Optical Materials 29 (2006) 403–406 www.elsevier.com/locate/optmat
Growth and spectral characterization of Yb3+:LiLa(WO4)2 crystal Xinyang Huang a
a,b
, Zhoubin Lin a, Zushu Hu a, Lizhen Zhang a, Taiju Tsuboi c, Guofu Wang a,*
Fujian Institute of Research on the Structural Matter, State Key Laboratory of Structural Chemistry, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China b Graduate School of Chinese Academy of Sciences, Beijing 100039, China c Faculty of Engineering, Kyoto Sangyo University, Kamigamo, Kita-Ku, Kyoto 603-8555, Japan Received 3 January 2005; accepted 22 July 2005 Available online 10 January 2006
Abstract The growth and spectroscopic properties of Yb3+:LiLa(WO4)2 crystal were investigated. Yb3+:LiLa(WO4)2 crystal with dimension B22 · 18 mm2 has been grown by the Czochralski method. Yb3+:LiLa(WO4)2 crystal has a larger FWHM of 10 nm at 976 nm. The absorption and emission cross-sections of Yb3+:LiLa(WO4)2 crystal are 2.46 · 1020 cm2 at 976 nm and 0.39 · 1020 cm2 at 1039 nm, respectively. The radiative lifetime and fluorescence lifetime of Yb3+:LiLa(WO4)2 crystal are 0.365 ms and 1.07 ms, respectively. 2005 Elsevier B.V. All rights reserved. PACS: 42.70.Hj; 78.20.e Keywords: A1. Optical microscopy; A2. Czochralski method; B1. Tungstate; B1. Solid-state laser materials
1. Introduction With the developments of the high-power InGaAs diode laser, Yb3+-doped solid-state laser materials have gained much interest. As well known, the trivalent Yb3+ as an active ion has only two electronic manifolds, i.e. the ground 2 F7/2 manifold and the excited 2F5/2 manifold, which are separated by approximately 10 000 cm1. There is no excited state absorption reducing effective laser cross-section, no up-conversion, low quantum defect, low thermal effect and longer lifetime. The small Stokes shift reduces the thermal loading of the material during laser operation and increases the laser efficiency. Yb3+ ion exhibits intense and broad absorption and emission band, which are suitable as diode-pumped femtosecond laser and tunable laser. The tungstate is an excellent laser gain media. For example, Yb3+:KY(WO4)2 (Yb:KYW), Yb3+:KGd(WO4)2 *
Corresponding author. Tel./fax: +86 591 3714636. E-mail address:
[email protected] (G. Wang).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.07.013
(Yb:KGW) and KYb(WO4)2 (KYbW) exhibit good chemical and physical properties [1–6]. However, since the most tungstate compounds have a phase transition, they cannot be grown directly from melt. The most tungstate crystals were only grown by the flux method and a few tungstate crystal can be grown by the top nucleated floating [7], which gives rise to a difficulty in the crystal growth. In order to find more efficient new Yb3+-doped materials, we synthesized a new tungstate crystal LiLa(WO4)2. LiLa(WO4)2 crystal belongs to the tetragonal system with the space group I41/a and unit cell parameters: a = ˚ , c = 11.580 A ˚ , Z = 2 [8]. LiLa(WO4)2 melts con5.311 A gruently at 1065 C [8], which can be grown by the Czochralski method. This paper reports the growth and the spectral characterization of Yb3+:LiLa(WO4)2 crystal. 2. Crystal growth Yb3+:LiLa(WO4)2 crystal was grown by the Czochralski method in a 2 kHZ frequency furnace. The raw materials
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X. Huang et al. / Optical Materials 29 (2006) 403–406
3. Spectral characterizations A sample of Yb3+:LiLa(WO4)2 with dimension 12 · 7.2 · 2.2 mm3 was cut from the as-grown crystal. The Yb3+ concentration in this sample was determined to be 4.38 at% by the electron probe microanalysis method. Thereby, the concentration of Yb3+ was calculated to be 2.63 · 1020 cm3. Absorption spectra were recorded using a Perkin–Elmer UV–VIS–NIR spectrophotometer (Lambda-35). The fluorescence spectrum and fluorescence lifetime were measured using an Edinbugh Instruments FL920 spectrophotometer. Fig. 2 shows absorption spectrum of Yb3+:LiLa(WO4)2 crystal at room temperature, which corresponds to 2F7/2 ! 2 F5/2 transition of Yb3+ ion. The strongest absorption band at 976 nm have a full-width at half-maximum (FWHM) of 10 nm. The absorption cross-section of 2F7/2 ! 2F5/2 transition was determined using the following formula:
Fig. 1. Yb3+:LiLa(WO4)2 crystal grown by the Czochralski method.
rabs ðkÞ ¼ 2:303 3+
of Yb :LiLa(WO4)2 crystals were prepared by means of the solid-state reaction. The chemicals used were Li2CO3 (99.9%), La2O3 (99.95%), WO3 (99.9%) and Yb2O3 (99.99%). Yb3+-doped LiLa(WO4)2 crystal was synthesized according to the following chemical reaction: 0:5Li2 CO3 þ 0:47La2 O3 þ 2WO3 þ 0:03Yb2 O3 ! LiLa0:94 Yb0:06 ðWO4 Þ2 þ 0:5CO2 " .
ð1Þ
These starting materials were thoroughly mixed and pressed to form pieces. The samples were held at 780 C for 24 h. Then, the synthesized raw materials of Yb3+: LiLa(WO4)2 with doping 6 at.% Yb3+ ions were melt in a platinum crucible of B50 · 60 mm2. Yb3+:LiLa(WO4)2 crystal was grown in air atmosphere. The melt was held at 1200 C for 2 h to emit the bubbles out of the melt. Yb3+:LiLa(WO4)2 crystal was grown at a pulling rate of 0.5–0.6 mm/h and a rotation rate of 10–35 r/min. When the growth ended, the crystal was drawn out of the melt surface and slowly cooled down to room temperature. Yb3+:LiLa(WO4)2 crystal with dimension B22 · 18 mm2 was obtained, as shown in Fig. 1.
ODðkÞ ¼ aðkÞ=N 0 ; lN 0
where OD(k) is the optical density at wavelength k, l is the crystal thickness, N0 is the concentration of Yb3+ in Yb3+:LiLa(WO4)2 crystal, and a(k) is the absorption coefficient at wavelength k. The absorption coefficient at wavelength of 976 nm is 5.8 cm1. Thereby, the absorption cross-section is 2.46 · 1020 cm2 at 976 nm. The fluorescence spectrum of Yb3+:LiLa(WO4)2 crystal is depicted in Fig. 3. The dominant feature of fluorescence is a broad band extending from 900 to 1050 nm. A Lorentz fit was applied to the fluorescence spectrum and four peaks were centered at 978.8, 998.9, 1012.8 and 1024 nm. Thus, the energy levels of Yb3+:LiLa(WO4)2 crystal can be obtained from the room temperature absorption and emission spectrum, as shown in Fig. 4. The overall splitting of 2 F7/2 manifold is 631.8 cm1, which is larger than that of Yb:KYW (568 cm1), Yb:KGW (535 cm1) and KYbW (555 cm1) [5,6]. Such large energy splitting will be available to lower laser thermal effect and reduce re-absorption losses. The emission cross-section rem(k) can be calculated from the absorption cross-section rabs(k) by the following formula [9–11]: 0.4
6
10K
5 0.3 4
Optical density
Absorption coefficient (cm -1 )
298K
3 2
0.2
0.1 1 0 800
(a)
ð2Þ
900
1000
Wavelength (nm)
1100
0.0 800
(b)
850
900
950
1000
1050
1100
Wavelength (nm)
Fig. 2. Absorption spectra of Yb3+:LiLa(WO4)2 crystal (a) at room temperature and (b) at 10 K (the dot line is Lorentz fitting line).
X. Huang et al. / Optical Materials 29 (2006) 403–406
405 10K
14
Fluorescent intensity (a.u.)
298K 42000 40000 38000 36000 34000
Fluorescence intensity (a.u.)
44000 12 10 8 6 4 2
32000 0 850
900
(a)
950
1000
1050
1000
1100
(b)
Wavelength (nm)
1100
Wavelength (nm)
Fig. 3. The Fluorescence spectra of Yb3+:LiLa(WO4)2 crystal (a) at room temperature and (b) 10 K (the dot line is Lorentz fitting line).
where c is the speed of light in vacuum, I(k) is the relative emission intensity, n is the crystal refractive index, sr is the radiative lifetime. Thus, the radiative lifetime can be obtained to be 0.365 ms. The fluorescence lifetime of Yb3+:LiLa(WO4)2 crystal was measured to be 1.07 ms. The fluorescence lifetime is longer than radiative lifetime, which is caused by the radiative trapping effect [13].
10672.4cm-1 2
F5/2
10395.0cm-1 10226.9cm-1 1013nm
974nm
465.1cm-1 355.2cm-1
2
F7/2
206.9cm-1
4. Laser performance parameters
0 Fig. 4. Energy levels of Yb3+ ions in Yb3+:LiLa(WO4)2 crystal.
rem ðkÞ ¼ rabs ðkÞ
Zl exp½ðEl hc=kÞ=kT ; Zu
ð3Þ
where El is the lowest excites state energy, k is the Boltzmann constant, Zl and Zu are the partition functions of lower and upper states, respectively, which was obtained from the following formula: X Zk ¼ d k expðEk =kT Þ; ð4Þ k
where dk are the appropriate energy-level degeneracy. In addition, the fundamental relationship between spontaneous and stimulated emission rates can be expressed by the Fuchtbauer–Ladenburg (F–L) formula [11,12]: 5
rem ðkÞ ¼
k 1 IðkÞ R ; 2 8pcn sr kIðkÞdk
ð5Þ
Based on the spectroscopic parameters mentioned above, the three important parameters bmin, Isat and Imin can be evaluated. bmin parameter, which is defined as minimum inversion fraction of Yb3+ that must be excited to balance the ions exactly with ground-state absorption at extraction wavelength, can be calculated by the following expression [9,11]: bmin ¼
rabs ðkext Þ . rabs ðkext Þ þ remi ðkext Þ
ð6Þ
The pump saturation intensity Isat can be obtained from the follows formula [9]: hc . ð7Þ I sat ¼ kp rabs sem The minimum pump intensity Imin used to indicate the relative usefulness of various Yb3+-doped laser materials as a figure of merit is obtained by the following formula [9,11]:
Table 1 Laser parameters and spectral feature of Yb3+-doped LiLa(WO4)2 crystal compared with those of other Yb3+-doped laser material Crystals
FWHM (nm)
sem (ms)
rabs (1020 cm2)
kem (nm)
rem (1020 cm2)
Isat (KW/cm2)
bmin (%)
Imin (KW/cm2)
References
LLW KYW KGW YSO FAP GCOB BCBF BLB
10 10 3.5
1.07 0.6 0.6 0.70 1.08 2.6 1.17 3.7
1.95 13.3 12 2.1 10 0.55 1.3 2.9
1039 1023 1025 1035 1043 1032 1034 1010
0.39 3 2.7 0.4 5.9 0.55 1.7 0.22
7.96 2.1 2.8 11.2 11.9 25.5 17 21.3
0.047
0.44
0.06 0.069 0.047 0.06 0.097 0.16
0.15 0.09 0.09 1.54 1.64 3.4
This work [2] [2] [14] [15] [16] [17] [18]
2.4 15 19 6.4
406
I min ¼ bmin I sat .
X. Huang et al. / Optical Materials 29 (2006) 403–406
ð8Þ
As a result, the values of these laser performance parameters bmin, Isat and Imin were obtained, which are listed in Table 1. In conclusion, Yb3+:LiLa(WO4)2 crystal with dimension B22 · 18 mm2 have been grown by the Czochralski method. The spectroscopic characterizations of Yb3+:LiLa(WO4)2 crystal were investigated. Yb3+:LiLa(WO4)2 crystal has a larger FWHM of 10 nm. The absorption and emission cross-sections of Yb3+:LiLa(WO4)2 crystal are 2.46 · 1020 cm2 at 976 nm and 0.39 · 1020 cm2 at 1039 nm, respectively. The radiative lifetime and fluorescence lifetime of Yb3+:LiLa(WO4)2 crystal are 0.365 ms and 1.07 ms, respectively. Therefore, the crystal Yb3+:LiLa(WO4)2 may be regarded as a laser material. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 50272066) and Key Project of Science and Technology of Fujian Province (2001F004), respectively. References [1] F. Brunner, G.J. Spu¨hler, J.A. Au, L. Krainer, F. Morier-Genoud, R. Paschotta, Opt. Lett. 25 (2000) 1119.
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