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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 3, Iss. 11 — Nov. 1, 2013
  • pp: 1931–1943
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Broadband fluorescence emission of Eu3+ doped germanotellurite glasses for fiber-based irradiation light sources

F. Wang, L. F. Shen, B. J. Chen, E. Y. B. Pun, and H. Lin  »View Author Affiliations


Optical Materials Express, Vol. 3, Issue 11, pp. 1931-1943 (2013)
http://dx.doi.org/10.1364/OME.3.001931


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Abstract

Eu3+ doped fiber-based germanotellurite (NZPGT) glasses with medium-low maximum phonon energy of 782 cm−1 have been fabricated and characterized. Judd-Ofelt intensity parameters Ω2 (6.25 × 10−20 cm2) and Ω4 (1.77 × 10−20 cm2) indicate a high asymmetrical and covalent environment around Eu3+ in the optical glasses. The spontaneous emission probability of the dominant transition 5D07F2 peaking at 612.5 nm and the corresponding maximum stimulated emission cross-section were derived to be 445.7 s−1 and 2.05 × 10−21 cm2, respectively, confirming the effectiveness of the red fluorescence emission. The quantum yield was derived to be 12% under 391 nm LED excitation, and approximately 88% photons have been demonstrated in wavelength range of 600−720 nm, indicating that Eu3+ doped NZPGT glasses under proper excitation conditions are promising optical materials for fiber-based irradiation light sources that are competent to activate diverse photodynamic therapy photosensitizers.

© 2013 Optical Society of America

1. Introduction

At present, lasers and light emitting diodes (LEDs) are widely used as PDT light sources due to their special advantages such as favorable directivity and high fluence rates [8

8. L. Brancaleon and H. Moseley, “Laser and non-laser light sources for photodynamic therapy,” Lasers Med. Sci. 17(3), 173–186 (2002). [CrossRef] [PubMed]

11

11. P. Babilas, E. Kohl, T. Maisch, H. Bäcker, B. Gross, A. L. Branzan, W. Bäumler, M. Landthaler, S. Karrer, and R. M. Szeimies, “In vitro and in vivo comparison of two different light sources for topical photodynamic therapy,” Br. J. Dermatol. 154(4), 712–718 (2006). [PubMed]

]. However, lasers with tremendous power density may mis-locate target diseased tissues, causing accidental damages to normal tissues. In terms of LEDs, low coupling efficiency with optical fiber catheters and narrow excited spectral bandwidth, to some extent, restrict their applications in PDT modality. Recently, investigations have been reported on the efficient fluorescence generated in rare-earth (RE) ions doped glass fibers and glass channel waveguides, which are deemed to be a new route for high-quality irradiation light sources for PDT modality [12

12. 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]

,13

13. 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]

]. As an active fluorescent center, Eu3+ emits intense fluorescence ranging from 570 to 720 nm wavelength region [14

14. I. V. Kityk, J. Wasylak, D. Dorosz, and J. Kucharski, “Eu3+-doped glass materials for red luminescence,” Opt. Laser Technol. 33(3), 157–160 (2001). [CrossRef]

17

17. C. E. Secu, R. F. Negrea, and M. Secu, “Eu3+ probe ion for rare-earth dopant site structure in sol-gel derived LiYF4 oxyfluoride glass-ceramic,” Opt. Mater. 35(12), 2456–2460 (2013). [CrossRef]

] that is within the scope of the maximum absorption regions of most PSs currently used in PDT modality or under clinical trials [7

7. S. Yano, S. Hirohara, M. Obata, Y. Hagiya, S.- Ogura, A. Ikeda, H. Kataoka, M. Tanaka, and T. Joh, “Current states and future views in photodynamic therapy,” J. Photochem. Photobiol. Chem. 12(1), 46–67 (2011). [CrossRef]

]. Among oxide glasses, tellurite glasses have low maximum phonon energy, high refractive index, and good rare-earth ion solubility [18

18. V. A. G. Rivera, S. P. A. Osorio, Y. Ledemi, D. Manzani, Y. Messaddeq, L. A. O. Nunes, and E. Marega Jr., “Localized surface plasmon resonance interaction with Er3+-doped tellurite glass,” Opt. Express 18(24), 25321–25328 (2010). [CrossRef] [PubMed]

], but exhibit low glass stability and mechanical strength for fabricating high-quality optical fibers. With the introduction of GeO2, tellurite glasses are expected to possess better glass stability and mechanical strength for fiber fabrication.

In this work, Eu3+ doped fiber-based germanotellurite (NZPGT) glasses with medium-low maximum phonon energy have been fabricated and characterized. The thermal stability range was derived to be 101 °C, indicating that NZPGT glasses are potential optical materials for fiber fabrication. Judd-Ofelt intensity parameters Ω2 (6.25 × 10−20 cm2) and Ω4 (1.77 × 10−20 cm2) indicate a high asymmetrical and covalent environment around Eu3+ in the optical glasses. The spontaneous emission probability of dominant transition 5D07F2 peaking at 612.5 nm and the corresponding maximum stimulated emission cross-section were derived to be 445.7 s−1 and 2.05 × 10−21 cm2, respectively, confirming the effectiveness of the red fluorescence emission in the optical glasses. The quantum yield was derived to be 12% under 391 nm LED excitation, and approximately 88% photons have been demonstrated in wavelength range of 600−720 nm, indicating that Eu3+ doped NZPGT glasses under proper excitation conditions are promising optical materials for fiber-based irradiation light sources to activate diverse PDT photosensitizers.

2. Experiments

According to the molar host composition 14Na2O−10ZnO−7PbO−19GeO2−50TeO2 (NZPGT), Eu3+ doped NZPGT glasses were prepared from high-purity Na2CO3, ZnO, PbO, GeO2, and TeO2 powders. Additional 0.2wt% and 1.2wt% Eu2O3 were introduced into the NZPGT glass composition based on the host weight. Firstly, the raw materials were well grinded in an agate mortar and preheated in pure Pt crucibles at 270 °C for 3 h. Then the glass melts were wobbled every ten minutes when melted at 880 °C for 30 min, and finally quenched in a preheated aluminum mold. Subsequently, the glass samples were immediately annealed at 270 °C for 3 h to diminish the inhomogeneity in the Eu3+ ion distribution caused by the quenching process, and slowly cooled down to room temperature. For optical measurements, the annealed glass samples were sliced and polished into pieces with parallel sides.

The infrared transmittance spectrum of 0.2wt% Eu2O3 doped NZPGT glasses was obtained by a Spectrum One-B Fourier transform IR (FTIR) spectrometer. The differential thermal analysis (DTA) scan was carried out by a WCR-2D differential thermal analyzer at a rate of 15 °C/min from room temperature to 900 °C. The density of 1.2wt% Eu2O3 doped NZPGT glasses was obtained to be 5.117 g∙cm−3 by the Archimedes method. The refractive indices were derived to be 1.9300 at 632.8 nm and 1.8849 at 1536 nm using a Metricon 2010 prism coupler, respectively. The refractive indices at all the other wavelengths can be solved by the Cauchy’s equation n=A+B/λ2 [19

19. E. Friedman and J. L. Miller, Photonics Rules of Thumb: Optics, Electro-Optics, Fiber Optics, and Lasers (McGraw-Hill, 2004), Chap. 10.

] with A = 1.8757 and B = 21751 nm2 for the coming Judd-Ofelt (J-O) analysis. The absorption spectrum was detected by a Perkin-Elmer UV-VIS-NIR Lambda 19 double beam spectrophotometer with wavelength accuracy of ± 0.15 nm for UV/VIS region and ± 0.6 nm for NIR region. The excitation and emission spectra were measured using a Jobin Yvon Fluorolog-3 spectrophotometer with spectral accuracy of 0.5 nm equipped with an R928 photomultiplier (PMT) tube as detector and a CW Xe-lamp as pump source. The spectral power distributions were measured using a 30 cm diameter integrating sphere equipped with an Ocean Optics USB4000 CCD detector with optical resolution of ~1.5 nm FWHM connected by a 400 μm-core optical fiber. The schematic diagram of the experimental setup for quantum yield measurement is depicted in Fig. 1.
Fig. 1 Schematic diagram of the experimental setup for quantum yield measurement.
The currents of commercially available 391 and 456 nm LEDs as pump sources were both fixed at 20 mA. A standard halogen lamp (EVERFINED062) was used for calibrating this measurement system and its spectral power distribution was obtained through fitting the factory data based on the blackbody radiation law. The LED pump sources were rounded by black tapes except the emitting surfaces were mounted in the integrating sphere. For quantitative characterization and analysis on absolute spectral properties of fluorescence, Eu3+ doped NZPGT glasses were put on the LED pump sources and covered their tops completely. All pictures were taken with a Sony SLT-α200 digital camera.

3. Results and discussion

3.1 Phonon energy and thermal properties of Eu3+ doped NZPGT glasses

The FTIR spectrum of 0.2wt% Eu2O3 doped NZPGT glasses is shown in the inset of Fig. 2.
Fig. 2 DTA curve of 0.2wt% Eu2O3 doped NZPGT glasses. Inset: FTIR spectrum of 0.2wt% Eu2O3 doped NZPGT glasses with a thickness of 2.28 mm.
The maximum phonon energy of Eu3+ doped NZPGT glasses, to some extent, can be derived by the IR transmission side band and was estimated to be 782 cm−1 by the empirical formula E=92.9+0.4257R, in which R is the wavenumber at 10% transmittance. The derived maximum phonon energy 782 cm−1 is ascribed to more distorted TeO4 due to the lengthening of one Te-O axial bond of the bipyramidal site [20

20. L. Petit, T. Cardinal, J. J. Videau, G. Le Flem, Y. Guyot, G. Boulon, M. Couzi, and T. Buffeteau, “Effect of the introduction of Na2B4O7 on erbium luminescence in tellurite glasses,” J. Non-Cryst. Solids 298(1), 76–88 (2002). [CrossRef]

,21

21. J. Ozdanova, H. Ticha, and L. Tichy, “Optical band gap and Raman spectra in some (Bi2O3)x(WO3)y(TeO2)100−x−y and (PbO)x(WO3)y(TeO2)100−x−y glasses,” J. Non-Cryst. Solids 355(45-47), 2318–2322 (2009). [CrossRef]

]. The DTA curve of 0.2wt% Eu2O3 doped NZPGT glasses is presented in Fig. 2. The transition temperature Tg, the crystallization onset temperature Tx, and the crystallization temperature Tc were derived to be 295, 396, and 416 °C, respectively. Generally, thermal stability range (ΔT = Tx−Tg) of core and cladding glasses should be larger than 100 °C to obtain wide operating temperature range and avoid crystallization during fiber drawing [22

22. N. Manikandan, A. Ryasnyanskiy, and J. Toulouse, “Thermal and optical properties of TeO2−ZnO−BaO glasses,” J. Non-Cryst. Solids 358(5), 947–951 (2012). [CrossRef]

24

24. X. Hu, G. Guery, J. Boerstler, J. D. Musgraves, D. Vanderveer, P. Wachtel, and K. Richardson, “Influence of Bi2O3 content on the crystallization behavior of TeO2−Bi2O3−ZnO glass system,” J. Non-Cryst. Solids 358(5), 952–958 (2012). [CrossRef]

]. Here, the ΔT of 0.2wt% Eu2O3 doped NZPGT glasses was derived to be 101 °C, indicating that Eu3+ doped NZPGT glasses are potential candidates for fiber materials.

In addition, another two critical thermal property parameters, i.e. the thermal stability parameter H and the Sadd-Poulain criterion S, are introduced to further evaluate the ability of NZPGT glasses against crystallization. The H parameter and the S criterion are defined as H=(TxTg)/TgandS=(TcTx)(TxTg)/Tg, respectively. For 0.2wt% Eu2O3 doped NZPGT glasses, the H parameter and the S criterion were derived to be 0.34 and 6.85, respectively, which is similar to the glass system K2O−Nb2O5−GeO2−TeO2 [25

25. G. Monteiro, L. F. Santos, J. C. G. Pereira, and R. M. Almeida, “Optical and spectroscopic properties of germanotellurite glasses,” J. Non-Cryst. Solids 357(14), 2695–2701 (2011). [CrossRef]

] and demonstrates that the introduction of GeO2 can extend the thermal stability range, and improve the chemical and thermal stability of the tellurite glasses for fiber drawing.

3.2 Spectral properties of Eu3+ doped NZPGT glasses

Fig. 3 (a) Absorption spectrum, (b) Excitation spectrum, (c) Emission spectrum, (d) Emission cross-section profiles of 1.2wt% Eu2O3 doped NZPGT glasses.
As labeled in Fig. 3(a), the absorption spectrum of 1.2wt% Eu2O3 doped NZPGT glasses presents five absorption bands that are attributed to the absorption transitions from the ground state 7F0 to the excited states 5L6, 5D3, 5D2, 5D1, and 7F6, respectively. The 7F05D1, 7F05D2, and 7F05D3 absorption bands are weak due to spin-forbidden rule. In contrast, the 7F05L6 and the 7F07F6 absorption transitions are stronger owing to spin-allowed rule. Doublet peaks at 534 nm (5D1) and 2082 nm (7F6), and shoulders on the broader peak at 394 nm (5L6) manifest the two thermally populated ground states 7F0,1. Because the closely spaced 7F0,1 ground levels have energy level differences on the order of kT, where k is Boltzmann’s constant and T is the absolute temperature, and they are easily thermal populated at room temperature unlike the other lanthanides that only have a single populated ground state [26

26. K. Driesen, V. K. Tikhomirov, and C. Görller-Walrand, “Eu3+ as a probe for rare-earth dopant site structure in nano-glass-ceramics,” J. Appl. Phys. 102(2), 024312–024317 (2007). [CrossRef]

]. For Eu3+, the population CJ (J = 0, 1, 2, 3, 4) of any level 7FJ (J = 0, 1, 2, 3, 4) is given by
CJC0=gJg0exp((EJE0)/kT),
(1)
where C0 and CJ are the thermal correction factors for the ground and excited states, respectively, gJ is the degeneracy of the level and is expressed as gJ=2J+1, EJ is energy of the upper levels [27

27. W. T. Carnall, P. R. Fields, and K. Rajnak, “Spectral intensities of the trivalent lanthanides and actinides in solution. II. Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, and Ho3+,” J. Chem. Phys. 49(10), 4412–4423 (1968). [CrossRef]

], and E0 is energy of the ground state. Given that T = 298 K, the fractional populations CJ/CJ were derived to be 0.6615 (7F0), 0.3166 (7F1), 0.0213 (7F2), 5.24 × 10−4 (7F3), and 5.52 × 10−6 (7F4), respectively.

The excitation spectrum for the red fluorescence emission of Eu3+ doped NZPGT glasses is shown in Fig. 3(b), which covers 350−600 nm spectral range and indicates that the red fluorescence at 612.5 nm originating from the emission transition 5D07F1 can be efficiently achieved under the excitation of commercially available UV/violet/blue/green laser diodes (LDs) and LEDs, together with Ar+ laser.

As labeled in Fig. 3(c), three main emission peaks at 591.5, 612.5, and 701.0 nm originate from Eu3+ characteristic excited level 5D0 to the lower lying levels 7F1,2,4 transitions, respectively, under 466.5 nm excitation. Other five minor emission peaks at 510.0, 536.0, 553.0, 578.5, and 652.5 nm are ascribed to 5D27F3, 5D27F4, 5D17F2, 5D07F0, and 5D07F3 transitions, respectively. The commonly observed 5D07F0,1,2,3,4 transitions were clearly detected, meanwhile, the infrequent transitions from the upper lying states 5D2 and 5D1 were also observed due to the medium-low maximum phonon energy (782 cm−1) of Eu3+ doped NZPGT glasses, though the intensities of the emission peaks are weak. The contributions of electron-phonon anharmonicities relate to temperature and cause effects such as emission peak shift, the quantum efficiency on the polarization of the pumping beam, and the non-polar third rank polar tensor like optical second harmonic generation [28

28. I. V. Kityk, J. Wasylak, D. Dorosz, J. Kucharski, S. Benet, and H. Kaddouri, “PbO−Bi2O3−Ga2O3−BaO glasses doped by Er3+ as novel materials for IR emission,” Opt. Laser Technol. 33(7), 511–514 (2001). [CrossRef]

,29

29. A. Wojciechowski, I. V. Kityk, G. Lakshminarayana, I. Fuks-Janczarek, J. Berdowski, E. Berdowska, and Z. Tylczyński, “Laser-induced optical effects in triglycine-zinc chloride single crystals,” Physica B 405(13), 2827–2830 (2010). [CrossRef]

]. For the Eu3+ doped germanotellurite glasses, there exist contributions of electron-phonon anharmonicities due to the special glass network structure and the low maximum phonon energy of the optical glasses containing heavy metal oxide.

The hypersensitive transition 5D07F2 of Eu3+ (an electric-dipole transition governed by the selection rules |ΔJ|=2,|ΔL|2,ΔS=1) is extremely sensitive to ligand symmetry and bond covalence [30

30. G. Lakshminarayana, E. M. Weis, A. C. Lira, U. Caldiño, D. J. Williams, and M. P. Hehlen, “Cross relaxation in rare-earth-doped oxyfluoride glasses,” J. Lumin. 139, 132–142 (2013). [CrossRef]

], and tends to be much more intense in non-symmetric sites and high bond covalence. Meanwhile, the 5D07F1 transition is independent on ligand symmetry and bond covalence due to magnetic dipole-allowed rule, which makes the integrated fluorescence intensity ratio of 5D07F2/5D07F1, i.e. the asymmetry ratio R, a good criterion to estimate the site asymmetry and the chemical bond covalency of Eu3+ in NZPGT glasses [31

31. M. Dejneka, E. Snitzer, and R. E. Riman, “Blue, green and red fluorescence and energy transfer of Eu3+ in fluoride glasses,” J. Lumin. 65(5), 227–245 (1995). [CrossRef]

33

33. Y. Gandhi, I. V. Kityk, M. G. Brik, P. R. Rao, and N. Veeraiah, “Influence of tungsten on the emission features of Nd3+, Sm3+ and Eu3+ ions in ZnF2−WO3−TeO2 glasses,” J. Alloy. Comp. 508(2), 278–291 (2010). [CrossRef]

]. Typicallly, higher value of R indicates lower ligand symmetry and higher bond covalency. Here, the R (3.58) of 1.2wt% Eu2O3 doped NZPGT glasses is higher than those of Eu3+ doped ZnO−TlO0.5−TeO2 (3.40) [34

34. V. P. Tuyen, T. Hayakawa, M. Nogami, J. R. Duclère, and P. Thomas, “Fluorescence line narrowing spectroscopy of Eu3+ in zinc-thallium-tellurite glass,” J. Solid State Chem. 183(11), 2714–2719 (2010). [CrossRef]

], CaO−La2O3−B2O3 (3.10) [35

35. R. Chakrabarti, M. Das, B. Karmakar, K. Annapurna, and S. Buddhudu, “Emission analysis of Eu3+:CaO−La2O3−B2O3 glass,” J. Non-Cryst. Solids 353(13-15), 1422–1426 (2007). [CrossRef]

], and PbF2−WO3−TeO2 glasses (2.78) [36

36. A. M. Babu, B. C. Jamalaiah, T. Suhasini, T. S. Rao, and L. R. Moorthy, “Optical properties of Eu3+ ions in lead tungstate tellurite glasses,” Solid State Sci. 13(3), 574–578 (2011). [CrossRef]

], which indicates that Eu3+ ions in NZPGT glasses occupy relatively lower ligand symmetry sites and higher bond covalence, and are beneficial for red fluorescence emission in the optical glasses.

J-O intensity parameters Ωt (t = 2, 4, 6) are important indicators to predict some radiative properties such as oscillator strengths, luminescence branching ratios, energy-transfer probabilities, and excited-state radiative lifetime [37

37. M. P. Hehlen, M. G. Brik, and K. W. Krämer, “50th anniversary of the Judd-Ofelt theory: An experimentalist's view of the formalism and its application,” J. Lumin. 136, 221–239 (2013). [CrossRef]

], which are usually derived from absorption spectrum. Nevertheless, they were calculated from the emission spectrum due to the special energy level structure of Eu3+ in present work. The 5D07F2,4,6 transitions originating from Eu3+ are electronic-dipole allowed, and the spontaneous emission probability Aed from initial manifold to final manifold is given using the following expression
Aed=64π4e2v33h(J+1)n(n2+2)29t=2,4,6Ωt<ΨJ||U(t)||ΨJ>2,
(2)
where h is the Planck constant, e is the electron charge, n is the refractive index, and v is the transition wavenumber. The term <ΨJ||U(t)||ΨJ>2 is the square of the matrix elements of the tensorial operator, which connects the initial state to the final state and is deemed to be independent of host matrix. The square of the reduced matrix elements of the tensorial operator of 5D07F1,2,4 transitions are referred to Ref [38

38. W. T. Carnall, P. R. Fields, and K. Rajnak, “Electronic energy levels of the trivalent lanthanide aquo ions. IV. Eu3+,” J. Chem. Phys. 49(10), 4450–4455 (1968). [CrossRef]

]. The 5D07F1 transition of Eu3+ is magnetic-dipole allowed, and the spontaneous emission probability of magnetic-dipole transition Amd can be derived by using the following expression
Amd=64π4v33h(2J+1)n3Smd,
(3)
where h is the Planck constant, v is the transition wavenumber, J is the total angular momentum of the excited state, n is the refractive index, and Smd is the magnetic-dipole line strength that is independent of the host medium. The value of Amd can be estimated using the relationship
Amd=(nn)3Amd,
(4)
where nand Amd were referred to Ref [31

31. M. Dejneka, E. Snitzer, and R. E. Riman, “Blue, green and red fluorescence and energy transfer of Eu3+ in fluoride glasses,” J. Lumin. 65(5), 227–245 (1995). [CrossRef]

] in this work. Due to selection rules and the special energy level structure of Eu3+, the J-O intensity parameters Ωt can be solved from the ratios of the integrated fluorescence intensity of 5D07F2,4,6 transitions to the integrated fluorescence intensity of 5D07F1 transition as follows:

IJ(v)dvImd(v)dv=AJAmd=64π4e2v33h(2J+1)n(n2+2)29AmdΩt<ΨJ||U(t)||ΨJ>2.
(5)

Using Ωt values, spontaneous transition probabilities Aij, branching ratios βij, and calculated radiative lifetime τrad were calculated and listed in Table 2.

Table 2. Spontaneous transition probabilities Aij, branching ratios βij, calculated radiative lifetime τrad, and maximum stimulated emission cross-sections σem-max of 5D0 in Eu3+ doped NZPGT glasses

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The β of 5D07F2 transition is dominant and is as high as 70.9%, indicating that the intense red fluorescence at 16327 cm−1, i.e. 612.5 nm, can be efficiently achieved in Eu3+ doped NZPGT glasses as irradiation light sources to activate diverse PDT photosensitizers.

Stimulated emission cross-section σem is a significant parameter to evaluate the energy extraction efficiency from RE doped optical materials, which can be solved using the Füchtbauer-Ladenburg formula
σem=Aij8πcn2×λ5ijI(λij)λijI(λij)dλ,
(6)
where Aij, λij, and I(λij) are the radiation transition probability, the wavelength, and the emission intensity from i to j state, respectively. c is the light speed in vacuum and n is the refractive index. The stimulated emission cross-section profiles originating from 5D07F1,2,4 transitions of Eu3+ doped NZPGT glasses are depicted in Fig. 3(d), and the maximum stimulated emission cross-sections σem-max of those transitions are listed in Table 2. Here, the σem-max of dominant emission transition 5D07F2 peaking at 612.5 nm was derived to be 2.05 × 10−21 cm2, which is larger than those of Eu3+ doped LiF−Li2CO3−H3BO3 (0.16 × 10−21 cm2) [45

45. K. K. Mahato, S. B. Rai, and A. Rai, “Optical studies of Eu3+ doped oxyfluoroborate glass,” Spectrochim. Acta A Mol. Biomol. Spectrosc. 60(4), 979–985 (2004). [CrossRef] [PubMed]

], ZnF2−PbO−TeO2 (0.23 × 10−21 cm2) [46

46. V. R. Kumar and N. Veeraiah, “Optical absorption and photoluminescence properties of Eu3+-doped ZnF2−PbO−TeO2 glasses,” J. Mater. Sci. 33(10), 2659–2662 (1998). [CrossRef]

], and K2O−SrO−Al2O3−P2O5 glasses (1.28 × 10−21 cm2) [43

43. K. Linganna and C. K. Jayasankar, “Optical properties of Eu3+ ions in phosphate glasses,” Spectrochim. Acta [A] 97, 788–797 (2012). [CrossRef]

], predicting that the red fluorescence can be efficiently extracted in Eu3+ doped NZPGT glasses as irradiation light sources to deliver sufficient energy intensity to activate diverse PDT photosensitizers.

3.3 Quantitative characterization and analysis on absolute spectral properties of fluorescence

Under different pump sources, 0.2wt% and 1.2wt% Eu2O3 doped NZPGT glasses vary in fluorescence colors and intensities as depicted in Fig. 4.
Fig. 4 Luminescence pictures of (a) 0.2wt% and (b) 1.2wt% Eu2O3 doped NZPGT glasses under 365 nm UV lamp radiation, and 1.2wt% Eu2O3 doped NZPGT glasses under (c) 391 and (d) 456 nm LED excitations.

As shown in Figs. 4(a) and 4(b), 1.2wt% Eu2O3 doped NZPGT glasses exhibit stronger fluorescence than 0.2wt% Eu2O3 doped NZPGT glasses under 365 nm UV lamp radiation due to a higher Eu3+ concentration. However, 1.2wt% Eu2O3 doped NZPGT glasses exhibit different colors under 391 and 456 nm LED excitations as depicted in Figs. 4(c) and 4(d). In fact, such a phenomenon results from polychromatic light, i.e. violet light mixing red light gets orange light and blue light mixing red light gets magenta light.

In order to essentially reveal the absolute spectral properties of fluorescence in 1.2wt% Eu2O3 doped NZPGT glasses, spectral power distributions, Pon and Pside, were recorded using the integrating sphere method with commercially available 391 and 456 nm LED as pump sources when the glass sample was located on (Pon, curve 1 in Figs. 5(a) and 5(b)) and aside (Pside, curve 2 in Figs. 5(a) and 5(b)) the LED pump sources. During the integrating sphere measurement for quantum yield, there exists light scattering from the glass sample. Nevertheless, the integrating sphere can reflect the scattering light repeatedly until it is evenly distributed. Therefore, the scattering of light as well as other parasitic problems is negligible here.
Fig. 5 Spectral power distributions (curve 1: Pon, the sample on the tops of the LEDs; curve 2: Pside, the sample aside the LEDs) of 1.2wt% Eu2O3 doped NZPGT glasses under (a) 391 and (b) 456 nm LED excitations. Inset of (a) and (b): details of spectral power distributions in wavelength range of 570−720 nm. Net absorption and emission photon distributions of 1.2wt% Eu2O3 doped NZPGT glasses under (c) 391 and (d) 456 nm LED excitations. Inset of (c) and (d): details of emission photon distributions in wavelength range of 570−720 nm.

The total radiant flux, ΦE, of the luminescence can be calculated by
ΦE=P(λ)dλ.
(7)
As shown in Figs. 5(a) and 5(b), the radiant fluxes of 1.2wt% Eu2O3 doped NZPGT glasses under 391 and 456 nm LED excitations were solved to be 1402 and 9329 μW in visible light wavelength range of 380−780 nm, respectively, when the glass sample was located on the tops of the LEDs. In fluorescence emission wavelength range of 570−720 nm, the radiant fluxes were solved to be 55 and 108 μW, respectively.

Based on absolute spectral power distributions, photon distributions N(v) can be derived by
N(v)=λ3hcP(λ),
(8)
where λ is the wavelength, v is the wavenumber, h is the Planck constant, c is the vacuum light velocity, and P(λ) is spectral power distribution. The corresponding Non and Nside can be derived from Eq. (8) with spectral power distributions Pon and Pside, respectively. The net absorption and emission photon distribution curves of 1.2wt% Eu2O3 doped NZPGT glasses were derived by subtracting the Nside component from the Non component as depicted in Figs. 5(c) and 5(d).

Quantum yield (QY) is being used as a selection criterion of luminesce materials for potential applications in solid-state lighting and is defined as the ratio of the number of emitted photons to the number of absorbed photons. Namely,
QY=emittedphotons/absorbedphotons=(EonEside)/(LsideLon),
(9)
where Eon and Eside were recorded photon numbers emitted from the sample, and Lon and Lside were recorded photon numbers emitted from the LED pump sources when the sample was located on and aside the LED pump sources, respectively. Lside−Lon and EonEside are net absorption photons and emission photons, respectively, evaluated by integrating the net absorption and emission photon distributions as shown in Figs. 5(c) and 5(d).

Fig. 6 Absorption cross-section profiles of 7F05L6 and 7F05D2 transitions in 1.2wt% Eu2O3 doped NZPGT glasses. Inset: net absorption and emission photon number stack column distributions of 1.2wt% Eu2O3 doped NZPGT glasses under 391 and 456 nm LED excitations.
As shown in the inset of Fig. 6, the QYs of 1.2wt% Eu2O3 doped NZPGT glasses under 391 and 456 nm LED excitations were calculated to be 12% and 4%, respectively, indicating that proper pump sources are crucial to obtain high external quantum yield in the optical glasses.

The absorption cross-section σabs(v) is calculated from the absorption spectrum by using the following formula
σabs(v)=2.303E(v)/N0d,
(10)
where E(v) is the absorbance, N0 is the Eu3+ ion concentration (ions/cm3), and d is the sample thickness. The QY of 1.2wt% Eu2O3 doped NZPGT glasses under 391 nm LED excitation is approximately three times larger than that of under 456 nm LED excitation, mainly because the maximum absorption cross-section σabs-max of 7F05L6 transition (5.11 × 10−21 cm2) peaking at 394 nm is much larger than that of 7F05D2 transition (1.63 × 10−21 cm2) peaking at 465 nm as depicted in Fig. 6, which reveals that Eu3+ are more beneficial in absorbing photons over NZPGT glass host under 391 nm LED excitation than that of under 456 nm LED excitation.

The broadband fluorescence ranging from 570 to 720 nm of 1.2wt% Eu2O3 doped NZPGT glasses under 391 and 456 nm LED excitations were divided into five equal parts with a step length of 30 nm, and the photon numbers of each wavelength interval were obtained by integrating the net emission photon distributions.

As a result, photon number percentages of different wavelength regions based on total emission photon numbers in wavelength range of 570−720 nm were labeled and shown in Figs. 7(a) and 7(b). Approximately 88% photons of the total fluorescence emitted from the glass sample ranging from 600 to 720 nm are qualified to match the maximum absorption regions of the Q1 bands of diverse clinical photosensitizers, such as Photofrin® (~630 nm), Metvix® (~630 nm), Foscan® (~652 nm), Laserphyfrin® (~654 nm), Photochlor® (~665 nm), Photosens® (~675 nm), and Visudyne® (~689 nm) [7

7. S. Yano, S. Hirohara, M. Obata, Y. Hagiya, S.- Ogura, A. Ikeda, H. Kataoka, M. Tanaka, and T. Joh, “Current states and future views in photodynamic therapy,” J. Photochem. Photobiol. Chem. 12(1), 46–67 (2011). [CrossRef]

,47

47. C. A. Morton, “Methyl aminolevulinate (Metvix) photodynamic therapy - practical pearls,” J. Dermatolog. Treat. 14(Suppl 3), 23–26 (2003). [PubMed]

,48

48. M. Khurana, H. A. Collins, A. Karotki, H. L. Anderson, D. T. Cramb, and B. C. Wilson, “Quantitative in vitro demonstration of two-photon photodynamic therapy using photofrin and visudyne,” Photochem. Photobiol. 83(6), 1441–1448 (2007). [CrossRef] [PubMed]

]. More than 55% and 20% photons have been demonstrated in wavelength range of 600−630 nm and 690−720 nm, respectively, indicating that Eu3+ doped NZPGT glasses as irradiation light sources are especially efficient for Photofrin®, Metvix®, and Visudyne®. Therefore, high quantum yield, intense broadband fluorescence, and foreseeable amplified spontaneous emission (ASE) fluorescence generated in Eu3+ doped NZPGT glass fibers under proper excitation conditions can deliver sufficient energy intensity to activate diverse PDT photosensitizers, producing a highly reactive oxygen species (1O2) and then resulting in cancer cell death via apoptosis or necrosis in PDT modality.
Fig. 7 Photon number percentages of 1.2wt% Eu2O3 doped NZPGT glasses under (a) 391 and (b) 456 nm LED excitations.

4. Conclusion

Eu3+ doped fiber-based germanotellurite (NZPGT) glasses with medium-low maximum phonon energy of 782 cm−1 have been fabricated and characterized. The derived Judd-Ofelt parameters Ω2 (6.25 × 10−20 cm2) and Ω4 (1.77 × 10−20 cm2) indicate a high asymmetrical and covalent environment around Eu3+ in the optical glasses. The spontaneous emission probability of the dominant transition 5D07F2 peaking at 612.5 nm and the corresponding maximum stimulated emission cross-section were derived to be 445.7 s−1 and 2.05 × 10−21 cm2, respectively, confirming the effectiveness of the red fluorescence emission. The quantum yield was derived to be 12% under 391 nm LED excitation, and approximately 88% photons have been demonstrated in wavelength range of 600−720 nm, indicating that Eu3+ doped NZPGT glasses under proper excitation conditions hold great promise for fiber-based irradiation light sources that are qualified to activate diverse photodynamic photosensitizers.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (61275075) and the Science and Technology Foundation of Liaoning Province, China (201202011).

References and links

1.

I. Amato, “Cancer therapy. Hope for a magic bullet that moves at the speed of light,” Science 262(5130), 32–33 (1993). [CrossRef] [PubMed]

2.

D. E. J. G. J. Dolmans, D. Fukumura, and R. K. Jain, “Photodynamic therapy for cancer,” Nat. Rev. Cancer 3(5), 380–387 (2003). [CrossRef] [PubMed]

3.

A. P. Castano, P. Mroz, and M. R. Hamblin, “Photodynamic therapy and anti-tumour immunity,” Nat. Rev. Cancer 6(7), 535–545 (2006). [CrossRef] [PubMed]

4.

Q. Chen, S. D. Shetty, L. Heads, F. Bolin, B. C. Wilson, M. S. Patterson, L. T. Sirls Ii, D. Schultz, J. C. Cerny, and F. W. Hetzel, “Photodynamic therapy in prostate cancer: optical dosimetry and response of normal tissue,” Proc. SPIE 1881, 231–235 (1993). [CrossRef]

5.

R. Richards-Kortum and E. Sevick-Muraca, “Quantitative optical spectroscopy for tissue diagnosis,” Annu. Rev. Phys. Chem. 47(1), 555–606 (1996). [CrossRef] [PubMed]

6.

J. P. Celli, B. Q. Spring, I. Rizvi, C. L. Evans, K. S. Samkoe, S. Verma, B. W. Pogue, and T. Hasan, “Imaging and photodynamic therapy: mechanisms, monitoring, and optimization,” Chem. Rev. 110(5), 2795–2838 (2010). [CrossRef] [PubMed]

7.

S. Yano, S. Hirohara, M. Obata, Y. Hagiya, S.- Ogura, A. Ikeda, H. Kataoka, M. Tanaka, and T. Joh, “Current states and future views in photodynamic therapy,” J. Photochem. Photobiol. Chem. 12(1), 46–67 (2011). [CrossRef]

8.

L. Brancaleon and H. Moseley, “Laser and non-laser light sources for photodynamic therapy,” Lasers Med. Sci. 17(3), 173–186 (2002). [CrossRef] [PubMed]

9.

T. S. Mang, “Lasers and light sources for PDT: past, present and future,” Photodiagn. Photodyn. 1(1), 43–48 (2004). [CrossRef]

10.

R. A. Weiss, D. H. McDaniel, R. G. Geronemus, M. A. Weiss, K. L. Beasley, G. M. Munavalli, and S. G. Bellew, “Clinical experience with light-emitting diode (LED) photomodulation,” Dermatol. Surg. 31(9 Pt 2), 1199–1205 (2005). [PubMed]

11.

P. Babilas, E. Kohl, T. Maisch, H. Bäcker, B. Gross, A. L. Branzan, W. Bäumler, M. Landthaler, S. Karrer, and R. M. Szeimies, “In vitro and in vivo comparison of two different light sources for topical photodynamic therapy,” Br. J. Dermatol. 154(4), 712–718 (2006). [PubMed]

12.

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]

13.

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]

14.

I. V. Kityk, J. Wasylak, D. Dorosz, and J. Kucharski, “Eu3+-doped glass materials for red luminescence,” Opt. Laser Technol. 33(3), 157–160 (2001). [CrossRef]

15.

A. H. Krumpel, E. V. D. Kolk, P. Dorenbos, P. Boutinaud, E. Cavalli, and M. Bettinelli, “Energy level diagram for lanthanide-doped lanthanum orthovanadate,” Mater. Sci. Eng. B-Adv. 146, 114–120 (2008).

16.

E. Cavalli, A. Belletti, R. Mahiou, and P. Boutinaud, “Luminescence properties of Ba2NaNb5O15 crystals activated with Sm3+, Eu3+, Tb3+ or Dy3+ ions,” J. Lumin. 130(4), 733–736 (2010). [CrossRef]

17.

C. E. Secu, R. F. Negrea, and M. Secu, “Eu3+ probe ion for rare-earth dopant site structure in sol-gel derived LiYF4 oxyfluoride glass-ceramic,” Opt. Mater. 35(12), 2456–2460 (2013). [CrossRef]

18.

V. A. G. Rivera, S. P. A. Osorio, Y. Ledemi, D. Manzani, Y. Messaddeq, L. A. O. Nunes, and E. Marega Jr., “Localized surface plasmon resonance interaction with Er3+-doped tellurite glass,” Opt. Express 18(24), 25321–25328 (2010). [CrossRef] [PubMed]

19.

E. Friedman and J. L. Miller, Photonics Rules of Thumb: Optics, Electro-Optics, Fiber Optics, and Lasers (McGraw-Hill, 2004), Chap. 10.

20.

L. Petit, T. Cardinal, J. J. Videau, G. Le Flem, Y. Guyot, G. Boulon, M. Couzi, and T. Buffeteau, “Effect of the introduction of Na2B4O7 on erbium luminescence in tellurite glasses,” J. Non-Cryst. Solids 298(1), 76–88 (2002). [CrossRef]

21.

J. Ozdanova, H. Ticha, and L. Tichy, “Optical band gap and Raman spectra in some (Bi2O3)x(WO3)y(TeO2)100−x−y and (PbO)x(WO3)y(TeO2)100−x−y glasses,” J. Non-Cryst. Solids 355(45-47), 2318–2322 (2009). [CrossRef]

22.

N. Manikandan, A. Ryasnyanskiy, and J. Toulouse, “Thermal and optical properties of TeO2−ZnO−BaO glasses,” J. Non-Cryst. Solids 358(5), 947–951 (2012). [CrossRef]

23.

X. Gai, T. Han, A. Prasad, S. Madden, D.-Y. Choi, R. Wang, D. Bulla, and B. Luther-Davies, “Progress in optical waveguides fabricated from chalcogenide glasses,” Opt. Express 18(25), 26635–26646 (2010). [CrossRef] [PubMed]

24.

X. Hu, G. Guery, J. Boerstler, J. D. Musgraves, D. Vanderveer, P. Wachtel, and K. Richardson, “Influence of Bi2O3 content on the crystallization behavior of TeO2−Bi2O3−ZnO glass system,” J. Non-Cryst. Solids 358(5), 952–958 (2012). [CrossRef]

25.

G. Monteiro, L. F. Santos, J. C. G. Pereira, and R. M. Almeida, “Optical and spectroscopic properties of germanotellurite glasses,” J. Non-Cryst. Solids 357(14), 2695–2701 (2011). [CrossRef]

26.

K. Driesen, V. K. Tikhomirov, and C. Görller-Walrand, “Eu3+ as a probe for rare-earth dopant site structure in nano-glass-ceramics,” J. Appl. Phys. 102(2), 024312–024317 (2007). [CrossRef]

27.

W. T. Carnall, P. R. Fields, and K. Rajnak, “Spectral intensities of the trivalent lanthanides and actinides in solution. II. Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, and Ho3+,” J. Chem. Phys. 49(10), 4412–4423 (1968). [CrossRef]

28.

I. V. Kityk, J. Wasylak, D. Dorosz, J. Kucharski, S. Benet, and H. Kaddouri, “PbO−Bi2O3−Ga2O3−BaO glasses doped by Er3+ as novel materials for IR emission,” Opt. Laser Technol. 33(7), 511–514 (2001). [CrossRef]

29.

A. Wojciechowski, I. V. Kityk, G. Lakshminarayana, I. Fuks-Janczarek, J. Berdowski, E. Berdowska, and Z. Tylczyński, “Laser-induced optical effects in triglycine-zinc chloride single crystals,” Physica B 405(13), 2827–2830 (2010). [CrossRef]

30.

G. Lakshminarayana, E. M. Weis, A. C. Lira, U. Caldiño, D. J. Williams, and M. P. Hehlen, “Cross relaxation in rare-earth-doped oxyfluoride glasses,” J. Lumin. 139, 132–142 (2013). [CrossRef]

31.

M. Dejneka, E. Snitzer, and R. E. Riman, “Blue, green and red fluorescence and energy transfer of Eu3+ in fluoride glasses,” J. Lumin. 65(5), 227–245 (1995). [CrossRef]

32.

C. E. Secu, D. Predoi, M. Secu, M. Cernea, and G. Aldica, “Structural investigations of sol-gel derived silicate gels using Eu3+ ion-probe luminescence,” Opt. Mater. 31(11), 1745–1748 (2009). [CrossRef]

33.

Y. Gandhi, I. V. Kityk, M. G. Brik, P. R. Rao, and N. Veeraiah, “Influence of tungsten on the emission features of Nd3+, Sm3+ and Eu3+ ions in ZnF2−WO3−TeO2 glasses,” J. Alloy. Comp. 508(2), 278–291 (2010). [CrossRef]

34.

V. P. Tuyen, T. Hayakawa, M. Nogami, J. R. Duclère, and P. Thomas, “Fluorescence line narrowing spectroscopy of Eu3+ in zinc-thallium-tellurite glass,” J. Solid State Chem. 183(11), 2714–2719 (2010). [CrossRef]

35.

R. Chakrabarti, M. Das, B. Karmakar, K. Annapurna, and S. Buddhudu, “Emission analysis of Eu3+:CaO−La2O3−B2O3 glass,” J. Non-Cryst. Solids 353(13-15), 1422–1426 (2007). [CrossRef]

36.

A. M. Babu, B. C. Jamalaiah, T. Suhasini, T. S. Rao, and L. R. Moorthy, “Optical properties of Eu3+ ions in lead tungstate tellurite glasses,” Solid State Sci. 13(3), 574–578 (2011). [CrossRef]

37.

M. P. Hehlen, M. G. Brik, and K. W. Krämer, “50th anniversary of the Judd-Ofelt theory: An experimentalist's view of the formalism and its application,” J. Lumin. 136, 221–239 (2013). [CrossRef]

38.

W. T. Carnall, P. R. Fields, and K. Rajnak, “Electronic energy levels of the trivalent lanthanide aquo ions. IV. Eu3+,” J. Chem. Phys. 49(10), 4450–4455 (1968). [CrossRef]

39.

M. A. K. Elfayoumi, M. Farouk, M. G. Brik, and M. M. Elokr, “Spectroscopic studies of Sm3+ and Eu3+ co-doped lithium borate glass,” J. Alloy. Comp. 492(1-2), 712–716 (2010). [CrossRef]

40.

Y. Dwivedi and S. B. Rai, “Optical properties of Eu3+ in oxyfluoroborate glass and its nanocrystalline glass,” Opt. Mater. 31(1), 87–93 (2008). [CrossRef]

41.

D. Uma Maheswari, J. Suresh Kumar, L. R. Moorthy, K. Jang, and M. Jayasimhadri, “Emission properties of Eu3+ ions in alkali tellurofluorophosphate glasses,” Physica B 403(10-11), 1690–1694 (2008). [CrossRef]

42.

T. G. V. M. Rao, A. Rupesh Kumar, K. Neeraja, N. Veeraiah, and M. Rami Reddy, “Optical and structural investigation of Eu3+ ions in Nd3+ co-doped magnesium lead borosilicate glasses,” J. Alloy. Comp. 557, 209–217 (2013). [CrossRef]

43.

K. Linganna and C. K. Jayasankar, “Optical properties of Eu3+ ions in phosphate glasses,” Spectrochim. Acta [A] 97, 788–797 (2012). [CrossRef]

44.

A. Ivankov, J. Seekamp, and W. Bauhofer, “Optical properties of Eu3+-doped zinc borate glasses,” J. Lumin. 121(1), 123–131 (2006). [CrossRef]

45.

K. K. Mahato, S. B. Rai, and A. Rai, “Optical studies of Eu3+ doped oxyfluoroborate glass,” Spectrochim. Acta A Mol. Biomol. Spectrosc. 60(4), 979–985 (2004). [CrossRef] [PubMed]

46.

V. R. Kumar and N. Veeraiah, “Optical absorption and photoluminescence properties of Eu3+-doped ZnF2−PbO−TeO2 glasses,” J. Mater. Sci. 33(10), 2659–2662 (1998). [CrossRef]

47.

C. A. Morton, “Methyl aminolevulinate (Metvix) photodynamic therapy - practical pearls,” J. Dermatolog. Treat. 14(Suppl 3), 23–26 (2003). [PubMed]

48.

M. Khurana, H. A. Collins, A. Karotki, H. L. Anderson, D. T. Cramb, and B. C. Wilson, “Quantitative in vitro demonstration of two-photon photodynamic therapy using photofrin and visudyne,” Photochem. Photobiol. 83(6), 1441–1448 (2007). [CrossRef] [PubMed]

OCIS Codes
(160.2290) Materials : Fiber materials
(160.2540) Materials : Fluorescent and luminescent materials
(160.2750) Materials : Glass and other amorphous materials
(160.5690) Materials : Rare-earth-doped materials

ToC Category:
Rare-Earth-Doped Materials

History
Original Manuscript: August 30, 2013
Revised Manuscript: October 11, 2013
Manuscript Accepted: October 15, 2013
Published: October 23, 2013

Citation
F. Wang, L. F. Shen, B. J. Chen, E. Y. B. Pun, and H. Lin, "Broadband fluorescence emission of Eu3+ doped germanotellurite glasses for fiber-based irradiation light sources," Opt. Mater. Express 3, 1931-1943 (2013)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-3-11-1931


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References

  1. I. Amato, “Cancer therapy. Hope for a magic bullet that moves at the speed of light,” Science262(5130), 32–33 (1993). [CrossRef] [PubMed]
  2. D. E. J. G. J. Dolmans, D. Fukumura, and R. K. Jain, “Photodynamic therapy for cancer,” Nat. Rev. Cancer3(5), 380–387 (2003). [CrossRef] [PubMed]
  3. A. P. Castano, P. Mroz, and M. R. Hamblin, “Photodynamic therapy and anti-tumour immunity,” Nat. Rev. Cancer6(7), 535–545 (2006). [CrossRef] [PubMed]
  4. Q. Chen, S. D. Shetty, L. Heads, F. Bolin, B. C. Wilson, M. S. Patterson, L. T. Sirls Ii, D. Schultz, J. C. Cerny, and F. W. Hetzel, “Photodynamic therapy in prostate cancer: optical dosimetry and response of normal tissue,” Proc. SPIE1881, 231–235 (1993). [CrossRef]
  5. R. Richards-Kortum and E. Sevick-Muraca, “Quantitative optical spectroscopy for tissue diagnosis,” Annu. Rev. Phys. Chem.47(1), 555–606 (1996). [CrossRef] [PubMed]
  6. J. P. Celli, B. Q. Spring, I. Rizvi, C. L. Evans, K. S. Samkoe, S. Verma, B. W. Pogue, and T. Hasan, “Imaging and photodynamic therapy: mechanisms, monitoring, and optimization,” Chem. Rev.110(5), 2795–2838 (2010). [CrossRef] [PubMed]
  7. S. Yano, S. Hirohara, M. Obata, Y. Hagiya, S.- Ogura, A. Ikeda, H. Kataoka, M. Tanaka, and T. Joh, “Current states and future views in photodynamic therapy,” J. Photochem. Photobiol. Chem.12(1), 46–67 (2011). [CrossRef]
  8. L. Brancaleon and H. Moseley, “Laser and non-laser light sources for photodynamic therapy,” Lasers Med. Sci.17(3), 173–186 (2002). [CrossRef] [PubMed]
  9. T. S. Mang, “Lasers and light sources for PDT: past, present and future,” Photodiagn. Photodyn.1(1), 43–48 (2004). [CrossRef]
  10. R. A. Weiss, D. H. McDaniel, R. G. Geronemus, M. A. Weiss, K. L. Beasley, G. M. Munavalli, and S. G. Bellew, “Clinical experience with light-emitting diode (LED) photomodulation,” Dermatol. Surg.31(9 Pt 2), 1199–1205 (2005). [PubMed]
  11. P. Babilas, E. Kohl, T. Maisch, H. Bäcker, B. Gross, A. L. Branzan, W. Bäumler, M. Landthaler, S. Karrer, and R. M. Szeimies, “In vitro and in vivo comparison of two different light sources for topical photodynamic therapy,” Br. J. Dermatol.154(4), 712–718 (2006). [PubMed]
  12. 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]
  13. 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]
  14. I. V. Kityk, J. Wasylak, D. Dorosz, and J. Kucharski, “Eu3+-doped glass materials for red luminescence,” Opt. Laser Technol.33(3), 157–160 (2001). [CrossRef]
  15. A. H. Krumpel, E. V. D. Kolk, P. Dorenbos, P. Boutinaud, E. Cavalli, and M. Bettinelli, “Energy level diagram for lanthanide-doped lanthanum orthovanadate,” Mater. Sci. Eng. B-Adv.146, 114–120 (2008).
  16. E. Cavalli, A. Belletti, R. Mahiou, and P. Boutinaud, “Luminescence properties of Ba2NaNb5O15 crystals activated with Sm3+, Eu3+, Tb3+ or Dy3+ ions,” J. Lumin.130(4), 733–736 (2010). [CrossRef]
  17. C. E. Secu, R. F. Negrea, and M. Secu, “Eu3+ probe ion for rare-earth dopant site structure in sol-gel derived LiYF4 oxyfluoride glass-ceramic,” Opt. Mater.35(12), 2456–2460 (2013). [CrossRef]
  18. V. A. G. Rivera, S. P. A. Osorio, Y. Ledemi, D. Manzani, Y. Messaddeq, L. A. O. Nunes, and E. Marega., “Localized surface plasmon resonance interaction with Er3+-doped tellurite glass,” Opt. Express18(24), 25321–25328 (2010). [CrossRef] [PubMed]
  19. E. Friedman and J. L. Miller, Photonics Rules of Thumb: Optics, Electro-Optics, Fiber Optics, and Lasers (McGraw-Hill, 2004), Chap. 10.
  20. L. Petit, T. Cardinal, J. J. Videau, G. Le Flem, Y. Guyot, G. Boulon, M. Couzi, and T. Buffeteau, “Effect of the introduction of Na2B4O7 on erbium luminescence in tellurite glasses,” J. Non-Cryst. Solids298(1), 76–88 (2002). [CrossRef]
  21. J. Ozdanova, H. Ticha, and L. Tichy, “Optical band gap and Raman spectra in some (Bi2O3)x(WO3)y(TeO2)100−x−y and (PbO)x(WO3)y(TeO2)100−x−y glasses,” J. Non-Cryst. Solids355(45-47), 2318–2322 (2009). [CrossRef]
  22. N. Manikandan, A. Ryasnyanskiy, and J. Toulouse, “Thermal and optical properties of TeO2−ZnO−BaO glasses,” J. Non-Cryst. Solids358(5), 947–951 (2012). [CrossRef]
  23. X. Gai, T. Han, A. Prasad, S. Madden, D.-Y. Choi, R. Wang, D. Bulla, and B. Luther-Davies, “Progress in optical waveguides fabricated from chalcogenide glasses,” Opt. Express18(25), 26635–26646 (2010). [CrossRef] [PubMed]
  24. X. Hu, G. Guery, J. Boerstler, J. D. Musgraves, D. Vanderveer, P. Wachtel, and K. Richardson, “Influence of Bi2O3 content on the crystallization behavior of TeO2−Bi2O3−ZnO glass system,” J. Non-Cryst. Solids358(5), 952–958 (2012). [CrossRef]
  25. G. Monteiro, L. F. Santos, J. C. G. Pereira, and R. M. Almeida, “Optical and spectroscopic properties of germanotellurite glasses,” J. Non-Cryst. Solids357(14), 2695–2701 (2011). [CrossRef]
  26. K. Driesen, V. K. Tikhomirov, and C. Görller-Walrand, “Eu3+ as a probe for rare-earth dopant site structure in nano-glass-ceramics,” J. Appl. Phys.102(2), 024312–024317 (2007). [CrossRef]
  27. W. T. Carnall, P. R. Fields, and K. Rajnak, “Spectral intensities of the trivalent lanthanides and actinides in solution. II. Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, and Ho3+,” J. Chem. Phys.49(10), 4412–4423 (1968). [CrossRef]
  28. I. V. Kityk, J. Wasylak, D. Dorosz, J. Kucharski, S. Benet, and H. Kaddouri, “PbO−Bi2O3−Ga2O3−BaO glasses doped by Er3+ as novel materials for IR emission,” Opt. Laser Technol.33(7), 511–514 (2001). [CrossRef]
  29. A. Wojciechowski, I. V. Kityk, G. Lakshminarayana, I. Fuks-Janczarek, J. Berdowski, E. Berdowska, and Z. Tylczyński, “Laser-induced optical effects in triglycine-zinc chloride single crystals,” Physica B405(13), 2827–2830 (2010). [CrossRef]
  30. G. Lakshminarayana, E. M. Weis, A. C. Lira, U. Caldiño, D. J. Williams, and M. P. Hehlen, “Cross relaxation in rare-earth-doped oxyfluoride glasses,” J. Lumin.139, 132–142 (2013). [CrossRef]
  31. M. Dejneka, E. Snitzer, and R. E. Riman, “Blue, green and red fluorescence and energy transfer of Eu3+ in fluoride glasses,” J. Lumin.65(5), 227–245 (1995). [CrossRef]
  32. C. E. Secu, D. Predoi, M. Secu, M. Cernea, and G. Aldica, “Structural investigations of sol-gel derived silicate gels using Eu3+ ion-probe luminescence,” Opt. Mater.31(11), 1745–1748 (2009). [CrossRef]
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  34. V. P. Tuyen, T. Hayakawa, M. Nogami, J. R. Duclère, and P. Thomas, “Fluorescence line narrowing spectroscopy of Eu3+ in zinc-thallium-tellurite glass,” J. Solid State Chem.183(11), 2714–2719 (2010). [CrossRef]
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