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Er3+:Ca9La(VO4)7
Author’s Accepted Manuscript Growth and Spectral Properties of a Promising Laser Crystal Yb3+/Er3+:Ca9La(VO4)7 Ming Li, Shijia Sun, Lizhen Zhang, Feifei Yuan, Yisheng Huang, Zhoubin Lin www.elsevier.com/locate/jcrysgro
PII: DOI: Reference:
S0022-0248(16)30349-9 http://dx.doi.org/10.1016/j.jcrysgro.2016.07.002 CRYS23447
To appear in: Journal of Crystal Growth Received date: 16 March 2016 Revised date: 4 July 2016 Accepted date: 5 July 2016 Cite this article as: Ming Li, Shijia Sun, Lizhen Zhang, Feifei Yuan, Yisheng Huang and Zhoubin Lin, Growth and Spectral Properties of a Promising Laser Crystal Yb3+/Er3+:Ca9La(VO4)7, Journal of Crystal Growth, http://dx.doi.org/10.1016/j.jcrysgro.2016.07.002 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 galley proof before it is published in its final citable 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.
Growth and Spectral Properties of a Promising Laser Crystal Yb3+/Er3+:Ca9La(VO4)7 Ming Lia,b, Shijia Suna, Lizhen Zhanga, Feifei Yuana, Yisheng Huanga, Zhoubin Lina,c*
a
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research
on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China b
University of Chinese Academy of Science, Beijing 100049, China
c
State Key Laboratory of Structural Chemistry, Fuzhou, 350002, China
*
Corresponding author: [email protected], Tel: +86-59163173415
Abstract A new crystal of Er3+/Yb3+:Ca9La(VO4)7 was grown successfully by the Czochralski method. Its spectral properties were investigated in detail. The crystal has a strong absorption band near 980 nm with a full-width at half-maximum of 33 nm, which means that it is very suitable for InGaAs laser diode pumping. Based on the Judd-Ofelt theory, the intensity parameters and radiative lifetime were obtained. The fluorescence lifetime of the 4I13/2 level of the Er3+ ion is 4.28 ms. The emission cross sections of the Er3+ ion at 1533 nm calculated by the Füchtbauer-Ladenburg method are 0.86 × 10-20 cm2 and 1.08 × 10-20 cm2 for σ- and π- polarization, respectively. The results indicate that Er3+/Yb3+:Ca9La(VO4)7 crystal is a potential 1.5-1.6 μm laser material.
Keywords: A2. Single crystal growth, A2. Czochralski method, B1. Vanadate
1
1. Introduction In recent years, a great deal of attention has been focused on Er3+-doped solid-state laser materials, which are the main source of 1.55 μm lasers. 1.55 μm lasers have wide applications in optical communications, remote sensing, light detecting and so on [1, 2]. So far, many Er3+-doped laser crystals have been found. But, due to their weak absorption around 810 and 980 nm, they are unfavorable for commercial AlGaAs or InGaAs laser diode (LD) pumping, which are the main pumping sources nowadays. In order to overcome this problem, many methods have been used. Among them, the most common and efficient method is by co-doping with Yb3+ ions. Yb3+ ions have a strong and broad absorption band around 980 nm, and can transfer the absorbed energy to the Er3+ ion efficiently. Many Er3+/Yb3+ co-doped crystals, such as LiNbO3 [3], GdAl3(BO3)4 [4], NaGd(MoO4)2 [5], YAl3(BO3)4 [6], YVO4 [7, 8]，SrY4(SiO4)3O [9] and Ca4YO(BO3)3 [10], have been investigated in detail. The results show that there is high efficiency energy transfer from Yb3+ to Er3+ ions. In some of these crystals, 1.55 μm laser output was realized by LD pumping, such as 2.0 W in YAl3(BO3)4 crystal [11], 1.8 W in GdAl3(BO3)4 crystal [12] and 270 mW in Ca4YO(BO3)3 crystal [13, 14]. Although some progress has been made, continuing efficiency improvements are required in 1.5-1.6 μm lasers. Therefore, it is valuable to search for new Er3+/Yb3+ co-doped crystals for 1.5-1.6 μm lasers. Inorganic vanadate materials usually have good physicochemical properties, many of them were used as laser media, the most famous is Nd3+:YVO4, which is an ideal commercial laser crystal. Ca9La(VO4)7 is a new inorganic vanadate compound, it belongs to trigonal system with space group R3c. In the structure, La3+ and Ca2+ ions occupy the same sites in statistical distribution, which leads to a partially disordered structure [15]. When rare earth active ions are doped into the crystal, this disordered structure will result in inhomogeneous broadening of the absorption bands, which are favorable to LD
2
pumping. Thus a new laser material fit for LD pumping may be obtained. The growth and spectral properties of Nd3+-doped Ca9La(VO4)7 crystal have been performed, and results showed that it is a good new laser crystal [16]. Therefore, when Er3+ and Yb3+ ions are co-doped into this crystal, a new crystal for 1.5-1.6 μm lasers is likely to be obtained. To our knowledge, there is no previous report of Er3+/Yb3+ co-doped Ca9La(VO4)7 crystals. It is reported that the usually used molar ratio range of Yb3+:Er3+ is (3-10):1 for Er3+/Yb3+-codoped materials [17, 18]. The Yb→Er energy transfer process occurs efficiently only when the Yb3+ concentration exceeds 3 at.%, and it usually reaches the upper limit when the Yb3+ concentration exceeds 10 at.% [19]. When the concentration of Yb3+ is 10 times higher than that of Er3+, up-conversion emission is usually observed [20], which is adverse to the required 1.5-1.6 μm laser emission. Besides, a high dopant concentration increases the risk of reabsorption, and affects the quality of the crystal significantly. Therefore, a doping concentration of 2 at.% Er3+ and 14 at.% Yb3+ was adopted for Ca9La(VO4)7 crystals. In this paper, the growth and polarized spectral properties of Er3+/Yb3+:Ca9La(VO4)7 crystals are reported.
2. Experimental procedure Er3+/Yb3+:Ca9La(VO4)7 melts congruently and can be grown by the Czochralski method. The raw materials used for crystal growth were synthesized by solid-state reaction. The chemicals used are CaCO3 (99.9%), La2O3 (99.99%), Yb2O3 (99.99%), Er2O3 (99.99%) and V2O5 (99.9%). These chemicals were weighed, mixed and ground well in an agate mortar, then extruded to form pellets. Then the pellets were sintered in air at 1000 °C for 48 h in a platinum crucible. These processes were repeated three times to assure the homogeneity of the materials. After that, X-ray powder diffraction (XRD) showed that the sample reacted completely to the single phase compound Er3+/Yb3+:Ca9La(VO4)7 .
3
The crystal was grown in an iridium crucible in a N2 atmosphere, in a 25 KHz mid-frequency induction furnace (DJL-400). The pulling rate is 1.0 mm/h and the rotation rate is 10 rpm. A [001] oriented Ca9La(VO4)7 crystal was used as the seed. The temperature-controller is a Eurotherm 818 with a control accuracy of 0.1 oC. At the end of the growth, the grown crystal was drawn out of the melt surface slowly and cooled to room temperature at a rate of 20 oC/h. The concentrations of Er3+ and Yb3+ ions in the grown crystal were measured by inductively coupled plasma atomic emission spectrometry. A piece of sample cut from the top of the grown crystal was used for the measurement. The X-ray powder diffraction pattern of the grown Er3+/Yb3+:Ca9La(VO4)7 crystal was measured on a D/max-rA diffractometer with CuKα radiation at a scan width of 0.02o in the range of 10-80o at room temperature. A wafer with dimensions of 6.0 × 6.0 × 1.2 mm3 along the optic axis (c- axis) was cut from the grown crystal, polished and used for spectral measurements. The polarized absorption spectra were recorded by a Perkin-Elmer UV-Vis-NIR spectrometer (Lambda-900) with the polarized incident light parallel (π- polarization) and perpendicular (σ- polarization) to the optic axis. The fluorescence spectra and lifetime were measured by an Edinburgh Instrument FLS920 spectrophotometer with a continuous Xe-flash lamp. All the measurements were carried out at room temperature.
3. Results and discussion 3.1. Crystal growth The grown Er3+/Yb3+:Ca9La(VO4)7 crystal with dimensions of 25 × 40 mm3 is shown in Fig. 1, the inset is the slice cut from the grown crystal before and after annealing. The crystal is free of cracks, which means that the Czochralski method is suitable for growing Er3+/Yb3+:Ca9La(VO4)7 crystals. After annealing at 1200 ºC for 24 h in air, the crystal shows a faint yellow color and improved transparency, which shows
4
that annealing is helpful to improve the quality of this crystal. The XRD patterns of the as-grown Er3+/Yb3+:Ca9La(VO4)7 and an undoped Ca9La(VO4)7 crystal (ICSD#: 85104) are shown together in Fig. 2. The diffraction peaks of the grown crystal match those of Ca9La(VO4)7 crystals very well, which means that the grown Er3+/Yb3+:Ca9La(VO4)7 crystal is isostructural to Ca9Y(VO4)7. The concentrations of Er3+ and Yb3+ ions in the grown crystal were measured to be 2.79 at% and 13.24 at%, respectively. The initial concentration of Er3+ and Yb3+ ions was 2.00 at% and 14.00 at%, respectively. Thus the segregation coefficients of Er3+ and Yb3+ ions in the grown crystal were calculated to be 1.39 and 0.95, respectively. The segregation coefficients of Er3+ and Yb3+ ions are sufficiently large that they can substitute easily for La3+ ions.
3.2. Spectroscopic properties The polarized absorption spectra of the Er3+/Yb3+:Ca9La(VO4)7 crystal are shown in Fig. 3. All the absorption bands except the band around 980 nm, which is caused by both the Yb3+ and Er3+ ions, are attributed to the transitions of the Er3+ ion. The strong and wide absorption band in the 880-1040 nm range corresponds to the 4I15/2→4I11/2 transition of the Er3+ ion and the 2F7/2→2F5/2 transition of the Yb3+ ion. The absorption cross sections at 980 nm are 1.15 × 10-20 cm2 and 0.76 × 10-20 cm2 for the σ- and πpolarizations, respectively with full-width at half-maximum (FWHM) of 33 nm. The broad and strong absorption band coincides well with the emission wavelength of commercial InGaAs LDs, which indicates that Er3+/Yb3+:Ca9La(VO4)7 crystal is very suitable for InGaAs LD pumping. In order to obtain the Er3+/Yb3+:Ca9La(VO4)7 crystal’s detailed spectroscopic parameters, such as intensity parameters, radiative lifetime, transition probabilities and branching ratios of the fluorescence transitions, Judd-Ofelt (J-O) theory was used to analyze the absorption spectra [21, 22]. J-O theory is widely used in the analysis of the
5
spectroscopic properties of rare earth elements both in glasses and crystals. The calculating procedures follow those of Ref. [23]. The calculated results are listed in Table 1. The radiative lifetime is 3.09 ms.
Table 1 Measured and calculated line strengths and oscillator strength parameters Excited states 4
I13/2 F9/2 2 H11/2 4 F7/2 4 G11/2 4
fexp (σ) fcal (σ) fexp (π) fcal (π) (10-6 cm2) (10-6 cm2) (10-6 cm2) (10-6 cm2) 1533 1.99 1.89 2.03 2.10 652 3.81 3.68 7.39 7.47 523 14.92 15.53 40.32 41.47 489 2.16 3.02 4.56 3.98 379 28.15 27.51 74.65 73.41 Ω2=10.92×10-20 cm2, Ω4=3.54×10-20 cm2, Ω6=3.75×10-20 cm2, RMS=0.98×10-6 cm2
(nm)
The polarized fluorescence spectra in the wavelength range 1400-1650 nm excited at 980 nm are presented in Fig. 4. The strong and broad fluorescence band corresponds to the 4I13/2→4I15/2 transition of the Er3+ ion. The FWHM of the emission bands are 40.8 nm for the π- polarization and 53.7 nm for the σ- polarization. The fluorescence decay curve of the 4I13/2 level of the Er3+ ion is shown in Fig. 5. The fluorescence lifetime is 4.28 ms. The
stimulated
emission
cross-sections
of
the
Er3+ ion,calculated
by
the
Füchtbauer-Ladenburg (F-L) formula [24], are 0.86 × 10-20 cm2 and 1.08 × 10-20 cm2 at 1533 nm for the σ and π- polarizations, respectively. The gain cross section σG(λ) is usually used to evaluate the possibility of laser output, and can be obtained from the calculated emission cross section and absorption cross section, according to the following equation [25]:
G ( ) em ( ) (1 ) abs ( )
(1)
where β is the population inversion ratio of the Er3+ ion, which is defined as ratio of the number of Er3+ ions in the 4I13/2 state to the total number of Er3+ ion. The gain cross sections as a function of β are shown in Fig. 6. Laser output is possible only when the gain cross section is greater than zero. From Fig. 6, it can be seen that when β = 0.2, laser
6
emission can be realized in a tuning range of 1575-1623 nm for π- polarization and 1578-1617 nm for σ- polarization. The spectral parameters of Er3+/Yb3+:Ca9La(VO4)7 crystals and some other Er3+/Yb3+ co-doped crystals are listed in Table 2. From these, it can be seen that the Er3+/Yb3+:Ca9La(VO4)7 crystal possesses a large FWHM of the absorption band, a big emission cross section and a long fluorescence lifetime similar to the other mature Er3+/Yb3+ co-doped crystals. The broad absorption band means that it is suitable for LD pumping. The big emission cross section and long fluorescence lifetime mean that the crystal will have a low laser threshold, and laser oscillation likely to be favorable in Er3+/Yb3+:Ca9La(VO4)7 crystal. Table 2 Spectral parameters of Er3+/Yb3+ co-doped Ca9La(VO4)7 and other crystals Nc (at.%)
α (cm-1)
FWHMa (nm)
Ca9La(VO4)7 [this work] GdVO4 [26] YAl3(BO3)4 [6,10] GdAl3(BO3)4 [12]
2.79 (Er) 13.24 (Yb) 2.4(Er) 22.3(Yb) 1.5(Er) 11(Yb) 1.1(Er)
1.73(π) 2.61(σ) 40 (π) 37.5 (σ) 16.6
33 (π) 33 (σ) 36 (π) 48 (σ) 27
4.58(π)
28(π)
20.7(Yb)
38.90(σ)
20(σ)
2.0(σ)
Sr3Lu2(BO3)4 [27]
0.87(Er) 18.63(Yb)
10
Ca4YO(BO3)3 [28]
2.0(Er) 20.0 (Yb)
33.5(E//X) 38.1(E//Y) 30.1(E//Z) 9.2(E//X) 8.0(E//Y) 9.9(E//Z)
1.63(E//X) 1.87(E//Y) 1.34(E//Z) 1.2(E//X) 2.1(E//Y) 1.9(E//Z)
Crystals
3
σem -20
τrad (ms)
τf (ms)
1.08 (π) 0.86 (σ) 0.411
3.09
4.28
5.9
4.5
1.5 (σ) 0.7(π) 1.0(π)
3.9
0.33
3.72
0.30
(10
2
cm )
4.48
0.67
6.0
1.26
4. Conclusions A Er3+/Yb3+:Ca9La(VO4)7 crystal with dimensions of 25 × 40 mm3 was grown successfully by the Czochralski method. The crystal has a strong broad absorption band at 980 nm with a FWHM of 33 nm that matches the emission wavelength of InGaAs LD
7
very well, which means that this crystal has strong potential for commercial InGaAs LD pumping. Based on J-O theory, the intensity parameters Ω2 = 10.92 × 10-20 cm2, Ω4 = 3.54 × 10-20 cm2, Ω6 = 3.75 × 10-20 cm2 and the radiative lifetime 3.09 ms were obtained. The emission bands near 1533 nm have a FWHM of 40.8 nm for the π- polarization and 53.7 nm for the σ- polarization. The maximum emission cross sections deduced by the F-L method at 1533 nm are 0.86 × 10-20 cm2 and 1.08 × 10-20 cm2 for σ- and π- polarizations, respectively. The fluorescence lifetime of the 4I13/2 level of Er3+ is 4.28 ms. The gain cross-sections σG(λ) show that when β = 0.2, laser emission can be realized in a tuning range of 1575-1623 nm for the π- polarization and 1578-1617 nm for the σ- polarization. In conclusion, the broad absorption and emission bands, large emission cross sections and fluorescence lifetime indicate that Er3+/Yb3+:Ca9La(VO4)7 crystals are a potential laser material for 1.5-1.6 μm lasers.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 61275177, 61475158 and 51302260) and the National Natural Science Foundation of Fujian Province (Nos. 2014H0052, 2015J01232 and 2016J01328).
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[24] B. Aull, H. Jenssen, Vibronic interactions in Nd-YAG resulting in nonreciprocity of absorption and stimulated-emission cross-sections, IEEE J. Quant. Electron 18 (1982) 925-930. [25] K. Ohta, H. Saito, M. Obara, Spectroscopic characterization of Tm3+-YVO4 crystal as an efficient diode pumped laser source near 2000-nm, J. Appl. Phys. 73 (1993) 3149-3152. [26] N.F. Zhuang, X.L. Hu, S.K. Gao, B. Zhao, J.L. Chen, J.Z. Chen, Spectral properties and energy transfer of Yb, Er :GdVO4 crystal, Appl. Phys. B 82 (2006) 607-613. [27] J.H. Huang, Y.J. Chen, X.H. Gong, Y.F. Lin, Z.D. Luo, Y.D. Huang, Spectral and laser properties of Er:Yb:Sr3Lu2(BO3)4 crystal at 1.5-1.6μm, Opt. Mater. Express 3 (2013) 1885-1892. [28] P. Wang, J.M. Dawes, P. Burns, J.A. Piper, H.J. Zhang, L. Zhu, X.L. Meng, Diode-pumped cw tunable Er3+: Yb3+: YCOB laser at 1.5-1.6 μm, Opt. Mater. 19 (2002) 383–387.
Fig. 1. Grown Er3+/Yb3+:Ca9La(VO4)7 crystal Fig. 2. XRD patterns of Er3+/Yb3+:Ca9La(VO4)7 and Ca9La(VO4)7 crystals Fig. 3. Polarized absorption spectra of a Er3+/Yb3+:Ca9La(VO4)7 crystal Fig. 4. Emission cross sections of a Er3+/Yb3+:Ca9La(VO4)7 crystal in the range 1400-1650 nm excited by 980 nm radiation Fig. 5. Fluorescence decay curve of the
4
I13/2 level of the Er3+ ion in a
Er3+/Yb3+:Ca9La(VO4)7 crystal Fig. 6. Gain cross sections of a Er3+/Yb3+:Ca9La(VO4)7 crystal
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Highlights
Er3+/Yb3+:Ca9La(VO4)7 crystal was grown successfully by the Czochralski method.
Er3+/Yb3+:Ca9La(VO4)7 crystal has large absorption and emission cross sections.
Laser oscillation likely to be favorable in Er3+/Yb3+:Ca9La(VO4)7 crystal.
Er3+/Yb3+:Ca9La(VO4)7 is a promising material for 1.55 μm lasers.
Fig. 1
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Fig. 2
Fig. 3
13
Fig. 4
14
Fig. 5
15
Fig. 6
16
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PII: DOI: Reference:
S0022-0248(16)30349-9 http://dx.doi.org/10.1016/j.jcrysgro.2016.07.002 CRYS23447
To appear in: Journal of Crystal Growth Received date: 16 March 2016 Revised date: 4 July 2016 Accepted date: 5 July 2016 Cite this article as: Ming Li, Shijia Sun, Lizhen Zhang, Feifei Yuan, Yisheng Huang and Zhoubin Lin, Growth and Spectral Properties of a Promising Laser Crystal Yb3+/Er3+:Ca9La(VO4)7, Journal of Crystal Growth, http://dx.doi.org/10.1016/j.jcrysgro.2016.07.002 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 galley proof before it is published in its final citable 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.
Growth and Spectral Properties of a Promising Laser Crystal Yb3+/Er3+:Ca9La(VO4)7 Ming Lia,b, Shijia Suna, Lizhen Zhanga, Feifei Yuana, Yisheng Huanga, Zhoubin Lina,c*
a
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research
on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China b
University of Chinese Academy of Science, Beijing 100049, China
c
State Key Laboratory of Structural Chemistry, Fuzhou, 350002, China
*
Corresponding author: [email protected], Tel: +86-59163173415
Abstract A new crystal of Er3+/Yb3+:Ca9La(VO4)7 was grown successfully by the Czochralski method. Its spectral properties were investigated in detail. The crystal has a strong absorption band near 980 nm with a full-width at half-maximum of 33 nm, which means that it is very suitable for InGaAs laser diode pumping. Based on the Judd-Ofelt theory, the intensity parameters and radiative lifetime were obtained. The fluorescence lifetime of the 4I13/2 level of the Er3+ ion is 4.28 ms. The emission cross sections of the Er3+ ion at 1533 nm calculated by the Füchtbauer-Ladenburg method are 0.86 × 10-20 cm2 and 1.08 × 10-20 cm2 for σ- and π- polarization, respectively. The results indicate that Er3+/Yb3+:Ca9La(VO4)7 crystal is a potential 1.5-1.6 μm laser material.
Keywords: A2. Single crystal growth, A2. Czochralski method, B1. Vanadate
1
1. Introduction In recent years, a great deal of attention has been focused on Er3+-doped solid-state laser materials, which are the main source of 1.55 μm lasers. 1.55 μm lasers have wide applications in optical communications, remote sensing, light detecting and so on [1, 2]. So far, many Er3+-doped laser crystals have been found. But, due to their weak absorption around 810 and 980 nm, they are unfavorable for commercial AlGaAs or InGaAs laser diode (LD) pumping, which are the main pumping sources nowadays. In order to overcome this problem, many methods have been used. Among them, the most common and efficient method is by co-doping with Yb3+ ions. Yb3+ ions have a strong and broad absorption band around 980 nm, and can transfer the absorbed energy to the Er3+ ion efficiently. Many Er3+/Yb3+ co-doped crystals, such as LiNbO3 [3], GdAl3(BO3)4 [4], NaGd(MoO4)2 [5], YAl3(BO3)4 [6], YVO4 [7, 8]，SrY4(SiO4)3O [9] and Ca4YO(BO3)3 [10], have been investigated in detail. The results show that there is high efficiency energy transfer from Yb3+ to Er3+ ions. In some of these crystals, 1.55 μm laser output was realized by LD pumping, such as 2.0 W in YAl3(BO3)4 crystal [11], 1.8 W in GdAl3(BO3)4 crystal [12] and 270 mW in Ca4YO(BO3)3 crystal [13, 14]. Although some progress has been made, continuing efficiency improvements are required in 1.5-1.6 μm lasers. Therefore, it is valuable to search for new Er3+/Yb3+ co-doped crystals for 1.5-1.6 μm lasers. Inorganic vanadate materials usually have good physicochemical properties, many of them were used as laser media, the most famous is Nd3+:YVO4, which is an ideal commercial laser crystal. Ca9La(VO4)7 is a new inorganic vanadate compound, it belongs to trigonal system with space group R3c. In the structure, La3+ and Ca2+ ions occupy the same sites in statistical distribution, which leads to a partially disordered structure [15]. When rare earth active ions are doped into the crystal, this disordered structure will result in inhomogeneous broadening of the absorption bands, which are favorable to LD
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pumping. Thus a new laser material fit for LD pumping may be obtained. The growth and spectral properties of Nd3+-doped Ca9La(VO4)7 crystal have been performed, and results showed that it is a good new laser crystal [16]. Therefore, when Er3+ and Yb3+ ions are co-doped into this crystal, a new crystal for 1.5-1.6 μm lasers is likely to be obtained. To our knowledge, there is no previous report of Er3+/Yb3+ co-doped Ca9La(VO4)7 crystals. It is reported that the usually used molar ratio range of Yb3+:Er3+ is (3-10):1 for Er3+/Yb3+-codoped materials [17, 18]. The Yb→Er energy transfer process occurs efficiently only when the Yb3+ concentration exceeds 3 at.%, and it usually reaches the upper limit when the Yb3+ concentration exceeds 10 at.% [19]. When the concentration of Yb3+ is 10 times higher than that of Er3+, up-conversion emission is usually observed [20], which is adverse to the required 1.5-1.6 μm laser emission. Besides, a high dopant concentration increases the risk of reabsorption, and affects the quality of the crystal significantly. Therefore, a doping concentration of 2 at.% Er3+ and 14 at.% Yb3+ was adopted for Ca9La(VO4)7 crystals. In this paper, the growth and polarized spectral properties of Er3+/Yb3+:Ca9La(VO4)7 crystals are reported.
2. Experimental procedure Er3+/Yb3+:Ca9La(VO4)7 melts congruently and can be grown by the Czochralski method. The raw materials used for crystal growth were synthesized by solid-state reaction. The chemicals used are CaCO3 (99.9%), La2O3 (99.99%), Yb2O3 (99.99%), Er2O3 (99.99%) and V2O5 (99.9%). These chemicals were weighed, mixed and ground well in an agate mortar, then extruded to form pellets. Then the pellets were sintered in air at 1000 °C for 48 h in a platinum crucible. These processes were repeated three times to assure the homogeneity of the materials. After that, X-ray powder diffraction (XRD) showed that the sample reacted completely to the single phase compound Er3+/Yb3+:Ca9La(VO4)7 .
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The crystal was grown in an iridium crucible in a N2 atmosphere, in a 25 KHz mid-frequency induction furnace (DJL-400). The pulling rate is 1.0 mm/h and the rotation rate is 10 rpm. A [001] oriented Ca9La(VO4)7 crystal was used as the seed. The temperature-controller is a Eurotherm 818 with a control accuracy of 0.1 oC. At the end of the growth, the grown crystal was drawn out of the melt surface slowly and cooled to room temperature at a rate of 20 oC/h. The concentrations of Er3+ and Yb3+ ions in the grown crystal were measured by inductively coupled plasma atomic emission spectrometry. A piece of sample cut from the top of the grown crystal was used for the measurement. The X-ray powder diffraction pattern of the grown Er3+/Yb3+:Ca9La(VO4)7 crystal was measured on a D/max-rA diffractometer with CuKα radiation at a scan width of 0.02o in the range of 10-80o at room temperature. A wafer with dimensions of 6.0 × 6.0 × 1.2 mm3 along the optic axis (c- axis) was cut from the grown crystal, polished and used for spectral measurements. The polarized absorption spectra were recorded by a Perkin-Elmer UV-Vis-NIR spectrometer (Lambda-900) with the polarized incident light parallel (π- polarization) and perpendicular (σ- polarization) to the optic axis. The fluorescence spectra and lifetime were measured by an Edinburgh Instrument FLS920 spectrophotometer with a continuous Xe-flash lamp. All the measurements were carried out at room temperature.
3. Results and discussion 3.1. Crystal growth The grown Er3+/Yb3+:Ca9La(VO4)7 crystal with dimensions of 25 × 40 mm3 is shown in Fig. 1, the inset is the slice cut from the grown crystal before and after annealing. The crystal is free of cracks, which means that the Czochralski method is suitable for growing Er3+/Yb3+:Ca9La(VO4)7 crystals. After annealing at 1200 ºC for 24 h in air, the crystal shows a faint yellow color and improved transparency, which shows
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that annealing is helpful to improve the quality of this crystal. The XRD patterns of the as-grown Er3+/Yb3+:Ca9La(VO4)7 and an undoped Ca9La(VO4)7 crystal (ICSD#: 85104) are shown together in Fig. 2. The diffraction peaks of the grown crystal match those of Ca9La(VO4)7 crystals very well, which means that the grown Er3+/Yb3+:Ca9La(VO4)7 crystal is isostructural to Ca9Y(VO4)7. The concentrations of Er3+ and Yb3+ ions in the grown crystal were measured to be 2.79 at% and 13.24 at%, respectively. The initial concentration of Er3+ and Yb3+ ions was 2.00 at% and 14.00 at%, respectively. Thus the segregation coefficients of Er3+ and Yb3+ ions in the grown crystal were calculated to be 1.39 and 0.95, respectively. The segregation coefficients of Er3+ and Yb3+ ions are sufficiently large that they can substitute easily for La3+ ions.
3.2. Spectroscopic properties The polarized absorption spectra of the Er3+/Yb3+:Ca9La(VO4)7 crystal are shown in Fig. 3. All the absorption bands except the band around 980 nm, which is caused by both the Yb3+ and Er3+ ions, are attributed to the transitions of the Er3+ ion. The strong and wide absorption band in the 880-1040 nm range corresponds to the 4I15/2→4I11/2 transition of the Er3+ ion and the 2F7/2→2F5/2 transition of the Yb3+ ion. The absorption cross sections at 980 nm are 1.15 × 10-20 cm2 and 0.76 × 10-20 cm2 for the σ- and πpolarizations, respectively with full-width at half-maximum (FWHM) of 33 nm. The broad and strong absorption band coincides well with the emission wavelength of commercial InGaAs LDs, which indicates that Er3+/Yb3+:Ca9La(VO4)7 crystal is very suitable for InGaAs LD pumping. In order to obtain the Er3+/Yb3+:Ca9La(VO4)7 crystal’s detailed spectroscopic parameters, such as intensity parameters, radiative lifetime, transition probabilities and branching ratios of the fluorescence transitions, Judd-Ofelt (J-O) theory was used to analyze the absorption spectra [21, 22]. J-O theory is widely used in the analysis of the
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spectroscopic properties of rare earth elements both in glasses and crystals. The calculating procedures follow those of Ref. [23]. The calculated results are listed in Table 1. The radiative lifetime is 3.09 ms.
Table 1 Measured and calculated line strengths and oscillator strength parameters Excited states 4
I13/2 F9/2 2 H11/2 4 F7/2 4 G11/2 4
fexp (σ) fcal (σ) fexp (π) fcal (π) (10-6 cm2) (10-6 cm2) (10-6 cm2) (10-6 cm2) 1533 1.99 1.89 2.03 2.10 652 3.81 3.68 7.39 7.47 523 14.92 15.53 40.32 41.47 489 2.16 3.02 4.56 3.98 379 28.15 27.51 74.65 73.41 Ω2=10.92×10-20 cm2, Ω4=3.54×10-20 cm2, Ω6=3.75×10-20 cm2, RMS=0.98×10-6 cm2
(nm)
The polarized fluorescence spectra in the wavelength range 1400-1650 nm excited at 980 nm are presented in Fig. 4. The strong and broad fluorescence band corresponds to the 4I13/2→4I15/2 transition of the Er3+ ion. The FWHM of the emission bands are 40.8 nm for the π- polarization and 53.7 nm for the σ- polarization. The fluorescence decay curve of the 4I13/2 level of the Er3+ ion is shown in Fig. 5. The fluorescence lifetime is 4.28 ms. The
stimulated
emission
cross-sections
of
the
Er3+ ion,calculated
by
the
Füchtbauer-Ladenburg (F-L) formula [24], are 0.86 × 10-20 cm2 and 1.08 × 10-20 cm2 at 1533 nm for the σ and π- polarizations, respectively. The gain cross section σG(λ) is usually used to evaluate the possibility of laser output, and can be obtained from the calculated emission cross section and absorption cross section, according to the following equation [25]:
G ( ) em ( ) (1 ) abs ( )
(1)
where β is the population inversion ratio of the Er3+ ion, which is defined as ratio of the number of Er3+ ions in the 4I13/2 state to the total number of Er3+ ion. The gain cross sections as a function of β are shown in Fig. 6. Laser output is possible only when the gain cross section is greater than zero. From Fig. 6, it can be seen that when β = 0.2, laser
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emission can be realized in a tuning range of 1575-1623 nm for π- polarization and 1578-1617 nm for σ- polarization. The spectral parameters of Er3+/Yb3+:Ca9La(VO4)7 crystals and some other Er3+/Yb3+ co-doped crystals are listed in Table 2. From these, it can be seen that the Er3+/Yb3+:Ca9La(VO4)7 crystal possesses a large FWHM of the absorption band, a big emission cross section and a long fluorescence lifetime similar to the other mature Er3+/Yb3+ co-doped crystals. The broad absorption band means that it is suitable for LD pumping. The big emission cross section and long fluorescence lifetime mean that the crystal will have a low laser threshold, and laser oscillation likely to be favorable in Er3+/Yb3+:Ca9La(VO4)7 crystal. Table 2 Spectral parameters of Er3+/Yb3+ co-doped Ca9La(VO4)7 and other crystals Nc (at.%)
α (cm-1)
FWHMa (nm)
Ca9La(VO4)7 [this work] GdVO4 [26] YAl3(BO3)4 [6,10] GdAl3(BO3)4 [12]
2.79 (Er) 13.24 (Yb) 2.4(Er) 22.3(Yb) 1.5(Er) 11(Yb) 1.1(Er)
1.73(π) 2.61(σ) 40 (π) 37.5 (σ) 16.6
33 (π) 33 (σ) 36 (π) 48 (σ) 27
4.58(π)
28(π)
20.7(Yb)
38.90(σ)
20(σ)
2.0(σ)
Sr3Lu2(BO3)4 [27]
0.87(Er) 18.63(Yb)
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Ca4YO(BO3)3 [28]
2.0(Er) 20.0 (Yb)
33.5(E//X) 38.1(E//Y) 30.1(E//Z) 9.2(E//X) 8.0(E//Y) 9.9(E//Z)
1.63(E//X) 1.87(E//Y) 1.34(E//Z) 1.2(E//X) 2.1(E//Y) 1.9(E//Z)
Crystals
3
σem -20
τrad (ms)
τf (ms)
1.08 (π) 0.86 (σ) 0.411
3.09
4.28
5.9
4.5
1.5 (σ) 0.7(π) 1.0(π)
3.9
0.33
3.72
0.30
(10
2
cm )
4.48
0.67
6.0
1.26
4. Conclusions A Er3+/Yb3+:Ca9La(VO4)7 crystal with dimensions of 25 × 40 mm3 was grown successfully by the Czochralski method. The crystal has a strong broad absorption band at 980 nm with a FWHM of 33 nm that matches the emission wavelength of InGaAs LD
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very well, which means that this crystal has strong potential for commercial InGaAs LD pumping. Based on J-O theory, the intensity parameters Ω2 = 10.92 × 10-20 cm2, Ω4 = 3.54 × 10-20 cm2, Ω6 = 3.75 × 10-20 cm2 and the radiative lifetime 3.09 ms were obtained. The emission bands near 1533 nm have a FWHM of 40.8 nm for the π- polarization and 53.7 nm for the σ- polarization. The maximum emission cross sections deduced by the F-L method at 1533 nm are 0.86 × 10-20 cm2 and 1.08 × 10-20 cm2 for σ- and π- polarizations, respectively. The fluorescence lifetime of the 4I13/2 level of Er3+ is 4.28 ms. The gain cross-sections σG(λ) show that when β = 0.2, laser emission can be realized in a tuning range of 1575-1623 nm for the π- polarization and 1578-1617 nm for the σ- polarization. In conclusion, the broad absorption and emission bands, large emission cross sections and fluorescence lifetime indicate that Er3+/Yb3+:Ca9La(VO4)7 crystals are a potential laser material for 1.5-1.6 μm lasers.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 61275177, 61475158 and 51302260) and the National Natural Science Foundation of Fujian Province (Nos. 2014H0052, 2015J01232 and 2016J01328).
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Fig. 1. Grown Er3+/Yb3+:Ca9La(VO4)7 crystal Fig. 2. XRD patterns of Er3+/Yb3+:Ca9La(VO4)7 and Ca9La(VO4)7 crystals Fig. 3. Polarized absorption spectra of a Er3+/Yb3+:Ca9La(VO4)7 crystal Fig. 4. Emission cross sections of a Er3+/Yb3+:Ca9La(VO4)7 crystal in the range 1400-1650 nm excited by 980 nm radiation Fig. 5. Fluorescence decay curve of the
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I13/2 level of the Er3+ ion in a
Er3+/Yb3+:Ca9La(VO4)7 crystal Fig. 6. Gain cross sections of a Er3+/Yb3+:Ca9La(VO4)7 crystal
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Highlights
Er3+/Yb3+:Ca9La(VO4)7 crystal was grown successfully by the Czochralski method.
Er3+/Yb3+:Ca9La(VO4)7 crystal has large absorption and emission cross sections.
Laser oscillation likely to be favorable in Er3+/Yb3+:Ca9La(VO4)7 crystal.
Er3+/Yb3+:Ca9La(VO4)7 is a promising material for 1.55 μm lasers.
Fig. 1
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Fig. 2
Fig. 3
13
Fig. 4
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Fig. 5
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Fig. 6
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