A new Yb-doped oxyorthosilicate laser crystal: Yb:Gd2SiO5

Solid State Communications 137 (2006) 451–455 www.elsevier.com/locate/ssc

A new Yb-doped oxyorthosilicate laser crystal: Yb:Gd2SiO5 Chengfeng Yan a,c,*, Guangjun Zhao a, Lianhan Zhang a, Jun Xu a,*, Xiaoying Liang a, Du Juan a,c, Wenxue Li b, Haifeng Pan b, Liangen Ding b, Heping Zeng b a

Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, People’s Republic of China b Key Laboratory of Optical and Magnetic Resonance Spectroscopy, Department of Physics, East China Normal University, Shanghai 200062, People’s Republic of China c Graduate School of Chinese Academy of Sciences, Beijing 100039, China Received 6 September 2005; received in revised form 25 October 2005; accepted 8 December 2005 by P. Sheng Available online 5 January 2006

Abstract A new Yb-doped oxyorthosilicate laser crystal, Yb:Gd2SiO5 (Yb:GSO), has been grown by the Czochralski (Cz) method. The crystal structure was determined by means of X-ray diffraction analysis. Room temperature absorption and fluorescence spectra of Yb3C ions in GSO crystal were measured. Then, spectroscopic parameters of Yb:GSO were calculated and compared with those of another Yb-doped oxyorthosilicate crystal Yb:YSO. Results indicated that Yb:GSO crystal seemed to be a very promising laser gain media in generating ultra-pulses and tunable solid state laser applications. As expected, the output power of 2.72 W at 1089 nm was achieved in Yb:GSO crystal with absorbed power of only 4.22 W at 976 nm, corresponding to the slope efficiency of 71.2% through the preliminary laser experiment. q 2005 Elsevier Ltd. All rights reserved. PACS: 42.70.Hj; 87.64.Ni; 81.10.Kh Keywords: A. Yb:Gd2SiO5; B. Czochralski method; C. Crystal structure; D. Absorption spectra; D. Fluorescence spectra; D. Laser

1. Introduction In the past decade, Ytterbium-doped solid state lasers have raised a considerable attention for the development of commercial high power InGaAs laser diodes in the 900– 1000 nm range [1]. However, a drawback of current Yb3C lasers is that Yb3C can only operate in a quasi-three-level scheme. The ground state manifold is also the laser terminal level, thermal populating of the terminal laser level causes strong re-absorption at the emission wavelengths, resulting in high laser pumping threshold power. In order to limit thermal population of the terminal level, a relatively strong crystal field is expected, increasing the Stark-splitting of the Yb3C2F7/2 ground manifold as large as possible. Thus, a low-symmetry crystal structure and multi-type of substitutional sits are essential for Ytterbium-doped host materials [2]. * Corresponding authors. Address: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, People’s Republic of China. Tel.: C86 2 1699 15174; fax: C86 2 1699 18485. E-mail addresses: [email protected] (C. Yan), [email protected]. cn (J. Xu).

0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.12.023

Recently, much attention have been attracted to Yb-doped oxyorthosilicates Yb:Y2SiO5 (Yb:YSO) [3,4] and Yb:Lu2SiO5 (Yb:LSO) [5]. Yb:YSO and Yb:LSO have the same lowsymmetry crystal structure of monoclinic C2/c space group and two non-equivalent crystallographic sites (coordinated with 7 and 6 oxygen atoms). Both crystals exhibit a large ground-state splitting and high absorption cross-sections and broad emission spectra. As expected, outstanding laser action were obtained in Yb:YSO and Yb:LSO including high optical conversions of more than 50% and large tunability. In this paper, we present a new Yb-doped oxyorthosilicate crystal, Yb:GSO. 5 at.% Yb:GSO has been grown by the Czochralski (Cz) method. The crystal structure was determined by means of X-ray diffraction analysis. Room temperature absorption and fluorescence spectra of Yb3C ions in GSO crystal were measured. Then, spectroscopic parameters of Yb:GSO were calculated and compared with those of Yb:YSO laser crystal. The preliminary laser experiment was performed on Yb:GSO laser material pumped under diode-end-pumping at room temperature. 2. Experiments The Yb:GSO single crystal was grown by Czochraski method in inductively heated iridium crucibles under nitrogen

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C. Yan et al. / Solid State Communications 137 (2006) 451–455

3. Results and discussion 3.1. Crystal structure

Fig. 1. The photograph of 5 at.% Yb:GSO crystal grown by Czochralski process.

ambient atmosphere. The starting materials are SiO2, Gd2O3 and Yb2O3 powder with at least 99.995% purity. Yb concentration in the melt was 5 at.% with respect to Gd. Nitrogen with purity of 5 N was used as the growth atmosphere. The pulling rate was 1–3 mm/h and the rotation rate of seed was 15–30 rpm. The GSO seed oriented along cleavage plane (100) is 4.5 mm diameter. After growth the Yb:GSO crystal was cooled to room temperature in 40 h. Finally, A Yb:GSO crystal boule with a size up to 60 mm in length and 30 mm in diameter was obtained (Fig. 1). The crystal boule was yellowy, high optical quality, cracking-free and none of inclusions. After air-annealing, it changed from yellowy to colorless. When the 20 mW 532 nm green laser beam passed through the crystal, the light beam was almost unseen by eyes, which indicated very few scattering particles in 5 at.% Yb:GSO crystal. The crystal structure was examined by D/MAX 2550 V powder X-ray diffractometer (XRD). The unpolarized absorption spectrum of Yb:GSO was measured by the UV/vis/NIR spectrophotometer (Model V-570, JASCO). Fluorescence spectrum was measured by TRIAX 550-type spectrophotometer made by Jobin–Yvon Company and the pumping source was a 940 nm laser. The fluorescence lifetime was measured by exciting the samples with a xenon lamp and detected by an S-1 photomultiplier tube. The laser experiment was performed with a 1.5 mm-long, 5 mm!6 mm of aperture, uncoated, 5 at.%-doped Yb:GSO crystal. All measurements were performed at room temperature.

The crystal structure of the as-grown 5 at.% Yb:GSO was examined by XRD. It demonstrates that symmetry of the Yb:GSO crystal still maintains the primitive monoclinic structure with space group of P21/c. In GSO host lattice, there are two non-equivalent crystallographic sites of Gd3C[6], later on labeled site Gd1 and site Gd2, which are coordinated with 7 and 9 oxygen atoms, respectively. From the structure point of view, both YSO and GSO hosts possess a low-symmetry monoclinic structure, a wide band gap, two non-equivalent distorted host sites and a large distribution of RE–O distances [2–8], which were shown in Table 1. Such characteristics are favourable for creating a strong crystal-field strength in YSO host as well as in GSO host. As Yb3C ion substitutes for Gd3C in GSO host, Yb3C has strong-field coupling effect with GSO host, the 4f13 shell of the Yb3C ion should be affected by the strong crystal-field and results in a large Stark-splitting of the Yb3C manifolds. We know that the Stark-splitting of 2F7/2 manifold at site Y1 is larger than that at site Y2 in YSO. In a similar way, Yb3C in site Gd1 has a larger crystal-field splitting than in site Gd2. Thus, Yb-doped GSO crystal is also a potentially useful quasi-fourlevel laser scheme, which results in easy population inversion and a low threshold value. 3.2. The absorption and LD-excited IR fluorescence spectra Room-temperature absorption and fluorescence spectra of Yb:GSO and Yb:YSO are shown in Fig. 2. They are very different. We can observe that Yb:YSO crystal exhibits four main intense absorption bands peaking at 977, 950, 917 and 899 nm, of which the absorption at 977 nm is the strongest. The absorption spectrum of Yb:GSO is also mainly composed of four strong bands around 974, 939, 921 and 897 nm, but the biggest intense absorption band is around 921 nm. These bands all correspond to the typical transitions from the ground state 2 F7/2 to the sublevels of 2F5/2 of Yb3C. The fluorescence spectrum of Yb:YSO, in addition to the zero-line at 977 nm, is mainly composed of four bands around 1003, 1040, 1056 and 1081 nm. The fluorescence intensity at 1003 nm is the strongest, but the fluorescence intensity at 1081 nm is the weakest. Differently, the fluorescence spectrum of Yb:GSO only includes four strong bands around 1013, 1030, 1048, and 1088 nm, The fluorescence intensity at 1030 and

Table 1 Structure parameters of GSO and YSO host [2–8] Host

Band gap (ev)

Space group

Label

Symmetry

Coordination number

˚) RE–Omin (A

˚) RE–Omax (A

Y2SiO5 (YSO)

w5.6

Gd2SiO5 (GSO)

w6.1

Monoclinic C2/c Monoclinic P21/c

Y1 Y2 Gd1 Gd2

C1 C1 Cs C3v

6 7 7 9

2.18 2.19 2.29 2.35

2.30 2.66 2.53 2.76

C. Yan et al. / Solid State Communications 137 (2006) 451–455 8

16 Yb-YSO 977 1003

6 4

899

Fluorescence 12 1035

917

950

8 1056

2

1081

FWHM=48 nm

0

4 0

8

16

Yb-GSO 1030

6

1088

921 4 897

974

1048

8

1013

939

12

4

2

Fluorescence intensity(a.u.)

Absorption

Absorption Coefficient (cm-1)

453

FWHM=71 nm 0

0 860 880 900 920 940 960 980 1000 1020 1040 1060 1080 1100 1120 1140

Wavelength(nm) Fig. 2. Room-temperature absorption and fluorescence spectra of Yb:GSO and Yb:YSO.

1088 nm is much stronger than others. They correspond to the transitions from the lowest level of 2F5/2 to the other levels in the 2F7/2 manifold except the lowest. The experimental fluorescence lifetime value of the excited manifold 2F5/2 of Yb3C has been measured to 1.74 ms for Yb:YSO and 1.76 ms for Yb:GSO. Compared with Yb:YSO, the overlapping between absorption and fluorescence spectra of Yb:GSO is much weaker, indicating higher quantum efficiency. As shown in Fig. 2, the absorption at 977 nm in YSO and 974 nm in GSO host belongs to the zero-line transition between the lowest levels of 2F7/2 and 2F5/2 manifolds. The emission at 1081 nm in Yb:YSO and 1088 nm in Yb:GSO is due to transition from the lowest levels of 2F5/2 manifold to the highest levels of 2F7/2 manifold. Obviously, one can estimate that the overall splitting of the ground-state manifold 2F7/2 in Yb:GSO reaches about 1076 cmK1, which is one of the largest known values and larger than that of Yb:YSO (about 985 cmK1). This proves that ytterbium experiences quite a much stronger crystalfield interaction in GSO host than in YSO host. Larger splitting of the fundamental manifold 2F7/2 is of critic importance to limit the thermal population of the terminal laser level. Thus, the thermal populating of thermal populating of the terminal laser level of the Yb3C 2F7/2 ground state in Yb:GSO is smaller than that of Yb:YSO, which can bring about smaller re-absorption losses and then to decrease the laser threshold in Yb:GSO. Yb:GSO crystal appears as an interesting laser medium approaching a quasi-fourlevel laser operating scheme like the Nd3C counterparts, which results in easy population inversion and a low threshold value. 3.3. Spectroscopic parameters The absorption cross-section of Yb3C is calculated according to the expression: sabs Z

a ; N

(1)

where a is the absorption coefficient of Yb3C, NZ8.0! 1020 ion/cm3 is the concentration of Yb3C ions in Yb:GSO, NZ9.6!1020 ion/cm3 is the concentration of Yb3C ions in Yb:YSO. The emission cross-section of Yb3C:2F5/2–2F7/2 transition can be calculated by Fu¨chtbauer–Ladenburg formula [9,10]: sem ðlÞ Z

l5 IðlÞ Ð ; 8pn ctrad lIðlÞdl 2

(2)

where I(l) is the intensity of corrected emission spectrum, trad is the radiative lifetime value of the excited manifold 2F5/2 of Yb3C, c is the velocity of light and n is the refractive index. The absorption cross section sabs and the emission cross section sem of 5 at.% Yb:GSO and 5 at.% Yb:YSO crystal were calculated and shown in Fig. 3. Then, their spectroscopic parameters were compared and listed in Table 2. Since broad and strong absorption bands are necessary to increase diode-pumping efficiency, because laser diodes typically emit in a 5 nm wide spectral range, and present a thermal shift of the peak wavelength [2]. As listed in Table 2, the absorption cross-section sabs at 921, 939, and 974 nm of Yb:GSO has been estimated at 0.60!10K20, 0.39!10K20, and 0.51!10K20 cm2, respectively. Their absorption bandwidth is about 26, 24 and 10 nm in turn. One can recognize here, the absorption bands of both Yb:GSO and Yb:YSO crystal are strong and broad enough to well match with the emission wavelength of efficient diode-pumping using high performance InGaAs laser diodes. According to Fig. 3, the largest emission cross-section of Yb:YSO is around 1003 nm, but the strong re-absorption losses consequently detrimentally affects the laser action around this band. Though the last band around 1081 nm exhibits a terminal laser level very few populated but its emission cross-section is very small, which is only about 0.12!10K20 cm2. While in Yb:GSO, four strong fluorescence bands around 1013, 1030,

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C. Yan et al. / Solid State Communications 137 (2006) 451–455 0.8 Yb:YSO 977

σabs

σem

0.6

Cross-section (x10–20cm2)

0.4

917

899

1003

950

1035

1056

0.2

1081

0.0 0.8 Yb:GSO

921 0.6

974

0.4

1048 1013

0.2 0.0 860

1088

1030

939

897

880

900

920

940

960

980

1000 1020 1040 1060 1080 1100

Wavelength (nm) Fig. 3. The absorption cross section sabs and the emission cross section sem of 5 at.% Yb:GSO and 5 at.% Yb:YSO. Table 2 Spectroscopic parameters of 5 at.% Yb:GSO and 5 at.% Yb:YSO Parameter

5 at.% Yb:YSO

5 at.% Yb:GSO

Main absorption peaks labs (nm) Absorption bandwidth (nm) Absorption cross-section sabs (10K20 cm2) Fluorescence peak (nm) Emission cross-section sem (10K20 cm2) Emission bandwidth (nm) Fluorescence lifetime (ms)

899, 917, 950, 977 15, 24, 31, 13 0.31, 0.28, 0.32, 0.64 977, 1003, 1035, 1056, 1081 0.24, 0.39, 0.23, 0.18, 0.12 48 1.74

897, 921, 939, 974 17, 26, 24, 10 0.33, 0.60, 0.39, 0.51 1013, 1030, 1048, 1088 0.17, 0.45, 0.31, 0.46 71 1.76

output coupler for 4.22 W of incident power at 976 nm, the low threshold pump power of laser operation was 380 mW and the slope efficiency evaluated at pump powers is up to 71.2%. 3000 Output power at 1090 nm (mW)

1048, and 1088 nm, can be all served as the possible efficient laser output, since their emission cross section sem is 0.17! 10K20, 0.45!10K20, 0.31!10K20 and 0.46!10K20 cm2, respectively. Particularly, in addition to the smallest thermal populating of the terminal laser level which brings about smallest re-absorption losses, the emission band around 1088 nm possesses the largest emission cross-section, which is most favorable for low threshold and high efficient laser operating around 1088 nm in Yb:GSO. It should be noted that, the particularly broad emission bandwidth (FWHM) of Yb:GSO crystal reaches about 71 nm, which is bigger than that of Yb:YSO (about 48 nm). Apparently, Yb:GSO crystal seemed to be a very promising laser gain media in generating ultra-pulses and tunable solid state laser applications when LD pumped at 940 and 980 nm.

2500

Yb:GSO: Thresholds: 380 mW Slope efficiency: 71.2% @ 1089 nm

2000 1500 1000 500

3.4. Laser performance The preliminary laser experiment was performed on Yb:GSO laser material pumped under diode-end-pumping at room temperature, as shown in Fig. 4. A maximal output power of 2.72 W at 1089 nm were obtained with a 12%

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Absorbed pump power at 980 nm (W)

Fig. 4. Output power versus absorbed power with 5 at.% Yb:GSO at room temperature with a 12% transmission output coupler.

C. Yan et al. / Solid State Communications 137 (2006) 451–455

4. Conclusion A new Yb-doped oxyorthosilicate laser crystal, Yb:Gd2SiO5 (Yb:GSO), has been grown by the Czochralski (Cz) method. Crystal structure of Yb:GSO is monoclinic symmetry with space group of P21/c. Ytterbium experiences quite a strong crystal-field interaction in GSO host and the overall splitting of the 2F7/2 manifold reaches about 1076 cmK1, which is one of the largest known values and larger than that of Yb:YSO (about 985 cmK1). Yb:GSO appears as an interesting laser medium approaching a quasi-four-level laser scheme, which results in easy population inversion and a low threshold value. Compared with Yb:YSO, Yb:GSO crystal has comparative absorption cross-sections and much broader emission bandwidth (about 71 nm). Such broad bands are similar to those of disordered materials such as glasses. The emission cross section sem of the fluorescence peaks of Yb:GSO are higher. Particularly, the emission band around 1088 nm possesses the largest emission cross-section, which is most favorable for low threshold and high efficient laser operating around 1088 nm in Yb:GSO. Through the preliminary laser experiment, a maximal output power of 2.72 W at 1089 nm were obtained with a 12% output coupler for 4.22 W of incident power at 976 nm, the low threshold pump power of laser

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operation was 380 mW and the slope efficiency evaluated at pump powers is up to 71.2%. Acknowledgements This work is supported by Science and Technology department of Shanghai (Grant 05JC14082), National Natural Science Fund (Grant No. 60544003). References [1] P. Lacovara, H.K. Choi, C.A. Wang, Opt. Lett. 16 (1991) 1089. ´ e, B. Vianal, J. Phys.: Condens. Matter 13 [2] P.H. Haumesser, R. Gaum (2001) 5427. [3] S. Campos, A. Denoyer, S. Jandl, B. Viana, D. Vivien, J. Phys.: Condens. Matter 16 (2004) 4579. [4] M. Jacquemet, C. Jacquemet, N. Janel, F. Druon, F. Balembois, Appl. Phys. B 78 (2004) 13. [5] M. Jacquemet, C. Jacquemet, N. Janel, F. Druon, F. Balembois, Appl. Phys. B 80 (2005) 171. [6] M. Jie, G. Zhao, J. Xu, J. Cryst. Growth 277 (2005) 175. [7] A.S.S. de Camargo, M.R. Davolos, L.A.O. Nunes, J. Phys.: Condens. Matter 14 (2002) 3353. [8] Y. Chen, B. Liu, C. Shi, M. Kirm, G. Zimmerer, J. Phys.: Condens. Matter 17 (2005) 1217. [9] J.A. Caird, A.J. Ramponi, P.R. Staver, J. Opt. Soc. Am. B 8 (1991) 1391. [10] L.D. Deloach, S.A. Payne, L.L. Chase, L.K. Smith, IEEE J. Quantum Electron. 29 (1993) 1179.