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Optical Materials Express

Optical Materials Express

  • Editor: David Hagan
  • Vol. 4, Iss. 5 — May. 1, 2014
  • pp: 1050–1056
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Up-conversion luminescence of Tb3+ ions in germanate glasses under diode-laser excitation of Yb3+

Marcin Kochanowicz, Dominik Dorosz, Jacek Zmojda, Jan Dorosz, Joanna Pisarska, and Wojciech A. Pisarski  »View Author Affiliations


Optical Materials Express, Vol. 4, Issue 5, pp. 1050-1056 (2014)
http://dx.doi.org/10.1364/OME.4.001050


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Abstract

Up-conversion luminescence processes of Tb3+ ions in GeO2-Ga2O3-BaO glass system were investigated under diode-laser excitation of Yb3+. Emission bands at 489, 543, 586 and 621nm corresponding to 5D47FJ (J = 6, 5, 4, 3) transitions and luminescence at 381, 415, 435 nm resulting from 5D3, 5G67FJ (J = 6, 5, 4) transitions of Tb3+ were observed. The highest up-conversion emission intensity was obtained for 0.7Yb2O3/0.7Tb2O3 co-doped lead-free germanate glass. The energy transfer coefficient was determined based on fitting of calculations and experimental results by least squares method. The energy transfer coefficient amounts to Cf = 1.5∙10−33 cm6/s while quantum efficiency of the Yb3+→Tb3+ energy transfer is 12%.

© 2014 Optical Society of America

1. Introduction

In this work, the effects of optical pumping by NIR diode-laser and cooperative energy transfer processes between Yb3+ and Tb3+ in germanate glass are presented. The results of the conducted research into optimization of Yb3+/Tb3+ dopants concentration in a glass from GeO2-Ga2O3-BaO system, ultimately aiming at maximization of up-conversion emission intensity, are discussed. The Yb3+→Tb3+ energy transfer efficiency is analyzed, along with the energy transfer coefficient Cf determination by means of the fitting method. To the best of our knowledge, cooperative energy transfer processes were not examined for lead-free germanate glasses containing Yb3+ and Tb3+ ions.

2. Experimental

Glasses with (1-y)50GeO2-25GaO-10BaO-15Na2O-0.7Yb2O3-yTb2O3, (y = 0.07, 0.15, 0.35, 0.7, 1.0) composition (in mol%) were melted from spectrally pure (99,99%) raw materials. The homogenized set was placed in a platinum crucible and melted in an electric furnace in T = 1500°C for 30 minutes. The molten glass was poured out onto a brass plate and then exposed to the process of annealing in air atmosphere at 610°C for 12 h to remove thermal strains. The glass samples were cutted and polished in order to carry out the optical measurements. Finally, a series of samples with the dimensions of 6x6x2mm3 were prepared. Emission spectra were measured at a station equipped with a Stellarnet Green-wave spectrometer and a pumping LIMO32-F200-DL980-LM laser diode (λp = 976 nm) with an optical fiber output having the maximum optical power P = 30W. A system PTI QuantaMaster QM40 coupled with tunable pulsed optical parametric oscillator (OPO), pumped by a third harmonic of a Nd:YAG laser (Opotek Opolette 355 LD) was used for luminescence decay measurements. The laser system was equipped with a double 200 mm monochromator, a multimode UV-VIS PMT (R928) and Hamamatsu H10330B-75 detectors controlled by a computer. Luminescence decay curves were recorded and stored by a PTI ASOC-10 [USB-2500] oscilloscope with an accuracy of ± 1 µs. The procedure of energy transfer coefficient determination was based on fitting of calculations and experimental results by least squares method. The calculated values were optimized by minimizing (simplex method) the sum of the squares o deviations of the calculated results, from experimental ones. The quality of the fits was characterized by the root-mean-squares deviation (RMS).

3. Results and discussion

Fig. 1 Emission spectra for Yb3+/Tb3+ co-doped germanate glasses. Inset shows dependence of upconversion emission intensity of 0.7Yb3+/0.7Tb3+ co-doped germanate glass on excitation power (λp = 976 nm).
Figure 1 presents emission spectra of Tb3+ ions in germanate glasses under excitation of Yb3+ by diode laser with λp = 976 nm, Ppump = 2W. Inset shows the logarithmic dependence of emission intensity on pumping radiation power. All possible transitions are schematized on the energy level diagram of Tb3+ and Yb3+ ions in germanate glass (Fig. 2).
Fig. 2 Simplified energy level diagram of Tb3+/Yb3+ ion and possible upconversion luminescence mechanisms.

Based on luminescence decay measurements for 2F5/2 state of Yb3+ ions in glass samples without and with Tb3+, the Yb3+→Tb3+ energy transfer efficiency was determined. Dependence of 2F5/2 (Yb3+) lifetime and the energy transfer efficiency with terbium concentration is presented in Fig. 4.
Fig. 4 Lifetime for 2F5/2 state of Yb3+ and energy transfer efficiency as a function of Tb3+ concentration.
The 2F5/2 lifetime of Yb3+ is reduced from 882 μs (0.7Yb2O3) to 751 μs in the presence of Tb3+ (0.7Yb2O3/1Tb2O3). However, maximum upconversion luminescence was obtained in glass co-doped with 0.7Yb2O3/0.7Tb2O3. Increase in terbium content up to 1mol% results in concentration quenching of luminescence.

Ignoring the reverse transfer (Tb3+→Yb3+), quantum efficiency of the Yb3+→Tb3+ energy transfer can be established according to the equation:
η=1τYbYbTb/τYb
(3)
Efficiency of cooperative Yb3→Tb3+ energy transfer increases with increasing Tb3+ concentration, because distance between the interacting lanthanide ions is reduced. To determine the energy transfer coefficient Cf, the luminescence dynamics of Yb3+/Tb3+ dopants configuration needs to be scrutinized. The following Eq. (4) describes cooperative energy transfer and up-conversion processes:
dnYb1dt=RYb1nYb1+WYbnYb2dnYb2dt=RYb1nYb1WYbnYb2CfnYb22nTb1dnTb1dt=CfnYb22nTb1+WTbnTb2dnTb2dt=WTbnTb2+2CfnYb22nTb1NYb=nYb1+nYb2NTb=nTb1+nTb2
(4)
where: Cf - upconversion energy transfer coefficient, RYb - pump rate of Yb3+, nYb1, nYb2 and nTb1, nTb2 - population densities of Yb3+ and Tb3+, respectively.

From literature data it is well known that the reverse energy transfer also call back transfer process is not neglected for some glasses containing Tb3+ and Yb3+ ions [31

31. E. Martins, C. B. de Araujo, J. R. Delben, A. S. L. Gomes, B. J. da Costa, and Y. Messaddeq, “Cooperative frequency upconversion in Yb3+–Tb3+ codoped fluoroindate glass,” Opt. Commun. 158(1-6), 61–64 (1998).

]. Here, the concentrations of rare earth ions are relatively low and participation of back transfer process in population of Yb3+ and Tb3+ levels is rather not very significant. The equations do not take into account the reverse energy transfer (Tb3+→Yb3+) whose coefficient is several orders of magnitude smaller than Cf. Similar phenomena were observed for silicate glasses, where the calculated back transfer coefficient (Cb) was seven orders smaller than Yb3+→Tb3+ energy transfer coefficient [32

32. T. Yamashita and Y. Ohishi, “Analysis of energy transfers between Tb3+ and Yb3+ codoped in borosilicate glasses,” JOSA B 26(4), 819–829 (2009). [CrossRef]

]. The standard procedure of energy transfer coefficient determination based on least squares fitting of theory and experimental results was used [32

32. T. Yamashita and Y. Ohishi, “Analysis of energy transfers between Tb3+ and Yb3+ codoped in borosilicate glasses,” JOSA B 26(4), 819–829 (2009). [CrossRef]

,33

33. T. Yamashita, Y. Ohishi, “Energy transfer and gain analysis for Tb3+-Yb3+ co-doped silicate glasses under the 0.98 µm excitation,” OSA / CLEO/QELS, 1–2 (2008).

]. The experimental and calculated results from decay curve of Tb3+:5D47F5 obtained for sample with 0.7Yb2O3/0.7Tb2O3 are presented in Fig. 5.
Fig. 5 Experimental and calculated decay curve of Tb3+: 5D47F5.

The 5D4 luminescence lifetime of Tb3+ is equal to 1.98 ms. The Yb3+→Tb3+ energy transfer coefficient Cf close to 1.5∙10−33 cm6/s was determined by means of the nonlinear least squares fitting method. Root mean square is 1.41∙10−5 [s], which proofs reliability of performed calculations. The Cf value is greater than in the case of silicate glasses [33

33. T. Yamashita, Y. Ohishi, “Energy transfer and gain analysis for Tb3+-Yb3+ co-doped silicate glasses under the 0.98 µm excitation,” OSA / CLEO/QELS, 1–2 (2008).

].

4. Conclusions

Up-conversion luminescence processes in GeO2-Ga2O3-BaO glass systems co-doped with Tb3+ and Yb3+ ions were studied. Emission bands due to 5D47FJ (J = 6, 5, 4, 3) and 5D3, 5G67FJ (J = 6, 5, 4) transitions of Tb3+ ions are observed under direct diode-laser excitation of Yb3+ by 976 nm line. Energy transfer coefficient in glass co-doped with 0.7Yb2O3/0.7Tb2O3 amounts to Cf = 1.5∙10−33 cm6/s while efficiency of the Yb3+→Tb3+ energy transfer is 12%.

Acknowledgments

This work was supported by National Science Centre (Poland) – grant No. N N515 512340

References and links

1.

D. Lande, S. S. Orlov, A. Akella, L. Hesselink, and R. R. Neurgaonkar, “Digital holographic storage system incorporating optical fixing,” Opt. Lett. 22(22), 1722–1724 (1997). [CrossRef] [PubMed]

2.

P. Xie and T. R. Gosnell, “Room-temperature upconversion fiber laser tunable in the red, orange, green, and blue spectral regions,” Opt. Lett. 20(9), 1014–1016 (1995). [CrossRef] [PubMed]

3.

Q. Duan, F. Qin, Z. Zhang, W. Cao, and W. Cao, “Quantum cutting mechanism in NaYF4:Tb3+, Yb3+.,” Opt. Lett. 37(4), 521–523 (2012). [CrossRef] [PubMed]

4.

Q. Y. Zhang, C. H. Yang, and Y. X. Pan, “Cooperative quantum cutting in one-dimentional (YbxGd1-x)Al3(BO3)4:Tb3+,” Appl. Phys. Lett. 90, 021107 (2007). [CrossRef]

5.

Q. Duan, F. Qin, D. Wang, W. Xu, J. Cheng, Z. Zhang, and W. Cao, “Quantum cutting mechanism in Tb3+-Yb3+ co-doped oxyfluoride glass,” J. Appl. Phys. 110(11), 113503 (2011). [CrossRef]

6.

X. Liu, S. Ye, Y. Qiao, G. Dong, B. Zhu, D. Chen, G. Lakshminarayana, and J. Qiu, “Cooperative downconversion and near-infrared luminescence of Tb3+–Yb3+ codoped lanthanum borogermanate glasses,” Appl. Phys. B 96(1), 51–55 (2009). [CrossRef]

7.

P. Molina, V. Vasyliev, E. G. Villora, and K. Shimamura, “Tb3+-Yb3+ cooperative down and up conversion processes in Tb0.81Ca0.19F2.81:Yb3+ single crystals,” J. Appl. Phys. 110(12), 123527 (2011). [CrossRef]

8.

Q. Duan, F. Qin, P. Wang, Z. Zhang, and W. Cao, “Upconversion emission efficiency of Tb3+-Yb3+ co-doped glass,” JOSA B 30(2), 456–459 (2013). [CrossRef]

9.

A. Lin, X. Liu, P. R. Watekar, H. Guo, B. Peng, W. Wei, M. Lu, W. T. Han, and J. Toulouse, “Intense green upconversion emission in Tb3+/Yb3+ codoped alumino-germano-silicate optical fibers,” Appl. Opt. 49(9), 1671–1675 (2010). [CrossRef] [PubMed]

10.

L. Huang, T. Yamashita, R. Jose, Y. Arai, T. Suzuki, and Y. Ohishi, “Intense ultraviolet emission from Tb3+ and Yb3+ codoped glass ceramic containing CaF2 nanocrystals,” Appl. Phys. Lett. 90(13), 131116 (2007). [CrossRef]

11.

D. Chen, Y. Yu, Y. Wang, P. Huang, and F. Weng, “Cooperative energy transfer up-conversion and quantum cutting down-conversion in Yb3+:TbF3 nanocrystals embedded glass ceramics,” J. Phys. Chem. C 113(16), 6406–6410 (2009). [CrossRef]

12.

M. V. D. Vermelho, P. V. dos Santos, M. T. de Araujo, A. S. Gouveia-Neto, F. C. Cassanjes, S. J. L. Ribeiro, and Y. Messaddeq, “Thermally enhanced cooperative energy-transfer frequency upconversion in terbium and ytterbium doped tellurite glass,” J. Lumin . 102-103, 762–767 (2003).

13.

Y. Arai, T. Yamashita, T. Suzuki, and Y. Ohishi, “Upconversion properties of Tb3+-Yb3+ codoped fluorophosphate glasses,” J. Appl. Phys. 105(8), 083105 (2009). [CrossRef]

14.

J. Ueda and S. Tanabe, “Sensitization mechanisms of 1μm luminescence in Tb3+-Yb3+ co-doped borate glasses,” Phys. Status Solidi., A Appl. Mater. Sci. 208(8), 1827–1832 (2011). [CrossRef]

15.

Q. Zhang, G. Chen, Y. Xu, X. Liu, B. Qian, G. Dong, Q. Zhou, J. Qiu, and D. Chen, “Abnormal upconversion luminescence from Yb3+ doped and Tb3+/Yb3+ codoped high silica glasses induced by intrinsic optical bistability,” Appl. Phys. B 98(2-3), 261–265 (2010). [CrossRef]

16.

I. R. Martín, A. C. Yanes, J. Mendez-Ramos, M. E. Torres, and V. D. Rodriguez, “Cooperative energy transfer in Yb3+–Tb3+ codoped silica sol-gel glasses,” J. Appl. Phys. 89(5), 2520–2524 (2001). [CrossRef]

17.

V. Scarnera, B. Richards, A. Jha, G. Jose, and C. Stacey, “Green up-conversion in Yb3+–Tb3+ and Yb3+–Tm3+–Tb3+ doped fluoro-germanate bulk glass and fibre,” Opt. Mater. 33(2), 159–163 (2010). [CrossRef]

18.

A. S. Gouveia-Neto, L. A. Bueno, A. C. M. Afonso, J. F. Nascimento, E. B. Costa, Y. Messaddeq, and S. J. L. Ribeiro, “Upconversion luminescence in Ho3+/Yb3+- and Tb3+/Yb3+-codoped fluorogermanate glass and glass ceramic,” J. Non-Cryst. Solids 354(2-9), 509–514 (2008). [CrossRef]

19.

M. Peng, N. Zhang, L. Wondraczek, J. Qiu, Z. Yang, and Q. Zhang, “Ultrabroad NIR luminescence and energy transfer in Bi and Er/Bi co-doped germanate glasses,” Opt. Express 19(21), 20799–20807 (2011). [CrossRef] [PubMed]

20.

B. Zhou, H. Lin, B. Chen, and E. Y. B. Pun, “Superbroadband near-infrared emission in Tm-Bi codoped sodium-germanium-gallate glasses,” Opt. Express 19(7), 6514–6523 (2011). [CrossRef] [PubMed]

21.

Y. Messaddeq, S. J. L. Ribeiro, E. Pecoraro, and E. M. Nascimento, “Glass for optical amplifier fiber,” US Patent No 7,773,647 B2 (Aug. 10, 2010)

22.

D. M. McPherson and S. C. Murray, “Germanate glass for mid-infrared medical optical fiber,” US Patent No 5,491,767 (Feb. 13, 1996)

23.

D. L. Yang, H. Gong, E. Y. B. Pun, X. Zhao, and H. Lin, “Rare-earth ions doped heavy metal germanium tellurite glasses for fiber lighting in minimally invasive surgery,” Opt. Express 18(18), 18997–19008 (2010). [CrossRef] [PubMed]

24.

B. J. Chen, L. F. Shen, E. Y. B. Pun, and H. Lin, “Sm3+-doped germanate glass channel waveguide as light source for minimally invasive photodynamic therapy surgery,” Opt. Express 20(2), 879–889 (2012). [CrossRef] [PubMed]

25.

J. Yang, B. J. Chen, E. Y. B. Pun, B. Zhai, and H. Lin, “Pr3+-doped heavy metal germanium tellurite glasses for irradiative light source in minimally invasive photodynamic therapy surgery,” Opt. Express 21(1), 1030–1040 (2013). [CrossRef] [PubMed]

26.

J. Wu, Z. Yao, J. Zong, and S. Jiang, “Highly efficient high-power thulium-doped germanate glass fiber laser,” Opt. Lett. 32(6), 638–640 (2007). [CrossRef] [PubMed]

27.

D. L. Yang, E. Y. B. Pun, B. J. Chen, and H. Lin, “Radiative transitions and optical gains in Er3+/Yb3+ codoped acid-resistant ion exchanged germanate glass channel waveguides,” JOSA B 26(2), 357–363 (2009). [CrossRef]

28.

J. Ding, Q. Zhang, J. Cheng, X. Liu, G. Lin, J. Qiu, and D. Chen, “Multicolor upconversion luminescence from RE3+-Yb3+ (RE = Er, Tm, Tb) codoped LaAlGe2O7 glasses,” J. Alloy. Comp. 495(1), 205–208 (2010). [CrossRef]

29.

R. K. Verma, D. K. Rai, and S. B. Rai, “Investigation of structural properties and its effect on optical properties: Yb3+/Tb3+ codoped in aluminum silicate glass,” J. Alloy. Comp. 509(18), 5591–5595 (2011). [CrossRef]

30.

N. K. Giri, D. K. Rai, and S. B. Rai, “UV-visible emission in Tb-Yb codoped tellurite glass on 980-nm excitation,” Appl. Phys. B 89(2-3), 345–348 (2007). [CrossRef]

31.

E. Martins, C. B. de Araujo, J. R. Delben, A. S. L. Gomes, B. J. da Costa, and Y. Messaddeq, “Cooperative frequency upconversion in Yb3+–Tb3+ codoped fluoroindate glass,” Opt. Commun. 158(1-6), 61–64 (1998).

32.

T. Yamashita and Y. Ohishi, “Analysis of energy transfers between Tb3+ and Yb3+ codoped in borosilicate glasses,” JOSA B 26(4), 819–829 (2009). [CrossRef]

33.

T. Yamashita, Y. Ohishi, “Energy transfer and gain analysis for Tb3+-Yb3+ co-doped silicate glasses under the 0.98 µm excitation,” OSA / CLEO/QELS, 1–2 (2008).

OCIS Codes
(160.3380) Materials : Laser materials
(160.4670) Materials : Optical materials
(160.5690) Materials : Rare-earth-doped materials

ToC Category:
Fluorescent and Luminescent Materials

History
Original Manuscript: February 25, 2014
Revised Manuscript: April 9, 2014
Manuscript Accepted: April 14, 2014
Published: April 30, 2014

Citation
Marcin Kochanowicz, Dominik Dorosz, Jacek Zmojda, Jan Dorosz, Joanna Pisarska, and Wojciech A. Pisarski, "Up-conversion luminescence of Tb3+ ions in germanate glasses under diode-laser excitation of Yb3+," Opt. Mater. Express 4, 1050-1056 (2014)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-4-5-1050


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References

  1. D. Lande, S. S. Orlov, A. Akella, L. Hesselink, and R. R. Neurgaonkar, “Digital holographic storage system incorporating optical fixing,” Opt. Lett.22(22), 1722–1724 (1997). [CrossRef] [PubMed]
  2. P. Xie and T. R. Gosnell, “Room-temperature upconversion fiber laser tunable in the red, orange, green, and blue spectral regions,” Opt. Lett.20(9), 1014–1016 (1995). [CrossRef] [PubMed]
  3. Q. Duan, F. Qin, Z. Zhang, W. Cao, and W. Cao, “Quantum cutting mechanism in NaYF4:Tb3+, Yb3+.,” Opt. Lett.37(4), 521–523 (2012). [CrossRef] [PubMed]
  4. Q. Y. Zhang, C. H. Yang, and Y. X. Pan, “Cooperative quantum cutting in one-dimentional (YbxGd1-x)Al3(BO3)4:Tb3+,” Appl. Phys. Lett.90, 021107 (2007). [CrossRef]
  5. Q. Duan, F. Qin, D. Wang, W. Xu, J. Cheng, Z. Zhang, and W. Cao, “Quantum cutting mechanism in Tb3+-Yb3+ co-doped oxyfluoride glass,” J. Appl. Phys.110(11), 113503 (2011). [CrossRef]
  6. X. Liu, S. Ye, Y. Qiao, G. Dong, B. Zhu, D. Chen, G. Lakshminarayana, and J. Qiu, “Cooperative downconversion and near-infrared luminescence of Tb3+–Yb3+ codoped lanthanum borogermanate glasses,” Appl. Phys. B96(1), 51–55 (2009). [CrossRef]
  7. P. Molina, V. Vasyliev, E. G. Villora, and K. Shimamura, “Tb3+-Yb3+ cooperative down and up conversion processes in Tb0.81Ca0.19F2.81:Yb3+ single crystals,” J. Appl. Phys.110(12), 123527 (2011). [CrossRef]
  8. Q. Duan, F. Qin, P. Wang, Z. Zhang, and W. Cao, “Upconversion emission efficiency of Tb3+-Yb3+ co-doped glass,” JOSA B30(2), 456–459 (2013). [CrossRef]
  9. A. Lin, X. Liu, P. R. Watekar, H. Guo, B. Peng, W. Wei, M. Lu, W. T. Han, and J. Toulouse, “Intense green upconversion emission in Tb3+/Yb3+ codoped alumino-germano-silicate optical fibers,” Appl. Opt.49(9), 1671–1675 (2010). [CrossRef] [PubMed]
  10. L. Huang, T. Yamashita, R. Jose, Y. Arai, T. Suzuki, and Y. Ohishi, “Intense ultraviolet emission from Tb3+ and Yb3+ codoped glass ceramic containing CaF2 nanocrystals,” Appl. Phys. Lett.90(13), 131116 (2007). [CrossRef]
  11. D. Chen, Y. Yu, Y. Wang, P. Huang, and F. Weng, “Cooperative energy transfer up-conversion and quantum cutting down-conversion in Yb3+:TbF3 nanocrystals embedded glass ceramics,” J. Phys. Chem. C113(16), 6406–6410 (2009). [CrossRef]
  12. M. V. D. Vermelho, P. V. dos Santos, M. T. de Araujo, A. S. Gouveia-Neto, F. C. Cassanjes, S. J. L. Ribeiro, and Y. Messaddeq, “Thermally enhanced cooperative energy-transfer frequency upconversion in terbium and ytterbium doped tellurite glass,” J. Lumin. 102-103, 762–767 (2003).
  13. Y. Arai, T. Yamashita, T. Suzuki, and Y. Ohishi, “Upconversion properties of Tb3+-Yb3+ codoped fluorophosphate glasses,” J. Appl. Phys.105(8), 083105 (2009). [CrossRef]
  14. J. Ueda and S. Tanabe, “Sensitization mechanisms of 1μm luminescence in Tb3+-Yb3+ co-doped borate glasses,” Phys. Status Solidi., A Appl. Mater. Sci.208(8), 1827–1832 (2011). [CrossRef]
  15. Q. Zhang, G. Chen, Y. Xu, X. Liu, B. Qian, G. Dong, Q. Zhou, J. Qiu, and D. Chen, “Abnormal upconversion luminescence from Yb3+ doped and Tb3+/Yb3+ codoped high silica glasses induced by intrinsic optical bistability,” Appl. Phys. B98(2-3), 261–265 (2010). [CrossRef]
  16. I. R. Martín, A. C. Yanes, J. Mendez-Ramos, M. E. Torres, and V. D. Rodriguez, “Cooperative energy transfer in Yb3+–Tb3+ codoped silica sol-gel glasses,” J. Appl. Phys.89(5), 2520–2524 (2001). [CrossRef]
  17. V. Scarnera, B. Richards, A. Jha, G. Jose, and C. Stacey, “Green up-conversion in Yb3+–Tb3+ and Yb3+–Tm3+–Tb3+ doped fluoro-germanate bulk glass and fibre,” Opt. Mater.33(2), 159–163 (2010). [CrossRef]
  18. A. S. Gouveia-Neto, L. A. Bueno, A. C. M. Afonso, J. F. Nascimento, E. B. Costa, Y. Messaddeq, and S. J. L. Ribeiro, “Upconversion luminescence in Ho3+/Yb3+- and Tb3+/Yb3+-codoped fluorogermanate glass and glass ceramic,” J. Non-Cryst. Solids354(2-9), 509–514 (2008). [CrossRef]
  19. M. Peng, N. Zhang, L. Wondraczek, J. Qiu, Z. Yang, and Q. Zhang, “Ultrabroad NIR luminescence and energy transfer in Bi and Er/Bi co-doped germanate glasses,” Opt. Express19(21), 20799–20807 (2011). [CrossRef] [PubMed]
  20. B. Zhou, H. Lin, B. Chen, and E. Y. B. Pun, “Superbroadband near-infrared emission in Tm-Bi codoped sodium-germanium-gallate glasses,” Opt. Express19(7), 6514–6523 (2011). [CrossRef] [PubMed]
  21. Y. Messaddeq, S. J. L. Ribeiro, E. Pecoraro, and E. M. Nascimento, “Glass for optical amplifier fiber,” US Patent No 7,773,647 B2 (Aug. 10, 2010)
  22. D. M. McPherson and S. C. Murray, “Germanate glass for mid-infrared medical optical fiber,” US Patent No 5,491,767 (Feb. 13, 1996)
  23. D. L. Yang, H. Gong, E. Y. B. Pun, X. Zhao, and H. Lin, “Rare-earth ions doped heavy metal germanium tellurite glasses for fiber lighting in minimally invasive surgery,” Opt. Express18(18), 18997–19008 (2010). [CrossRef] [PubMed]
  24. B. J. Chen, L. F. Shen, E. Y. B. Pun, and H. Lin, “Sm3+-doped germanate glass channel waveguide as light source for minimally invasive photodynamic therapy surgery,” Opt. Express20(2), 879–889 (2012). [CrossRef] [PubMed]
  25. J. Yang, B. J. Chen, E. Y. B. Pun, B. Zhai, and H. Lin, “Pr3+-doped heavy metal germanium tellurite glasses for irradiative light source in minimally invasive photodynamic therapy surgery,” Opt. Express21(1), 1030–1040 (2013). [CrossRef] [PubMed]
  26. J. Wu, Z. Yao, J. Zong, and S. Jiang, “Highly efficient high-power thulium-doped germanate glass fiber laser,” Opt. Lett.32(6), 638–640 (2007). [CrossRef] [PubMed]
  27. D. L. Yang, E. Y. B. Pun, B. J. Chen, and H. Lin, “Radiative transitions and optical gains in Er3+/Yb3+ codoped acid-resistant ion exchanged germanate glass channel waveguides,” JOSA B26(2), 357–363 (2009). [CrossRef]
  28. J. Ding, Q. Zhang, J. Cheng, X. Liu, G. Lin, J. Qiu, and D. Chen, “Multicolor upconversion luminescence from RE3+-Yb3+ (RE = Er, Tm, Tb) codoped LaAlGe2O7 glasses,” J. Alloy. Comp.495(1), 205–208 (2010). [CrossRef]
  29. R. K. Verma, D. K. Rai, and S. B. Rai, “Investigation of structural properties and its effect on optical properties: Yb3+/Tb3+ codoped in aluminum silicate glass,” J. Alloy. Comp.509(18), 5591–5595 (2011). [CrossRef]
  30. N. K. Giri, D. K. Rai, and S. B. Rai, “UV-visible emission in Tb-Yb codoped tellurite glass on 980-nm excitation,” Appl. Phys. B89(2-3), 345–348 (2007). [CrossRef]
  31. E. Martins, C. B. de Araujo, J. R. Delben, A. S. L. Gomes, B. J. da Costa, and Y. Messaddeq, “Cooperative frequency upconversion in Yb3+–Tb3+ codoped fluoroindate glass,” Opt. Commun.158(1-6), 61–64 (1998).
  32. T. Yamashita and Y. Ohishi, “Analysis of energy transfers between Tb3+ and Yb3+ codoped in borosilicate glasses,” JOSA B26(4), 819–829 (2009). [CrossRef]
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