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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 5 — Mar. 11, 2013
  • pp: 6020–6027
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Efficient three-photon luminescence with strong polarization dependence from a scintillating silicate glass co-doped with Gd3+ and Tb3+

Guang-Can Li, Cheng-Yun Zhang, Hai-Dong Deng, Guang-Yin Liu, Sheng Lan, Qi- Qian, Zhong-Min Yang, and Achanta Venu Gopal  »View Author Affiliations


Optics Express, Vol. 21, Issue 5, pp. 6020-6027 (2013)
http://dx.doi.org/10.1364/OE.21.006020


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Abstract

Efficient three-photon luminescence (3PL) from a scintillating silicate glass co-doped with Gd3+ and Tb3+ was generated by using a focused femtosecond laser beam at 800 nm. Four emission bands centered at 496, 541, 583, and 620 nm were identified as the electronic transitions between the energy levels of Tb3+ followed by three-photon absorption (3PA) in Gd3+ and Tb3+ and the resonant energy transfer from Gd3+ to Tb3+. More interestingly, a strong polarization dependence of the 3PL was observed and it is ascribed to the polarization dependent 3PA in Gd3+ and Tb3+ and/or the angular distribution of photogenerated electrons in the glass.

© 2013 OSA

1. Introduction

Owing to their potential applications in high-energy and nuclear physics, medical imaging, and industry, scintillators and especially rare-earth-ion-doped glasses have attracted great interest in the last two decades because such glasses can be fabricated at a low cost and in large quantities [1

1. G. Zanella, R. Zannoni, R. Dall’Igna, B. Locardi, P. Polato, M. Bettinelli, and A. Marigo, “A new cerium scintillating glass for X-ray detection,” Nucl. Instr. Meth. A 345(1), 198–201 (1994). [CrossRef]

5

5. M. Nikl and C. Pedrini, “Photoluminescence of heavily doped CeF3:Cd2+ single crystals,” Solid State Commun. 90(3), 155–159 (1994). [CrossRef]

]. More importantly, optical fibers made of these glasses will further broaden their applications in the fields of thermal neutron detection and radiography, nondestructive testing, and radiation dosimetry [6

6. M. Nikl, “Scintillation detectors for X-rays,” Meas. Sci. Technol. 17(4), R37–R54 (2006). [CrossRef]

8

8. W. P. Siegmund, P. Nass, J. P. Fabre, W. Flegel, V. Zacek, G. Martellotti, and G. Wilquet, “Glasses as active and passive components for scintillating fiber detectors,” SPIE 1737, 2–13 (1993). [CrossRef]

].

2. Experimental details

The experimental setup used to study the MPL of the glass and its polarization dependence is schematically shown in Fig. 1
Fig. 1 Experimental setup used to generate the MPL from the scintillating glass and to investigate its dependence on the polarization of the fs laser light.
. The 800-nm fs laser light with a repetition rate of 76 MHz and a pulse duration of ~120 fs delivered by a Ti: sapphire oscillator (Mira 900, Coherent) was introduced into an inverted microscope (Observer A1, Zeiss), reflected by a dichroic mirror and focused on the glass, which has been polished, by using a 40 × objective lens (NA = 0.65). The diameter of the excitation spot was estimated to be ~4.4 μm. The luminescence from the glass was collected by the same objective lens and delivered to a spectrometer (SR-500I-B1, Andor) equipped with a charge-coupled device (CCD) (DU970N, Andor) for analysis. The polarization of the fs laser light was modified by using a half-wave (λ/2) plate while its intensity was adjusted by using an attenuator.

3. Results and discussion

3.1 Single-photon-induced luminescence from the glass

Before studying the MPL of the glass, we first examined the emission and excitation spectra of the glass under single photon excitation by using a fluorescence spectrometer (F-4500, Hitachi). They are shown in Fig. 2(a)
Fig. 2 (a) Emission and excitation spectra of the glass under single photon excitation and its MPL spectra under different excitation densities at 800 nm. (b) Logarithmic plots of the PL peak intensity versus excitation density for the four different emission bands along with fits.
by the black solid curve and dark blue dashed curve, respectively. In the emission spectrum obtained by single photon excitation at 253 nm, one can see four narrow emission bands centered at 496, 541, 583, and 620 nm which have been observed in glasses of different compositions co-doped with Tb3+ and Gd3+ under the excitation of ultraviolet or X ray [16

16. H. C. Aspinall, “Chiral lanthanide complexes: coordination chemistry and applications,” Chem. Rev. 102(6), 1807–1850 (2002). [CrossRef] [PubMed]

19

19. M. C. Downer and A. Bivas, “Third- and fourth-order analysis of the intensities and polarization dependence of two-photon absorption lines of Gd3+ in LaF3 and aqueous solution,” Phys. Rev. B 28(7), 3677–3696 (1983). [CrossRef]

]. They have been attributed to the electronic transitions between the energy levels of Tb3+ (i.e., 5D47F6, 5D47F5, 5D47F4, and 5D47F3), as depicted in Fig. 3
Fig. 3 Energy level diagrams for Gd3+ and Tb3+ showing the 3PA in Gd3+ and Tb3+ followed by nonradiative relaxation (NR), resonant energy transfer (RET) from Gd3+ to Tb3+, and the electronic transitions between the energy levels of Tb3+ corresponding to the four emission bands observed in the MPL spectrum.
. Although Gd3+ was doped in the glass in a large amount, we did not see any emission from Gd3+. In the excitation spectrum obtained by monitoring the luminescence intensity at 549 nm, one can see a broad band with a strong intensity ranging from ~240 to ~280 nm, implying a high efficiency for three-photon absorption (3PA) by using fs laser at ~800 nm.

3.2 Three-photon-induced luminescence from the glass

In Fig. 2(a), we also show the MPL spectra of the glass obtained at 800 nm for different excitation densities by colored curves. The excitation densities given in Fig. 2 are average power densities obtained by dividing the average powers of the fs laser with the area of the excitation spot. The corresponding peak power densities can be readily deduced and they are several orders of magnitude larger than the average power densities. Similarly, four narrow emission bands centered at 496, 541, 583, and 620 nm are observed. However, the relative intensities of the four emission bands (2.96: 7.81: 1.13: 1) are found to be different from those observed in single photon excitation (5.90: 14.31: 1.56: 1).

In order to gain a deep insight into the nonlinear process involved in the MPL of the glass, the excitation density dependence of the luminescence intensity at the central wavelength of each emission band was analyzed and the results are presented in Fig. 2(b). The fitting of the excitation density dependent luminescence gives a power index of ~3 for all emission bands, indicating that 3PA is responsible for the generation of the luminescence. As far as 3PA processes are concerned, there are three possible ways. One is the stepwise excitation to a high-energy level by continuously absorbing three photons. This process requires the existence of equally separated energy levels with energy interval equal to the one-photon energy of the pump light. Another is one-photon excitation to a real level followed by a two-photon absorption (2PA) or two-photon excitation to a real level followed by one-photon absorption. The third is the simultaneous absorption of three photons. From the energy level diagrams of Gd3+ and Tb3+ presented in Fig. 3, it can be clearly seen that there are not two energy levels whose energy interval equals to the one- or two-photon energy of the pump light. It is further confirmed by the absorption spectrum of the 0.5-mm-thick glass shown in Fig. 4
Fig. 4 Absorption spectrum of the glass in the ultraviolet, visible and near infrared regions. The red and green arrows indicate the absorptions in Gd3+ due to the electronic transitions 8S7/26IJ and 8S7/26P7/2 while the blue one indicates the absorption in Tb3+ due to the electronic transition 7F65D4. The insets show the magnified spectral regions where the small absorption peaks can be identified. The vertical coordinate for each inset has been changed from linear to logarithmic in order to show clearly the absorption peaks.
. In the insets of Fig. 4 where the magnified violet and visible spectral regions are shown, one can identify three tiny peaks which can be attributed to the absorptions in Gd3+ and Tb3+. The red and green arrows indicate the absorptions in Gd3+ due to the electronic transitions 8S7/26IJ (J = 11/2, 13/2, 15/2) and 8S7/26P7/2, respectively. The corresponding wavelengths appear at ~273 nm and ~312 nm. The blue arrow indicates the absorption in Tb3+ due to the electronic transition 7F65D4, corresponding to a wavelength of ~490 nm. On the other hand, the energy of three photons, which corresponds to a central wavelength of ~267 nm, matches approximately the energy interval for the electronic transition 8S7/2JI6 in Gd3+ (~273 nm) and the electronic transition 4f8 → 4f75d1 in Tb3+ (~281 nm) [16

16. H. C. Aspinall, “Chiral lanthanide complexes: coordination chemistry and applications,” Chem. Rev. 102(6), 1807–1850 (2002). [CrossRef] [PubMed]

]. Therefore, it is thought that the three-photon luminescence (3PL) of the glass originates from the simultaneous absorption of three photons. In the 3PL shown in Fig. 2(a), all the emission bands can be attributed to the electronic transitions between the energy levels of Tb3+ and emissions originating from the electronic transitions between the energy levels of Gd3+ are not observed. This phenomenon suggests the transfer of electrons generated via 3PA from Gd3+ to Tb3+ when these two ions are located close together. This mechanism has been proposed earlier to interpret the luminescence from the glass generated by single photon excitation [16

16. H. C. Aspinall, “Chiral lanthanide complexes: coordination chemistry and applications,” Chem. Rev. 102(6), 1807–1850 (2002). [CrossRef] [PubMed]

18

18. J. Fu, M. Kobayashi, and J. M. Parker, “Terbium-activated heavy scintillating glasses,” J. Lumin. 128(1), 99–104 (2008). [CrossRef]

]. From Fig. 4, it can be seen that part of the pump energy may be lost due to the electronic absorption in the glass, reducing the efficiency of 3PL. In fact, the absorption at a short wavelength of ~267 nm cannot be avoided even for undoped glass. A possible way to reduce such absorption is to shift the pump light to a longer wavelength of ~820 nm (or ~936 nm). By doing so, we can resonantly induce the electric transition 8S7/26IJ (or 8S7/26P7/2) in Gd3+, which corresponds to a wavelength of ~273 nm (or ~312 nm), through 3PA.

3.3 Polarization dependence of the three-photon-induced luminescence

Keeping the physical mechanism for the 3PL in mind, let us examine the polarization dependence of the 3PL. In experiments, we changed the polarization of the fs laser by rotating the λ/2 plate placed outside the microscope. The reason why we did the experiments by using an inverted microscope is that the system shown in Fig. 1 provides high excitation density (small focused laser spot) as well as efficient collection of the generated luminescence. Similarly experiment was also carried out without using the microscope and polarization dependent luminescence was also demonstrated. However, the polarization dependence is not as strong as that observed by using the microscope because of the low excitation density and collection efficiency.

Initially, the laser light is horizontally polarized (θ = 0°) and it corresponds to a s-polarized wave for the dichroic mirror. If we make it vertically polarized (θ = 90°) by rotating the λ/2 plate by 45°, then it corresponds to a p-polarized wave. For both s- and p-polarized waves, their linear polarization states are preserved after being reflected by the dichroic mirror and focused by the objective lens. Accordingly, linearly polarized light along 0° (Ey′) or 90° (Ex′) can be obtained on the focused plane (see Fig. 1). In order to see the effects of polarization on the 3PL of the glass, we chose to generate an elliptically polarized light by utilizing the reflection phase difference between the s- and p-polarized waves. It was achieved by changing the polarization of the incident light before the dichroic mirror. The polarization states on the focused plane measured for different polarization angles (θ = 0°, 30°, 45°, 60°, and 90°) by using the combination of a Glan-Taylor polarizer and a power meter are shown in Fig. 5(a)
Fig. 5 (a) Polarization states of the fs laser light with polarization angles of 0°, 30°, 45°, 60°, and 90° after being reflected by the dichroic mirror. (b) Polarization dependent 3PL intensities at the central wavelengths of the four emission bands. (c) Excitation polarization dependent 3PL intensities at the central wavelengths of the four emission bands plotted in a polar coordinate.
. Two linearly polarized beams along 0° and 90° and three elliptically polarized beams with different ellipticities are clearly identified.

In order to clearly show the polarization dependent 3PL from the glass, we have plotted the 3PL intensity as a function of polarization in a polar coordinate, as shown in Fig. 5(c). Another feature that is worth noting in Fig. 5(b) is the small difference in the 3PL intensity for the two linear polarizations perpendicular to each other. It is found that the 3PL intensity for the polarization at 0° is about 1.1 times of that at 90°. This feature is not obvious for the emission bands with weak intensities (λ = 490, 591, and 623 nm) but it becomes significant for the emission band with the strongest intensity (λ = 549 nm). Actually, the reflectance of the dichromic mirror for s- and p-polarized waves is generally different. In our case, this difference is quite small and it cannot be resolved when we compared the power of the s- and p-polarized waves after the dichromic mirror (see Fig. 5(a)). However, the third power dependence of the 3PL on excitation density makes it possible for discriminating the small difference because it is quite sensitive to the small change in the excitation intensity. In addition, the efficient 3PL further enlarges the difference which can be easily identified in Fig. 5(b). Based on the difference in 3PL intensity, it is deduced that the power of p-polarized wave is about 1.03 times that of s-polarized one after being reflected by the dichroic mirror.

4. Summary

In summary, we have investigated the MPL from a scintillating silicate glass co-doped with Gd3+ and Tb3+ by using a focused fs laser light at 800 nm. The efficient upconversion luminescence appearing in the visible region was demonstrated to be 3PL and the physical mechanism is thought to be the 3PA in Gd3+ and Tb3+ followed by RET from Gd3+ to Tb3+. A strong polarization dependence of 3PL was observed and it is attributed to the polarization dependent 3PA in Gd3+ and Tb3+ and/or the angular distribution of photogenerated electrons in the glass. The efficient MPL from rare-earth-ion-doped glass is expected to find applications in various fields of science and technology.

Acknowledgments

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 10974060, 51171066 and 11111120068), the Ministry of Education (Grant No. 20114407110002) and the program for high-level professionals in the universities of Guangdong province, China.

References and links

1.

G. Zanella, R. Zannoni, R. Dall’Igna, B. Locardi, P. Polato, M. Bettinelli, and A. Marigo, “A new cerium scintillating glass for X-ray detection,” Nucl. Instr. Meth. A 345(1), 198–201 (1994). [CrossRef]

2.

D. F. Anderson, “Properties of the high-density scintillator cerium fluoride,” EEE Trans. Nucl. Sci. 36(1), 137–140 (1989). [CrossRef]

3.

G. Zanella and R. Zannoni, “The detective quantum efficiency of an imaging detector,” Nucl. Instr. Meth. A 359(3), 474–477 (1995). [CrossRef]

4.

C. Pedrini, B. Moine, J. C. Gacon, and B. Jacquier, “One- and two-photon spectroscopy of Ce3+ ions in LaF3-CeF3 mixed crystals,” J. Phys. Condens. Matter 4(24), 5461–5470 (1992). [CrossRef]

5.

M. Nikl and C. Pedrini, “Photoluminescence of heavily doped CeF3:Cd2+ single crystals,” Solid State Commun. 90(3), 155–159 (1994). [CrossRef]

6.

M. Nikl, “Scintillation detectors for X-rays,” Meas. Sci. Technol. 17(4), R37–R54 (2006). [CrossRef]

7.

M. C. Aznar, C. E. Andersen, L. Bøtter-Jensen, S. Å. J. Bäck, S. Mattsson, F. Kjaer-Kristoffersen, and J. Medin, “Real-time optical-fibre luminescence dosimetry for radiotherapy: physical characteristics and applications in photon beams,” Phys. Med. Biol. 49(9), 1655–1669 (2004). [CrossRef] [PubMed]

8.

W. P. Siegmund, P. Nass, J. P. Fabre, W. Flegel, V. Zacek, G. Martellotti, and G. Wilquet, “Glasses as active and passive components for scintillating fiber detectors,” SPIE 1737, 2–13 (1993). [CrossRef]

9.

J. A. Mares, M. Nikl, K. Nitsch, N. Solovieva, A. Krasnikov, and S. Zazubovich, “A role of Gd3+ in scintillating process in Tb-doped Na-Gd phosphate glasses,” J. Lumin. 94–95, 321–324 (2001). [CrossRef]

10.

D. He, C. Yu, J. Cheng, S. Li, and L. Hu, “Energy transfer between Gd3+ and Tb3+ in phosphate glass,” J. Rare Earths 29(1), 48–51 (2011). [CrossRef]

11.

G. H. Dicke, Spectra and Energy Levels of Rare-Earth Ions (Wiley, 1968).

12.

S. Zhang, B. Zhu, S. Zhou, S. Xu, and J. Qiu, “Multi-photon absorption upconversion luminescence of a Tb3+-doped glass excited by an infrared femtosecond laser,” Opt. Express 15(11), 6883–6888 (2007). [CrossRef] [PubMed]

13.

A.-H. Li, Z. R. Zheng, Q. Lü, L. Sun, W.-Z. Wu, W.-L. Liu, Y.-Q. Yang, and T.-Q. Lü, “Two-photon-excited luminescence in a Tb3+ -doped lithium niobate crystal pumped by a near-infrared femtosecond laser,” Opt. Lett. 33(9), 1014–1016 (2008). [CrossRef] [PubMed]

14.

K. Hanaoka, K. Kikuchi, S. Kobayashi, and T. Nagano, “Time-Resolved long-lived luminescence imaging method employing luminescent lanthanide probes with a new microscopy system,” J. Am. Chem. Soc. 129(44), 13502–13509 (2007). [CrossRef] [PubMed]

15.

K. L. Wong, W. M. Kwok, W. T. Wong, D. L. Phillips, and K. W. Cheah, “Green and red three-photon upconversion from polymeric lanthanide(III) complexes,” Angew. Chem. Int. Ed. Engl. 43(35), 4659–4662 (2004). [CrossRef] [PubMed]

16.

H. C. Aspinall, “Chiral lanthanide complexes: coordination chemistry and applications,” Chem. Rev. 102(6), 1807–1850 (2002). [CrossRef] [PubMed]

17.

S. Wang, Q. Qian, Q. Y. Zhang, Z. M. Yang, and Z. H. Jiang, “Gd3+-sensitized Tb3+-doped scintillating silicate glasses,” J. Inor. Materi. 24(4), 773–777 (2009). [CrossRef]

18.

J. Fu, M. Kobayashi, and J. M. Parker, “Terbium-activated heavy scintillating glasses,” J. Lumin. 128(1), 99–104 (2008). [CrossRef]

19.

M. C. Downer and A. Bivas, “Third- and fourth-order analysis of the intensities and polarization dependence of two-photon absorption lines of Gd3+ in LaF3 and aqueous solution,” Phys. Rev. B 28(7), 3677–3696 (1983). [CrossRef]

20.

R. Francini, U. M. Grassano, S. Boiko, G. G. Tarasov, and A. Scacco, “Anisotropy of two-photon excited f-f transitions of Eu2+ in KMgF3,” J. Chem. Phys. 110(1), 457–464 (1999). [CrossRef]

21.

M. C. Downer, C. D. Cordero-Montalvo, and H. Crosswhite, “Study of new 4f7 levels of Eu2+ in GaF2 and SrF2 using two-photon absorption spectroscopy,” Phys. Rev. B 28(9), 4931–4943 (1983). [CrossRef]

22.

H. J. Seo, B. K. Moon, and T. Tsuboi, “Two-photon excitation spectroscopy of 4f7-4f7 transitions of Eu2+ ions doped in a KMgF3 crystal,” Phys. Rev. B 62(19), 12688–12695 (2000). [CrossRef]

23.

P. G. Kazansky, H. Inouye, T. Mitsuyu, K. Miura, J. Qiu, K. Hirao, and F. Starrost, “Anomalous anisotropic light scattering in Ge-doped silica glass,” Phys. Rev. Lett. 82(10), 2199–2202 (1999). [CrossRef]

OCIS Codes
(160.5690) Materials : Rare-earth-doped materials
(190.4180) Nonlinear optics : Multiphoton processes
(190.7220) Nonlinear optics : Upconversion

ToC Category:
Materials

History
Original Manuscript: December 21, 2012
Revised Manuscript: February 18, 2013
Manuscript Accepted: February 23, 2013
Published: March 4, 2013

Citation
Guang-Can Li, Cheng-Yun Zhang, Hai-Dong Deng, Guang-Yin Liu, Sheng Lan, Qi- Qian, Zhong-Min Yang, and Achanta Venu Gopal, "Efficient three-photon luminescence with strong polarization dependence from a scintillating silicate glass co-doped with Gd3+ and Tb3+," Opt. Express 21, 6020-6027 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-5-6020


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References

  1. G. Zanella, R. Zannoni, R. Dall’Igna, B. Locardi, P. Polato, M. Bettinelli, and A. Marigo, “A new cerium scintillating glass for X-ray detection,” Nucl. Instr. Meth. A345(1), 198–201 (1994). [CrossRef]
  2. D. F. Anderson, “Properties of the high-density scintillator cerium fluoride,” EEE Trans. Nucl. Sci.36(1), 137–140 (1989). [CrossRef]
  3. G. Zanella and R. Zannoni, “The detective quantum efficiency of an imaging detector,” Nucl. Instr. Meth. A359(3), 474–477 (1995). [CrossRef]
  4. C. Pedrini, B. Moine, J. C. Gacon, and B. Jacquier, “One- and two-photon spectroscopy of Ce3+ ions in LaF3-CeF3 mixed crystals,” J. Phys. Condens. Matter4(24), 5461–5470 (1992). [CrossRef]
  5. M. Nikl and C. Pedrini, “Photoluminescence of heavily doped CeF3:Cd2+ single crystals,” Solid State Commun.90(3), 155–159 (1994). [CrossRef]
  6. M. Nikl, “Scintillation detectors for X-rays,” Meas. Sci. Technol.17(4), R37–R54 (2006). [CrossRef]
  7. M. C. Aznar, C. E. Andersen, L. Bøtter-Jensen, S. Å. J. Bäck, S. Mattsson, F. Kjaer-Kristoffersen, and J. Medin, “Real-time optical-fibre luminescence dosimetry for radiotherapy: physical characteristics and applications in photon beams,” Phys. Med. Biol.49(9), 1655–1669 (2004). [CrossRef] [PubMed]
  8. W. P. Siegmund, P. Nass, J. P. Fabre, W. Flegel, V. Zacek, G. Martellotti, and G. Wilquet, “Glasses as active and passive components for scintillating fiber detectors,” SPIE1737, 2–13 (1993). [CrossRef]
  9. J. A. Mares, M. Nikl, K. Nitsch, N. Solovieva, A. Krasnikov, and S. Zazubovich, “A role of Gd3+ in scintillating process in Tb-doped Na-Gd phosphate glasses,” J. Lumin.94–95, 321–324 (2001). [CrossRef]
  10. D. He, C. Yu, J. Cheng, S. Li, and L. Hu, “Energy transfer between Gd3+ and Tb3+ in phosphate glass,” J. Rare Earths29(1), 48–51 (2011). [CrossRef]
  11. G. H. Dicke, Spectra and Energy Levels of Rare-Earth Ions (Wiley, 1968).
  12. S. Zhang, B. Zhu, S. Zhou, S. Xu, and J. Qiu, “Multi-photon absorption upconversion luminescence of a Tb3+-doped glass excited by an infrared femtosecond laser,” Opt. Express15(11), 6883–6888 (2007). [CrossRef] [PubMed]
  13. A.-H. Li, Z. R. Zheng, Q. Lü, L. Sun, W.-Z. Wu, W.-L. Liu, Y.-Q. Yang, and T.-Q. Lü, “Two-photon-excited luminescence in a Tb3+ -doped lithium niobate crystal pumped by a near-infrared femtosecond laser,” Opt. Lett.33(9), 1014–1016 (2008). [CrossRef] [PubMed]
  14. K. Hanaoka, K. Kikuchi, S. Kobayashi, and T. Nagano, “Time-Resolved long-lived luminescence imaging method employing luminescent lanthanide probes with a new microscopy system,” J. Am. Chem. Soc.129(44), 13502–13509 (2007). [CrossRef] [PubMed]
  15. K. L. Wong, W. M. Kwok, W. T. Wong, D. L. Phillips, and K. W. Cheah, “Green and red three-photon upconversion from polymeric lanthanide(III) complexes,” Angew. Chem. Int. Ed. Engl.43(35), 4659–4662 (2004). [CrossRef] [PubMed]
  16. H. C. Aspinall, “Chiral lanthanide complexes: coordination chemistry and applications,” Chem. Rev.102(6), 1807–1850 (2002). [CrossRef] [PubMed]
  17. S. Wang, Q. Qian, Q. Y. Zhang, Z. M. Yang, and Z. H. Jiang, “Gd3+-sensitized Tb3+-doped scintillating silicate glasses,” J. Inor. Materi.24(4), 773–777 (2009). [CrossRef]
  18. J. Fu, M. Kobayashi, and J. M. Parker, “Terbium-activated heavy scintillating glasses,” J. Lumin.128(1), 99–104 (2008). [CrossRef]
  19. M. C. Downer and A. Bivas, “Third- and fourth-order analysis of the intensities and polarization dependence of two-photon absorption lines of Gd3+ in LaF3 and aqueous solution,” Phys. Rev. B28(7), 3677–3696 (1983). [CrossRef]
  20. R. Francini, U. M. Grassano, S. Boiko, G. G. Tarasov, and A. Scacco, “Anisotropy of two-photon excited f-f transitions of Eu2+ in KMgF3,” J. Chem. Phys.110(1), 457–464 (1999). [CrossRef]
  21. M. C. Downer, C. D. Cordero-Montalvo, and H. Crosswhite, “Study of new 4f7 levels of Eu2+ in GaF2 and SrF2 using two-photon absorption spectroscopy,” Phys. Rev. B28(9), 4931–4943 (1983). [CrossRef]
  22. H. J. Seo, B. K. Moon, and T. Tsuboi, “Two-photon excitation spectroscopy of 4f7-4f7 transitions of Eu2+ ions doped in a KMgF3 crystal,” Phys. Rev. B62(19), 12688–12695 (2000). [CrossRef]
  23. P. G. Kazansky, H. Inouye, T. Mitsuyu, K. Miura, J. Qiu, K. Hirao, and F. Starrost, “Anomalous anisotropic light scattering in Ge-doped silica glass,” Phys. Rev. Lett.82(10), 2199–2202 (1999). [CrossRef]

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