 4 (2015) 054012.
Fig. 6. The 1.06 μm laser output power of Nd:GSAG crystal before and after irradiated with different doses gamma-ray versus the absorbed pump power.
4. Conclusion In this work, Nd:GSAG crystal was irradiated by 60Co gamma-ray with different doses to investigate the irradiation effect on its spectral and laser properties. After exposure with the irradiation dose as high as 20 Mrad, the absorption at around 808 nm and emission at 1060 nm of the crystal are almost unaffected. With the increasing of the irradiation doses, the lifetime of the 4F3/2 → 4I11/2 transition only increases slightly, which can be explained by the generation of defects in the crystal after irradiation. Using LD pumping, 1.06 μm continuous-wave laser output was realized in Nd:GSAG crystal with high efficiency. Importantly, the laser performance is only decreased slightly after 20 Mrad gamma-ray irradiation. All the results indicate that Nd:GSAG crystal is of great potential for application in harsh radiant environment. Declaration of Competing Interest The authors declare that there are no conflict of interest. Acknowledgements The University Natural Science Research Project of Anhui Province, China (KJ2019ZD06), Startup Foundation for Advanced Talents of Anhui University of Technology, National Natural Science Foundation
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