- Email: [email protected]
The bulk crystal growth, spectral analyses and rate equation model of Yb,Er:SrLaGa3O7 crystal
Author’s Accepted Manuscript The bulk crystal growth, spectral analyses and rate equation model of Yb,Er:SrLaGa3O7 crystal Baotong Zhang, Yan Wang, Jianfu Li, Yunyun Liu, Zhaojie Zhu, Zhenyu You, Chaoyang Tu www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(17)30080-7 http://dx.doi.org/10.1016/j.jlumin.2017.05.034 LUMIN14758
To appear in: Journal of Luminescence Received date: 16 January 2017 Revised date: 10 May 2017 Accepted date: 10 May 2017 Cite this article as: Baotong Zhang, Yan Wang, Jianfu Li, Yunyun Liu, Zhaojie Zhu, Zhenyu You and Chaoyang Tu, The bulk crystal growth, spectral analyses and rate equation model of Yb,Er:SrLaGa3O7 crystal, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2017.05.034 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.
The bulk crystal growth, spectral analyses and rate equation model of Yb,Er:SrLaGa3O7 crystal Baotong Zhang1,2, Yan Wang1,*, Jianfu Li1, Yunyun Liu1,3, Zhaojie Zhu1, Zhenyu You1, and Chaoyang Tu1,* 1
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou City, Fujian Province, 350002 P. R. China 2
College of Materials Science and Engineering, Fujian Normal University, Fuzhou City, Fujian Province, 350007 P. R. China 3
University of Chinese Academy of Sciences, Beijing 100039, P.R.China *Corresponding authors: [email protected] & [email protected]
ABSTRACT: Yb3+ and Er3+ co-doped SrLaGa3O7 (abbr. as Yb,Er:SLGO) bulk crystal with dimensions of Φ 20×30 mm was grown successfully by the Czochralski (Cz) method for the first time. The absorption and fluorescence spectra as well as the fluorescence decay curves of Yb,Er:SLGO crystal were measured and investigated. The emission cross-section and fluorescence lifetimes have been obtained at the ~2.7 µm wavelength. Based on Yb3+-Er3+ energy level diagrams, the rate equation model was built and discussed. It is found that 2.7~3.0 µm emission has been improved in Yb,Er:SLGO crystal under excitation of 980 nm, owing to the sensitization of the co-dopant Yb3+ and the resonant energy transfer from Yb3+ to Er3+. A noteworthy result is that Yb,Er:SLGO crystal is an excellent mid-infrared laser gain material. Keywords: Yb,Er:SLGO single crystal; Crystal growth; Mid-infrared spectroscopy; Rate equation model
1. Introduction During the last decades, 2.7~3.0 μm mid-infrared (MIR) solid-state lasers based on the 4I11/2→4I13/2 transition of Er3+ have drawn a lot of attention owing to their applications in the fields of dentistry, lasers surgery, remote sensors, environment atmosphere
exploration and military fields etc[1-3]. In addition, 2.7~3.0 μm laser is also attractive as a pump source for far-infrared waveband optical parametric oscillation (OPO) or optical parametric generation (OPG) laser system[4,5]. Lasing at 2.7~3.0 μm can be obtained through the 4I11/2→4I13/2 transition of Er3+, and several Er3+ activated crystals with good laser performances have been reported, such as Er:GGG[6-8], Er:YSGG[7-9], Er:YAG[8,9], Er,Pr:GYSGG[10], Er:CaF2[11-13], Er:SrF2[13] etc. ABC3O7 (A=Ca, Sr, Ba; B=La, Gd; and C=Ga, Al) is melilite structure with tetragonal system and P 4 21m space group[14-19]. In this work, SrLaGa3O7(abbr. as SLGO) crystal as one member of ABC3O7 is chosen as the host matrix. Its lattice parameters are a=8.06Å, c=5.34Å and its density is 5.251g/cm3[19]. Compared with other popular gallate crystals such as GGG, YSGG crystals, there are at least two advantages in it: one is that SLGO crystal has a relative low phonon energy (of ~750 cm−1) [20], which is favorable for reducing the multiphonon relaxation rate of the Er3+: 4
I11/2→4I13/2 transition and thus beneficial to decrease the laser threshold and improve the
2.7~3.0 μm laser output power and efficiency[21]; the other is its disordered structure in which Sr2+ and La3+ ions are randomly distributed in a 1:1 ratio, once Er3+ ions are doped into the host, they can substitute both for Sr2+ and La3+ ions, producing a large inhomogeneous broadening of absorption and emission spectra and it’s very favorable for achievements of tunable or ultra fast lasers [22]. More importantly, SLGO crystal has a relatively lower melting point (about 1588℃)[19], which makes it easier to obtain large-sized crystals with higher optical quality by using the Czochralski (CZ) method. In our previous job, Er3+ doped SLGO crystal has been reported as 2.7~3.0 µm laser gain media [19]. However, the absorption is somewhat weak whitin the ~980 nm pumping waveband, and thus makes the mid-infrared emissions not strong enough. Since the energy levels of Yb3+ match with Er3+ ions very well, Yb3+ could be introduced into Er3+ activated gallate crystals to improve the ~980 nm absorption, and then transfers energy from Yb3+ to Er3+, finally enhances the 2.7~3.0 μm emission[23-25]. In this work, we report on the bulk growth, spectral properties and the rate equation model of Yb,Er:SLGO crystal, with focus on the potential application of this crystal for lasing amplification in the wavelength range 2.7~3.0 μm.
2. Experimental details The crystal growth of SrLaGa3O7 doped with 5at% Yb and 20at% Er was carried out in DJL-400 furnace (NCIREO, China). The adopted raw materials were 99.9% purity SrCO3, 99.99% purity Yb2O3, Ga2O3, La2O3 and Er2O3 commercial powders, which were dried before experiment. They were weighed out according to the compositional formula, with 1~2wt% extra Ga2O3() to compensate Ga loss owing to its evaporation. The raw materials were mixed, ground carefully, and compressed into cylinder tablets under a pressure of 20 Mpa. Then the tablets were heated up in a Muffle furnace till 1100 °C and thermal insulation for 10 h to completely decompose the SrCO3, and then heated to 1200℃ and maintaining at this temperature for 24 h. The sintered tablets were milled again and pressed into tablets, sintered at 1250℃ for 24 h. We can confirm the finally synthesized poly-crystalline compounds by using X-ray diffraction (XRD, Miniflex 600 X-ray diffractometer) method at room temperature. The crystal growth was carried out in a DJL-400 furnace with 98%N2 and 2%O2 atmosphere protection, and an Ir crucible with the diameter of 50 mm and the height of 30 mm was used. The used seed was <001>direction SLGO crystal. The crystal rotation speed was kept at 10~15 rpm and the pulling rate varied from 1.0 to 3.0 mm/h. Finally, the grown crystal was annealed to room temperature at a rate of 10~25 K/h. Also, the Yb,Er:SLGO single crystal was confirmed by XRD using Cu Kα ( λ=1.54056Å ) in the 2θ range of 10-70° with a graphite monochromator at room temperature. The dopant concentrations of various rare earth ions in Yb,Er:SLGO crystal were measured by the method of Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). Three optically polished sample which was cut along the a-axis from the Yb,Er:SLGO crystal with dimensions of 5.0 mm×5.0 mm×1.0 mm (a×c×a) was used for spectroscopic analysis. The room temperature absorption spectrum was measured by using a Perkin-Elmer UV-VIS-NIR Spectrometer (Lambda-950). The room temperature up-conversion emission spectrum was carried out by Edinburgh Instruments FLS920 spectrophotometer with 980 LD pump source. The room temperature near-infrared (NIR) emission spectrum and decay curve were collected on an Edinburgh Instruments FLS980 spectrophotometer by using 980 LD pump source too. The room temperature mid-infrared (MIR) emission spectrum and the decay curve
were
measured by using an Edinburgh
Instruments
FSP920c
Spectrophotometer. To obtain comparable results, all measurements of samples were carried out under the same measurement conditions.
3. Results and discussion Fig. 1 shows the photo of grown Yb,Er:SLGO crystal which is pink and transparent, with high-optical-quality. As seen in figure 2, the XRD pattern of the Yb,Er:SLGO crystal powder is obtained, and it is well consistent with the standard JCPDF file [No.45-0637] for SLGO crystal, which means that there is no phase transformation in the Yb,Er:SLGO crystal. The doping concentrations of Yb3+, Er3+ and La3+ in the Yb,Er:SLGO crystal were measured to be 4.20×1019, 2.78×1020 and 5.45×1021 ionscm-3, respectively.
Figure 1. The grown Yb,Er:SLGO crystal
Figure 2. XRD pattern of Yb,Er:SLGO crystal
At the room temperature, figure 3 displays the absorption spectrum, up-conversion, NIR and MIR emission spectra of Yb,Er: SLGO crystal. As seen in the Fig. 3(a), there are mainly nine absorption bands which the peaks centered at 379, 407, 452, 488, 523, 651, 802, 980 and 1536 nm, corresponding to the transitions from Er3+: 4I15/2 to 4G11/2+2K15/2, 2
H9/2, 4F5/2+4F3/2, 4F7/2, 2H11/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2, respectively. Owing to the
existence of Yb3+, Yb,Er:SLGO crystal exhibits stronger and wider absorption bands centered at 980nm with a full width at half maximum (FWHM) of 28 nm in the range of 900 to 1000 nm as compared with Er:SLGO, which correspond to 2F7/2→2F5/2 of Yb3+ and 4I15/2→4I11/2 of Er3+ jointly, and such a broad absorption band makes Yb,Er:SLGO crystal very suitable for the commercial InGaAs laser diode pumping. The NIR fluorescence spectrum of Yb,Er:SLGO crystal in the wavelength range from 1450 to 1630 nm is shown in figure 3(b) under excitation of 980 nm. As seen from it, the strong
emissions with peaks around 1536 and 1546 nm are observed in this crystal, corresponding to the transition of Er3+: 4I13/2→4I15/2. As seen in Fig. 3(c), the MIR fluorescence spectrum of Yb,Er:SLGO crystal within 2500~3000 nm is obtained under pumping of 980 nm. There is a strong emission with peak centered around at 2720 nm due to Er3+: 4I11/2→4I13/2 in Yb,Er:SLGO crystal. This is the target emission which would lead to the achievement of ~2.7 µm laser in Yb,Er:SLGO crystal. The green up-conversion emission band centered at about 549 nm and red emission band centered at 676 nm can be observed in figure 3(d), which correspond to Er3+: (2H11/2, 4S3/2)→4I15/2 and 4F9/2→4I15/2 transitions, respectively. And it is clear from Fig. 3(d) that the green emission is much stronger than that of the red emission by more than 7 times.
Figure 3. Absorption (a), near-infrared (b), mid-infrared (c), and upconversion emission spectra (d) of Yb,Er: SrLaGa3O7 crystal measured at room temperature.
Fig. 4 displays the decay curves of Er3+: 4I13/2 and 4I11/2 energy levels in Yb,Er:SLGO crystal. The decay curves were collected on the Edinburgh Instruments FLS980 with the monitoring wavelength of 1546nm and the excitation wavelength of 980 nm. The decay curve of 4I13/2 energy level in Yb,Er:SLGO crystal is singly-exponential decaying, while
that of 4I11/2 level shows double-exponential decay behavior which suggests the existence of energy transfer process in Yb,Er:SLGO crystal. The curves are fitted and the values of fluorescence lifetime are shown in Fig. 4. And the fluorescence life-times are 9.88 ms and 0.72 ms, respectively. It is obvious that the longer lifetime of laser lower level 4I13/2 than upper level 4I11/2 would make populations on the 4I11/2 level cannot relax quickly enough to maintain the necessary population inversion, and then cause the 4I11/2→4I13/2 lasing transition to self-terminate automatically. However the up-conversion processes and inner energy transfer are advantageous to the population inversion.
Figure 4. The fluorescence decay curves of Er3+:4I13/2 and 4I11/2 multiplets of Yb,Er:SLGO crystal measured at room temperature
The emission cross sections can be calculated by the F-L equation [26,27]: em ( )
5 I ( ) . 8 cn 2 r I ( )d
(1)
Where I ( ) / I ( )d is the normalized line shape function of the experimental emission spectrum, β is the fluorescence branching ratio, c is the speed of light, n is the refractive index and τr is the radiative lifetime. Based on the Judd-Ofelt theory [28, 29], the values of β and τr of 4I11/2→ 4I13/2 transition in Yb,Er:SLGO crystal were calculated to be 0.16 and 6.73 ms, respectively, and thus the maximum emission cross section is estimated to be 2.1610-19 cm2 at 2720 nm, which is much larger than Er:SLGO (1.5310-19 cm2) [19], Er:SrGdGa3O7 (1.6410-19 cm2) [30] and Er:YSGG crystal
(1.0210-19 cm2) [31]. And similary, with the calculated necessary parameters of 4I13/2→ 4
I15/2 transition (β=1, τr= 4.408 ms), the maximum emission cross section of Yb,Er:SLGO
crystal is estimated to be 2.9510-20 cm2 at 1546 nm, which is much lower than that at 2720 nm. The above obtained spectroscopic parameters prove that Yb,Er:SLGO crystal is a better candidate for achievement of ~2.7 µm laser than Er:SLGO crystal. The theory of rate equation is based on Einstein’ phenomenological theory, which doesn’t involve the details of interaction between light and matter, as well as the mechanical processes, therefore, it is kind of simplified quantum theory. The rate equation model is used to describe and characterize the density of populations in various atoms’ energy levels, as well as the change rule of the total number of photons v.s. time in medium, so this model is very helpful in simulation of laser properties [32-34]. The laser/emission of Er3+ sensitized by Yb3+ in SLGO crystal involves many complicated dynamic processes, such as the energy transfer, up-conversion, cross-relaxation and spontaneous decay etc. The co-dopant Yb3+ is meant to improve the absorption of pump energy and thus accelerate the decay rate of the second excited state of Er3+. In order to analyze and simulate the laser action of Yb,Er:SLGO crystal, the rate equation model is built on the energy level transitions of Yb3+ and Er3+. Figure 5 shows the energy level diagram of Yb-Er level transition of Yb,Er:SLGO crystal, and the energy levels locating upper of 4S3/2 level are omitted in the following simulation.
Figure 5. The energy level diagram of Yb-Er level transitions of Yb,Er:SLGO crystal
Herein, Ny1 and Ny0 are the populations’ density of Yb3+ on the levels of 2F5/2 and 2F7/2, respectively. N0, N1, N2, N3, N4, N5 and N6 are assigned to be the populations of Er3+ on the levels of 4I15/2, 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2 and 4F7/2, respectively. Therefore, the rate equations of energy level transitions of Yb3+ and Er3+ under 4S3/2 level in Yb,Er:SLGO crystal are expressed as follows: dN y1
Wp N y 0
N y1
KN 0 N y1 K ' N y 0 N 2 K1 N y1 N1 K 2 N y1 N 2 dt y dN6 N W06 N0 6 W22 ( N 2 2 N0 N6 ) K 2 N y1 N 2 dt 6 dN5 N N W05 N0 5 6 W50 ( N5 N0 N3 N1 ) dt 5 6 N dN 4 N W04 N0 4 5 K1 N 4' N1 dt 4 5 dN3 N N W03 N0 3 4 W50 ( N5 N0 N3 N1 ) W11 ( N12 N 0 N3 ) dt 3 4 N dN 2 N W02 N 0 21 ( N 2 N1 ) 2 3 K ' N y 0 N 2 KN 0 N y1 dt 2 3
(2) (3) (4) (5) (6)
(7)
2W22 ( N 2 N 0 N 6 ) K 2Wy1 N 2 2
dN1 N N W01 N0 1 21 ( N 2 N1 ) 2 2W11 ( N12 N0 N3 ) W50 ( N5 N0 N3 N1 ) dt 1 2 cl p N d ' 21 ( N2 N1 ) C 2 dt l (n 1)l 2
(8) (9)
N y 0 N y1 N y 0
(10)
N0 N1 N2 N3 N4 N5 N6 N0 N1 N2
(11)
Herein, Wp is the pumping ratio of Yb3+ from ground energy level 2F7/2 to excited energy level 2F5/2, W0i (i=1,2,3···6) is the excited absorption transition ratio of Er3+ from ground energy level 4I15/2 to several excited energy levels, W26 is the excited absorption transition ratio of Er3+ from energy level 4I11/2 to 4F7/2, φ is the photon flux density, σ21 is the emission cross section of Er3+ from energy level 4I11/2 to 4I13/2, α and β are the Boltzmann distribution coefficient of stark energy levels of 4I11/2 and 4I13/2, respectively. l, lp and l’ are the length of laser gain media, pump area and the resonant cavity, respectively. ρ is the total loss inside resonant cavity, CN2 / 2 represents the contribution of spontaneous radiation to the photon flux density inside resonant cavity. y is the
lifetime of Yb3+: 2F5/2, i (i=1,2,3···6) is the lifetime of Er3+: 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2 and 4F7/2, respectively. K and K’ are the energy transfer coefficient and reversing energy transfer coefficient of Yb3+:2F5/2 to Er: 4I11/2 respectively. K1 and K2 are the energy transfer coefficients between Er3+ and Yb3+. W11, W22 and W50 are the energy transfer ratio of up-conversion and cross-relax, respectively. Considering 1, 2 1, 2 , 3 , 4 , and the contribution of spontaneous radiation to the photon flux density inside resonant cavity could be omitted, furthermore, the excited absorption of Er3+ energy levels except for 4I11/2 and Yb3+ energy levels except for 2F7/2 could be omitted because the 980 nm pumping source is used to achieve 2.7 µm laser in Yb,Er:SLGO crystal, that means W01=W03=W04=W05=W06=0, and in the steady states dNi d =0, 0 , so the rate equations can be abbreviated as follows: dt dt Wp N y 0
N y1
y
KN 0 N y1 K ' N y 0 N 2 K1 N y1 N1 K 2 N y1 N 2 0
W02 N 0 21 ( N 2 N1 )
N2
2
K ' N y 0 N 2 KN 0 N y1
(12)
(13)
W11 N W22 N 2 K 2Wy1 N 2 K1 N1 N y1 0 2 1
2
N2
N (14) 1 2W11 N12 W22 N 2 2 21 ( N2 N1 ) K 2 N y1 N2 0 2 1 (15) N2 N1 / Then, based on the above equations, the photon flux density can be expressed as
follows:
1
(W02 N0 Wp N y 0 )(2 p 2 )
Herein, p
1
21 2
W22
2 212
N1 (
2 W 1 p2 2 22 ) N 2 21 1 1
(16)
ln(r1r2 ) W22 p0 , here r1 and r2 are the reflectivity of input ; 2l p W11
mirror and output mirror, respectively. When the pumping ratio is big enough, the first item of the equation of the photon flux density plays the main role, and the other items could be omitted, so the photon flux density is expressed as:
1
(W02 N0 Wp N y 0 )(2 p 2 )
(17)
And the laser output power is: 1 1 hc Pout ln( ) Vmod e 2l p r2 L The pumping absorption power is: hc Pabs (W02 N 0 Wp N y 0 ) Vpump
p
(18)
(19)
Herein, l and p are the laser wavelength and pump light wavelength; Vmod e and
Vpump are the mode volume and pump volume of laser gain media, respectively. The laser quantum efficiency is expressed as: 1 ln( ) p Vmod e P r2 out Pabs 2l p (W02 N 0 Wp N y 0 ) L Vpump
(20)
As for Yb,Er:SLGO crystal, the values of the necessary parameters are list as follows: the emission cross-section σ21 is 2.1610-19 cm2, the fluorescence lifetimes of Er: 4
I13/2 ( 1 ) and Er: 4I11/2 ( 2 ) are 0.72 ms and 9.88 ms, respectively. α and β are calculated to
be 0.23 and 0.12, respectively. And the chosen values of W11 and W22 are 1.3×10-15 cm3/s and 3.7×10-15 cm3/s, respectively [35]. For a numerical simulation of Yb,Er:SLGO laser pumped by 980 nm, we find that the laser quantum efficiency increases quickly with the increment of pump rate firstly, then increases slowly, and ultimately it reaches to a fixed value 2 p 2 , which is the high limit of the quantum efficiency. This is can be explained as follows: when the pump rate is big enough, the photon flux density as expressed in Eq. 17 is reasonable, therefore, the laser quantum efficiency is approximately equal to
2 p 2 =1.23. The weaker intensity of 2.7~3.0 μm emission in Er:SLGO crystal indicates that the nonradiative transition is the main route for depopulating 4I11/2 [19]. However, once Yb3+ ions are introduced into Er: SLGO crystal, the 980 nm pump energy is mainly absorbed by Yb3+ via the transition 2F7/2→2F5/2. Yb3+ ions in the 2F5/2 level transferred their energy to Er3+: 4I9/2 and 4I11/2. The energy transfer process from Yb3+ to Er3+ can increase the population of Er3+:4I11/2, which could enhance the 2.7~3.0 μm emission corresponding to Er3+: 4I11/2→4I13/2 and thus complete the population inversion for laser operation.
4. Conclusions
Yb,Er:SLGO bulk crystal has been grown successfully by the Czochralski method. The absorption and fluorescence spectra as well as the fluorescence decay curves of it were measured and investigated. Based on Yb3+-Er3+ energy level diagrams, the rate equation model was built and the emission properties were discussed. It is found that the population in the 4I11/2 state of Er3+ in Yb,Er:SLGO crystal under excitation of 980 nm can be increased by co-doping with Yb3+. This is due to the resonant energy transfer from Yb3+ to Er3+, and this process is especially useful for the Er:SLGO crystal with low absorption in ~980 nm pumping waveband. Improved 2.7~3.0 µm emission has been obtained in Yb,Er:SLGO crystal, and based on the above analyses of rate equation model, we can conclude that this crystal is a potential laser gain medium for 2.7~3.0 µm laser.
Acknowledgments This work is supported by National Nature Science Foundation of China (Grant No. 51472240, 61675204 and 11304313), The National Key Research and Development Program
of
China
(Grant
No.
2016YFB0701002),
the Strategic Priority Research Program of the Chinese Academy of Science (Grant No. XDB20010200), Key Laboratory of Functional Crystal Materials and Device of Ministry of Education of Shandong University (Grant No. JG1403), State Key Laboratory of Rare Earth Resource Utilization of Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (Grant No. RERU2015018), State Key Laboratory of Structure Chemistry of Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (Grant No.20160012).
References [1] [2]
R. Won, Nature photon. 5 (2011) 457. C. Apel, J. Meister, R.S. Ioana, R. Franzen, P.Hering, N. Gutknecht, Lasers Med. Sci. 17 (2002) 246.
[3]
I.T. Sorokina, K.L. Vodopyanov(Eds.), Berlin Heidelberg: Springer-Verlag 89 (2003) 144.
[4]
K.L. Vodopyanov, J. Opt. Soc. Am. B. 16 (1999) 1579.
[5]
K.L. Vodopyanov, F. Ganikhanov, J.P. Maffetone, I. Zwieback, W.Ruderman,
Opt. Lett. 25 (2000) 841. [6]
Z. You, Y. Wang, J. Xu, Z. Zhu, J. Li, H. Wang, C. Tu, Opt. Lett. 40 (2015) 3846.
[7]
B.J. Dinerman, Ph.D. Dissertation, Boston, USA, 1994.
[8]
B.J. Dinerman, P.F. Moulton, Opt. Lett. 19 (1994) 1143.
[9]
E. Arbabzadah, S. Chard, H. Amrania, C. Phillips, M. Damzen, Opt. Express 19 (2011) 25860.
[10]
J. Chen, D. Sun, J. Luo, H. Zhang, R. Dou, J. Xiao, Q. Zhang, S. Yin, Opt. Express 21 (2013) 23425.
[11]
W. Ma, L. Su, X. Xu, J. Wang, D. Jiang, L. Zheng, X. Fan, C. Li, L. Liu, J. Xu, Opt. Mater. Express 6 (2016) 409.
[12]
C. Li, J. Liu, S. Jiang, S. Xu, W. Ma, J. Wang, X. Xu, L. Su, Opt. Mater. Express 6 (2016) 1570.
[13]
T.T. Basiev, Y.V. Orlovskii, M.V. Polyachenkova, P.P. Fedorov, S.V. Kuznetsov, V.A. Konyushkin, V.V. Osiko, O.K. Alimov, A.Y. Dergachev, Quantum Electronics 36 (2006) 591.
[14]
Y. Zhang, X. Yin, H. Yu, H. Cong, H. Zhang, J. Wang, I. Robert, Boughton, Cryst. Growth Des. 12 (2012) 622.
[15]
B.S. Teisseire, B. Viana, A.M. Lejus, D. Vivien, J. Lumin. 72-74 (1997) 971.
[16]
W. Ryba-Romanowski, S.
o a , G. Dominiak-Dzik and M. Berkowski,
Materials Science and Engineering B 15 (1992) 217. [17]
W.Ryba-Romanowski, S.
o a , W.A. Pisarski., G. Dominiak-Dzik, M.
Berkowski and . a ac o s a, J. Phys. Chem. Solids 58 (1997) 639. [18]
W. Ryba-Romanowski, S.
oa
and W.A. Pisarski, Spectrochimica Acta A 54
(1998) 2071. [19]
Y. Wang, J. Li, Z. Zhu, Z.You, J. Xu, C. Tu, Cryst. Growth Des. 16 (2016) 2289 .
[20]
J. Hanuza, K. Hermanowicz, M. Maczka and W. Ryba-Romanowski, J. Mol. Struct. 325 (1994) 111.
[21]
B. Zhang, Y. Wang, J. Li, Z. You, H. Xia, Z. Zhu, J. Xu, C.Tu, Chin. J. Appl. Chem. 33 (2016) 994.
[22]
Y. Wang, J. Li, Z. Yu, H. Xia, Z. Zhu, J. Xu, C. Tu, Scientia Sinica Technologica
46 (2016) 894. [23]
Y. Wang, Z. You, J. Li, Z. Zhu, C. Tu, Mater. Res. Bull. 48 (2013) 2969 .
[24]
J. Chen, D. Sun, J. Luo, J. Xiao, H. Kang, H. Zhang, M. Cheng, Q. Zhang, S. Yin, Opt. Lett. 38 (2013) 1218.
[25]
H. Xia, J. Feng, Y. Ji, Y. Sun, Y. Wang, Z. Jia, C. Tu, J. Quant. Spectrosc. Radiat. Transf. 173(2016) 7.
[26]
Y. Wang, B. Zhang, J. Li, Z. Zhu, Z. You, C. Tu, J. Lumin. 183 (2017) 201.
[27]
Y. Wang, J. Li, Z. You, Z. Zhu, J. Xu, C. Tu, J. Quant. Spectrosc. Radiat. Transf. 149 (2014) 253.
[28]
B.R. Judd, Phys. Rev. 127 (1962) 750.
[29]
G. S. Ofelt, J. Chem. Phys. 37 (1962) 511.
[30]
H. Xia, J. Feng, Y. Wang, J. Li, Z. Jia, C. Tu, Sci. Rep. 5 (2015) 13988.
[31]
Y. Wang, J. Li, Z. You, Z. Zhu, J. Xu, C. Tu, Opt. Express 23 (2015) 18554.
[32]
L. Huang, Q. Guo, J. Luo, S. Wang, J. Zhou, Z. Wu, Chin. J. Quant. Elect. 29 (2012) 45 .
[33]
A. Hu, Q. Guo, S. Wang, L. Huang, Z. Wu, J. Zhou, Infrar. Laser Eng. 41 (2012) 1168.
[34]
C. StrohhÖfer, P.G. Kik, A. Polman, J. Appl. Phys. 88 (2000) 4486.
[35]
S. Georgescu, O. Toma, H. Totia, IEEE Quant. Elect. 39 (2003) 722.
Graphical abstract The bulk crystal of 5at% Yb3+ and 20at% Er3+ co-doped SrLaGa3O7 was grown successfully by the Czochralski method. The spectroscopic properties and the rate equation model were presented and analyzed systematically.
PII: DOI: Reference:
S0022-2313(17)30080-7 http://dx.doi.org/10.1016/j.jlumin.2017.05.034 LUMIN14758
To appear in: Journal of Luminescence Received date: 16 January 2017 Revised date: 10 May 2017 Accepted date: 10 May 2017 Cite this article as: Baotong Zhang, Yan Wang, Jianfu Li, Yunyun Liu, Zhaojie Zhu, Zhenyu You and Chaoyang Tu, The bulk crystal growth, spectral analyses and rate equation model of Yb,Er:SrLaGa3O7 crystal, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2017.05.034 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.
The bulk crystal growth, spectral analyses and rate equation model of Yb,Er:SrLaGa3O7 crystal Baotong Zhang1,2, Yan Wang1,*, Jianfu Li1, Yunyun Liu1,3, Zhaojie Zhu1, Zhenyu You1, and Chaoyang Tu1,* 1
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou City, Fujian Province, 350002 P. R. China 2
College of Materials Science and Engineering, Fujian Normal University, Fuzhou City, Fujian Province, 350007 P. R. China 3
University of Chinese Academy of Sciences, Beijing 100039, P.R.China *Corresponding authors: [email protected] & [email protected]
ABSTRACT: Yb3+ and Er3+ co-doped SrLaGa3O7 (abbr. as Yb,Er:SLGO) bulk crystal with dimensions of Φ 20×30 mm was grown successfully by the Czochralski (Cz) method for the first time. The absorption and fluorescence spectra as well as the fluorescence decay curves of Yb,Er:SLGO crystal were measured and investigated. The emission cross-section and fluorescence lifetimes have been obtained at the ~2.7 µm wavelength. Based on Yb3+-Er3+ energy level diagrams, the rate equation model was built and discussed. It is found that 2.7~3.0 µm emission has been improved in Yb,Er:SLGO crystal under excitation of 980 nm, owing to the sensitization of the co-dopant Yb3+ and the resonant energy transfer from Yb3+ to Er3+. A noteworthy result is that Yb,Er:SLGO crystal is an excellent mid-infrared laser gain material. Keywords: Yb,Er:SLGO single crystal; Crystal growth; Mid-infrared spectroscopy; Rate equation model
1. Introduction During the last decades, 2.7~3.0 μm mid-infrared (MIR) solid-state lasers based on the 4I11/2→4I13/2 transition of Er3+ have drawn a lot of attention owing to their applications in the fields of dentistry, lasers surgery, remote sensors, environment atmosphere
exploration and military fields etc[1-3]. In addition, 2.7~3.0 μm laser is also attractive as a pump source for far-infrared waveband optical parametric oscillation (OPO) or optical parametric generation (OPG) laser system[4,5]. Lasing at 2.7~3.0 μm can be obtained through the 4I11/2→4I13/2 transition of Er3+, and several Er3+ activated crystals with good laser performances have been reported, such as Er:GGG[6-8], Er:YSGG[7-9], Er:YAG[8,9], Er,Pr:GYSGG[10], Er:CaF2[11-13], Er:SrF2[13] etc. ABC3O7 (A=Ca, Sr, Ba; B=La, Gd; and C=Ga, Al) is melilite structure with tetragonal system and P 4 21m space group[14-19]. In this work, SrLaGa3O7(abbr. as SLGO) crystal as one member of ABC3O7 is chosen as the host matrix. Its lattice parameters are a=8.06Å, c=5.34Å and its density is 5.251g/cm3[19]. Compared with other popular gallate crystals such as GGG, YSGG crystals, there are at least two advantages in it: one is that SLGO crystal has a relative low phonon energy (of ~750 cm−1) [20], which is favorable for reducing the multiphonon relaxation rate of the Er3+: 4
I11/2→4I13/2 transition and thus beneficial to decrease the laser threshold and improve the
2.7~3.0 μm laser output power and efficiency[21]; the other is its disordered structure in which Sr2+ and La3+ ions are randomly distributed in a 1:1 ratio, once Er3+ ions are doped into the host, they can substitute both for Sr2+ and La3+ ions, producing a large inhomogeneous broadening of absorption and emission spectra and it’s very favorable for achievements of tunable or ultra fast lasers [22]. More importantly, SLGO crystal has a relatively lower melting point (about 1588℃)[19], which makes it easier to obtain large-sized crystals with higher optical quality by using the Czochralski (CZ) method. In our previous job, Er3+ doped SLGO crystal has been reported as 2.7~3.0 µm laser gain media [19]. However, the absorption is somewhat weak whitin the ~980 nm pumping waveband, and thus makes the mid-infrared emissions not strong enough. Since the energy levels of Yb3+ match with Er3+ ions very well, Yb3+ could be introduced into Er3+ activated gallate crystals to improve the ~980 nm absorption, and then transfers energy from Yb3+ to Er3+, finally enhances the 2.7~3.0 μm emission[23-25]. In this work, we report on the bulk growth, spectral properties and the rate equation model of Yb,Er:SLGO crystal, with focus on the potential application of this crystal for lasing amplification in the wavelength range 2.7~3.0 μm.
2. Experimental details The crystal growth of SrLaGa3O7 doped with 5at% Yb and 20at% Er was carried out in DJL-400 furnace (NCIREO, China). The adopted raw materials were 99.9% purity SrCO3, 99.99% purity Yb2O3, Ga2O3, La2O3 and Er2O3 commercial powders, which were dried before experiment. They were weighed out according to the compositional formula, with 1~2wt% extra Ga2O3() to compensate Ga loss owing to its evaporation. The raw materials were mixed, ground carefully, and compressed into cylinder tablets under a pressure of 20 Mpa. Then the tablets were heated up in a Muffle furnace till 1100 °C and thermal insulation for 10 h to completely decompose the SrCO3, and then heated to 1200℃ and maintaining at this temperature for 24 h. The sintered tablets were milled again and pressed into tablets, sintered at 1250℃ for 24 h. We can confirm the finally synthesized poly-crystalline compounds by using X-ray diffraction (XRD, Miniflex 600 X-ray diffractometer) method at room temperature. The crystal growth was carried out in a DJL-400 furnace with 98%N2 and 2%O2 atmosphere protection, and an Ir crucible with the diameter of 50 mm and the height of 30 mm was used. The used seed was <001>direction SLGO crystal. The crystal rotation speed was kept at 10~15 rpm and the pulling rate varied from 1.0 to 3.0 mm/h. Finally, the grown crystal was annealed to room temperature at a rate of 10~25 K/h. Also, the Yb,Er:SLGO single crystal was confirmed by XRD using Cu Kα ( λ=1.54056Å ) in the 2θ range of 10-70° with a graphite monochromator at room temperature. The dopant concentrations of various rare earth ions in Yb,Er:SLGO crystal were measured by the method of Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). Three optically polished sample which was cut along the a-axis from the Yb,Er:SLGO crystal with dimensions of 5.0 mm×5.0 mm×1.0 mm (a×c×a) was used for spectroscopic analysis. The room temperature absorption spectrum was measured by using a Perkin-Elmer UV-VIS-NIR Spectrometer (Lambda-950). The room temperature up-conversion emission spectrum was carried out by Edinburgh Instruments FLS920 spectrophotometer with 980 LD pump source. The room temperature near-infrared (NIR) emission spectrum and decay curve were collected on an Edinburgh Instruments FLS980 spectrophotometer by using 980 LD pump source too. The room temperature mid-infrared (MIR) emission spectrum and the decay curve
were
measured by using an Edinburgh
Instruments
FSP920c
Spectrophotometer. To obtain comparable results, all measurements of samples were carried out under the same measurement conditions.
3. Results and discussion Fig. 1 shows the photo of grown Yb,Er:SLGO crystal which is pink and transparent, with high-optical-quality. As seen in figure 2, the XRD pattern of the Yb,Er:SLGO crystal powder is obtained, and it is well consistent with the standard JCPDF file [No.45-0637] for SLGO crystal, which means that there is no phase transformation in the Yb,Er:SLGO crystal. The doping concentrations of Yb3+, Er3+ and La3+ in the Yb,Er:SLGO crystal were measured to be 4.20×1019, 2.78×1020 and 5.45×1021 ionscm-3, respectively.
Figure 1. The grown Yb,Er:SLGO crystal
Figure 2. XRD pattern of Yb,Er:SLGO crystal
At the room temperature, figure 3 displays the absorption spectrum, up-conversion, NIR and MIR emission spectra of Yb,Er: SLGO crystal. As seen in the Fig. 3(a), there are mainly nine absorption bands which the peaks centered at 379, 407, 452, 488, 523, 651, 802, 980 and 1536 nm, corresponding to the transitions from Er3+: 4I15/2 to 4G11/2+2K15/2, 2
H9/2, 4F5/2+4F3/2, 4F7/2, 2H11/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2, respectively. Owing to the
existence of Yb3+, Yb,Er:SLGO crystal exhibits stronger and wider absorption bands centered at 980nm with a full width at half maximum (FWHM) of 28 nm in the range of 900 to 1000 nm as compared with Er:SLGO, which correspond to 2F7/2→2F5/2 of Yb3+ and 4I15/2→4I11/2 of Er3+ jointly, and such a broad absorption band makes Yb,Er:SLGO crystal very suitable for the commercial InGaAs laser diode pumping. The NIR fluorescence spectrum of Yb,Er:SLGO crystal in the wavelength range from 1450 to 1630 nm is shown in figure 3(b) under excitation of 980 nm. As seen from it, the strong
emissions with peaks around 1536 and 1546 nm are observed in this crystal, corresponding to the transition of Er3+: 4I13/2→4I15/2. As seen in Fig. 3(c), the MIR fluorescence spectrum of Yb,Er:SLGO crystal within 2500~3000 nm is obtained under pumping of 980 nm. There is a strong emission with peak centered around at 2720 nm due to Er3+: 4I11/2→4I13/2 in Yb,Er:SLGO crystal. This is the target emission which would lead to the achievement of ~2.7 µm laser in Yb,Er:SLGO crystal. The green up-conversion emission band centered at about 549 nm and red emission band centered at 676 nm can be observed in figure 3(d), which correspond to Er3+: (2H11/2, 4S3/2)→4I15/2 and 4F9/2→4I15/2 transitions, respectively. And it is clear from Fig. 3(d) that the green emission is much stronger than that of the red emission by more than 7 times.
Figure 3. Absorption (a), near-infrared (b), mid-infrared (c), and upconversion emission spectra (d) of Yb,Er: SrLaGa3O7 crystal measured at room temperature.
Fig. 4 displays the decay curves of Er3+: 4I13/2 and 4I11/2 energy levels in Yb,Er:SLGO crystal. The decay curves were collected on the Edinburgh Instruments FLS980 with the monitoring wavelength of 1546nm and the excitation wavelength of 980 nm. The decay curve of 4I13/2 energy level in Yb,Er:SLGO crystal is singly-exponential decaying, while
that of 4I11/2 level shows double-exponential decay behavior which suggests the existence of energy transfer process in Yb,Er:SLGO crystal. The curves are fitted and the values of fluorescence lifetime are shown in Fig. 4. And the fluorescence life-times are 9.88 ms and 0.72 ms, respectively. It is obvious that the longer lifetime of laser lower level 4I13/2 than upper level 4I11/2 would make populations on the 4I11/2 level cannot relax quickly enough to maintain the necessary population inversion, and then cause the 4I11/2→4I13/2 lasing transition to self-terminate automatically. However the up-conversion processes and inner energy transfer are advantageous to the population inversion.
Figure 4. The fluorescence decay curves of Er3+:4I13/2 and 4I11/2 multiplets of Yb,Er:SLGO crystal measured at room temperature
The emission cross sections can be calculated by the F-L equation [26,27]: em ( )
5 I ( ) . 8 cn 2 r I ( )d
(1)
Where I ( ) / I ( )d is the normalized line shape function of the experimental emission spectrum, β is the fluorescence branching ratio, c is the speed of light, n is the refractive index and τr is the radiative lifetime. Based on the Judd-Ofelt theory [28, 29], the values of β and τr of 4I11/2→ 4I13/2 transition in Yb,Er:SLGO crystal were calculated to be 0.16 and 6.73 ms, respectively, and thus the maximum emission cross section is estimated to be 2.1610-19 cm2 at 2720 nm, which is much larger than Er:SLGO (1.5310-19 cm2) [19], Er:SrGdGa3O7 (1.6410-19 cm2) [30] and Er:YSGG crystal
(1.0210-19 cm2) [31]. And similary, with the calculated necessary parameters of 4I13/2→ 4
I15/2 transition (β=1, τr= 4.408 ms), the maximum emission cross section of Yb,Er:SLGO
crystal is estimated to be 2.9510-20 cm2 at 1546 nm, which is much lower than that at 2720 nm. The above obtained spectroscopic parameters prove that Yb,Er:SLGO crystal is a better candidate for achievement of ~2.7 µm laser than Er:SLGO crystal. The theory of rate equation is based on Einstein’ phenomenological theory, which doesn’t involve the details of interaction between light and matter, as well as the mechanical processes, therefore, it is kind of simplified quantum theory. The rate equation model is used to describe and characterize the density of populations in various atoms’ energy levels, as well as the change rule of the total number of photons v.s. time in medium, so this model is very helpful in simulation of laser properties [32-34]. The laser/emission of Er3+ sensitized by Yb3+ in SLGO crystal involves many complicated dynamic processes, such as the energy transfer, up-conversion, cross-relaxation and spontaneous decay etc. The co-dopant Yb3+ is meant to improve the absorption of pump energy and thus accelerate the decay rate of the second excited state of Er3+. In order to analyze and simulate the laser action of Yb,Er:SLGO crystal, the rate equation model is built on the energy level transitions of Yb3+ and Er3+. Figure 5 shows the energy level diagram of Yb-Er level transition of Yb,Er:SLGO crystal, and the energy levels locating upper of 4S3/2 level are omitted in the following simulation.
Figure 5. The energy level diagram of Yb-Er level transitions of Yb,Er:SLGO crystal
Herein, Ny1 and Ny0 are the populations’ density of Yb3+ on the levels of 2F5/2 and 2F7/2, respectively. N0, N1, N2, N3, N4, N5 and N6 are assigned to be the populations of Er3+ on the levels of 4I15/2, 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2 and 4F7/2, respectively. Therefore, the rate equations of energy level transitions of Yb3+ and Er3+ under 4S3/2 level in Yb,Er:SLGO crystal are expressed as follows: dN y1
Wp N y 0
N y1
KN 0 N y1 K ' N y 0 N 2 K1 N y1 N1 K 2 N y1 N 2 dt y dN6 N W06 N0 6 W22 ( N 2 2 N0 N6 ) K 2 N y1 N 2 dt 6 dN5 N N W05 N0 5 6 W50 ( N5 N0 N3 N1 ) dt 5 6 N dN 4 N W04 N0 4 5 K1 N 4' N1 dt 4 5 dN3 N N W03 N0 3 4 W50 ( N5 N0 N3 N1 ) W11 ( N12 N 0 N3 ) dt 3 4 N dN 2 N W02 N 0 21 ( N 2 N1 ) 2 3 K ' N y 0 N 2 KN 0 N y1 dt 2 3
(2) (3) (4) (5) (6)
(7)
2W22 ( N 2 N 0 N 6 ) K 2Wy1 N 2 2
dN1 N N W01 N0 1 21 ( N 2 N1 ) 2 2W11 ( N12 N0 N3 ) W50 ( N5 N0 N3 N1 ) dt 1 2 cl p N d ' 21 ( N2 N1 ) C 2 dt l (n 1)l 2
(8) (9)
N y 0 N y1 N y 0
(10)
N0 N1 N2 N3 N4 N5 N6 N0 N1 N2
(11)
Herein, Wp is the pumping ratio of Yb3+ from ground energy level 2F7/2 to excited energy level 2F5/2, W0i (i=1,2,3···6) is the excited absorption transition ratio of Er3+ from ground energy level 4I15/2 to several excited energy levels, W26 is the excited absorption transition ratio of Er3+ from energy level 4I11/2 to 4F7/2, φ is the photon flux density, σ21 is the emission cross section of Er3+ from energy level 4I11/2 to 4I13/2, α and β are the Boltzmann distribution coefficient of stark energy levels of 4I11/2 and 4I13/2, respectively. l, lp and l’ are the length of laser gain media, pump area and the resonant cavity, respectively. ρ is the total loss inside resonant cavity, CN2 / 2 represents the contribution of spontaneous radiation to the photon flux density inside resonant cavity. y is the
lifetime of Yb3+: 2F5/2, i (i=1,2,3···6) is the lifetime of Er3+: 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2 and 4F7/2, respectively. K and K’ are the energy transfer coefficient and reversing energy transfer coefficient of Yb3+:2F5/2 to Er: 4I11/2 respectively. K1 and K2 are the energy transfer coefficients between Er3+ and Yb3+. W11, W22 and W50 are the energy transfer ratio of up-conversion and cross-relax, respectively. Considering 1, 2 1, 2 , 3 , 4 , and the contribution of spontaneous radiation to the photon flux density inside resonant cavity could be omitted, furthermore, the excited absorption of Er3+ energy levels except for 4I11/2 and Yb3+ energy levels except for 2F7/2 could be omitted because the 980 nm pumping source is used to achieve 2.7 µm laser in Yb,Er:SLGO crystal, that means W01=W03=W04=W05=W06=0, and in the steady states dNi d =0, 0 , so the rate equations can be abbreviated as follows: dt dt Wp N y 0
N y1
y
KN 0 N y1 K ' N y 0 N 2 K1 N y1 N1 K 2 N y1 N 2 0
W02 N 0 21 ( N 2 N1 )
N2
2
K ' N y 0 N 2 KN 0 N y1
(12)
(13)
W11 N W22 N 2 K 2Wy1 N 2 K1 N1 N y1 0 2 1
2
N2
N (14) 1 2W11 N12 W22 N 2 2 21 ( N2 N1 ) K 2 N y1 N2 0 2 1 (15) N2 N1 / Then, based on the above equations, the photon flux density can be expressed as
follows:
1
(W02 N0 Wp N y 0 )(2 p 2 )
Herein, p
1
21 2
W22
2 212
N1 (
2 W 1 p2 2 22 ) N 2 21 1 1
(16)
ln(r1r2 ) W22 p0 , here r1 and r2 are the reflectivity of input ; 2l p W11
mirror and output mirror, respectively. When the pumping ratio is big enough, the first item of the equation of the photon flux density plays the main role, and the other items could be omitted, so the photon flux density is expressed as:
1
(W02 N0 Wp N y 0 )(2 p 2 )
(17)
And the laser output power is: 1 1 hc Pout ln( ) Vmod e 2l p r2 L The pumping absorption power is: hc Pabs (W02 N 0 Wp N y 0 ) Vpump
p
(18)
(19)
Herein, l and p are the laser wavelength and pump light wavelength; Vmod e and
Vpump are the mode volume and pump volume of laser gain media, respectively. The laser quantum efficiency is expressed as: 1 ln( ) p Vmod e P r2 out Pabs 2l p (W02 N 0 Wp N y 0 ) L Vpump
(20)
As for Yb,Er:SLGO crystal, the values of the necessary parameters are list as follows: the emission cross-section σ21 is 2.1610-19 cm2, the fluorescence lifetimes of Er: 4
I13/2 ( 1 ) and Er: 4I11/2 ( 2 ) are 0.72 ms and 9.88 ms, respectively. α and β are calculated to
be 0.23 and 0.12, respectively. And the chosen values of W11 and W22 are 1.3×10-15 cm3/s and 3.7×10-15 cm3/s, respectively [35]. For a numerical simulation of Yb,Er:SLGO laser pumped by 980 nm, we find that the laser quantum efficiency increases quickly with the increment of pump rate firstly, then increases slowly, and ultimately it reaches to a fixed value 2 p 2 , which is the high limit of the quantum efficiency. This is can be explained as follows: when the pump rate is big enough, the photon flux density as expressed in Eq. 17 is reasonable, therefore, the laser quantum efficiency is approximately equal to
2 p 2 =1.23. The weaker intensity of 2.7~3.0 μm emission in Er:SLGO crystal indicates that the nonradiative transition is the main route for depopulating 4I11/2 [19]. However, once Yb3+ ions are introduced into Er: SLGO crystal, the 980 nm pump energy is mainly absorbed by Yb3+ via the transition 2F7/2→2F5/2. Yb3+ ions in the 2F5/2 level transferred their energy to Er3+: 4I9/2 and 4I11/2. The energy transfer process from Yb3+ to Er3+ can increase the population of Er3+:4I11/2, which could enhance the 2.7~3.0 μm emission corresponding to Er3+: 4I11/2→4I13/2 and thus complete the population inversion for laser operation.
4. Conclusions
Yb,Er:SLGO bulk crystal has been grown successfully by the Czochralski method. The absorption and fluorescence spectra as well as the fluorescence decay curves of it were measured and investigated. Based on Yb3+-Er3+ energy level diagrams, the rate equation model was built and the emission properties were discussed. It is found that the population in the 4I11/2 state of Er3+ in Yb,Er:SLGO crystal under excitation of 980 nm can be increased by co-doping with Yb3+. This is due to the resonant energy transfer from Yb3+ to Er3+, and this process is especially useful for the Er:SLGO crystal with low absorption in ~980 nm pumping waveband. Improved 2.7~3.0 µm emission has been obtained in Yb,Er:SLGO crystal, and based on the above analyses of rate equation model, we can conclude that this crystal is a potential laser gain medium for 2.7~3.0 µm laser.
Acknowledgments This work is supported by National Nature Science Foundation of China (Grant No. 51472240, 61675204 and 11304313), The National Key Research and Development Program
of
China
(Grant
No.
2016YFB0701002),
the Strategic Priority Research Program of the Chinese Academy of Science (Grant No. XDB20010200), Key Laboratory of Functional Crystal Materials and Device of Ministry of Education of Shandong University (Grant No. JG1403), State Key Laboratory of Rare Earth Resource Utilization of Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (Grant No. RERU2015018), State Key Laboratory of Structure Chemistry of Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (Grant No.20160012).
References [1] [2]
R. Won, Nature photon. 5 (2011) 457. C. Apel, J. Meister, R.S. Ioana, R. Franzen, P.Hering, N. Gutknecht, Lasers Med. Sci. 17 (2002) 246.
[3]
I.T. Sorokina, K.L. Vodopyanov(Eds.), Berlin Heidelberg: Springer-Verlag 89 (2003) 144.
[4]
K.L. Vodopyanov, J. Opt. Soc. Am. B. 16 (1999) 1579.
[5]
K.L. Vodopyanov, F. Ganikhanov, J.P. Maffetone, I. Zwieback, W.Ruderman,
Opt. Lett. 25 (2000) 841. [6]
Z. You, Y. Wang, J. Xu, Z. Zhu, J. Li, H. Wang, C. Tu, Opt. Lett. 40 (2015) 3846.
[7]
B.J. Dinerman, Ph.D. Dissertation, Boston, USA, 1994.
[8]
B.J. Dinerman, P.F. Moulton, Opt. Lett. 19 (1994) 1143.
[9]
E. Arbabzadah, S. Chard, H. Amrania, C. Phillips, M. Damzen, Opt. Express 19 (2011) 25860.
[10]
J. Chen, D. Sun, J. Luo, H. Zhang, R. Dou, J. Xiao, Q. Zhang, S. Yin, Opt. Express 21 (2013) 23425.
[11]
W. Ma, L. Su, X. Xu, J. Wang, D. Jiang, L. Zheng, X. Fan, C. Li, L. Liu, J. Xu, Opt. Mater. Express 6 (2016) 409.
[12]
C. Li, J. Liu, S. Jiang, S. Xu, W. Ma, J. Wang, X. Xu, L. Su, Opt. Mater. Express 6 (2016) 1570.
[13]
T.T. Basiev, Y.V. Orlovskii, M.V. Polyachenkova, P.P. Fedorov, S.V. Kuznetsov, V.A. Konyushkin, V.V. Osiko, O.K. Alimov, A.Y. Dergachev, Quantum Electronics 36 (2006) 591.
[14]
Y. Zhang, X. Yin, H. Yu, H. Cong, H. Zhang, J. Wang, I. Robert, Boughton, Cryst. Growth Des. 12 (2012) 622.
[15]
B.S. Teisseire, B. Viana, A.M. Lejus, D. Vivien, J. Lumin. 72-74 (1997) 971.
[16]
W. Ryba-Romanowski, S.
o a , G. Dominiak-Dzik and M. Berkowski,
Materials Science and Engineering B 15 (1992) 217. [17]
W.Ryba-Romanowski, S.
o a , W.A. Pisarski., G. Dominiak-Dzik, M.
Berkowski and . a ac o s a, J. Phys. Chem. Solids 58 (1997) 639. [18]
W. Ryba-Romanowski, S.
oa
and W.A. Pisarski, Spectrochimica Acta A 54
(1998) 2071. [19]
Y. Wang, J. Li, Z. Zhu, Z.You, J. Xu, C. Tu, Cryst. Growth Des. 16 (2016) 2289 .
[20]
J. Hanuza, K. Hermanowicz, M. Maczka and W. Ryba-Romanowski, J. Mol. Struct. 325 (1994) 111.
[21]
B. Zhang, Y. Wang, J. Li, Z. You, H. Xia, Z. Zhu, J. Xu, C.Tu, Chin. J. Appl. Chem. 33 (2016) 994.
[22]
Y. Wang, J. Li, Z. Yu, H. Xia, Z. Zhu, J. Xu, C. Tu, Scientia Sinica Technologica
46 (2016) 894. [23]
Y. Wang, Z. You, J. Li, Z. Zhu, C. Tu, Mater. Res. Bull. 48 (2013) 2969 .
[24]
J. Chen, D. Sun, J. Luo, J. Xiao, H. Kang, H. Zhang, M. Cheng, Q. Zhang, S. Yin, Opt. Lett. 38 (2013) 1218.
[25]
H. Xia, J. Feng, Y. Ji, Y. Sun, Y. Wang, Z. Jia, C. Tu, J. Quant. Spectrosc. Radiat. Transf. 173(2016) 7.
[26]
Y. Wang, B. Zhang, J. Li, Z. Zhu, Z. You, C. Tu, J. Lumin. 183 (2017) 201.
[27]
Y. Wang, J. Li, Z. You, Z. Zhu, J. Xu, C. Tu, J. Quant. Spectrosc. Radiat. Transf. 149 (2014) 253.
[28]
B.R. Judd, Phys. Rev. 127 (1962) 750.
[29]
G. S. Ofelt, J. Chem. Phys. 37 (1962) 511.
[30]
H. Xia, J. Feng, Y. Wang, J. Li, Z. Jia, C. Tu, Sci. Rep. 5 (2015) 13988.
[31]
Y. Wang, J. Li, Z. You, Z. Zhu, J. Xu, C. Tu, Opt. Express 23 (2015) 18554.
[32]
L. Huang, Q. Guo, J. Luo, S. Wang, J. Zhou, Z. Wu, Chin. J. Quant. Elect. 29 (2012) 45 .
[33]
A. Hu, Q. Guo, S. Wang, L. Huang, Z. Wu, J. Zhou, Infrar. Laser Eng. 41 (2012) 1168.
[34]
C. StrohhÖfer, P.G. Kik, A. Polman, J. Appl. Phys. 88 (2000) 4486.
[35]
S. Georgescu, O. Toma, H. Totia, IEEE Quant. Elect. 39 (2003) 722.
Graphical abstract The bulk crystal of 5at% Yb3+ and 20at% Er3+ co-doped SrLaGa3O7 was grown successfully by the Czochralski method. The spectroscopic properties and the rate equation model were presented and analyzed systematically.