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Growth and spectroscopic properties of Yb:Lu1.5Y1.5Al5O12 mixed crystal
Optical Materials 33 (2010) 112–115
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Growth and spectroscopic properties of Yb:Lu1.5Y1.5Al5O12 mixed crystal Shishu Cheng a,b, Xiaodong Xu a,⇑, Dongzhen Li a,b, Dahua Zhou a,b, Feng Wu a, Zhiwei Zhao a, Jun Xu c a
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China c Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China b
a r t i c l e
i n f o
Article history: Received 30 March 2010 Received in revised form 19 August 2010 Accepted 25 August 2010
Keywords: A1. Spectral properties A2. Czochralski method B1. Yttrium compounds B3. Solid state lasers
a b s t r a c t Yb3+-doped (Lu0.5Y0.5)3Al5O12 (Yb:LuYAG) single crystal has been grown by the Czochralski method. The segregation coefficient of Yb3+ was studied by the inductively coupled plasma atomic emission spectrometry (ICP-AES) method. The crystal structure has been determined by X-ray diffraction analysis. The absorption and emission spectra and fluorescence lifetime of Yb:LuYAG crystal were measured at room temperature. The spectroscopic parameters of Yb:LuYAG crystal were compared with those of Yb:YAG and Yb:LuAG crystals with the same doping level. The results indicate that Yb:LuYAG crystals are potential candidates for high-power ultrashort pulse lasers. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Recent development of InGaAs laser diodes has stimulated interest in Yb3+-doped solid state materials as gain media for diode-pumped high-power lasers emitting in the spectral range near 1 lm [1–3], and Yb3+-doped materials have been recently reported to be applied in laser cooling of the solids [4]. Broad gain bandwidth of these materials has stimulated their applications for generation of ultrashort pulses in mode-locked regime. YAG and LuAG are excellent host materials and possess many features for high average power laser applications [5–10]. However, a narrow emission band at the main emission peak limits shorter pulse laser output. For further shortening the modelocked pulses, it is necessary to broaden the gain bandwidth of the material. One of the possible ways is to use the mixed crystals due to their disordered natures resulting in inhomogeneous broadening of fluorescence lines, with expectations of improving the laser performance in mode-locked regimes [11–14]. Optical grade single crystals of (LuxY1-x)3Al5O12 (LuYAG) solid solution series can be easily grown by Czochralski method and the solidification points, lattice parameters, effective segregation coefficients, refractive indices and thermal properties of LuYAG crystals were reported [15]. Tm3+-doped LuYAG mixed crystals are attractive media for laser-radar in the 2 lm region because its laser wavelength is superior to that of Tm:LuAG and Tm:YAG crystals [16]. Ytterbium ion in LuYAG mixed crystals is expected
⇑ Corresponding author. Tel.: +86 21 69918484; fax: +86 21 69918607. E-mail address: [email protected] (X. Xu). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.08.020
to exhibit a wider emission band due to disordered crystal-field in the crystals. In this paper, Yb:Lu1.5Y1.5Al5O12 crystal was grown by Czochralski method for the first time to our knowledge. The spectroscopic properties of Yb:LuYAG were measured and compared with those of Yb:YAG and Yb:LuAG with the same Yb3+ concentration.
2. Experiments Yb:Lu1.5Y1.5Al5O12 single crystal with 10 at.% doped concentration was grown by the Czochralski technique. The start materials used were Yb2O3, Lu2O3, Y2O3 and Al2O3 with at least 99.995% purity. The detailed crystal growth procedure was similar with that of Yb:YAG crystals described elsewhere [17] and it can also be referred in [15], which reported the growth of pure LuYAG crystal. Because the solidification point of Lu1.5Y1.5Al5O12 crystal is 1970 °C, which is lower than that of LuAG crystal (2010 °C), but higher than that of YAG crystal (1930 °C), we chose the <1 1 1>-oriented LuAG crystal as seed. Samples for spectroscopic measurements were cut from the boule and surfaces perpendicular to the <1 1 1>-growth axis were polished. The thickness of the sample is 1.2 mm. The absorption spectra were measured in the wavelength range from 300 to 1200 nm using a Lambda 900 spectrophotometer (Perkin-Elmer Company). The luminescence spectrum of the sample was recorded by a spectrofluorometer (Fluorolog-3, Jobin Yvon, Edision, USA) equipped with a Hamamatsu R928 photomultiplier tube. A 940 nm continuous wave diode laser was used as the excitation source. The decay time was measured using a computer controlled
S. Cheng et al. / Optical Materials 33 (2010) 112–115
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transient digitizer. All measurements were performed at room temperature. The actual concentrations of Yb3+, Y3+ and Lu3+ ions in the crystals were measured by Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES). The sample was cut from the upper part of the as-grown Yb:LuYAG crystal, and then was ground to powder in an agate mortar for measurement. The crystal structure of as-grown Yb:LuYAG single crystal was analyzed by X-ray diffraction (XRD) using Cu Ka radiation (Ultima IV diffractometer, Rigaku, Japan) at a scan width of 0.02° within 2h = 10–90°. Fine ground powder of the as-grown Yb:LuYAG single crystal was used as the sample. 3. Results and discussion The Yb:LuYAG single crystal of good optical quality up to 40 mm in length and 30 mm in diameter is shown in Fig. 1, and the crystal boule was cyan in color and free from cracks, inclusions and scattering centers. The cyan color could be removed by annealing the samples in air atmosphere at 1200 °C for 24 h. The crystals became navy blue after annealed in hydrogen atmosphere at 1000 °C for 24 h.The segregation coefficient for Yb3+ in Yb:LuYAG crystal can be calculated according to the following formula [17]:
K m ¼ C t =C 0
ð1Þ 3+
where Ct is the Yb concentration at the growth starting position in the crystal; C0 is the initial Yb3+ concentration in the melt. The segregation coefficient of Yb3+ in Yb:LuYAG crystal was calculated to be 1.10. The segregation coefficient for Y ions is 0.85, which is not far from that in Lu1.5Y1.5Al5O12 crystal (0.89) [15]. It is widely known that for bulk crystals grown by Czochralski technique, the transversal dopant distribution is almost uniform, but if the segregation coefficient is not equal to 1, the longitudinal distribution will vary a lot in different parts of the boule. The actual concentration of Yb3+ in different parts can be calculated by the equation:
C s ¼ C 0 K m ð1 gÞK m 1
ð2Þ
where g is the crystallized fraction of the melt, Km is the segregation coefficient and C0 is the initial Yb3+ concentration in the melt. The structure of Yb:LuYAG has been determined by X-ray diffraction analysis, which is shown in Fig. 2. The result reveals the Yb:LuYAG crystal crystallizes in cubic with space group Ia3d and has the cell parameters: a = 1.1949 nm, V = 1.7061 nm3. The cell parameter is much smaller than that of Lu1.5Y1.5Al5O12 crystal (a = 1.1958 nm) [15]. The radius of Yb3+ ion (0.0985 nm) is not far from the radius of Lu3+ ion (0.0977 nm), but smaller than that of Y3+ (0.1019 nm). The Yb3+ ions of smaller radius are in the place of Y3+ ions of larger radius resulting in smaller cell parameter [18].
Fig. 1. The photograph of as-grown Yb:LuYAG crystal boule.
Fig. 2. Powder XRD pattern of Yb:LuYAG crystal.
Fig. 3 shows the absorption spectra of as-grown and after annealed Yb:LuYAG crystal in the visible wavelength region at room temperature. There are two absorption bands locating at wavelengths 372 and 610 nm, respectively, in as-grown and H2-annealed Yb:LuYAG crystals. The absorption peak at 372 nm corresponds to the f ? d electron transition of Yb2+ ions in the crystal, while the absorption peak at 610 nm is due to the Re-F color centers in the crystal [19]. The crystal was grown in an inert atmosphere, which brought a lot of oxygen vacancies and formed Re-F color centers. Yb2+ and Re-F color centers are detrimental to the intrinsic spectroscopic performances of Yb:LuYAG. After H2annealing, the color center absorption peaks increased in intensity, and the main band position moved a little to the longer wavelength. On the other hand, the absorption bands disappeared after air-annealing, as Yb2+ ions have been oxidized to be Yb3+ and oxygen vacancies have been filled, consequently eliminating the color centers. The absorption spectrum of Yb:LuYAG crystal in the range of 850–1050 nm is presented in Fig. 4. The absorption bands of Yb3+ ion were centered at 916, 939, 969 and 1029 nm, corresponding to the 2F7/2 ? 2F5/2 transition of Yb3+. The multi-peak absorption is attributed to the crystal-field splitting in the host material. The maximum absorption band at 939 nm has a FWHM (full width at half maximum) of 25 nm, which is suitable for InGaAs diode-laser pumping. The absorption coefficients at 939 and 969 nm are 10.04 and 6.40 cm1, respectively. The absorption cross-sections are 0.66 1020 cm2 and 0.42 1020 cm2 at 939 and 969 nm in turn. A wide FWHM means the laser crystal is less sensitive to diode
Fig. 3. Absorption spectra of as-grown and after annealed Yb:LuYAG crystal in the visible wavelength region.
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S. Cheng et al. / Optical Materials 33 (2010) 112–115
Fig. 4. Absorption spectrum of Yb:LuYAG crystal in the range of 850–1050 nm. Fig. 6. Decay curve of the 2F5/2 manifold of Yb:LuYAG crystal.
wavelength specification and the output power of the laser would be more stable. Therefore, the FWHM of absorption band at pump wavelength is one of the important parameters for laser crystal. Fig. 5 shows the fluorescence spectrum of the Yb:LuYAG crystal. There is a strong emission peak locating at 1030 nm, and the emission bandwidth is 12.5 nm. Wide emission bandwidth is useful for generating femtosecond mode-locked pulses. Fluorescence decay curve is shown in Fig. 6. The fluorescence lifetime was measured to be 1.36 ms. The measurement of the intrinsic lifetime of Yb3+ ions is difficult because the reabsorption effect lengthens the measured lifetime strongly. In our experiment, the sample is as thin as 1.2 mm, which can reduce the reabsorption effect to a certain extent. From the absorption and fluorescence spectra, the resulting Stark energy-level diagram of Yb3+ in Yb:LuYAG crystal-field at room temperature can be determined and presented in Fig. 7. The ground-state energy-level splitting, given by DE = Ezl Eext, was calculated to be DE = 769 cm1 and the zero-line energy Ezl is 10,320 cm1. The emission cross-section of Yb:2F5/2 ? 2F7/2 transition can be calculated by the Fuchtbauer–Ladenburg formula [20]:
rem ðkÞ ¼
k5 IðkÞ R 8pn2 csr IðkÞkdk
Energy(cm-1) 11000
2F
10917 10650 10320
5/2
10000 2000
939nm
1030nm
1000
2F 0
769 611 390
7/2
0
Yb:LuYAG Fig. 7. Stark energy-level diagram of the Yb:LuYAG crystal.
2
F5/2 and
2
F7/2 manifolds of Yb3+ in
ð3Þ
where I(k) is the intensity of the emission spectrum, sr the radiative lifetime value of the excited manifold 2F5/2 of Yb3+, c the velocity of light and n the refractive index which is equal to 1.83. The emission cross-section rem spectrum is shown in Fig. 8. The rem at 1030 nm is 1.24 1020 cm2 as calculated from Eq. (3).
Fig. 8. Emission cross-section spectrum of Yb:LuYAG crystal.
Fig. 5. Fluorescence spectrum of Yb:LuYAG crystal excited by 940 nm.
Spectroscopic properties of some Yb3+-doped garnet family hosts are presented in Table 1, where Yb:LuYAG is compared to other crystals. The emission bandwidth of Yb:LuAG is 1.25 times as wide as that of Yb:YAG and Yb:LuAG crystals, which is favorable for tunable and ultrashort pulse laser operations. Moreover, the absorption bandwidth and absorption cross-section are almost equal to those of the Yb:YAG and Yb:LuAG crystals. The fluorescence lifetime of Yb:LuYAG crystal is much bigger than that of
S. Cheng et al. / Optical Materials 33 (2010) 112–115 Table 1 Spectroscopic parameters of Yb:LuYAG, Yb:YAG and Yb:LuAG crystals with 10 at.% doped concentration. Materials
Yb:LuYAG (10 at.%)
Yb:YAG (10 at.%)
Main absorption peaks (nm) Absorption bandwidth (nm) Main emission peaks (nm)
916, 939, 969, 1029 22 (at 939 nm)
915, 941, 969, 918, 939, 1030 969, 1030 20 (at 941 nm) 22 (at 939 nm)
969, 1007, 1030, 1047 Emission bandwidth (nm) 12.5 (at 1030 nm) rabs (1020 cm2) 0.66 rem (1020 cm2) 1.24 s (ms) 1.36
968, 1006, 1029, 1047 10 (at 1030 nm) 0.66 1.52 1.21
Yb:LuAG (10 at.%)
969, 1006, 1030, 1048 10 (at 1030 nm) 0.67 1.75 0.99
Yb:YAG and Yb:LuAG crystals, but the emission cross-section of Yb:LuYAG crystal is smaller than that of Yb:YAG and Yb:LuAG crystals. 4. Conclusion Yb:LuYAG single crystal with dimensions up to U30 40 mm3 has been grown successfully by the Czochralski technique. The crystals were cyan and free from crack, inclusions and precipitations. The segregation coefficients of Yb3+ and Y3+ in the LuYAG host lattice are 1.10 and 0.85.The absorption and emission spectra and fluorescence lifetime of Yb:LuYAG crystal were measured at room temperature. The absorption, emission cross-section and fluorescence lifetime have been estimated as 0.66 1020 cm2, 1.24 1020 cm2 and 1.36 ms, respectively. The spectroscopic parameters of Yb:LuYAG crystal were compared with those of Yb:YAG and Yb:YAG crystal with the same doping level. The results
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indicate that Yb:LuYAG crystals are potential candidates for ultrashort pulse lasers. Acknowledgements This work is partially supported by National Natural Science Foundation of China (No. 60938001, 61078054). References [1] W.F. Krupke, IEEE. J. Sel. Top. Quantum Electron. 6 (2000) 1287. [2] E.C. Honea, R.J. Beach, S.C. Mitchell, J.A. Skidmore, M.A. Emanuel, S.B. Sutton, S.A. Payne, P.V. Avizonis, R.S. Monroe, D.G. Harris, Opt. Lett. 25 (2000) 805. [3] Y. Cheng, X. Xu, J. Xu, C. Zhao, X. Yang, X. Liang, S. Zhou, IEEE J. Quantum Electron. 45 (2009) 1571. [4] Denis V. Seletskiy, Seth D. Melgaard, Stefano Bigotta, Alberto Di Lieto, Mauro Tonelli, Mansoor Sheik-Bahae, Nat. Photonics 4 (2010) 161. [5] A. Brenier, Y. Guyot, H. Canibano, G. Boulon, A. Rodenas, D. Jaque, A. Eganyan, A.G. Petrosyan, J. Opt. Soc. Am. B 23 (2006) 676. [6] J. Dong, K. Ueda, A.A. Kaminskii, Opt. Lett. 32 (2007) 3266. [7] J. He, X. Liang, J. Li, H. Yu, X. Xu, Z. Zhao, J. Xu, Z. Xu, Opt. Express 17 (2009) 11537. [8] Q. Liu, M. Gong, F. Liu, W. Gong, C. Li, Opt. Lett. 30 (2005) 726. [9] X. Xu, Z. Zhao, P. Song, G. Zhou, J. Xu, P. Deng, J. Opt. Soc. Am. B 21 (2004) 543. [10] D. Sangla, N. Aubry, A. Nehari, A. Brenier, O. Tillement, K. Lebbou, F. Balembois, P. Georges, D. Perrodin, J. Didierjean, J.M. Fourmigue, J. Cryst. Growth 312 (2009) 125. [11] V.E. Kisel, N.A. Tolstik, A.E. Troshin, N.V. Kuleshov, V.N. Matrosov, T.A. Matrosova, M.I. Kupchenko, F. Brunner, R. Paschotta, F. Morier-Genoud, U. Keller, Appl. Phys. B 85 (2006) 581. [12] J. Liu, H. Zhang, X. Mateos, W. Han, V. Petrov, Opt. Lett. 33 (2008) 1810. [13] J. Dong, K. Ueda, A.A. Kaminskii, Opt. Express 16 (2008) 5241. [14] Y. Xue, C. Wang, J. Li, Q. Liu, L. Chai, J. Wang, H. Zhang, IEEE J. Quantum Electron. 44 (2008) 686. [15] Y. Kuwano, K. Suda, N. Ishizawa, T. Yamada, J. Cryst. Growth 260 (2004) 159. [16] J.D. Kmetec, T.S. Kubo, T.J. Kane, Opt. Lett. 19 (1994) 186. [17] X. Xu, Z. Zhao, J. Xu, P. Deng, J. Cryst. Growth 255 (2003) 338. [18] Y. Yu, Z. Wu, S. Zhang, J. Alloy Compd. 302 (2000) 204. [19] P. Yang, P. Deng, Z. Yin, J. Lumin. 97 (2002) 51. [20] S.A. Pain, L.L. Chase, L.K. Smith, W.L. Kway, W.F. Krupke, IEEE J. Quantum Electron. 28 (1992) 2619.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Growth and spectroscopic properties of Yb:Lu1.5Y1.5Al5O12 mixed crystal Shishu Cheng a,b, Xiaodong Xu a,⇑, Dongzhen Li a,b, Dahua Zhou a,b, Feng Wu a, Zhiwei Zhao a, Jun Xu c a
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China c Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China b
a r t i c l e
i n f o
Article history: Received 30 March 2010 Received in revised form 19 August 2010 Accepted 25 August 2010
Keywords: A1. Spectral properties A2. Czochralski method B1. Yttrium compounds B3. Solid state lasers
a b s t r a c t Yb3+-doped (Lu0.5Y0.5)3Al5O12 (Yb:LuYAG) single crystal has been grown by the Czochralski method. The segregation coefficient of Yb3+ was studied by the inductively coupled plasma atomic emission spectrometry (ICP-AES) method. The crystal structure has been determined by X-ray diffraction analysis. The absorption and emission spectra and fluorescence lifetime of Yb:LuYAG crystal were measured at room temperature. The spectroscopic parameters of Yb:LuYAG crystal were compared with those of Yb:YAG and Yb:LuAG crystals with the same doping level. The results indicate that Yb:LuYAG crystals are potential candidates for high-power ultrashort pulse lasers. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Recent development of InGaAs laser diodes has stimulated interest in Yb3+-doped solid state materials as gain media for diode-pumped high-power lasers emitting in the spectral range near 1 lm [1–3], and Yb3+-doped materials have been recently reported to be applied in laser cooling of the solids [4]. Broad gain bandwidth of these materials has stimulated their applications for generation of ultrashort pulses in mode-locked regime. YAG and LuAG are excellent host materials and possess many features for high average power laser applications [5–10]. However, a narrow emission band at the main emission peak limits shorter pulse laser output. For further shortening the modelocked pulses, it is necessary to broaden the gain bandwidth of the material. One of the possible ways is to use the mixed crystals due to their disordered natures resulting in inhomogeneous broadening of fluorescence lines, with expectations of improving the laser performance in mode-locked regimes [11–14]. Optical grade single crystals of (LuxY1-x)3Al5O12 (LuYAG) solid solution series can be easily grown by Czochralski method and the solidification points, lattice parameters, effective segregation coefficients, refractive indices and thermal properties of LuYAG crystals were reported [15]. Tm3+-doped LuYAG mixed crystals are attractive media for laser-radar in the 2 lm region because its laser wavelength is superior to that of Tm:LuAG and Tm:YAG crystals [16]. Ytterbium ion in LuYAG mixed crystals is expected
⇑ Corresponding author. Tel.: +86 21 69918484; fax: +86 21 69918607. E-mail address: [email protected] (X. Xu). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.08.020
to exhibit a wider emission band due to disordered crystal-field in the crystals. In this paper, Yb:Lu1.5Y1.5Al5O12 crystal was grown by Czochralski method for the first time to our knowledge. The spectroscopic properties of Yb:LuYAG were measured and compared with those of Yb:YAG and Yb:LuAG with the same Yb3+ concentration.
2. Experiments Yb:Lu1.5Y1.5Al5O12 single crystal with 10 at.% doped concentration was grown by the Czochralski technique. The start materials used were Yb2O3, Lu2O3, Y2O3 and Al2O3 with at least 99.995% purity. The detailed crystal growth procedure was similar with that of Yb:YAG crystals described elsewhere [17] and it can also be referred in [15], which reported the growth of pure LuYAG crystal. Because the solidification point of Lu1.5Y1.5Al5O12 crystal is 1970 °C, which is lower than that of LuAG crystal (2010 °C), but higher than that of YAG crystal (1930 °C), we chose the <1 1 1>-oriented LuAG crystal as seed. Samples for spectroscopic measurements were cut from the boule and surfaces perpendicular to the <1 1 1>-growth axis were polished. The thickness of the sample is 1.2 mm. The absorption spectra were measured in the wavelength range from 300 to 1200 nm using a Lambda 900 spectrophotometer (Perkin-Elmer Company). The luminescence spectrum of the sample was recorded by a spectrofluorometer (Fluorolog-3, Jobin Yvon, Edision, USA) equipped with a Hamamatsu R928 photomultiplier tube. A 940 nm continuous wave diode laser was used as the excitation source. The decay time was measured using a computer controlled
S. Cheng et al. / Optical Materials 33 (2010) 112–115
113
transient digitizer. All measurements were performed at room temperature. The actual concentrations of Yb3+, Y3+ and Lu3+ ions in the crystals were measured by Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES). The sample was cut from the upper part of the as-grown Yb:LuYAG crystal, and then was ground to powder in an agate mortar for measurement. The crystal structure of as-grown Yb:LuYAG single crystal was analyzed by X-ray diffraction (XRD) using Cu Ka radiation (Ultima IV diffractometer, Rigaku, Japan) at a scan width of 0.02° within 2h = 10–90°. Fine ground powder of the as-grown Yb:LuYAG single crystal was used as the sample. 3. Results and discussion The Yb:LuYAG single crystal of good optical quality up to 40 mm in length and 30 mm in diameter is shown in Fig. 1, and the crystal boule was cyan in color and free from cracks, inclusions and scattering centers. The cyan color could be removed by annealing the samples in air atmosphere at 1200 °C for 24 h. The crystals became navy blue after annealed in hydrogen atmosphere at 1000 °C for 24 h.The segregation coefficient for Yb3+ in Yb:LuYAG crystal can be calculated according to the following formula [17]:
K m ¼ C t =C 0
ð1Þ 3+
where Ct is the Yb concentration at the growth starting position in the crystal; C0 is the initial Yb3+ concentration in the melt. The segregation coefficient of Yb3+ in Yb:LuYAG crystal was calculated to be 1.10. The segregation coefficient for Y ions is 0.85, which is not far from that in Lu1.5Y1.5Al5O12 crystal (0.89) [15]. It is widely known that for bulk crystals grown by Czochralski technique, the transversal dopant distribution is almost uniform, but if the segregation coefficient is not equal to 1, the longitudinal distribution will vary a lot in different parts of the boule. The actual concentration of Yb3+ in different parts can be calculated by the equation:
C s ¼ C 0 K m ð1 gÞK m 1
ð2Þ
where g is the crystallized fraction of the melt, Km is the segregation coefficient and C0 is the initial Yb3+ concentration in the melt. The structure of Yb:LuYAG has been determined by X-ray diffraction analysis, which is shown in Fig. 2. The result reveals the Yb:LuYAG crystal crystallizes in cubic with space group Ia3d and has the cell parameters: a = 1.1949 nm, V = 1.7061 nm3. The cell parameter is much smaller than that of Lu1.5Y1.5Al5O12 crystal (a = 1.1958 nm) [15]. The radius of Yb3+ ion (0.0985 nm) is not far from the radius of Lu3+ ion (0.0977 nm), but smaller than that of Y3+ (0.1019 nm). The Yb3+ ions of smaller radius are in the place of Y3+ ions of larger radius resulting in smaller cell parameter [18].
Fig. 1. The photograph of as-grown Yb:LuYAG crystal boule.
Fig. 2. Powder XRD pattern of Yb:LuYAG crystal.
Fig. 3 shows the absorption spectra of as-grown and after annealed Yb:LuYAG crystal in the visible wavelength region at room temperature. There are two absorption bands locating at wavelengths 372 and 610 nm, respectively, in as-grown and H2-annealed Yb:LuYAG crystals. The absorption peak at 372 nm corresponds to the f ? d electron transition of Yb2+ ions in the crystal, while the absorption peak at 610 nm is due to the Re-F color centers in the crystal [19]. The crystal was grown in an inert atmosphere, which brought a lot of oxygen vacancies and formed Re-F color centers. Yb2+ and Re-F color centers are detrimental to the intrinsic spectroscopic performances of Yb:LuYAG. After H2annealing, the color center absorption peaks increased in intensity, and the main band position moved a little to the longer wavelength. On the other hand, the absorption bands disappeared after air-annealing, as Yb2+ ions have been oxidized to be Yb3+ and oxygen vacancies have been filled, consequently eliminating the color centers. The absorption spectrum of Yb:LuYAG crystal in the range of 850–1050 nm is presented in Fig. 4. The absorption bands of Yb3+ ion were centered at 916, 939, 969 and 1029 nm, corresponding to the 2F7/2 ? 2F5/2 transition of Yb3+. The multi-peak absorption is attributed to the crystal-field splitting in the host material. The maximum absorption band at 939 nm has a FWHM (full width at half maximum) of 25 nm, which is suitable for InGaAs diode-laser pumping. The absorption coefficients at 939 and 969 nm are 10.04 and 6.40 cm1, respectively. The absorption cross-sections are 0.66 1020 cm2 and 0.42 1020 cm2 at 939 and 969 nm in turn. A wide FWHM means the laser crystal is less sensitive to diode
Fig. 3. Absorption spectra of as-grown and after annealed Yb:LuYAG crystal in the visible wavelength region.
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S. Cheng et al. / Optical Materials 33 (2010) 112–115
Fig. 4. Absorption spectrum of Yb:LuYAG crystal in the range of 850–1050 nm. Fig. 6. Decay curve of the 2F5/2 manifold of Yb:LuYAG crystal.
wavelength specification and the output power of the laser would be more stable. Therefore, the FWHM of absorption band at pump wavelength is one of the important parameters for laser crystal. Fig. 5 shows the fluorescence spectrum of the Yb:LuYAG crystal. There is a strong emission peak locating at 1030 nm, and the emission bandwidth is 12.5 nm. Wide emission bandwidth is useful for generating femtosecond mode-locked pulses. Fluorescence decay curve is shown in Fig. 6. The fluorescence lifetime was measured to be 1.36 ms. The measurement of the intrinsic lifetime of Yb3+ ions is difficult because the reabsorption effect lengthens the measured lifetime strongly. In our experiment, the sample is as thin as 1.2 mm, which can reduce the reabsorption effect to a certain extent. From the absorption and fluorescence spectra, the resulting Stark energy-level diagram of Yb3+ in Yb:LuYAG crystal-field at room temperature can be determined and presented in Fig. 7. The ground-state energy-level splitting, given by DE = Ezl Eext, was calculated to be DE = 769 cm1 and the zero-line energy Ezl is 10,320 cm1. The emission cross-section of Yb:2F5/2 ? 2F7/2 transition can be calculated by the Fuchtbauer–Ladenburg formula [20]:
rem ðkÞ ¼
k5 IðkÞ R 8pn2 csr IðkÞkdk
Energy(cm-1) 11000
2F
10917 10650 10320
5/2
10000 2000
939nm
1030nm
1000
2F 0
769 611 390
7/2
0
Yb:LuYAG Fig. 7. Stark energy-level diagram of the Yb:LuYAG crystal.
2
F5/2 and
2
F7/2 manifolds of Yb3+ in
ð3Þ
where I(k) is the intensity of the emission spectrum, sr the radiative lifetime value of the excited manifold 2F5/2 of Yb3+, c the velocity of light and n the refractive index which is equal to 1.83. The emission cross-section rem spectrum is shown in Fig. 8. The rem at 1030 nm is 1.24 1020 cm2 as calculated from Eq. (3).
Fig. 8. Emission cross-section spectrum of Yb:LuYAG crystal.
Fig. 5. Fluorescence spectrum of Yb:LuYAG crystal excited by 940 nm.
Spectroscopic properties of some Yb3+-doped garnet family hosts are presented in Table 1, where Yb:LuYAG is compared to other crystals. The emission bandwidth of Yb:LuAG is 1.25 times as wide as that of Yb:YAG and Yb:LuAG crystals, which is favorable for tunable and ultrashort pulse laser operations. Moreover, the absorption bandwidth and absorption cross-section are almost equal to those of the Yb:YAG and Yb:LuAG crystals. The fluorescence lifetime of Yb:LuYAG crystal is much bigger than that of
S. Cheng et al. / Optical Materials 33 (2010) 112–115 Table 1 Spectroscopic parameters of Yb:LuYAG, Yb:YAG and Yb:LuAG crystals with 10 at.% doped concentration. Materials
Yb:LuYAG (10 at.%)
Yb:YAG (10 at.%)
Main absorption peaks (nm) Absorption bandwidth (nm) Main emission peaks (nm)
916, 939, 969, 1029 22 (at 939 nm)
915, 941, 969, 918, 939, 1030 969, 1030 20 (at 941 nm) 22 (at 939 nm)
969, 1007, 1030, 1047 Emission bandwidth (nm) 12.5 (at 1030 nm) rabs (1020 cm2) 0.66 rem (1020 cm2) 1.24 s (ms) 1.36
968, 1006, 1029, 1047 10 (at 1030 nm) 0.66 1.52 1.21
Yb:LuAG (10 at.%)
969, 1006, 1030, 1048 10 (at 1030 nm) 0.67 1.75 0.99
Yb:YAG and Yb:LuAG crystals, but the emission cross-section of Yb:LuYAG crystal is smaller than that of Yb:YAG and Yb:LuAG crystals. 4. Conclusion Yb:LuYAG single crystal with dimensions up to U30 40 mm3 has been grown successfully by the Czochralski technique. The crystals were cyan and free from crack, inclusions and precipitations. The segregation coefficients of Yb3+ and Y3+ in the LuYAG host lattice are 1.10 and 0.85.The absorption and emission spectra and fluorescence lifetime of Yb:LuYAG crystal were measured at room temperature. The absorption, emission cross-section and fluorescence lifetime have been estimated as 0.66 1020 cm2, 1.24 1020 cm2 and 1.36 ms, respectively. The spectroscopic parameters of Yb:LuYAG crystal were compared with those of Yb:YAG and Yb:YAG crystal with the same doping level. The results
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indicate that Yb:LuYAG crystals are potential candidates for ultrashort pulse lasers. Acknowledgements This work is partially supported by National Natural Science Foundation of China (No. 60938001, 61078054). References [1] W.F. Krupke, IEEE. J. Sel. Top. Quantum Electron. 6 (2000) 1287. [2] E.C. Honea, R.J. Beach, S.C. Mitchell, J.A. Skidmore, M.A. Emanuel, S.B. Sutton, S.A. Payne, P.V. Avizonis, R.S. Monroe, D.G. Harris, Opt. Lett. 25 (2000) 805. [3] Y. Cheng, X. Xu, J. Xu, C. Zhao, X. Yang, X. Liang, S. Zhou, IEEE J. Quantum Electron. 45 (2009) 1571. [4] Denis V. Seletskiy, Seth D. Melgaard, Stefano Bigotta, Alberto Di Lieto, Mauro Tonelli, Mansoor Sheik-Bahae, Nat. Photonics 4 (2010) 161. [5] A. Brenier, Y. Guyot, H. Canibano, G. Boulon, A. Rodenas, D. Jaque, A. Eganyan, A.G. Petrosyan, J. Opt. Soc. Am. B 23 (2006) 676. [6] J. Dong, K. Ueda, A.A. Kaminskii, Opt. Lett. 32 (2007) 3266. [7] J. He, X. Liang, J. Li, H. Yu, X. Xu, Z. Zhao, J. Xu, Z. Xu, Opt. Express 17 (2009) 11537. [8] Q. Liu, M. Gong, F. Liu, W. Gong, C. Li, Opt. Lett. 30 (2005) 726. [9] X. Xu, Z. Zhao, P. Song, G. Zhou, J. Xu, P. Deng, J. Opt. Soc. Am. B 21 (2004) 543. [10] D. Sangla, N. Aubry, A. Nehari, A. Brenier, O. Tillement, K. Lebbou, F. Balembois, P. Georges, D. Perrodin, J. Didierjean, J.M. Fourmigue, J. Cryst. Growth 312 (2009) 125. [11] V.E. Kisel, N.A. Tolstik, A.E. Troshin, N.V. Kuleshov, V.N. Matrosov, T.A. Matrosova, M.I. Kupchenko, F. Brunner, R. Paschotta, F. Morier-Genoud, U. Keller, Appl. Phys. B 85 (2006) 581. [12] J. Liu, H. Zhang, X. Mateos, W. Han, V. Petrov, Opt. Lett. 33 (2008) 1810. [13] J. Dong, K. Ueda, A.A. Kaminskii, Opt. Express 16 (2008) 5241. [14] Y. Xue, C. Wang, J. Li, Q. Liu, L. Chai, J. Wang, H. Zhang, IEEE J. Quantum Electron. 44 (2008) 686. [15] Y. Kuwano, K. Suda, N. Ishizawa, T. Yamada, J. Cryst. Growth 260 (2004) 159. [16] J.D. Kmetec, T.S. Kubo, T.J. Kane, Opt. Lett. 19 (1994) 186. [17] X. Xu, Z. Zhao, J. Xu, P. Deng, J. Cryst. Growth 255 (2003) 338. [18] Y. Yu, Z. Wu, S. Zhang, J. Alloy Compd. 302 (2000) 204. [19] P. Yang, P. Deng, Z. Yin, J. Lumin. 97 (2002) 51. [20] S.A. Pain, L.L. Chase, L.K. Smith, W.L. Kway, W.F. Krupke, IEEE J. Quantum Electron. 28 (1992) 2619.