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Growth, spectroscopy, and laser performance of a radiation-resistant Cr,Yb,Ho,Pr:GYSGG crystal for 2.84 µm mid-infrared laser
Journal of Luminescence 194 (2018) 636–640
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Growth, spectroscopy, and laser performance of a radiation-resistant Cr,Yb, Ho,Pr:GYSGG crystal for 2.84 µm mid-infrared laser
MARK
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Huili Zhanga,b, Dunlu Suna, , Jianqiao Luoa,c, Fang Penga, Zhongqing Fanga,b, Xuyao Zhaoa,b, Cong Quana,b, Maojie Chenga, Qingli Zhanga, Shaotang Yina a The Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, Anhui Province, PR China b University of Science and Technology of China, Hefei 230022, PR China c State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute, Hefei 230037, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Cr,Yb,Ho,Pr:GYSGG crystal Czochralski method Spectroscopic properties Mid-infrared laser
A new Cr3+, Yb3+, Ho3+, and Pr3+ co-doped GYSGG (Gd2YSc2Ga3O12) single crystal with a size of Φ 26 mm × 80 mm was grown successfully using the Czochralski method for the first time. Absorption spectrum is studied, and main absorption bands are centered at 449, 638, and 934 nm. The fluorescence spectrum shows a broad emission band around 2.8 µm, indicating the Cr3+ and Yb3+ ions can act as the sensitizer for the Ho3+ ions. The fluorescence lifetimes of the 5I6 and 5I7 levels of the Ho3+ ions are fitted to be 0.39 and 5.31 ms, respectively. By comparison with those of the Cr,Yb,Ho:GYSGG crystal, the Pr3+ ions acting as the deactivator can decrease efficiently the lifetime of the lower laser level 5I7 of the Ho3+ ions. Additionally, a flash lamp pumping 2.84 µm laser with maximum average power of 257 mW is achieved. These results indicate the Cr,Yb,Ho,Pr:GYSGG crystal is a new promising radiation-resistant laser gain medium applied in radiant environment.
1. Introduction Solid state lasers operating in the 2.7–3.1 µm have aroused much interest in recent years due to their wide applications in medical procedures such as dentistry and surgery since this wavelength region overlaps well with the vibration absorption band of water [1–3]. In addition, 2.7–3.1 µm lasers can act as a pumping source for the optical parametric oscillator (OPO) or optical parametric generation (OPG) to realize 3–19 µm infrared lasers output [4,5], which have important applications in the fields of atmospheric detection, poisonous gas detection, and optoelectronic countermeasures, et al. [6]. The 2.7–3.1 µm lasers have been realized on some laser crystals doped with Er3+ or Ho3+ as the activator ions, such as Er:YAG (Y3Al5O12) [2], Cr,Er:YSGG (Y3Sc2Ga3O12) [1,3,7], Ho:YAP (YAlO3) [8], Cr,Yb,Ho:YSGG [9], et al. The Ho3+ ions acting as the activator exhibit a broadband spectrum, which benefits to realize multiple-wavelength laser and ultra-short pulse laser output. However, it is well known that there are two main problems for the Ho3+ single-doped laser crystals. One is that the lifetime of the 5I7 level is longer than that of the 5I6 level, thus the lifetimes ratio between the 5I7 level and the 5I6 level is relatively large, which is disadvantageous to realize the population inversion. Therefore, a method of utilizing Pr3+ as the deactivator ions can be
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taken into consideration to relieve this problem by depopulating the Ho3+ ions at the 5I7 level, as reported in Yb,Ho,Pr:YAP [10], Cr,Tm,Ho,Pr:LuYAG (Lu3−xYxAl5O12) [11], Ho,Pr:LiLuF4 [12], et al. The other is that the Ho3+ single-doped laser crystals exhibit low absorption when pumped by currently well-developed laser diode (LD) or flash lamp. In order to solve this problem, co-doping proper ions as the sensitizer for the Ho3+ ions was put forward. Yb3+ ions or Cr3+,Yb3+ ions can act as the sensitizer to absorb and transfer pumping energy to the Ho3+ activator ions [9,13], which can increase the pumping efficiency and make them suitable to be pumped by LD or flash lamp. Particularly, it was found that the energy transfer efficiency of Cr3+ → Yb3+ → Ho3+ (> 90%) is greatly higher than that of Cr3+ → Ho3+ (6%) when the Ho3+ ions concentration is 5 × 10–19 cm−3 in the YSGG crystal [9]. Therefore, in this study, we choose Cr3+ and Yb3+ as the sensitizer and Pr3+ as the deactivator in Ho3+ activated laser crystal. As the lasers can be used in outer space and some other environments with strong radiation, the irradiation damages of the laser gain media should be carefully considered [14]. Unfortunately, above mentioned holmium laser crystals commonly have weak or no resistance to the radiation, so it is important and necessary to develop novel crystals which possess good radiation-resistant ability and can be applied in the radiant environment. The GSGG crystal was reported to
Corresponding author. E-mail address: [email protected] (D. Sun).
http://dx.doi.org/10.1016/j.jlumin.2017.09.026 Received 11 May 2017; Received in revised form 8 September 2017; Accepted 11 September 2017 Available online 12 September 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 194 (2018) 636–640
H. Zhang et al.
have an ability to withstand the strong radiation [15], but its phonon energy is higher than that of the YSGG host crystal, resulting in a relatively high non-radiative transition probability and an increase of the laser threshold. In recent years, our group has made extensive researches on a new host crystal GYSGG (Gd3−xYxSc2Ga3O12) [16–19], which can be obtained by replacing some of Gd3+ with Y3+ in GSGG. The new host GYSGG possesses many advantages such as an excellent dual-wavelength laser property in the Nd:GYSGG crystal [20–22]. Especially, 100 Mrad gamma-ray radiation has almost no influence on the laser performance of the Er3+ doped GYSGG crystals, suggesting a strong radiation-resistant property [16,18]. It was reported that the Gd3+ ions simultaneously with the Sc3+ ions can make the crystals have higher covalent property and bond order [23] and increase the resistant ability for the formation of the color centers. Moreover, a special bond-chain structure among Gd3+, Y3+, and Sc3+ may be formed in the GYSGG crystal which leads to its stronger radiation-resistant ability. Additionally, the mixing of Gd3+ and Y3+ in the GYSGG makes the crystal structure more disordered and the crystal-field interaction weaker [24]. As a result, the emission bands of activator ions in the GYSGG crystals might be broadened, which is beneficial to realize the ultra-short pulse laser output. However, the novel radiationresistant GYSGG crystal co-doped with the Cr3+, Yb3+, Ho3+, and Pr3+ ions for mid-infrared lasers has not been reported up to now. In this work, a Cr,Yb,Ho,Pr:GYSGG single crystal was grown successfully by the Czochralski (Cz) method for the first time. The absorption spectrum, fluorescence spectrum and level lifetimes are measured and discussed. Meantime, we investigate the laser performances of this crystal pumped by a flash lamp with different repetition rates and output mirror transmissions.
Fig. 1. Photograph of the as-grown Cr,Yb,Ho,Pr:GYSGG single crystal and processed laser rod.
spectrum in the range of 2700–3100 nm and the fluorescence decay curves with the excitation sources of a 450 nm LD and an OPO (Opolette 355 I), respectively. All spectroscopic measurements were conducted at room temperature. In the laser experiment, the pumping source is a single xenon flash lamp with an arc length of 75 mm and an inner diameter of 4 mm. A plane-plane Cr,Yb,Ho,Pr:GYSGG crystal rod with dimensions of Φ 4 mm × 72 mm was taken as the laser gain medium, and the effective length of the crystal rod pumped by the flash lamp is 60 mm. Both end faces of the laser rod were polished and coated anti-reflection (AR) at 2.7–3 µm. The xenon flash lamp and crystal rod were both placed into a closecoupled diffusing ceramic cavity and cooled with the circulating water, which was maintained at 19 °C. A simple plane-parallel resonator formed by two mirrors with a cavity-length of 192 mm was employed to demonstrate mid-infrared lasers, as shown in Fig. 2. M1 is a plane mirror with reflectivity of 100% around 2.8 µm. Three plane mirrors with different transmissions of 5%, 15%, or 30% around 2.8 µm were used as the output mirrors (M2), respectively. The laser output energy was measured by an energy meter (OPHIR PE50-DIF-C), and the laser spectrum was recorded by the fluorescence spectrophotometer (Edinburgh FLSP-920).
2. Experimental details 2.1. Crystal growth Using the Cz method, a Cr,Yb,Ho,Pr:GYSGG single crystal was grown from a melt of congruent composition with 2 at% Cr3+, 50 at% Yb3+, 0.42 at% Ho3+, and 0.1 at% Pr3+. The Cr2O3 (4N), Yb2O3 (4N5), Ho2O3 (5N), Pr6O11 (5N), Gd2O3 (4N), Y2O3 (5N), Sc2O3 (4N), and Ga2O3 (5N) oxide powders were used as the starting materials and weighed accurately according to the structural formula Cr0.04Yb1.5Ho0.0126Pr0.003Gd0.4844YSc1.96Ga3O12. Accordingly, Ga2O3 was overweighed by 1.5 wt% to compensate for the evaporation loss during the crystal growth. The powders were well mixed, pressed into tablets, then sintered at 1250 °C for 24 h in a muffle furnace with air atmosphere in order to react completely and synthesize polycrystalline material. The crystal growth was carried out in a JGD-60 furnace (CETC26th, China) with an automatic diameter controlled (ADC) growth system operated at an up-weighing mode. An iridium crucible with dimensions of Φ 60 mm × 50 mm was used in high purity argon atmosphere to prevent oxidization. A 〈111〉-orientation pure GYSGG crystal rod was used as a seed, and the pulling rate and rotation speed were 1.5–2.5 mm/h and 1–5 rpm, respectively. A Cr,Yb,Ho,Pr:GYSGG crystal with a high quality was obtained successfully, and the crystal dimensions were about Φ 26 mm × 80 mm. The as-grown crystal was annealed in air atmosphere at 1550 °C for 72 h. A photograph of the asgrown Cr,Yb,Ho,Pr:GYSGG crystal and processed laser rod is shown in Fig. 1.
3. Results and discussion 3.1. Absorption spectrum Fig. 3 shows the absorption spectrum from 320 to 3000 nm of the Cr,Yb,Ho,Pr:GYSGG crystal. The absorption bands are mainly located in visible and near-infrared regions. Two wide absorption bands in visible region are centered at 449 and 638 nm, respectively, corresponding to the typical absorption transitions of 4A2 → 4T1 and 4A2 → 4T2 of the Cr3+ ions, as indexed and shown in the inset of Fig. 3. The full width at half maximum (FWHM) of the absorption bands centered at 449 and 638 nm are 68 and 80 nm, and the corresponding absorption coefficients are 11.1 and 5.7 cm-1, respectively. These two broad absorption bands (449 and 638 nm) overlap well with the emission bands of a flash lamp, which makes the crystal more suitable to be pumped by the xenon lamp. In addition, several sharp absorption peaks located near 934 nm are observed in the near-infrared region, corresponding to the
2.2. Characterizations Samples with the thickness of 2.25 mm were cut perpendicularly to the growth orientation 〈111〉 and optically polished on both faces for the spectroscopic measurements. The absorption spectrum with a resolution of 1 nm was measured from 320 to 3000 nm by a Perkin-Elmer UV–VIS–NIR spectrophotometer (Lambda 950). A fluorescence spectrometer (Edinburgh FLSP920) was used to record the fluorescence
Fig. 2. Schematic of the xenon lamp pumped Cr,Yb,Ho,Pr:GYSGG laser.
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Fig. 5. Schematic of energy transfer processes among Cr3+, Yb3+, Ho3+, and Pr3+ ions in the Cr,Yb,Ho,Pr:GYSGG crystal.
Fig. 3. Absorption spectrum of the Cr,Yb,Ho,Pr:GYSGG crystal. Inset: enlarged spectrum in the range of 320–800 nm.
curve is acquired and shown in the inset of Fig. 4. The maximum emission cross-section is located at 2840 nm and as large as 1.24 × 1020 cm2, which is beneficial to realize low-threshold and high-efficiency laser output. The presence of the emission peak at 2840 nm implies that the energy transfer from the Cr3+ to Ho3+ ions can be conducted successfully with the Yb3+ ions acting as an intermediate medium. The energy transfer mechanisms are exhibited in Fig. 5. ET and CR are the abbreviations of energy transfer and cross-relaxation, respectively. Pumping energy of the flash lamp can be absorbed by the Cr3+ ions jumping from the ground state 4A2 to the excited state 4T1 or 4T2, then relaxing to the 2E level. Subsequently, the Cr3+ ions at the 2E level transfer energy to the excited level 2F5/2 of the Yb3+ ions. Additionally, the Yb3+ ions can be excited from the ground state 2F7/2 to the excited state 2F5/2 by absorbing the pumping energy of the high-power InGaAs LD. Then, the Yb3+ ions at the excited level 2F5/2 interact with the Ho3+ ions through ET and CR processes, resulting in the population gathering of the Ho3+ ions at the upper laser level 5I6. Ultimately, the ET3 and ET4 processes occur from the Ho3+ ions to the deactivator Pr3+ ions, which influence slightly the lifetime of the upper laser level 5I6 while reduce obviously that of the lower laser level 5I7. It reveals that the doping of the deactivator Pr3+ ions is beneficial to inhibit effectively the self-termination problem and realize population inversions between the upper laser level 5I6 and the lower laser level 5I7 of the Ho3+ ions.
typical 2F7/2 → 2F5/2 absorption transition of the Yb3+ ions. The concentration of the doped Yb3+ ions is as high as 50 at% in the Cr,Yb,Ho,Pr:GYSGG crystal, which results in the absorption coefficient of the absorption peak at 934 nm as large as 39.6 cm−1. This strong absorption matches well with the emission wavelength of a high-power InGaAs LD. Therefore, both of the flash lamp and LD can be used as the pumping source of the Cr,Yb,Ho,Pr:GYSGG crystal, which is a potential advantage. 3.2. Fluorescence spectrum The fluorescence spectrum of the Cr,Yb,Ho,Pr:GYSGG crystal in the range of 2700–3100 nm was recorded by exciting with a 450 nm LD and presented in Fig. 4. Many fluorescence peaks are observed within 2.7–3.1 µm, arising from the Stark sub-levels transitions from the 5I6 to 5 I7 level of the Ho3+ ions, and the strongest emission peak is centered at 2840 nm with a FWHM of about 20 nm. In addition, the stimulated emission cross-section was calculated by the Füchtbauer-Ladenburg equation [25]
σem (λ ) =
βλ5I (λ ) ∫ λI (λ ) dλ
8πn2cτr
(1)
where λ is the emission wavelength, I (λ) is the fluorescence intensity, n is the refractive index, c is the velocity of light, and τr is the radiative lifetime of the upper laser level. The stimulated emission cross-section
3.3. Fluorescence lifetimes Fig. 6 shows the fluorescence decay curves of the upper laser level I6 and lower laser level 5I7 of the Ho3+ ions in the Cr,Yb,Ho:GYSGG and Cr,Yb,Ho,Pr:GYSGG crystals, which were recorded at 1204 nm and 2092 nm emission peaks under excitation of the OPO pulse lasers, respectively. The decay curves of the upper and lower laser levels in these two crystals both exhibit a single exponential decay behavior. The lifetimes of the upper laser level 5I6 and the lower laser level 5I7 of the Ho3+ ions in the Cr,Yb,Ho,Pr:GYSGG crystal are fitted to be 0.39 ms and 5.31 ms; those are 0.45 ms and 8.33 ms in the Cr,Yb,Ho:GYSGG crystal, respectively. We note that the upper level lifetime of the Ho3+ ions in the Cr,Yb,Ho,Pr:GYSGG crystal decreases slightly. However, the lower level lifetime of the Ho3+ ions in the Cr,Yb,Ho,Pr:GYSGG crystal becomes much shorter than that in the Cr,Yb,Ho:GYSGG crystal, mainly owing to the energy transfer via 5I7 (Ho3+) → 3F2 (Pr3+) + 3H6 (Pr3+), as shown in Fig. 5. Based on the level lifetimes, the energy transfer efficiencies from the Ho3+ to Pr3+ ions can be calculated according to the equation [26] 5
Fig. 4. Fluorescence spectrum of the Cr,Yb,Ho,Pr:GYSGG crystal excited by a 450 nm LD. Inset: emission cross-section curve in the range of 2700–3100 nm.
η = 1 − τDA/ τD 638
(2)
Journal of Luminescence 194 (2018) 636–640
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Fig. 6. Fluorescence decay curves of the Cr,Yb,Ho:GYSGG and Cr,Yb,Ho,Pr:GYSGG crystals. (a) 1204 nm decay (5I6 → 5I8); (b) 2092 nm decay (5I7 → 5I8).
Fig. 7. Output energy versus pump energy operated at 1 Hz and 5 Hz with different output coupler transmissions. (a) T = 5%, (b) T = 15%, (c) T = 30%.
where τDA is the level lifetime of the Ho3+ ions in the presence of the Pr3+ ions, and τD is the level lifetime of the Ho3+ ions in the absence of the Pr3+ ions. Based on Eq. (2) and the measured level lifetimes of these two crystals, the energy transfer efficiencies of the ET3 and ET4 processes from the Ho3+ to Pr3+ ions are obtained to be 13.3% and 36.3%, respectively. It means that the lifetime of the lower laser level 5I7 can decrease quicker than that of the upper laser level 5I6 in the Cr,Yb,Ho,Pr:GYSGG crystal. Therefore, the introducing of the deactivator Pr3+ ions can weaken the self-termination effect and is beneficial
to the population inversions between the 5I6 and 5I7 levels of the Ho3+ ions. Additionally, as the energy transfer efficiency from the activator to deactivator ions is highly dependent on the concentrations of the doped ions [19], a suitable concentration ratio between the Ho3+ and Pr3+ ions can keep the lifetime of the upper laser level 5I6 as long as possible while reduce obviously that of the lower laser level 5I7. Thus, the concentration ratio will be optimized in the future research in order to improve the performances of the 2.8–3.1 µm lasers.
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the laser performances of the Cr,Yb,Ho,Pr:GYSGG crystal. Acknowledgments This work was financially supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB1102301), the National Natural Science Foundation of China (Grant Nos. 51272254, 61405206, and 51502292), and the Open Research Fund of State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute (Grant No. SKL2015KF01). References [1] A. Högele, G. Hörbe, H. Lubatschowski, H. Welling, W. Ertmer, 2.70 µm Cr,Er:YSGG laser with high output energy and FTIR-Q-switch, Opt. Commun. 125 (1996) 90–94. [2] A. Zajac, M. Skorczakowski, J. Swiderski, P. Nyga, Electrooptically Q-switched mid-infrared Er:YAG laser for medical applications, Opt. Express 12 (2004) 5125–5130. [3] M. Tempus, W. Luthy, H.P. Weber, V.G. Ostroumov, I.A. Shcherbakov, 2.79 μm YSGG:Cr:Er laser pumped at 790 nm, IEEE J. Quantum Electron. 30 (1994) 2608–2611. [4] K.L. Vodopyanov, F. Ganikhanov, J.P. Maffetone, I. 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Fig. 8. Laser spectrum of the Cr,Yb,Ho,Pr:GYSGG crystal.
3.4. Laser performance The output energy of Cr,Yb,Ho,Pr:GYSGG crystal versus the pump energy operated at 1 Hz and 5 Hz with different output coupler transmissions of 5%, 15%, and 30% are shown in Fig. 7, respectively. The cavity length of 192 mm was maintained throughout the entire experiment. The laser thresholds are about 36.0, 52.2, and 82.1 J with output coupler transmissions of 5%, 15% and 30% at a repetition rate of 1 Hz, respectively, and those are 29.0, 43.7, and 71.4 J at the repetition rate of 5 Hz. As can be seen from the Fig. 7, the single-pulse energy at the repetition of 5 Hz is larger than that at the repetition of 1 Hz under the same pump energy. When the output coupler transmission is 5%, a maximum average power of 257 mW is obtained at a repetition rate of 5 Hz, corresponding to the electrical-to-optical efficiency of 0.033% and the slope efficiency of 0.040%. Fig. 8 presents the laser spectrum of the Cr,Yb,Ho,Pr:GYSGG crystal, suggesting that the 2.84 µm laser output is realized for the first time on this laser crystal. By contract, the laser output energy and efficiency are still far lower than those of the Cr,Er:YSGG and Cr,Yb,Ho:YSGG crystals [7,9]. Four factors including the crystal quality and length, the concentrations of the doped ions, and the cavity design are the main influences on the laser performances of the Cr,Yb,Ho,Pr:GYSGG crystal. Therefore, we will enhance the crystal quality, make the crystal rod longer, and optimize the concentrations of the doped ions and the cavity design to improve the laser output energy and efficiency in the future. 4. Conclusions We demonstrate the growth, spectroscopy, and laser performance of the Cr,Yb,Ho,Pr:GYSGG radiation-resistant crystal grown by the Cz method for the first time. The absorption coefficient at 449 nm and 934 nm are 11.1 cm−1 and 39.6 cm−1, respectively. The stimulated emission cross section at 2840 nm is 1.24 × 10-20 cm2. The lifetimes of the 5I6 and 5I7 levels of the Ho3+ ions are 0.39 ms and 5.31 ms, respectively, indicating the Pr3+ ions can decrease the lifetime of the lower laser level 5I7 while influence slightly that of the upper laser level 5 I6. The pulse laser at 2.84 µm is realized with the flash lamp pumping. The minimum laser threshold is about 29.0 J owing to the doping of the deactivator Pr3+ ions. The maximum average power of 257 mW is obtained at a repetition rate of 5 Hz, corresponding to the electrical-tooptical efficiency of 0.033% and the slope efficiency of 0.040%. These results indicate that the Cr,Yb,Ho,Pr:GYSGG crystal is a new potential radiation-resistant gain medium for mid-infrared laser which can be applied in radiant environment. In the next work, we will enhance the crystal quality, make the crystal rod longer, and optimize the concentrations of the doped ions and the cavity design to further improve 640
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Growth, spectroscopy, and laser performance of a radiation-resistant Cr,Yb, Ho,Pr:GYSGG crystal for 2.84 µm mid-infrared laser
MARK
⁎
Huili Zhanga,b, Dunlu Suna, , Jianqiao Luoa,c, Fang Penga, Zhongqing Fanga,b, Xuyao Zhaoa,b, Cong Quana,b, Maojie Chenga, Qingli Zhanga, Shaotang Yina a The Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, Anhui Province, PR China b University of Science and Technology of China, Hefei 230022, PR China c State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute, Hefei 230037, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Cr,Yb,Ho,Pr:GYSGG crystal Czochralski method Spectroscopic properties Mid-infrared laser
A new Cr3+, Yb3+, Ho3+, and Pr3+ co-doped GYSGG (Gd2YSc2Ga3O12) single crystal with a size of Φ 26 mm × 80 mm was grown successfully using the Czochralski method for the first time. Absorption spectrum is studied, and main absorption bands are centered at 449, 638, and 934 nm. The fluorescence spectrum shows a broad emission band around 2.8 µm, indicating the Cr3+ and Yb3+ ions can act as the sensitizer for the Ho3+ ions. The fluorescence lifetimes of the 5I6 and 5I7 levels of the Ho3+ ions are fitted to be 0.39 and 5.31 ms, respectively. By comparison with those of the Cr,Yb,Ho:GYSGG crystal, the Pr3+ ions acting as the deactivator can decrease efficiently the lifetime of the lower laser level 5I7 of the Ho3+ ions. Additionally, a flash lamp pumping 2.84 µm laser with maximum average power of 257 mW is achieved. These results indicate the Cr,Yb,Ho,Pr:GYSGG crystal is a new promising radiation-resistant laser gain medium applied in radiant environment.
1. Introduction Solid state lasers operating in the 2.7–3.1 µm have aroused much interest in recent years due to their wide applications in medical procedures such as dentistry and surgery since this wavelength region overlaps well with the vibration absorption band of water [1–3]. In addition, 2.7–3.1 µm lasers can act as a pumping source for the optical parametric oscillator (OPO) or optical parametric generation (OPG) to realize 3–19 µm infrared lasers output [4,5], which have important applications in the fields of atmospheric detection, poisonous gas detection, and optoelectronic countermeasures, et al. [6]. The 2.7–3.1 µm lasers have been realized on some laser crystals doped with Er3+ or Ho3+ as the activator ions, such as Er:YAG (Y3Al5O12) [2], Cr,Er:YSGG (Y3Sc2Ga3O12) [1,3,7], Ho:YAP (YAlO3) [8], Cr,Yb,Ho:YSGG [9], et al. The Ho3+ ions acting as the activator exhibit a broadband spectrum, which benefits to realize multiple-wavelength laser and ultra-short pulse laser output. However, it is well known that there are two main problems for the Ho3+ single-doped laser crystals. One is that the lifetime of the 5I7 level is longer than that of the 5I6 level, thus the lifetimes ratio between the 5I7 level and the 5I6 level is relatively large, which is disadvantageous to realize the population inversion. Therefore, a method of utilizing Pr3+ as the deactivator ions can be
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taken into consideration to relieve this problem by depopulating the Ho3+ ions at the 5I7 level, as reported in Yb,Ho,Pr:YAP [10], Cr,Tm,Ho,Pr:LuYAG (Lu3−xYxAl5O12) [11], Ho,Pr:LiLuF4 [12], et al. The other is that the Ho3+ single-doped laser crystals exhibit low absorption when pumped by currently well-developed laser diode (LD) or flash lamp. In order to solve this problem, co-doping proper ions as the sensitizer for the Ho3+ ions was put forward. Yb3+ ions or Cr3+,Yb3+ ions can act as the sensitizer to absorb and transfer pumping energy to the Ho3+ activator ions [9,13], which can increase the pumping efficiency and make them suitable to be pumped by LD or flash lamp. Particularly, it was found that the energy transfer efficiency of Cr3+ → Yb3+ → Ho3+ (> 90%) is greatly higher than that of Cr3+ → Ho3+ (6%) when the Ho3+ ions concentration is 5 × 10–19 cm−3 in the YSGG crystal [9]. Therefore, in this study, we choose Cr3+ and Yb3+ as the sensitizer and Pr3+ as the deactivator in Ho3+ activated laser crystal. As the lasers can be used in outer space and some other environments with strong radiation, the irradiation damages of the laser gain media should be carefully considered [14]. Unfortunately, above mentioned holmium laser crystals commonly have weak or no resistance to the radiation, so it is important and necessary to develop novel crystals which possess good radiation-resistant ability and can be applied in the radiant environment. The GSGG crystal was reported to
Corresponding author. E-mail address: [email protected] (D. Sun).
http://dx.doi.org/10.1016/j.jlumin.2017.09.026 Received 11 May 2017; Received in revised form 8 September 2017; Accepted 11 September 2017 Available online 12 September 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
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have an ability to withstand the strong radiation [15], but its phonon energy is higher than that of the YSGG host crystal, resulting in a relatively high non-radiative transition probability and an increase of the laser threshold. In recent years, our group has made extensive researches on a new host crystal GYSGG (Gd3−xYxSc2Ga3O12) [16–19], which can be obtained by replacing some of Gd3+ with Y3+ in GSGG. The new host GYSGG possesses many advantages such as an excellent dual-wavelength laser property in the Nd:GYSGG crystal [20–22]. Especially, 100 Mrad gamma-ray radiation has almost no influence on the laser performance of the Er3+ doped GYSGG crystals, suggesting a strong radiation-resistant property [16,18]. It was reported that the Gd3+ ions simultaneously with the Sc3+ ions can make the crystals have higher covalent property and bond order [23] and increase the resistant ability for the formation of the color centers. Moreover, a special bond-chain structure among Gd3+, Y3+, and Sc3+ may be formed in the GYSGG crystal which leads to its stronger radiation-resistant ability. Additionally, the mixing of Gd3+ and Y3+ in the GYSGG makes the crystal structure more disordered and the crystal-field interaction weaker [24]. As a result, the emission bands of activator ions in the GYSGG crystals might be broadened, which is beneficial to realize the ultra-short pulse laser output. However, the novel radiationresistant GYSGG crystal co-doped with the Cr3+, Yb3+, Ho3+, and Pr3+ ions for mid-infrared lasers has not been reported up to now. In this work, a Cr,Yb,Ho,Pr:GYSGG single crystal was grown successfully by the Czochralski (Cz) method for the first time. The absorption spectrum, fluorescence spectrum and level lifetimes are measured and discussed. Meantime, we investigate the laser performances of this crystal pumped by a flash lamp with different repetition rates and output mirror transmissions.
Fig. 1. Photograph of the as-grown Cr,Yb,Ho,Pr:GYSGG single crystal and processed laser rod.
spectrum in the range of 2700–3100 nm and the fluorescence decay curves with the excitation sources of a 450 nm LD and an OPO (Opolette 355 I), respectively. All spectroscopic measurements were conducted at room temperature. In the laser experiment, the pumping source is a single xenon flash lamp with an arc length of 75 mm and an inner diameter of 4 mm. A plane-plane Cr,Yb,Ho,Pr:GYSGG crystal rod with dimensions of Φ 4 mm × 72 mm was taken as the laser gain medium, and the effective length of the crystal rod pumped by the flash lamp is 60 mm. Both end faces of the laser rod were polished and coated anti-reflection (AR) at 2.7–3 µm. The xenon flash lamp and crystal rod were both placed into a closecoupled diffusing ceramic cavity and cooled with the circulating water, which was maintained at 19 °C. A simple plane-parallel resonator formed by two mirrors with a cavity-length of 192 mm was employed to demonstrate mid-infrared lasers, as shown in Fig. 2. M1 is a plane mirror with reflectivity of 100% around 2.8 µm. Three plane mirrors with different transmissions of 5%, 15%, or 30% around 2.8 µm were used as the output mirrors (M2), respectively. The laser output energy was measured by an energy meter (OPHIR PE50-DIF-C), and the laser spectrum was recorded by the fluorescence spectrophotometer (Edinburgh FLSP-920).
2. Experimental details 2.1. Crystal growth Using the Cz method, a Cr,Yb,Ho,Pr:GYSGG single crystal was grown from a melt of congruent composition with 2 at% Cr3+, 50 at% Yb3+, 0.42 at% Ho3+, and 0.1 at% Pr3+. The Cr2O3 (4N), Yb2O3 (4N5), Ho2O3 (5N), Pr6O11 (5N), Gd2O3 (4N), Y2O3 (5N), Sc2O3 (4N), and Ga2O3 (5N) oxide powders were used as the starting materials and weighed accurately according to the structural formula Cr0.04Yb1.5Ho0.0126Pr0.003Gd0.4844YSc1.96Ga3O12. Accordingly, Ga2O3 was overweighed by 1.5 wt% to compensate for the evaporation loss during the crystal growth. The powders were well mixed, pressed into tablets, then sintered at 1250 °C for 24 h in a muffle furnace with air atmosphere in order to react completely and synthesize polycrystalline material. The crystal growth was carried out in a JGD-60 furnace (CETC26th, China) with an automatic diameter controlled (ADC) growth system operated at an up-weighing mode. An iridium crucible with dimensions of Φ 60 mm × 50 mm was used in high purity argon atmosphere to prevent oxidization. A 〈111〉-orientation pure GYSGG crystal rod was used as a seed, and the pulling rate and rotation speed were 1.5–2.5 mm/h and 1–5 rpm, respectively. A Cr,Yb,Ho,Pr:GYSGG crystal with a high quality was obtained successfully, and the crystal dimensions were about Φ 26 mm × 80 mm. The as-grown crystal was annealed in air atmosphere at 1550 °C for 72 h. A photograph of the asgrown Cr,Yb,Ho,Pr:GYSGG crystal and processed laser rod is shown in Fig. 1.
3. Results and discussion 3.1. Absorption spectrum Fig. 3 shows the absorption spectrum from 320 to 3000 nm of the Cr,Yb,Ho,Pr:GYSGG crystal. The absorption bands are mainly located in visible and near-infrared regions. Two wide absorption bands in visible region are centered at 449 and 638 nm, respectively, corresponding to the typical absorption transitions of 4A2 → 4T1 and 4A2 → 4T2 of the Cr3+ ions, as indexed and shown in the inset of Fig. 3. The full width at half maximum (FWHM) of the absorption bands centered at 449 and 638 nm are 68 and 80 nm, and the corresponding absorption coefficients are 11.1 and 5.7 cm-1, respectively. These two broad absorption bands (449 and 638 nm) overlap well with the emission bands of a flash lamp, which makes the crystal more suitable to be pumped by the xenon lamp. In addition, several sharp absorption peaks located near 934 nm are observed in the near-infrared region, corresponding to the
2.2. Characterizations Samples with the thickness of 2.25 mm were cut perpendicularly to the growth orientation 〈111〉 and optically polished on both faces for the spectroscopic measurements. The absorption spectrum with a resolution of 1 nm was measured from 320 to 3000 nm by a Perkin-Elmer UV–VIS–NIR spectrophotometer (Lambda 950). A fluorescence spectrometer (Edinburgh FLSP920) was used to record the fluorescence
Fig. 2. Schematic of the xenon lamp pumped Cr,Yb,Ho,Pr:GYSGG laser.
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Fig. 5. Schematic of energy transfer processes among Cr3+, Yb3+, Ho3+, and Pr3+ ions in the Cr,Yb,Ho,Pr:GYSGG crystal.
Fig. 3. Absorption spectrum of the Cr,Yb,Ho,Pr:GYSGG crystal. Inset: enlarged spectrum in the range of 320–800 nm.
curve is acquired and shown in the inset of Fig. 4. The maximum emission cross-section is located at 2840 nm and as large as 1.24 × 1020 cm2, which is beneficial to realize low-threshold and high-efficiency laser output. The presence of the emission peak at 2840 nm implies that the energy transfer from the Cr3+ to Ho3+ ions can be conducted successfully with the Yb3+ ions acting as an intermediate medium. The energy transfer mechanisms are exhibited in Fig. 5. ET and CR are the abbreviations of energy transfer and cross-relaxation, respectively. Pumping energy of the flash lamp can be absorbed by the Cr3+ ions jumping from the ground state 4A2 to the excited state 4T1 or 4T2, then relaxing to the 2E level. Subsequently, the Cr3+ ions at the 2E level transfer energy to the excited level 2F5/2 of the Yb3+ ions. Additionally, the Yb3+ ions can be excited from the ground state 2F7/2 to the excited state 2F5/2 by absorbing the pumping energy of the high-power InGaAs LD. Then, the Yb3+ ions at the excited level 2F5/2 interact with the Ho3+ ions through ET and CR processes, resulting in the population gathering of the Ho3+ ions at the upper laser level 5I6. Ultimately, the ET3 and ET4 processes occur from the Ho3+ ions to the deactivator Pr3+ ions, which influence slightly the lifetime of the upper laser level 5I6 while reduce obviously that of the lower laser level 5I7. It reveals that the doping of the deactivator Pr3+ ions is beneficial to inhibit effectively the self-termination problem and realize population inversions between the upper laser level 5I6 and the lower laser level 5I7 of the Ho3+ ions.
typical 2F7/2 → 2F5/2 absorption transition of the Yb3+ ions. The concentration of the doped Yb3+ ions is as high as 50 at% in the Cr,Yb,Ho,Pr:GYSGG crystal, which results in the absorption coefficient of the absorption peak at 934 nm as large as 39.6 cm−1. This strong absorption matches well with the emission wavelength of a high-power InGaAs LD. Therefore, both of the flash lamp and LD can be used as the pumping source of the Cr,Yb,Ho,Pr:GYSGG crystal, which is a potential advantage. 3.2. Fluorescence spectrum The fluorescence spectrum of the Cr,Yb,Ho,Pr:GYSGG crystal in the range of 2700–3100 nm was recorded by exciting with a 450 nm LD and presented in Fig. 4. Many fluorescence peaks are observed within 2.7–3.1 µm, arising from the Stark sub-levels transitions from the 5I6 to 5 I7 level of the Ho3+ ions, and the strongest emission peak is centered at 2840 nm with a FWHM of about 20 nm. In addition, the stimulated emission cross-section was calculated by the Füchtbauer-Ladenburg equation [25]
σem (λ ) =
βλ5I (λ ) ∫ λI (λ ) dλ
8πn2cτr
(1)
where λ is the emission wavelength, I (λ) is the fluorescence intensity, n is the refractive index, c is the velocity of light, and τr is the radiative lifetime of the upper laser level. The stimulated emission cross-section
3.3. Fluorescence lifetimes Fig. 6 shows the fluorescence decay curves of the upper laser level I6 and lower laser level 5I7 of the Ho3+ ions in the Cr,Yb,Ho:GYSGG and Cr,Yb,Ho,Pr:GYSGG crystals, which were recorded at 1204 nm and 2092 nm emission peaks under excitation of the OPO pulse lasers, respectively. The decay curves of the upper and lower laser levels in these two crystals both exhibit a single exponential decay behavior. The lifetimes of the upper laser level 5I6 and the lower laser level 5I7 of the Ho3+ ions in the Cr,Yb,Ho,Pr:GYSGG crystal are fitted to be 0.39 ms and 5.31 ms; those are 0.45 ms and 8.33 ms in the Cr,Yb,Ho:GYSGG crystal, respectively. We note that the upper level lifetime of the Ho3+ ions in the Cr,Yb,Ho,Pr:GYSGG crystal decreases slightly. However, the lower level lifetime of the Ho3+ ions in the Cr,Yb,Ho,Pr:GYSGG crystal becomes much shorter than that in the Cr,Yb,Ho:GYSGG crystal, mainly owing to the energy transfer via 5I7 (Ho3+) → 3F2 (Pr3+) + 3H6 (Pr3+), as shown in Fig. 5. Based on the level lifetimes, the energy transfer efficiencies from the Ho3+ to Pr3+ ions can be calculated according to the equation [26] 5
Fig. 4. Fluorescence spectrum of the Cr,Yb,Ho,Pr:GYSGG crystal excited by a 450 nm LD. Inset: emission cross-section curve in the range of 2700–3100 nm.
η = 1 − τDA/ τD 638
(2)
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Fig. 6. Fluorescence decay curves of the Cr,Yb,Ho:GYSGG and Cr,Yb,Ho,Pr:GYSGG crystals. (a) 1204 nm decay (5I6 → 5I8); (b) 2092 nm decay (5I7 → 5I8).
Fig. 7. Output energy versus pump energy operated at 1 Hz and 5 Hz with different output coupler transmissions. (a) T = 5%, (b) T = 15%, (c) T = 30%.
where τDA is the level lifetime of the Ho3+ ions in the presence of the Pr3+ ions, and τD is the level lifetime of the Ho3+ ions in the absence of the Pr3+ ions. Based on Eq. (2) and the measured level lifetimes of these two crystals, the energy transfer efficiencies of the ET3 and ET4 processes from the Ho3+ to Pr3+ ions are obtained to be 13.3% and 36.3%, respectively. It means that the lifetime of the lower laser level 5I7 can decrease quicker than that of the upper laser level 5I6 in the Cr,Yb,Ho,Pr:GYSGG crystal. Therefore, the introducing of the deactivator Pr3+ ions can weaken the self-termination effect and is beneficial
to the population inversions between the 5I6 and 5I7 levels of the Ho3+ ions. Additionally, as the energy transfer efficiency from the activator to deactivator ions is highly dependent on the concentrations of the doped ions [19], a suitable concentration ratio between the Ho3+ and Pr3+ ions can keep the lifetime of the upper laser level 5I6 as long as possible while reduce obviously that of the lower laser level 5I7. Thus, the concentration ratio will be optimized in the future research in order to improve the performances of the 2.8–3.1 µm lasers.
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the laser performances of the Cr,Yb,Ho,Pr:GYSGG crystal. Acknowledgments This work was financially supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB1102301), the National Natural Science Foundation of China (Grant Nos. 51272254, 61405206, and 51502292), and the Open Research Fund of State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute (Grant No. SKL2015KF01). References [1] A. Högele, G. Hörbe, H. Lubatschowski, H. Welling, W. Ertmer, 2.70 µm Cr,Er:YSGG laser with high output energy and FTIR-Q-switch, Opt. Commun. 125 (1996) 90–94. [2] A. Zajac, M. Skorczakowski, J. Swiderski, P. Nyga, Electrooptically Q-switched mid-infrared Er:YAG laser for medical applications, Opt. Express 12 (2004) 5125–5130. [3] M. Tempus, W. Luthy, H.P. Weber, V.G. Ostroumov, I.A. Shcherbakov, 2.79 μm YSGG:Cr:Er laser pumped at 790 nm, IEEE J. Quantum Electron. 30 (1994) 2608–2611. [4] K.L. Vodopyanov, F. Ganikhanov, J.P. Maffetone, I. 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Fig. 8. Laser spectrum of the Cr,Yb,Ho,Pr:GYSGG crystal.
3.4. Laser performance The output energy of Cr,Yb,Ho,Pr:GYSGG crystal versus the pump energy operated at 1 Hz and 5 Hz with different output coupler transmissions of 5%, 15%, and 30% are shown in Fig. 7, respectively. The cavity length of 192 mm was maintained throughout the entire experiment. The laser thresholds are about 36.0, 52.2, and 82.1 J with output coupler transmissions of 5%, 15% and 30% at a repetition rate of 1 Hz, respectively, and those are 29.0, 43.7, and 71.4 J at the repetition rate of 5 Hz. As can be seen from the Fig. 7, the single-pulse energy at the repetition of 5 Hz is larger than that at the repetition of 1 Hz under the same pump energy. When the output coupler transmission is 5%, a maximum average power of 257 mW is obtained at a repetition rate of 5 Hz, corresponding to the electrical-to-optical efficiency of 0.033% and the slope efficiency of 0.040%. Fig. 8 presents the laser spectrum of the Cr,Yb,Ho,Pr:GYSGG crystal, suggesting that the 2.84 µm laser output is realized for the first time on this laser crystal. By contract, the laser output energy and efficiency are still far lower than those of the Cr,Er:YSGG and Cr,Yb,Ho:YSGG crystals [7,9]. Four factors including the crystal quality and length, the concentrations of the doped ions, and the cavity design are the main influences on the laser performances of the Cr,Yb,Ho,Pr:GYSGG crystal. Therefore, we will enhance the crystal quality, make the crystal rod longer, and optimize the concentrations of the doped ions and the cavity design to improve the laser output energy and efficiency in the future. 4. Conclusions We demonstrate the growth, spectroscopy, and laser performance of the Cr,Yb,Ho,Pr:GYSGG radiation-resistant crystal grown by the Cz method for the first time. The absorption coefficient at 449 nm and 934 nm are 11.1 cm−1 and 39.6 cm−1, respectively. The stimulated emission cross section at 2840 nm is 1.24 × 10-20 cm2. The lifetimes of the 5I6 and 5I7 levels of the Ho3+ ions are 0.39 ms and 5.31 ms, respectively, indicating the Pr3+ ions can decrease the lifetime of the lower laser level 5I7 while influence slightly that of the upper laser level 5 I6. The pulse laser at 2.84 µm is realized with the flash lamp pumping. The minimum laser threshold is about 29.0 J owing to the doping of the deactivator Pr3+ ions. The maximum average power of 257 mW is obtained at a repetition rate of 5 Hz, corresponding to the electrical-tooptical efficiency of 0.033% and the slope efficiency of 0.040%. These results indicate that the Cr,Yb,Ho,Pr:GYSGG crystal is a new potential radiation-resistant gain medium for mid-infrared laser which can be applied in radiant environment. In the next work, we will enhance the crystal quality, make the crystal rod longer, and optimize the concentrations of the doped ions and the cavity design to further improve 640