- Email: [email protected]
Spectral properties and highly efficient continuous-wave laser operation in Nd, Gd:CaF2 crystals
Accepted Manuscript Spectral properties and highly efficient continuous-wave laser operation in Nd, Gd:CaF2 crystals Shaozhao Wang, Dapeng Jiang, Qinghui Wu, Siyuan Pang, Jingya Wang, Xiaobo Qian, Jie Liu, Bingchu Mei, Liangbi Su PII:
S0925-8388(18)34605-X
DOI:
https://doi.org/10.1016/j.jallcom.2018.12.058
Reference:
JALCOM 48699
To appear in:
Journal of Alloys and Compounds
Received Date: 17 October 2018 Revised Date:
27 November 2018
Accepted Date: 4 December 2018
Please cite this article as: S. Wang, D. Jiang, Q. Wu, S. Pang, J. Wang, X. Qian, J. Liu, B. Mei, L. Su, Spectral properties and highly efficient continuous-wave laser operation in Nd, Gd:CaF2 crystals, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2018.12.058. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Spectral properties and highly efficient continuous-wave laser operation in Nd, Gd:CaF2 crystals
1
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of
Technology, Wuhan 430070, China 2
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of
SC
Ceramics, Chinese Academy of Sciences, Shanghai 201899, China 3
Shandong provincial key laboratory of optics and photonic device, College of Physics and Electronics, Shandong
Normal University, Jinan 250014, China
Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences,
M AN U
4
RI PT
Shaozhao Wang, 1,2 Dapeng Jiang, 2, * Qinghui Wu, 2 Siyuan Pang, 2 Jingya Wang, 2 Xiaobo Qian, 2 Jie Liu, 3 Bingchu Mei 1 and Liangbi Su 2,4, *
Beijing 100049, China
AC C
EP
TE D
*[email protected] (D. Jiang); [email protected] (L. Su)
ACCEPTED MANUSCRIPT
Spectral properties and highly efficient continuous-wave laser operation in Nd, Gd:CaF2 crystals
RI PT
Shaozhao Wang, 1,2 Dapeng Jiang, 2, * Qinghui Wu, 2 Siyuan Pang, 2 Jingya Wang, 2 Xiaobo Qian, 2 Jie Liu, 3 Bingchu Mei 1 and Liangbi Su 2,4, *
M AN U
SC
1 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China 4 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China 3 Shandong provincial key laboratory of optics and photonic device, College of Physics and Electronics, Shandong Normal University, Jinan 250014, China 4 Synthetic Single Crystal Research Center, CAS Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China *[email protected] (D. Jiang); [email protected] (L. Su)
Abstract: Spectral properties of Nd, Gd:CaF2 crystals were investigated. In comparison with Nd:CaF2, the spectral parameters of Nd, Gd:CaF2 were altered in a large scale. LD-pumped true CW laser has been demonstrated in the crystals. The slope efficiency with the value of 45% in 0.5 at.% Nd, 5 at.% Gd:CaF2 was achieved, it is the highest slope efficiency in Nd doped CaF2 single crystals by LD-pumping. The system is a promising candidate for highly efficient lasers.
1. Introduction
TE D
Keywords: Laser crystal; Spectral properties; Continuous-wave laser
AC C
EP
Neodymium-doped fluorite has received much attention since Nd, Y:CaF2 achieved continuous-wave (CW) laser operation [1]. These materials possess suitable emission crosssections and long upper-level lifetimes [2-4]. Its broad gain linewidths [1,5-9] support pulses of sub-100 fs. Also, the real four-level system of Nd3+ makes it easy to realize population inversion. Besides, fluorites, with low nonlinear refractive index [10], could be grown in a large size. Furthermore, it is suitable to be pumped by high-power LD because of its highthermal conductivity [11]. Therefore, Nd-doped fluorites are potential gain media for generating ultrashort and ultrahigh peak power lasers. In recent years, researchers have achieved a multitude of advances in this field. Doualan et al. reported a CW laser operation in Nd, Lu:CaF2 with slope efficiency of 20% pumped with a laser diode (LD) [12]. In Nd, Y:CaF2 crystal, Qin et al. achieved 103 fs using a diodepumped passively mode-locked technique [5]. When pumped with a CW Ti:Sapphire laser, Wei et al. demonstrated 332 fs from passively mode-locked pulse in Nd, Y:SrF2 crystal [6]. Also in Nd, Y:SrF2 crystal, Jelínek et al. reported a true CW laser operation with output power of 380 mW and slope efficiency of 28% [7]. Zhang et al. demonstrated CW laser operation in Nd, Y:CaF2 media with output power of 901 mW and slope efficiency of 30.1% [8]. Most recently, Wang et al. presented a CW laser with output power of 1.33 W and slope efficiency of 40.3% in Nd, La:CaF2 [9]. Ma et al. achieved CW laser with slope efficiency up to 43.5% in Nd, Y:SrF2 by LD-pumping [13]. Zhang et al. demonstrated a dual-wavelength synchronously mode locked Nd, Gd:CaF2 laser for the first time [14].
ACCEPTED MANUSCRIPT
M AN U
SC
2. Spectra properties and continuous-wave operation
RI PT
In this study, we report on diode-pumped, true CW laser operation in Nd, Gd:CaF2 gains, which grown by temperature gradient technique (TGT) method. By optimizing the Gd3+doping concentration, a broad fluorescence linewidth has been yielded in Nd, Gd:CaF2 crystal, which is even broader than the gains linewidth of Nd:glass [15]. The highest slope efficiency of 45% in 0.5 at.% Nd, 5 at.% Gd:CaF2 single crystal was demonstrated. To the best of our knowledge, it is the highest CW laser efficiency obtained in Nd-doped CaF2 single crystals.
Fig.1. Crystal structure of alkaline earth fluorides, the green and purple red balls represent divalent alkaline earth metal and fluorine ions, respectively
AC C
EP
TE D
Fig. 1 shows the crystal structure of alkaline earth fluorides. Cubic crystals of CaF2 of space group -Fm3m belong to this kind of multicenter system. It can be presented as a kind of statistical formation of centers with Nd3+ ions, forming at heterovalent substitution of trivalent cations by Ca2+ ions in these crystals by the following scheme: TR3+ + F- Ca2+ + VF-. The vacancies in this case reduced the coordination number of TR3+ cations in its three or four neighbor polyhedra. In Nd doped fluorides system, there are mainly paired rhombic M ([Nd3+-Nd3+]-Fi-2) and N ([Nd3+-Nd3+]2-Fi-4) centers, and high-symmetry tetragonal L (Nd3+-Fi-) centers. M and N centers are luminescence quenched, but L centers are luminescence nonquenched. With addition of regulating ions Re3+ (Re3+ = Y3+, La3+, Gd3+, Lu3+, etc.), some L′ (Fi--Nd3+-Fi-), M′ ([Nd3+-Re3+]-Fi-2) and N′ ([Nd3+-Re3+]2-Fi-4, [Nd3+-Re3+3]-Fi-4, [Nd3+3-Re3+]Fi-4) centers will be formed in the crystal [16,17]. Consequently, rare earth ions easily form a variety of cluster structures in alkaline earth fluorides, which makes these laser crystals possess broad absorption and emission bands.
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
Fig. 2. Absorption spectra of the crystals, recorded by a Jasco V-570 UV/VIS/NIR spectro-photometer. The insert graph represents the absorption cross section at 790 nm.
AC C
EP
TE D
The room-temperature absorption spectra of 0.5 at.% Nd, x at.% Gd:CaF2 [(O, x = 0), (A, x = 2), (B, x = 5), (C, x = 8)] are shown in Fig. 2, with all the absorption bands labeled with their corresponding energy levels. The actual concentrations of dopants were measured by the inductively coupled plasma optical emission spectroscopy (ICP-OES). Moreover, the segregation coefficients of Nd3+ in O, A, B and C are 0.92, 1.22, 1.28 and 1.09, respectively. 4 The insert graph shows the detail of the absorption band corresponding to the 4I9/2 F5/2 + 2 H9/2 absorption transition, which is usually used for diode pumping. For the O crystal, the absorption band is peaking at 791 nm. When GdF3 is introduced in the crystal, one main peak at 791 nm and a shoulder peak at 795 nm are observed, which is similar to Nd, Y:CaF2 [8] and Nd, La:CaF2 [9]. Meanwhile, the absorption cross section at 791 nm decreases with increase in Gd-doped concentration, they are 2.17 × 10-20 cm2, 2.10 × 10-20 cm2, 2.04 × 10-20 cm2 and 1.49 × 10-20 cm2 in O, A, B and C sample, respectively. Consequently, the absorption spectra of Nd3+ ions can be significantly altered by co-doping with regulating ions Gd3+ as a result of the formation of new [Nd3+- Gd3+] clusters, which have more or less complicated structures. From the absorption spectra, we can conclude that co-doping Nd:CaF2 with Gd3+ ions has obvious influence on the local environment of neodymium.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 3. Emission spectra of the samples excited at 791 nm by a FLSP980 time-resolved fluorescence spectrometer.
AC C
EP
TE D
Fig. 3 displays the emission spectra of the samples at room-temperature, which are corrected to the spectral sensitivity of the measuring devices. Emission intensity is sharply enhanced with the regulating ions Gd3+, which is suitable for femtosecond pulses. As shown 4 I11/2 of O crystal is very weak compared with the in Fig. 3, the emission intensity of 4F3/2 other three, as a result of emission quenching and high-symmetry local lattice of neodymium ions [16]. In the Nd:CaF2 system, there are mainly M and N centers, and very few nonquenched L centers. Since M and N centers are luminescence quenched, fluorescence intensity of O sample would be very low. With addition of regulating ions Gd3+, some L′, M′ and N′ centers would be formed, [Nd3+-Nd3+] quenching pairs would be transformed to [Nd3+Gd3+] non-quenching ones [17]. As the Gd3+-concentration increases, the intensity of the emission peak at 1064 nm decreases, and the A medium has the highest emission intensity. Since the binding energy of [Gd3+-Fi-] and [Nd3+-Fi-] are different and unknown, the ratios of Gd3+/Nd3+ are crucial to achieve the strongest emission intensity. In comparison with B and C media, it is certain that at Gd3+/Nd3+ ratio of 4 in A sample more L, M and N centers are transformed to L′, M′ and N′ centers. 4 The emission band 4F3/2 I11/2 of O crystal consists of two bands, peaking at 1061nm and 1089 nm. For the mixed crystals, however, the emission spectrum consists of only one band peaking at 1064 nm with a shoulder at 1052 nm, which is similar to Nd, Y:CaF2 but have broader bandwidth of 34 nm [1]. On the one hand, with regulating ions Gd3+ optical centers are redistributed [18], leading to fluorescence lines differ from single-doped ones, as presented in Fig. 3. On the other hand, one Nd3+ in the [Nd3+-Nd3+] pair is substituted by Gd3+, the symmetry of the centers is nearly the same, energy levels of Nd3+ in the [Nd3+-Gd3+] pair differ from that in the [Nd3+-Nd3+] pair, and it induces the appearance of new lines and some old ones are killed [19]. These results indicate that Gd3+ can effectively reduce lattice symmetry and emission quenching of neodymium ions.
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 4. Fluorescence decay curves of the samples [(a) O; (b) A, B, C)] recorded by TDS 3052 oscilloscope with flash lamp at 1064 nm and fitted by exponential function.
M AN U
Fluorescence decay curves recorded at 1064 nm are presented in Fig. 4. For O sample, the decay curve is double exponential. The emitters in O medium are a long lifetime (899.8 µs) of L and short (7.6 µs) of M centers, which is similar to 0.5 at.% Nd:SrF2 crystal [16]. In the mixed crystals, the symmetry of the optical centers is manipulated to be lowered by gadolinium and lifetimes shortened. It is 630.2 µs, 596.9 µs and 543.8 µs of A, B and C crystal, respectively. The value agrees well with that in Nd:glasses, which definitely indicates that the quenching effect is greatly suppressed by co-doping with Gd3+. We therefore believe that weak quenching and serous local lattice distortion manipulated by GdF3 contribute to the shorter lifetimes in the mixed crystals.
EP
TE D
3. Laser properties of Nd, Gd:CaF2 crystal under LD pumping
Fig. 5. Schematic of CW-laser setup.
AC C
The CW laser operation was carried out with the setup presented in Fig. 4. The uncoated Nd, Gd:CaF2 disordered crystal with the dimension of 3 mm × 3 mm × 5 mm was pumped by a fiber coupled semiconductor laser at 791 nm with a fiber core diameter of 400 µm and numerical aperture (NA) of 0.22. An optics coupling system of 1:0.8 was used to focus the pump laser on the crystal. In order to remove the heat and reduce the thermal effect, the crystal was mounted in a Cu holder whose temperature was stabilized at 12 °C by cooling water. The experiment employed a simple plane-concave resonator whose length was 192 mm for reducing the loss and obtaining high averaged output power. The input plane mirror M1 was high reflection at 1.06 µm. The output concave mirror M2 had a radius of 200 mm with the transmission of 3%. The averaged output power was measured by the power meter (30A-SH-V1, made in Israel).
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 6. Averaged output power versus absorbed pump power, where η represents the slope efficiency.
4. Conclusions
TE D
M AN U
We did not achieve laser output in the O sample because of the serious quenching effect of Nd3+. As shown in Fig. 6, CW laser operations at 1.06 µm were demonstrated in both A, B and C crystals. The maximum averaged output power and slope efficiency of A, B, and C specimens are respectively 1.21 W at the absorbed pump power of 3.54 W and 37.1%, 1.34 W at 3.11 W and 45%, 1.25 W at 3.02 W and 43.8%. They are absorbed at 68.1%, 60.1%, and 63.5% of the pump power, respectively. Moreover, the laser thresholds are 0.48 W, 0.45 W and 0.29 W, which is surely benefited from the low phonon energy and long fluorescence lifetime of Nd:CaF2 single crystal. The 0.5 at.% Nd, 5 at.% Gd:CaF2 crystal is proved to have the best laser performance among these three crystals, which CW laser slope efficiency is much higher than that of Nd, Y:CaF2 [8] and Nd, La:CaF2 crystals [9]. In order to protect the crystals from destroying, the experiment researches were done in a low incident pump power level. Moreover, if the crystals were coated for antireflection at the lasing wavelength and the pump wavelength, the averaged output power was expected to enhance.
EP
In conclusion, spectral parameters, namely absorption cross section, bandwidth, emission intensity, and lifetime of 0.5 at.% Nd, x at.% Gd:CaF2 (x = 2, 5, 8) crystals can be changed in a large scale by co-doping Gd3+. LD-pumped, true CW laser with slope efficiency up to 45% has been performed. To the best of our knowledge, this is the highest CW efficiency obtained in Nd-doped CaF2 single crystal by LD-pumping. The system is promising alternative for highly efficient lasers and thus calls for further investigations.
AC C
Acknowledgment
This work was financially supported by the National Natural Science Foundation of China (Grants Nos. 61635012 and 51432007), the Strategic Priority Program of the Chinese Academy of Sciences (Grant No. XDB16030000), and Shanghai science and Technology Commission (Grant No. 16520721300) References
[1] L.B. Su, Q.G. Wang, H.J. Li, G. Brasse, P. Camy, J.L. Doualan, A. Braud, R. Moncorgé, Y.Y. Zhan, L.H. Zheng, X.B. Qian, and J. Xu, Laser Phys. Lett. 10 (2013) 035804. [2] N.P. Barnes, D.J. Gettemy, L. Esterowitz, and R.E. Allen, IEEE J. Quantum Electron. 23 (1987) 1434. [3] T.T. Basiev, V.B. Sigachev, M.E. Doroshenko, A.G. Papashvili, and V.V. Osiko, Proc. SPIE 2498 (1995) 179.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[4] D.P. Jiang, Y.Y. Zhan, Q. Zhang, F.K. Ma, L.B. Su, F. Tang, X.B. Qian, and J. Xu, Cryst. Eng. Commun. 17 (2015) 7398. [5] Z.P. Qin, G.Q. Xie, J. Ma, W.Y. Ge, P. Yuan, L.J. Qian, L.B. Su, D.P. Jiang, F.K. Ma, Q. Zhang, Y.X. Cao, and J. Xu, Opt. Lett. 39 (2014) 1737. [6] L. Wei, H.N. Han, W.L. Tian, J.X. Liu, Z.H. Wang, Z. Zhu, Y.L. Jia, L.B. Su, J. Xu, and Z.Y. Wei, Appl. Phys. Express 7 (2014) 092704. [7] M. Jelínek, V. Kubeček, L.B. Su, D.P. Jiang, F.K. Ma, Q. Zhang, Y.X. Cao, and J. Xu, Laser Phys. Lett. 11 (2014) 055001. [8] Q. Zhang, L.B. Su, D.P. Jiang, F.K. Ma, Z.P. Qin, G.Q. Xie, J.G. Zheng, Q.H. Deng, W.G. Zheng, L.J. Qian, and J. Xu, Chin. Opt. Lett. 13 (2015) 071402. [9] S.Z. Wang, D.P. Jiang, Q.H. Wu, Z.D. Cao, S.Y. Pang, J.Y. Wang, X.B. Qian, J. Liu, B.C. Mei, and L.B. Su, Laser Phys. 29 (2019) 015801 [10] Y. Guo, S.B. Lu, L.B. Su, C.J. Zhao, H. Zhang, and S.C. Wen, Appl. Opt. 54 (2015) 953. [11] W.F. Krupke, M.D. Shinn, J.E. Marion, J.A. Caird, and S.E. Stokowski, J. Opt. Soc. Am. B 3 (1986) 102. [12] J.L. Doualan, L.B. Su, G. Brasse, A. Benayad, V. Menard, Y.Y. Zhan, A. Braud, P. Camy, J. Xu, and R. Moncorgé, J. Opt. Soc. Am. B 30 (2013) 3018. [13] F.K. Ma, D.P. Jiang, L.B. Su, J.Y. Wang, W. Cai, J. Liu, J.G. Zhang, W.G. Zheng, J. Xu, and Y. Liu, Opt. Lett. 41 (2016) 501. [14] F. Zhang, Y.J. Wu, J. Liu, S.Y. Pang, F.K. Ma, D.P. Jiang, Q.H Wu, and L.B. Su, Optics and Laser Technology. 100 (2018) 294. [15] D. Kopf, F.X. Kartner, and U. Keller, Opt. Lett. 20 (1995) 1169. [16] O.K. Alimov, T.T. Basiev, M.E. Doroshenko, P.P. Fedorov, V. A. Konyushkin, A.N. Nakladov, and V.V. Osiko, Opt. Mater. 34 (2012) 799. [17] F.K. Ma, Q. Zhang, D.P. Jiang, L.B. Su, Y.J. Shao, J.Y. Wang, F. Tang, J. Xu, P. Solarz, W. RybaRomanowski, R. Lisiecki, and B. Macalik, Laser Phys. 24 (2014) 105703. [18] K.S. Bagdasarov, Y.K. Voronko, A.A. Kaminskii, L.V. Krotova, and V.V. Osiko, Stat. Sol. b 12 (1965) 905.
ACCEPTED MANUSCRIPT
Highlights
AC C
EP
TE D
M AN U
SC
RI PT
1. High optical quality Nd, Gd:CaF2 crystals were grown by temperature gradient technique. 2. Compared with Nd:CaF2, the spectral parameters of Nd, Gd:CaF2 were altered in a large scale. 3. The highest slope efficiency of 45% was achieved in 0.5 at.% Nd, 5 at.% Gd:CaF2.
S0925-8388(18)34605-X
DOI:
https://doi.org/10.1016/j.jallcom.2018.12.058
Reference:
JALCOM 48699
To appear in:
Journal of Alloys and Compounds
Received Date: 17 October 2018 Revised Date:
27 November 2018
Accepted Date: 4 December 2018
Please cite this article as: S. Wang, D. Jiang, Q. Wu, S. Pang, J. Wang, X. Qian, J. Liu, B. Mei, L. Su, Spectral properties and highly efficient continuous-wave laser operation in Nd, Gd:CaF2 crystals, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2018.12.058. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Spectral properties and highly efficient continuous-wave laser operation in Nd, Gd:CaF2 crystals
1
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of
Technology, Wuhan 430070, China 2
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of
SC
Ceramics, Chinese Academy of Sciences, Shanghai 201899, China 3
Shandong provincial key laboratory of optics and photonic device, College of Physics and Electronics, Shandong
Normal University, Jinan 250014, China
Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences,
M AN U
4
RI PT
Shaozhao Wang, 1,2 Dapeng Jiang, 2, * Qinghui Wu, 2 Siyuan Pang, 2 Jingya Wang, 2 Xiaobo Qian, 2 Jie Liu, 3 Bingchu Mei 1 and Liangbi Su 2,4, *
Beijing 100049, China
AC C
EP
TE D
*[email protected] (D. Jiang); [email protected] (L. Su)
ACCEPTED MANUSCRIPT
Spectral properties and highly efficient continuous-wave laser operation in Nd, Gd:CaF2 crystals
RI PT
Shaozhao Wang, 1,2 Dapeng Jiang, 2, * Qinghui Wu, 2 Siyuan Pang, 2 Jingya Wang, 2 Xiaobo Qian, 2 Jie Liu, 3 Bingchu Mei 1 and Liangbi Su 2,4, *
M AN U
SC
1 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China 4 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China 3 Shandong provincial key laboratory of optics and photonic device, College of Physics and Electronics, Shandong Normal University, Jinan 250014, China 4 Synthetic Single Crystal Research Center, CAS Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China *[email protected] (D. Jiang); [email protected] (L. Su)
Abstract: Spectral properties of Nd, Gd:CaF2 crystals were investigated. In comparison with Nd:CaF2, the spectral parameters of Nd, Gd:CaF2 were altered in a large scale. LD-pumped true CW laser has been demonstrated in the crystals. The slope efficiency with the value of 45% in 0.5 at.% Nd, 5 at.% Gd:CaF2 was achieved, it is the highest slope efficiency in Nd doped CaF2 single crystals by LD-pumping. The system is a promising candidate for highly efficient lasers.
1. Introduction
TE D
Keywords: Laser crystal; Spectral properties; Continuous-wave laser
AC C
EP
Neodymium-doped fluorite has received much attention since Nd, Y:CaF2 achieved continuous-wave (CW) laser operation [1]. These materials possess suitable emission crosssections and long upper-level lifetimes [2-4]. Its broad gain linewidths [1,5-9] support pulses of sub-100 fs. Also, the real four-level system of Nd3+ makes it easy to realize population inversion. Besides, fluorites, with low nonlinear refractive index [10], could be grown in a large size. Furthermore, it is suitable to be pumped by high-power LD because of its highthermal conductivity [11]. Therefore, Nd-doped fluorites are potential gain media for generating ultrashort and ultrahigh peak power lasers. In recent years, researchers have achieved a multitude of advances in this field. Doualan et al. reported a CW laser operation in Nd, Lu:CaF2 with slope efficiency of 20% pumped with a laser diode (LD) [12]. In Nd, Y:CaF2 crystal, Qin et al. achieved 103 fs using a diodepumped passively mode-locked technique [5]. When pumped with a CW Ti:Sapphire laser, Wei et al. demonstrated 332 fs from passively mode-locked pulse in Nd, Y:SrF2 crystal [6]. Also in Nd, Y:SrF2 crystal, Jelínek et al. reported a true CW laser operation with output power of 380 mW and slope efficiency of 28% [7]. Zhang et al. demonstrated CW laser operation in Nd, Y:CaF2 media with output power of 901 mW and slope efficiency of 30.1% [8]. Most recently, Wang et al. presented a CW laser with output power of 1.33 W and slope efficiency of 40.3% in Nd, La:CaF2 [9]. Ma et al. achieved CW laser with slope efficiency up to 43.5% in Nd, Y:SrF2 by LD-pumping [13]. Zhang et al. demonstrated a dual-wavelength synchronously mode locked Nd, Gd:CaF2 laser for the first time [14].
ACCEPTED MANUSCRIPT
M AN U
SC
2. Spectra properties and continuous-wave operation
RI PT
In this study, we report on diode-pumped, true CW laser operation in Nd, Gd:CaF2 gains, which grown by temperature gradient technique (TGT) method. By optimizing the Gd3+doping concentration, a broad fluorescence linewidth has been yielded in Nd, Gd:CaF2 crystal, which is even broader than the gains linewidth of Nd:glass [15]. The highest slope efficiency of 45% in 0.5 at.% Nd, 5 at.% Gd:CaF2 single crystal was demonstrated. To the best of our knowledge, it is the highest CW laser efficiency obtained in Nd-doped CaF2 single crystals.
Fig.1. Crystal structure of alkaline earth fluorides, the green and purple red balls represent divalent alkaline earth metal and fluorine ions, respectively
AC C
EP
TE D
Fig. 1 shows the crystal structure of alkaline earth fluorides. Cubic crystals of CaF2 of space group -Fm3m belong to this kind of multicenter system. It can be presented as a kind of statistical formation of centers with Nd3+ ions, forming at heterovalent substitution of trivalent cations by Ca2+ ions in these crystals by the following scheme: TR3+ + F- Ca2+ + VF-. The vacancies in this case reduced the coordination number of TR3+ cations in its three or four neighbor polyhedra. In Nd doped fluorides system, there are mainly paired rhombic M ([Nd3+-Nd3+]-Fi-2) and N ([Nd3+-Nd3+]2-Fi-4) centers, and high-symmetry tetragonal L (Nd3+-Fi-) centers. M and N centers are luminescence quenched, but L centers are luminescence nonquenched. With addition of regulating ions Re3+ (Re3+ = Y3+, La3+, Gd3+, Lu3+, etc.), some L′ (Fi--Nd3+-Fi-), M′ ([Nd3+-Re3+]-Fi-2) and N′ ([Nd3+-Re3+]2-Fi-4, [Nd3+-Re3+3]-Fi-4, [Nd3+3-Re3+]Fi-4) centers will be formed in the crystal [16,17]. Consequently, rare earth ions easily form a variety of cluster structures in alkaline earth fluorides, which makes these laser crystals possess broad absorption and emission bands.
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
Fig. 2. Absorption spectra of the crystals, recorded by a Jasco V-570 UV/VIS/NIR spectro-photometer. The insert graph represents the absorption cross section at 790 nm.
AC C
EP
TE D
The room-temperature absorption spectra of 0.5 at.% Nd, x at.% Gd:CaF2 [(O, x = 0), (A, x = 2), (B, x = 5), (C, x = 8)] are shown in Fig. 2, with all the absorption bands labeled with their corresponding energy levels. The actual concentrations of dopants were measured by the inductively coupled plasma optical emission spectroscopy (ICP-OES). Moreover, the segregation coefficients of Nd3+ in O, A, B and C are 0.92, 1.22, 1.28 and 1.09, respectively. 4 The insert graph shows the detail of the absorption band corresponding to the 4I9/2 F5/2 + 2 H9/2 absorption transition, which is usually used for diode pumping. For the O crystal, the absorption band is peaking at 791 nm. When GdF3 is introduced in the crystal, one main peak at 791 nm and a shoulder peak at 795 nm are observed, which is similar to Nd, Y:CaF2 [8] and Nd, La:CaF2 [9]. Meanwhile, the absorption cross section at 791 nm decreases with increase in Gd-doped concentration, they are 2.17 × 10-20 cm2, 2.10 × 10-20 cm2, 2.04 × 10-20 cm2 and 1.49 × 10-20 cm2 in O, A, B and C sample, respectively. Consequently, the absorption spectra of Nd3+ ions can be significantly altered by co-doping with regulating ions Gd3+ as a result of the formation of new [Nd3+- Gd3+] clusters, which have more or less complicated structures. From the absorption spectra, we can conclude that co-doping Nd:CaF2 with Gd3+ ions has obvious influence on the local environment of neodymium.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 3. Emission spectra of the samples excited at 791 nm by a FLSP980 time-resolved fluorescence spectrometer.
AC C
EP
TE D
Fig. 3 displays the emission spectra of the samples at room-temperature, which are corrected to the spectral sensitivity of the measuring devices. Emission intensity is sharply enhanced with the regulating ions Gd3+, which is suitable for femtosecond pulses. As shown 4 I11/2 of O crystal is very weak compared with the in Fig. 3, the emission intensity of 4F3/2 other three, as a result of emission quenching and high-symmetry local lattice of neodymium ions [16]. In the Nd:CaF2 system, there are mainly M and N centers, and very few nonquenched L centers. Since M and N centers are luminescence quenched, fluorescence intensity of O sample would be very low. With addition of regulating ions Gd3+, some L′, M′ and N′ centers would be formed, [Nd3+-Nd3+] quenching pairs would be transformed to [Nd3+Gd3+] non-quenching ones [17]. As the Gd3+-concentration increases, the intensity of the emission peak at 1064 nm decreases, and the A medium has the highest emission intensity. Since the binding energy of [Gd3+-Fi-] and [Nd3+-Fi-] are different and unknown, the ratios of Gd3+/Nd3+ are crucial to achieve the strongest emission intensity. In comparison with B and C media, it is certain that at Gd3+/Nd3+ ratio of 4 in A sample more L, M and N centers are transformed to L′, M′ and N′ centers. 4 The emission band 4F3/2 I11/2 of O crystal consists of two bands, peaking at 1061nm and 1089 nm. For the mixed crystals, however, the emission spectrum consists of only one band peaking at 1064 nm with a shoulder at 1052 nm, which is similar to Nd, Y:CaF2 but have broader bandwidth of 34 nm [1]. On the one hand, with regulating ions Gd3+ optical centers are redistributed [18], leading to fluorescence lines differ from single-doped ones, as presented in Fig. 3. On the other hand, one Nd3+ in the [Nd3+-Nd3+] pair is substituted by Gd3+, the symmetry of the centers is nearly the same, energy levels of Nd3+ in the [Nd3+-Gd3+] pair differ from that in the [Nd3+-Nd3+] pair, and it induces the appearance of new lines and some old ones are killed [19]. These results indicate that Gd3+ can effectively reduce lattice symmetry and emission quenching of neodymium ions.
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 4. Fluorescence decay curves of the samples [(a) O; (b) A, B, C)] recorded by TDS 3052 oscilloscope with flash lamp at 1064 nm and fitted by exponential function.
M AN U
Fluorescence decay curves recorded at 1064 nm are presented in Fig. 4. For O sample, the decay curve is double exponential. The emitters in O medium are a long lifetime (899.8 µs) of L and short (7.6 µs) of M centers, which is similar to 0.5 at.% Nd:SrF2 crystal [16]. In the mixed crystals, the symmetry of the optical centers is manipulated to be lowered by gadolinium and lifetimes shortened. It is 630.2 µs, 596.9 µs and 543.8 µs of A, B and C crystal, respectively. The value agrees well with that in Nd:glasses, which definitely indicates that the quenching effect is greatly suppressed by co-doping with Gd3+. We therefore believe that weak quenching and serous local lattice distortion manipulated by GdF3 contribute to the shorter lifetimes in the mixed crystals.
EP
TE D
3. Laser properties of Nd, Gd:CaF2 crystal under LD pumping
Fig. 5. Schematic of CW-laser setup.
AC C
The CW laser operation was carried out with the setup presented in Fig. 4. The uncoated Nd, Gd:CaF2 disordered crystal with the dimension of 3 mm × 3 mm × 5 mm was pumped by a fiber coupled semiconductor laser at 791 nm with a fiber core diameter of 400 µm and numerical aperture (NA) of 0.22. An optics coupling system of 1:0.8 was used to focus the pump laser on the crystal. In order to remove the heat and reduce the thermal effect, the crystal was mounted in a Cu holder whose temperature was stabilized at 12 °C by cooling water. The experiment employed a simple plane-concave resonator whose length was 192 mm for reducing the loss and obtaining high averaged output power. The input plane mirror M1 was high reflection at 1.06 µm. The output concave mirror M2 had a radius of 200 mm with the transmission of 3%. The averaged output power was measured by the power meter (30A-SH-V1, made in Israel).
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 6. Averaged output power versus absorbed pump power, where η represents the slope efficiency.
4. Conclusions
TE D
M AN U
We did not achieve laser output in the O sample because of the serious quenching effect of Nd3+. As shown in Fig. 6, CW laser operations at 1.06 µm were demonstrated in both A, B and C crystals. The maximum averaged output power and slope efficiency of A, B, and C specimens are respectively 1.21 W at the absorbed pump power of 3.54 W and 37.1%, 1.34 W at 3.11 W and 45%, 1.25 W at 3.02 W and 43.8%. They are absorbed at 68.1%, 60.1%, and 63.5% of the pump power, respectively. Moreover, the laser thresholds are 0.48 W, 0.45 W and 0.29 W, which is surely benefited from the low phonon energy and long fluorescence lifetime of Nd:CaF2 single crystal. The 0.5 at.% Nd, 5 at.% Gd:CaF2 crystal is proved to have the best laser performance among these three crystals, which CW laser slope efficiency is much higher than that of Nd, Y:CaF2 [8] and Nd, La:CaF2 crystals [9]. In order to protect the crystals from destroying, the experiment researches were done in a low incident pump power level. Moreover, if the crystals were coated for antireflection at the lasing wavelength and the pump wavelength, the averaged output power was expected to enhance.
EP
In conclusion, spectral parameters, namely absorption cross section, bandwidth, emission intensity, and lifetime of 0.5 at.% Nd, x at.% Gd:CaF2 (x = 2, 5, 8) crystals can be changed in a large scale by co-doping Gd3+. LD-pumped, true CW laser with slope efficiency up to 45% has been performed. To the best of our knowledge, this is the highest CW efficiency obtained in Nd-doped CaF2 single crystal by LD-pumping. The system is promising alternative for highly efficient lasers and thus calls for further investigations.
AC C
Acknowledgment
This work was financially supported by the National Natural Science Foundation of China (Grants Nos. 61635012 and 51432007), the Strategic Priority Program of the Chinese Academy of Sciences (Grant No. XDB16030000), and Shanghai science and Technology Commission (Grant No. 16520721300) References
[1] L.B. Su, Q.G. Wang, H.J. Li, G. Brasse, P. Camy, J.L. Doualan, A. Braud, R. Moncorgé, Y.Y. Zhan, L.H. Zheng, X.B. Qian, and J. Xu, Laser Phys. Lett. 10 (2013) 035804. [2] N.P. Barnes, D.J. Gettemy, L. Esterowitz, and R.E. Allen, IEEE J. Quantum Electron. 23 (1987) 1434. [3] T.T. Basiev, V.B. Sigachev, M.E. Doroshenko, A.G. Papashvili, and V.V. Osiko, Proc. SPIE 2498 (1995) 179.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[4] D.P. Jiang, Y.Y. Zhan, Q. Zhang, F.K. Ma, L.B. Su, F. Tang, X.B. Qian, and J. Xu, Cryst. Eng. Commun. 17 (2015) 7398. [5] Z.P. Qin, G.Q. Xie, J. Ma, W.Y. Ge, P. Yuan, L.J. Qian, L.B. Su, D.P. Jiang, F.K. Ma, Q. Zhang, Y.X. Cao, and J. Xu, Opt. Lett. 39 (2014) 1737. [6] L. Wei, H.N. Han, W.L. Tian, J.X. Liu, Z.H. Wang, Z. Zhu, Y.L. Jia, L.B. Su, J. Xu, and Z.Y. Wei, Appl. Phys. Express 7 (2014) 092704. [7] M. Jelínek, V. Kubeček, L.B. Su, D.P. Jiang, F.K. Ma, Q. Zhang, Y.X. Cao, and J. Xu, Laser Phys. Lett. 11 (2014) 055001. [8] Q. Zhang, L.B. Su, D.P. Jiang, F.K. Ma, Z.P. Qin, G.Q. Xie, J.G. Zheng, Q.H. Deng, W.G. Zheng, L.J. Qian, and J. Xu, Chin. Opt. Lett. 13 (2015) 071402. [9] S.Z. Wang, D.P. Jiang, Q.H. Wu, Z.D. Cao, S.Y. Pang, J.Y. Wang, X.B. Qian, J. Liu, B.C. Mei, and L.B. Su, Laser Phys. 29 (2019) 015801 [10] Y. Guo, S.B. Lu, L.B. Su, C.J. Zhao, H. Zhang, and S.C. Wen, Appl. Opt. 54 (2015) 953. [11] W.F. Krupke, M.D. Shinn, J.E. Marion, J.A. Caird, and S.E. Stokowski, J. Opt. Soc. Am. B 3 (1986) 102. [12] J.L. Doualan, L.B. Su, G. Brasse, A. Benayad, V. Menard, Y.Y. Zhan, A. Braud, P. Camy, J. Xu, and R. Moncorgé, J. Opt. Soc. Am. B 30 (2013) 3018. [13] F.K. Ma, D.P. Jiang, L.B. Su, J.Y. Wang, W. Cai, J. Liu, J.G. Zhang, W.G. Zheng, J. Xu, and Y. Liu, Opt. Lett. 41 (2016) 501. [14] F. Zhang, Y.J. Wu, J. Liu, S.Y. Pang, F.K. Ma, D.P. Jiang, Q.H Wu, and L.B. Su, Optics and Laser Technology. 100 (2018) 294. [15] D. Kopf, F.X. Kartner, and U. Keller, Opt. Lett. 20 (1995) 1169. [16] O.K. Alimov, T.T. Basiev, M.E. Doroshenko, P.P. Fedorov, V. A. Konyushkin, A.N. Nakladov, and V.V. Osiko, Opt. Mater. 34 (2012) 799. [17] F.K. Ma, Q. Zhang, D.P. Jiang, L.B. Su, Y.J. Shao, J.Y. Wang, F. Tang, J. Xu, P. Solarz, W. RybaRomanowski, R. Lisiecki, and B. Macalik, Laser Phys. 24 (2014) 105703. [18] K.S. Bagdasarov, Y.K. Voronko, A.A. Kaminskii, L.V. Krotova, and V.V. Osiko, Stat. Sol. b 12 (1965) 905.
ACCEPTED MANUSCRIPT
Highlights
AC C
EP
TE D
M AN U
SC
RI PT
1. High optical quality Nd, Gd:CaF2 crystals were grown by temperature gradient technique. 2. Compared with Nd:CaF2, the spectral parameters of Nd, Gd:CaF2 were altered in a large scale. 3. The highest slope efficiency of 45% was achieved in 0.5 at.% Nd, 5 at.% Gd:CaF2.