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Visible luminescence in Yb3+-doped gadolinium gallium garnets
Materials Science and Engineering B 137 (2007) 20–23
Visible luminescence in Yb3+-doped gadolinium gallium garnets Benxue Jiang a,b , Zhiwei Zhao a,∗ , Xiaodong Xu a,b , Pingxin Song a,b , Xiaodan Wang a,b , Jun Xu a a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, PR China b Graduate School of Chinese Academy of Science, Beijing 100039, PR China Received 8 January 2006; received in revised form 26 September 2006; accepted 30 September 2006
Abstract In this paper, some results on visible luminescence performed on Yb3+ -doped gadolinium gallium garnets under 165 and 940 nm excitation were presented. The upconversion luminescence was ascribed to Yb3+ cooperative luminescence and the presence of rare earth impurity ions. The gain cross-sections of Yb:GGG crystal as a function of excited-state population fraction β were studied. Emission spectra under 165 nm at 20 K showed there was no charge transfer luminescence in Yb:GGG. © 2006 Elsevier B.V. All rights reserved. Keywords: Yb:GGG; Yb,Cr:GGG; Cooperative luminescence; Charge transfer luminescence
1. Introduction Yb3+ ion spectroscopy have received considerable attention for laser application over the past years [1,2]. The main interest of Yb3+ ion lies in its very simple energy level diagram which leads to very low quantum defects, reducing thermal loads and preventing undesired effects such as upconversion and excited state absorption and weak concentration quenching. Owing to this unique configuration, Yb3+ presents remarkable properties. The thermal conductivity of Yb:GGG is larger than that of Yb:YAG with Yb doping level more than 4 at.% [3,4]. Owing to this unique configuration, Yb:GGG crystal presents remarkable properties. Cooperative luminescence is a special type of upconversion in which two interacting ions in the excited state return to the ground state simultaneously, emitting one photon of the sum of the energies of the single ion transitions. It was observed by Nakazawa and Shionoya in 1970 for Yb3+ in YbPO4 [5]. Malinowski et al. [6], reported blue cooperative emission centered at 484 nm in Yb:YAG planar epitaxial waveguides and found that some of the visible emission peaks were exactly at half wavelength of the IR peaks.
∗
Corresponding author. Tel.: +86 2169915174. E-mail addresses: [email protected] (B. Jiang), [email protected] (Z. Zhao). 0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.09.027
In this paper, we have measured the visible luminescence spectra of Yb:GGG crystal as a function of Yb concentration and the emission spectra at low temperature. The causing of the visible luminescence of Yb:GGG was discussed. 2. Experimental Yb:GGG crystal was grown by the RF-heating Cz method. The crystal growth process was described in Ref. [7]. Samples for spectroscopic measurements were cut out of boules and the surfaces perpendicular to the growth axis were polished. The thickness of the samples was 0.5 mm. Room temperature absorption spectra were measured with a JASCO V-570 UV–vis-NIR spectrophotometer. The fluorescence spectra were acquired by a Triax 550 spectrophotometer with an InGaAs laser diode as the pump source (excited at 940 nm). The pump power was 700 mW. 3. Results and discussion Fig. 1 presents the absorption spectra of Yb:YAG and Yb:GGG crystals, attributed to the 2 F7/2 –2 F5/2 Yb3+ transition. The main absorption peak positions of Yb:GGG move to longer wavelength direction compared to Yb:YAG. The main absorption bands of Yb:GGG are centered at 932, 944, 971, 1025 nm while Yb:YAG are centered at 915, 939, 968, 1031 nm. So Yb:GGG has a small quantum defect (between pumping and laser wavelength) which contributes to weak ther-
B. Jiang et al. / Materials Science and Engineering B 137 (2007) 20–23
Fig. 1. Absorption spectra of Yb:GGG and Yb:YAG single crystal at room temperature.
mal effects. There is self-absorption (having absorption at laser wavelength) at the lasing wavelength of 1.03 mm in the crystals at room temperature.Fig. 2 shows, for Yb:GGG, room temperature gain cross-section σ g , obtained for different values of excited-state population fraction β(β = N2 /N). The gain crosssection is defined as σ g = βσ e − (1 − β)σ a and the absorption and emission cross-section curves, σ e and σ a , respectively, are given for the values β = 0 and 1, respectively. The absorption and emission cross-section of Yb can be estimated according to the following expression [18]: 2.303 lg(I/I0 ) and dc Zl Ezl − (hc/λ) σem (λ) = σabs (λ) exp Zu KT σabs (λ) =
Absorption and emission cross-section peaks are located near 945 and 1031 nm, with values of 6 × 10−21 and 11 × 10−21 cm2 . It is worth noting that the emission spectrum extends up to nearly 1050 nm, which is promising for the development of broadly tunable laser sources, as well as for the design of femtosecond oscillators or amplifiers. We assumed that the radiative lifetime
Fig. 2. Gain cross-section (σ g ) of Yb:GGG for different values of population inversion ratio β = N2 /N.
21
Fig. 3. Emission spectra under 165 nm excitations of Yb:GGG obtained at 20 K and room temperature.
was 0.8 ms, which is the values measured by Belovolov et al. [4] (while in our sample the measured lifetime are 1.7 and 0.6 ms for 10% and 30% Yb concentration because of radiation trapping and quenching by impurities ions such as Er and Ho which will be discussed later). These results are encouraging in terms of energy storage and laser power capacity. We have studied the emission spectra under 269, 275 and 313 nm excitations of Yb:GGG obtained at room temperature which shows that Yb:GGG cannot be suitable for scintillator applications because of the overlapping of charge transfer absorption of Yb3+ ions with that of Gd3+ ions [7]. Fig. 3 shows the emission spectra under 165 nm excitations of Yb:GGG obtained at 20 K and room temperature, and also we cannot get the charge transfer luminescence of Yb ions as obtained in many other Yb doped crystals, such as Yb:YAG and Yb:YAP (a large emission band centered at 340 nm and a weak band centered at 500 nm). The emission peak 560 nm corresponds to Tm3+ , Er3+ , and Ho3+ ions. Fig. 4 shows the emission spectrum of 10 and 30 at.% Yb:GGG crystal at room temperature. The spectra are dominated by an emission band at 1030 nm with smaller peaks on the high- and low-wavelength side. The emission bands can be
Fig. 4. Infrared luminescence spectra of Yb:GGG single crystal under 940 nm excitations at room temperature.
22
B. Jiang et al. / Materials Science and Engineering B 137 (2007) 20–23
Fig. 6. Energy level of Yb/Tm/Er/Ho and possible transition pathway. Fig. 5. Visible luminescence spectrum of Yb:GGG and Cr,Yb:GGG crystals at room temperature.
assigned to transitions between the 2 F5/2 excited state and different crystal components of the 2 F7/2 ground state for Yb3+ in GGG. The visible fluorescence spectra of 10, 28 and 30 at.% Yb doped GGG crystal and Cr,Yb:GGG crystals under 940 nm excitations at room temperature is shown in Fig. 5. The energy level scheme of Yb3+ ion is simple with only the 2 F7/2 ground state and the 2 F5/2 excited state, separated by some 10,000 cm−1 , and no energy levels in the visible. The 4f electrons of Yb3+ have a relatively strong interaction with their surroundings and the interaction between the 4f13 configurations of neighboring Yb3+ is strong in comparison with other rare earth ions, the strong interaction gives rise to a high cooperative luminescence transition probability. The blue (486 nm) emission could result from the cooperative process corresponding to the simultaneous radiative relaxation of a pair of excited Yb3+ ions accompanied by the emission of a visible photon in the following manner [8]: Yb(2 F5/2 ) + Yb(2 F5/2 ) → 2Yb(2 F7/2 ) + hυ, the photon energy of the cooperative luminescence is nearly exactly twice the energy of the normal (single-ion) luminescence [18]. The starting materials used in this study are with a purity of 99.99% and 99.999%. As we know rare earth elements are indeed chemically related, so it is difficult to separate them from each other. Thus, impurities are inevitable. Fig. 6 shows the energy level of Yb, Er, Ho. We can see that many resonant energy transfers are possible between trivalent rare earth ions. According to the energy matching conditions, the possible upconversion mechanisms for the green and red emissions are discussed based on the energy level of Er3+ and Yb3+ presented in Fig. 6 [9–13,17,18]. The green (542 and 555 nm) and red (653 nm) emissions were simultaneously observed. The green (542 and 555 nm) and red (653 nm) upconversion luminescence were identified from the 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 , and 4 3+ 4F 9/2 → I15/2 transitions of Er , respectively. The upconversion luminescence of Ho3+ ions can be 5 I6 –5 S2 absorption (539 nm) transition followed by the 5 S2 –5 I8 emission
transition. The transition 5 F5 –5 I8 of Ho3+ [14–17] ions may be responsible for the upconversion luminescence centered at around 669 nm in Fig. 5. The upconversion luminescence of these ions is excited by infrared light through two- or three-step energy transfer from Yb3+ ions in general. It should be pointed out that the upconversion luminescence in Yb:GGG crystal is detrimental to the IR laser operation due to the loss of excited energy. Because the Cr3+ ions have a strong absorption bands at the visible range, when we doped Cr3+ ions into Yb:GGG crystal the visible luminescence was greatly weakened. Utilizing the above cooperative sensitization mechanism, Yb:GGG crystal also can be served as the excellent host for Ho3+ or Er3+ ions co-doping for visible upconversion laser operation. From Fig. 5 we can see with the increase of Yb ions, the concentration of Er, Ho ions was also increased and this leads to visible fluorescence intensity increase. 4. Conclusion The visible luminescence of Yb3+ -doped gadolinium gallium garnets at room temperature was measured. The blue (486 nm) emission was cooperative luminescence and the emission bands centered at 538 and 669 nm were ascribed to the presence of the Er3+ , and Ho3+ , etc. impurities. With the increase of Yb3+ concentration the visible fluorescence intensity increase and we also find that Cr3+ ions can greatly weaken the visible luminescence of Yb:GGG crystal when pumped by 940 nm LD. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
P. Lacovara, H.K. Choi, C.A. Wang, et al., Opt. Lett. 16 (1991) 1089. H. Qiu, P. Yang, J. Dong, et al., Mater. Lett. 55 (2002) 1. S. Chenais, F. Druon, F. Balembois, et al., Opt. Mater. 22 (2003) 99–106. M.I. Belovolov, E.M. Dianov, M.I. Timoschechkin et al., CLEO Europe (1996) 43. E. Nakazawa, S. Shionoya, Phys. Rev. Lett. 21 (1970) 1710. M. Malinowski, M. Kaczkan, R. Piramidowicz, et al., J. Lumin. 94–95 (2001) 29. B. Jiang, Z. Zhao, X. Xu, et al., J. Cryst. Growth 277 (2005) 186–191. E. Nakazawa, S. Shionoya, Phys. Rev. Lett. 21 (1970) 1710. L. Laversenne, C. Goutaudier, Y. Guyot, M.T. Cohen-Adad, G. Boulon, J. Alloy. Compd. 341 (2002) 214. P.Z. Yang, P.Z. Deng, Z.W. Yin, J. Lumin. 97 (2002) 51.
B. Jiang et al. / Materials Science and Engineering B 137 (2007) 20–23 [11] M. Ito, C. Goutaudier, Y. Guyot, et al., J. Phys.: Condens. Matter. 16 (2004) 1501. [12] Z.X. Cheng, S.J. Zhang, F. Song, et al., J. Phys. Chem. Solids 63 (2002) 2011. [13] R.H. Page, K.I. Schaffers, P.A. Waide, et al., J. Opt. Soc. Am. B 15 (1998) 996. [14] Th. Rothacher, W. Luthy, H.P. Weber, Opt. Commun. 155 (1998) 68.
23
[15] A.V. Kir’yanov, V. Aboites, A.M. Belovolov, et al., Opt. Express 10 (2002) 832. [16] A.V. Kir’yanova, V. Aboitesa, A.M. Belovolovb, et al., J. Lumin. 102–103 (2003) 715. [17] F. Ermeneux, C. Goutaudier, R. Moncorge, et al., Opt. Mater. 8 (1997) 83. [18] X. Xu, Z. Zhao, P. Song, et al., Physica B 357 (2005) 365–369.
Visible luminescence in Yb3+-doped gadolinium gallium garnets Benxue Jiang a,b , Zhiwei Zhao a,∗ , Xiaodong Xu a,b , Pingxin Song a,b , Xiaodan Wang a,b , Jun Xu a a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, PR China b Graduate School of Chinese Academy of Science, Beijing 100039, PR China Received 8 January 2006; received in revised form 26 September 2006; accepted 30 September 2006
Abstract In this paper, some results on visible luminescence performed on Yb3+ -doped gadolinium gallium garnets under 165 and 940 nm excitation were presented. The upconversion luminescence was ascribed to Yb3+ cooperative luminescence and the presence of rare earth impurity ions. The gain cross-sections of Yb:GGG crystal as a function of excited-state population fraction β were studied. Emission spectra under 165 nm at 20 K showed there was no charge transfer luminescence in Yb:GGG. © 2006 Elsevier B.V. All rights reserved. Keywords: Yb:GGG; Yb,Cr:GGG; Cooperative luminescence; Charge transfer luminescence
1. Introduction Yb3+ ion spectroscopy have received considerable attention for laser application over the past years [1,2]. The main interest of Yb3+ ion lies in its very simple energy level diagram which leads to very low quantum defects, reducing thermal loads and preventing undesired effects such as upconversion and excited state absorption and weak concentration quenching. Owing to this unique configuration, Yb3+ presents remarkable properties. The thermal conductivity of Yb:GGG is larger than that of Yb:YAG with Yb doping level more than 4 at.% [3,4]. Owing to this unique configuration, Yb:GGG crystal presents remarkable properties. Cooperative luminescence is a special type of upconversion in which two interacting ions in the excited state return to the ground state simultaneously, emitting one photon of the sum of the energies of the single ion transitions. It was observed by Nakazawa and Shionoya in 1970 for Yb3+ in YbPO4 [5]. Malinowski et al. [6], reported blue cooperative emission centered at 484 nm in Yb:YAG planar epitaxial waveguides and found that some of the visible emission peaks were exactly at half wavelength of the IR peaks.
∗
Corresponding author. Tel.: +86 2169915174. E-mail addresses: [email protected] (B. Jiang), [email protected] (Z. Zhao). 0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.09.027
In this paper, we have measured the visible luminescence spectra of Yb:GGG crystal as a function of Yb concentration and the emission spectra at low temperature. The causing of the visible luminescence of Yb:GGG was discussed. 2. Experimental Yb:GGG crystal was grown by the RF-heating Cz method. The crystal growth process was described in Ref. [7]. Samples for spectroscopic measurements were cut out of boules and the surfaces perpendicular to the growth axis were polished. The thickness of the samples was 0.5 mm. Room temperature absorption spectra were measured with a JASCO V-570 UV–vis-NIR spectrophotometer. The fluorescence spectra were acquired by a Triax 550 spectrophotometer with an InGaAs laser diode as the pump source (excited at 940 nm). The pump power was 700 mW. 3. Results and discussion Fig. 1 presents the absorption spectra of Yb:YAG and Yb:GGG crystals, attributed to the 2 F7/2 –2 F5/2 Yb3+ transition. The main absorption peak positions of Yb:GGG move to longer wavelength direction compared to Yb:YAG. The main absorption bands of Yb:GGG are centered at 932, 944, 971, 1025 nm while Yb:YAG are centered at 915, 939, 968, 1031 nm. So Yb:GGG has a small quantum defect (between pumping and laser wavelength) which contributes to weak ther-
B. Jiang et al. / Materials Science and Engineering B 137 (2007) 20–23
Fig. 1. Absorption spectra of Yb:GGG and Yb:YAG single crystal at room temperature.
mal effects. There is self-absorption (having absorption at laser wavelength) at the lasing wavelength of 1.03 mm in the crystals at room temperature.Fig. 2 shows, for Yb:GGG, room temperature gain cross-section σ g , obtained for different values of excited-state population fraction β(β = N2 /N). The gain crosssection is defined as σ g = βσ e − (1 − β)σ a and the absorption and emission cross-section curves, σ e and σ a , respectively, are given for the values β = 0 and 1, respectively. The absorption and emission cross-section of Yb can be estimated according to the following expression [18]: 2.303 lg(I/I0 ) and dc Zl Ezl − (hc/λ) σem (λ) = σabs (λ) exp Zu KT σabs (λ) =
Absorption and emission cross-section peaks are located near 945 and 1031 nm, with values of 6 × 10−21 and 11 × 10−21 cm2 . It is worth noting that the emission spectrum extends up to nearly 1050 nm, which is promising for the development of broadly tunable laser sources, as well as for the design of femtosecond oscillators or amplifiers. We assumed that the radiative lifetime
Fig. 2. Gain cross-section (σ g ) of Yb:GGG for different values of population inversion ratio β = N2 /N.
21
Fig. 3. Emission spectra under 165 nm excitations of Yb:GGG obtained at 20 K and room temperature.
was 0.8 ms, which is the values measured by Belovolov et al. [4] (while in our sample the measured lifetime are 1.7 and 0.6 ms for 10% and 30% Yb concentration because of radiation trapping and quenching by impurities ions such as Er and Ho which will be discussed later). These results are encouraging in terms of energy storage and laser power capacity. We have studied the emission spectra under 269, 275 and 313 nm excitations of Yb:GGG obtained at room temperature which shows that Yb:GGG cannot be suitable for scintillator applications because of the overlapping of charge transfer absorption of Yb3+ ions with that of Gd3+ ions [7]. Fig. 3 shows the emission spectra under 165 nm excitations of Yb:GGG obtained at 20 K and room temperature, and also we cannot get the charge transfer luminescence of Yb ions as obtained in many other Yb doped crystals, such as Yb:YAG and Yb:YAP (a large emission band centered at 340 nm and a weak band centered at 500 nm). The emission peak 560 nm corresponds to Tm3+ , Er3+ , and Ho3+ ions. Fig. 4 shows the emission spectrum of 10 and 30 at.% Yb:GGG crystal at room temperature. The spectra are dominated by an emission band at 1030 nm with smaller peaks on the high- and low-wavelength side. The emission bands can be
Fig. 4. Infrared luminescence spectra of Yb:GGG single crystal under 940 nm excitations at room temperature.
22
B. Jiang et al. / Materials Science and Engineering B 137 (2007) 20–23
Fig. 6. Energy level of Yb/Tm/Er/Ho and possible transition pathway. Fig. 5. Visible luminescence spectrum of Yb:GGG and Cr,Yb:GGG crystals at room temperature.
assigned to transitions between the 2 F5/2 excited state and different crystal components of the 2 F7/2 ground state for Yb3+ in GGG. The visible fluorescence spectra of 10, 28 and 30 at.% Yb doped GGG crystal and Cr,Yb:GGG crystals under 940 nm excitations at room temperature is shown in Fig. 5. The energy level scheme of Yb3+ ion is simple with only the 2 F7/2 ground state and the 2 F5/2 excited state, separated by some 10,000 cm−1 , and no energy levels in the visible. The 4f electrons of Yb3+ have a relatively strong interaction with their surroundings and the interaction between the 4f13 configurations of neighboring Yb3+ is strong in comparison with other rare earth ions, the strong interaction gives rise to a high cooperative luminescence transition probability. The blue (486 nm) emission could result from the cooperative process corresponding to the simultaneous radiative relaxation of a pair of excited Yb3+ ions accompanied by the emission of a visible photon in the following manner [8]: Yb(2 F5/2 ) + Yb(2 F5/2 ) → 2Yb(2 F7/2 ) + hυ, the photon energy of the cooperative luminescence is nearly exactly twice the energy of the normal (single-ion) luminescence [18]. The starting materials used in this study are with a purity of 99.99% and 99.999%. As we know rare earth elements are indeed chemically related, so it is difficult to separate them from each other. Thus, impurities are inevitable. Fig. 6 shows the energy level of Yb, Er, Ho. We can see that many resonant energy transfers are possible between trivalent rare earth ions. According to the energy matching conditions, the possible upconversion mechanisms for the green and red emissions are discussed based on the energy level of Er3+ and Yb3+ presented in Fig. 6 [9–13,17,18]. The green (542 and 555 nm) and red (653 nm) emissions were simultaneously observed. The green (542 and 555 nm) and red (653 nm) upconversion luminescence were identified from the 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 , and 4 3+ 4F 9/2 → I15/2 transitions of Er , respectively. The upconversion luminescence of Ho3+ ions can be 5 I6 –5 S2 absorption (539 nm) transition followed by the 5 S2 –5 I8 emission
transition. The transition 5 F5 –5 I8 of Ho3+ [14–17] ions may be responsible for the upconversion luminescence centered at around 669 nm in Fig. 5. The upconversion luminescence of these ions is excited by infrared light through two- or three-step energy transfer from Yb3+ ions in general. It should be pointed out that the upconversion luminescence in Yb:GGG crystal is detrimental to the IR laser operation due to the loss of excited energy. Because the Cr3+ ions have a strong absorption bands at the visible range, when we doped Cr3+ ions into Yb:GGG crystal the visible luminescence was greatly weakened. Utilizing the above cooperative sensitization mechanism, Yb:GGG crystal also can be served as the excellent host for Ho3+ or Er3+ ions co-doping for visible upconversion laser operation. From Fig. 5 we can see with the increase of Yb ions, the concentration of Er, Ho ions was also increased and this leads to visible fluorescence intensity increase. 4. Conclusion The visible luminescence of Yb3+ -doped gadolinium gallium garnets at room temperature was measured. The blue (486 nm) emission was cooperative luminescence and the emission bands centered at 538 and 669 nm were ascribed to the presence of the Er3+ , and Ho3+ , etc. impurities. With the increase of Yb3+ concentration the visible fluorescence intensity increase and we also find that Cr3+ ions can greatly weaken the visible luminescence of Yb:GGG crystal when pumped by 940 nm LD. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
P. Lacovara, H.K. Choi, C.A. Wang, et al., Opt. Lett. 16 (1991) 1089. H. Qiu, P. Yang, J. Dong, et al., Mater. Lett. 55 (2002) 1. S. Chenais, F. Druon, F. Balembois, et al., Opt. Mater. 22 (2003) 99–106. M.I. Belovolov, E.M. Dianov, M.I. Timoschechkin et al., CLEO Europe (1996) 43. E. Nakazawa, S. Shionoya, Phys. Rev. Lett. 21 (1970) 1710. M. Malinowski, M. Kaczkan, R. Piramidowicz, et al., J. Lumin. 94–95 (2001) 29. B. Jiang, Z. Zhao, X. Xu, et al., J. Cryst. Growth 277 (2005) 186–191. E. Nakazawa, S. Shionoya, Phys. Rev. Lett. 21 (1970) 1710. L. Laversenne, C. Goutaudier, Y. Guyot, M.T. Cohen-Adad, G. Boulon, J. Alloy. Compd. 341 (2002) 214. P.Z. Yang, P.Z. Deng, Z.W. Yin, J. Lumin. 97 (2002) 51.
B. Jiang et al. / Materials Science and Engineering B 137 (2007) 20–23 [11] M. Ito, C. Goutaudier, Y. Guyot, et al., J. Phys.: Condens. Matter. 16 (2004) 1501. [12] Z.X. Cheng, S.J. Zhang, F. Song, et al., J. Phys. Chem. Solids 63 (2002) 2011. [13] R.H. Page, K.I. Schaffers, P.A. Waide, et al., J. Opt. Soc. Am. B 15 (1998) 996. [14] Th. Rothacher, W. Luthy, H.P. Weber, Opt. Commun. 155 (1998) 68.
23
[15] A.V. Kir’yanov, V. Aboites, A.M. Belovolov, et al., Opt. Express 10 (2002) 832. [16] A.V. Kir’yanova, V. Aboitesa, A.M. Belovolovb, et al., J. Lumin. 102–103 (2003) 715. [17] F. Ermeneux, C. Goutaudier, R. Moncorge, et al., Opt. Mater. 8 (1997) 83. [18] X. Xu, Z. Zhao, P. Song, et al., Physica B 357 (2005) 365–369.