Tb3+ doped zinc–sodium–aluminosilicate glasses

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Optical spectroscopy and optical waveguide fabrication in Eu3 þ and Eu3 þ /Tb3 þ doped zinc-sodium-aluminosilicate glasses U. Caldiño, A. Speghini, S Berneschi, M. Bettinelli, M. Brenci, E. Pasquini, S. Pelli, G.C. Righini

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Cite this article as: U. Caldiño, A. Speghini, S Berneschi, M. Bettinelli, M. Brenci, E. Pasquini, S. Pelli, G.C. Righini, Optical spectroscopy and optical waveguide fabrication in Eu3 þ and Eu3 þ /Tb3 þ doped zinc-sodium-aluminosilicate glasses, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2013.11.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Optical and spectroscopic properties of 2.0% Eu(PO3)3 singly doped and 5.0% Tb(PO3)3-2.0%

Eu(PO3)3

codoped

zinc-sodium-aluminosilicate

glasses

were

investigated. Reddish-orange light emission, with x = 0.64 and y = 0.36 CIE1931 chromaticity coordinates, is obtained in the europium singly doped glass excited at 393 nm. Such chromaticity coordinates are close to those (0.67,0.33) standard of the National Television System Committee for the red phosphor. When the sodium-zincaluminosilicate glass is co-doped with Tb3+ and Eu3+ reddish-orange light emission, with (0.61,0.37) CIE1931 chromaticity coordinates, is obtained upon Tb3+ excitation at 344 nm. This reddish-orange luminescence is generated mainly by 5D0 → 7F1 and 5D0 →7F2 emissions of Eu3+, europium being sensitized by terbium through a non-radiative energy transfer. From an analysis of the Tb3+ emission decay curves it is inferred that the Tb3+ → Eu3+ energy transfer might take place between Tb3+ and Eu3+ clusters through a short-range interaction mechanism, so that an electric dipole-quadrupole interaction appears to be the most probable transfer mechanism. The efficiency of this energy transfer is about 62% upon excitation at 344 nm. In the singly doped and codoped glasses multimode optical waveguides were successfully produced by Ag+-Na+ ion exchange, and they could be characterized at various wavelengths.

Optical spectroscopy and optical waveguide fabrication in Eu3+ and Eu3+/Tb3+ doped zinc-sodium-aluminosilicate glasses U. Caldiñoa1, A. Speghinib,c, S. Berneschic, M. Bettinellib, M. Brencic, E. Pasquinic,d, S. Pellic, G.C. Righinic,e a

Departamento de Física, Universidad Autónoma Metropolitana-Iztapalapa, P.O. Box 55-534, 09340 México, D.F., México b Dipartimento di Biotecnologie, Università di Verona, and INSTM, UdR Verona, Strada Le Grazie 15, I37314 Verona, Italy c Istituto di Fisica Applicata Nello Carrara, C.N.R., Via Madonna del Piano 10, 50019 Sesto Fiorentino (Firenze), Italy d Dipartimento di Fisica e Astronomia, Università di Firenze, Via Sansone 1, 50019 Sesto Fiorentino (Firenze), Italy e Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Piazza del Viminale 2, 00184 Roma, Italy

Abstract Optical and spectroscopic properties of 2.0% Eu(PO3)3 singly doped and 5.0% Tb(PO3)3-2.0%

Eu(PO3)3

codoped

zinc-sodium-aluminosilicate

glasses

were

investigated. Reddish-orange light emission, with x = 0.64 and y = 0.36 CIE1931 chromaticity coordinates, is obtained in the europium singly doped glass excited at 393 nm. Such chromaticity coordinates are close to those (0.67,0.33) standard of the National Television System Committee for the red phosphor. When the sodium-zincaluminosilicate glass is co-doped with Tb3+ and Eu3+, reddish-orange light emission, with (0.61,0.37) CIE1931 chromaticity coordinates, is obtained upon Tb3+ excitation at 344 nm. This reddish-orange luminescence is generated mainly by 5D0 → 7F1 and 5D0 →7F2 emissions of Eu3+, europium being sensitized by terbium through a non-radiative energy transfer. From an analysis of the Tb3+ emission decay curves it is inferred that the Tb3+ → Eu3+ energy transfer might take place between Tb3+ and Eu3+ clusters through a short-range interaction mechanism, so that an electric dipole-quadrupole interaction appears to be the most probable transfer mechanism. The efficiency of this energy transfer is about 62% upon excitation at 344 nm. In the singly doped and codoped glasses multimode optical waveguides were successfully produced by Ag+-Na+ ion exchange, and they could be characterized at various wavelengths.

1

Author to whom all correspondence should be addressed. E-mail: [email protected]

1. Introduction Glasses are very versatile materials as they can be doped with rare-earth ions, and so they have greatly contributed to the development of optical amplifiers, lasers, optical waveguides and white-light-emitting devices. The latter ones have gained considerable attention for applications in both liquid crystal monitor screens and white light emitting diodes (W-LEDs). W-LEDs are of long lifetime, reliable, safe, energy saving and environmental-friendly, which make them quite versatile and attractive for the replacement of conventional incandescent and fluorescent lamps. One of the most usual methods to develop W-LEDs is to couple an UV emitting LED, such as AlGaN-based LEDs [1], with a glass phosphor [2]. Glass phosphors represent a good alternative for W-LEDs as they can be produced at a relatively low cost. Among the many different combinations of glasses and rare earth and transition metal ions that have been investigated so far, oxide glasses doped with two (Ce3+/Tb3+) [2] or three (Ce3+/Dy3+/Mn2+) [3] luminescent ions have already proved to generate white light emission. On the other hand, there is a clear interest to develop integrated optical devices [4], such as optical amplifiers for telecommunications applications and compact structures for displays and solar energy managing. One of the most effective fabrication techniques of optical waveguides is ion-exchange [5], so that ion-exchanged waveguides have been successfully produced in rare earth doped glasses, especially in silicate [6,7] and phosphate [8] glasses. Zinc phosphate glasses activated by Tb3+/Eu3+ ions have shown very good spectroscopic properties [9]. Phosphate glasses, however, have lower chemical resistance than silicates, so that silica-based glass phosphors are more suitable for ion-exchange processes as they are chemically much more robust than phosphate glasses. Considering the importance of finding efficient conversion phosphors for LED applications and for the design of integrated optical devices utilizing the ion-exchange process, here we report on the spectroscopic and optical properties of zinc-sodium-aluminosilicate glasses doped either with Eu3+ alone and codoped with Tb3+ and Eu3+, as well on fabrication tests of optical waveguides by Ag+-Na+ ionexchange. 2. Experimental The molar compositions of the glasses investigated are: 60SiO2-18Na2O-12K2O-5ZnO5Al2O3-2Eu2O3 (ZN2Eu), 60SiO2-18Na2O-12K2O-5ZnO-5Al2O3-5Tb2O3 (ZN5Tb) and 60SiO2-18Na2O-12K2O-5ZnO-5Al2O3-5Tb2O3-2Eu2O3 (ZN5Tb2Eu). A suitable amount

of reagent grade powders with analytical quality of SiO2, Na2CO3, K2CO3, ZnO, Al2O3, Eu(NO3)3 and Tb(NO3)3 were mixed and placed into a platinum crucible. The melt was held at 1450 oC for 2 hours, and then quickly poured onto a copper plate kept at room temperature. The glasses were annealed for 4 hours at 500 oC to attain thermal and structural stability. Luminescence spectra and decay time curves were recorded by means of a Horiba Jobin-Yvon Fluorolog 3-22 spectrofluorometer equipped with a phosphorimeter. Refractive index was measured at 4 different wavelengths (635, 980, 1300 and 1550 nm) by a semi-automatic system (COMPASSO) developed in-house, based on dark-line spectroscopy. A Schott SF14 glass (n = 1.75581 @ 635 nm) was used as coupling prism. The accuracy in the measurements of the bulk refractive index is better than 1×10-3. The same system was used to measure the effective indices of the guided modes after ion-exchange process, with accuracy of 5×10-4. Among the many techniques for the fabrication of optical waveguides, we have chosen the ion-exchange process [6]; indeed, it is a very simple and effective technique that can be easily used in glasses containing a sufficient concentration of alkali ions. In this investigation we employed the Ag+-Na+ ion exchange, which produces a higher refractive index difference than the K+-Na+ exchange. We fabricated multimode planar waveguides to obtain accurate values of ion-exchange parameters, such as diffusion constant ( DAg + ) and diffusion depth (d), which are necessary for an accurate design of integrated optical devices. The glass substrates were immersed in a stainless steel recipient containing a dilute AgNO3 salt melt (AgNO3:NaNO3 = 0.5:99.5 mol%) after standard cleaning procedure and 10 min preheating. The temperature was monitored by a thermocouple with a precision of ±1 °C. The ion-exchange temperature and diffusion time were fixed at 325 °C and 150 s. The modal characteristics of the resulting waveguides were measured by the COMPASSO instrument. The refractive index profile of the ion-exchanged layer was determined by means of the inverse WKB method [10] using the modal effective indices. 3. Results and discussion 3.1. Optical properties and ion-exchange The refractive indices of the two bulk glasses are n = 1.5362 and n = 1.5736 (± 1×10-4) for the singly and doubly doped samples, respectively. These values refer to 635 nm wavelength, but indices were measured at five wavelenghts (635, 830, 980, 1300 and

1550 nm), so that it was possible to calculate the chromatic dispersion by using the approximated formula, n 2 (λ ) = 1 + λ2 ( A1λ2 + A2 ) , where λ is expressed in nm. For the singly doped glass the values of coefficients are A1 = 0.760 and A2 = −9.73 × 10 3 ; for the doubly-doped glass the values are A1 = 0.698 and A2 = −8.46 × 103 . The ion-exchange parameters in the two glasses are very similar, too. Referring to the ZN2Eu sample, after the diluted silver exchange at 325°C for 150 seconds, we obtained:

N = 9 , Δn = 0.051 ± 0.001 , d = 7.0 ± 0.2 μm, DAg + = 0.082 ± 0.003 μm2/s,

where N is the number of guided modes at 635 nm, Δn is the difference between surface refractive index and substrate glass refractive index, d = 2 D t is the diffusion depth of the silver ions (i.e. the equivalent waveguide thickness), and DAg+ is the diffusion constant of silver ions. The reconstructed Gaussian index profile is portrayed in Fig. 1 for the Eu3+-singly doped glass. By using these data it would be easy to properly design the guiding structure (single- or multi-mode) most suitable for each specific application. 3.2. Eu3+-doped zinc-sodium-aluminosilicate glass The emission spectrum of the Eu3+-singly doped zinc-sodium-aluminosilicate glass (ZN2Eu) excited into the most intense absorption band of europium (7F0 → 5L7 at 393 nm) is shown in Fig. 2, and it consists of emission bands of europium 4f 6 → 4f 6 transitions from 5D1 and 5D0 levels to 7FJ manifolds: 5D1 → 7F0 (526 nm), 7F1 (535 nm), 7

F2 (551 nm) and 5D0 → 7F0 (578 nm), 7F1 (591 nm), 7F2 (610 nm), 7F3 (653 nm), 7F4

(704 nm), 7F5 (744 nm) and 7F6 (808 nm). The most intense emission (5D0 → 7F2 transition) leads to the bright reddish-orange emission usually observed in Eu3+activated phosphors. The initial population of the 5L7 level relaxes non-radiatively to the 5

D3 level, and then sequentially to the lower 5D2, 5D1 and 5D0 levels (Fig. 3). Emissions

from the 5D1 level are negligible relative to those from the 5D0 level, since the 5D1 → 5

D0 multiphonon relaxation is quite active, the energy gap between them being only

1747 cm-1. This energy gap was estimated from the excitation spectrum monitored at (5D0→ 7F2) 610 nm emission wavelength (Fig. 4). Emissions from the 5D3 and 5D2 levels are not observed due to the effect of cross-relaxation from 5D2 to 5D1 levels and from 5

D3 to 5D2 levels, so that such cross-relaxation channels are: 5D2 + 7F1 → 5D1 + 7F4 and

5

D3 + 7F0 → 5D2 + 7F4 [11], see Fig. 3. Considering that the 5D0 → 7F6 emission band is

centered at 808 nm, then the 5D0 level is separated from the next lower lying level (7F6) by around 12376 cm-1, and therefore the multiphonon relaxation between these levels is negligible. The Eu3+ energy level scheme shown in Fig. 3 was obtained from the emission and excitation spectra displayed in Figs. 2 and 4, respectively. The global emission generated from the ZN2Eu glass (excited at 393 nm) was characterized by its chromaticity coordinates in a CIE1931 diagram. The coordinates resulted to be x = 0.64 and y = 0.36, which are in the reddish-orange region and they are close to those (0.67,0.33) standard of the National Television System Committee for the red phosphor. The excitation spectrum of ZN2Eu glass (monitored at 610 nm, within the europium 5

D0 → 7F2 emission) consists of several bands originated from intra-4f forbidden

transitions of Eu3+ (Fig. 4): 7F0 → 5F5,5I6,7,8 (287 nm), 7F0 → 5F1,2,3,5I4 (295 nm), 7F0 → 5

I5 (298 nm),7F1 → 5I5 (304 nm), 7F0 → 5H6 (320 nm), 7F0 → 5H3,7 (327 nm), 7F0 → 5D4

(362 nm), 7F0 → 5L10 (364 nm), 7F0 → 5G2,3,4,5 (376 nm), 7F0 → 5L8 (381 nm), 7F0 → 5L7 (392 nm), 7F1 → 5L7 (402 nm), 7F0 → 5D3,5L6 (414 nm), 7F0 → 5D2 (463 nm), 7F1 → 5D2 (472 nm), 7F0 → 5D1 (525 nm), 7F1 → 5D1 (532 nm), 7F0 → 5D0 (578 nm) and 7F1 → 5D0 (587 nm). The decay time curve of the 5D0 level was recorded monitoring the 5D0 → 7F2 emission (610 nm) after 392 nm excitation. The decay curve follows a single exponential function with a lifetime value of 3.4±0.1 ms (Fig. 5). Thus, the long lifetime is in agreement with the nature forbidden of the intra-4f transitions of Eu3+. 3.3. Tb3+-and Eu3+-doped zinc-sodium-aluminosilicate glass Fig. 6 shows the emission spectrum of the Tb3+/Eu3+-codoped zinc-sodiumaluminosilicate glass (ZN5Tb2Eu) excited into the Tb3+ (7F6 → 5G3) excitation band [6] at 344 nm. Excitation at 344 nm fits the requirements of AlGaN based LEDs [12]. From the europium energy level scheme and excitation spectrum (Figs. 3 and 4) it can be seen that the europium alone cannot be excited at 344 nm wavelength (29070 cm-1). However, emissions from the europium 5D0 level to 7FJ manifolds (J = 0, 1, 2, 3, 4) are observed in addition to 5D4 → 7F6 and 5D4 → 7F5 emissions of terbium. It can also be observed that Tb3+ emissions from the 5D3 level are quenched by non-radiative relaxation from the 5D3 to the 5D4 level. This non-radiative relaxation is promoted by the excitation from the 7F6 to the 7F0 level via a cross-relaxation process [9]. The energy difference between 5D3 and 5D4 levels is the same as that between 7F0 to 7F6 levels, and

as a consequence the 5D3 to 7FJ transitions of materials with high Tb3+ concentration are quenched by energy transfer between identical centres, 5D3+7F6 → 5D4+7F0. Then, only 5

D4 to 7FJ emissions are observed. The global emission generated from the ZN5Tb2Eu glass (excited at 344 nm) was

characterized by its chromaticity coordinates in a CIE1931 diagram. The coordinates resulted to be x = 0.61 and y = 0.37, which are in the reddish-orange region, slightly yellow-shifted with respect to the ZN2Eu glass. The excitation spectrum monitored at 704 nm, within the europium 5D0 → 7F4 emission band, shows terbium 4f 8 → 4f 8 transitions (Fig. 7). Thus, the europium excitation through terbium and the presence of terbium excitation bands in the excitation spectrum of europium reveal a Tb3+ → Eu3+ energy transfer [13,14]. Moreover, the terbium 5D4 → 7F5 and 5D4 → 7F4 emissions overlap the europium 7F1 → 5

D1 and (7F0,7F1) → 5D0 absorptions, respectively, as it can be appreciated from the

spectra shown in Fig. 8. Considering that the terbium 5D3 level emissions are quenched by cross relaxation, the Tb3+ → Eu3+ energy transfer could then be: 5D4+7F1 → 7F5+5D1 and 5D4+(7F0, 7F1) → 7F4+5D0 (Fig. 3). The Tb3+-energy-level scheme was taken from the terbium excitation spectrum previously reported [6]. Lifetime measurements of the terbium emission from the 5D4 level in the ZN5Tb and ZN5Tb2Eu glasses excited at 376 and 344 nm, respectively, were performed monitoring the terbium 5D4 → 7F5 emission at 542 nm. In the ZN5Tb glass the 5D4 level emission decay follows a single-exponential function with a lifetime value of 3.2±0.1 ms. In the codoped glass the 5D4 level decays non-exponentially, and an average decay time, τ

= ∫ tI (t )dt

∫ I (t )dt , of 1.2±0.1 ms is obtained. This faster decay reveals a

non-radiative energy transfer from Tb3+ to Eu3+. The 5D4 → 7F5 emission decay time curves recorded for the ZN5Tb and ZN5Tb2Eu glasses are portrayed in Fig. 9. In the ZN5Tb2Eu glass the decay time curve of the terbium 5D4 → 7F5 emission was analyzed to determine some energy transfer spectroscopic parameters, such as the dominant mechanism of Tb3+ → Eu3+ energy transfer and the RC energy transfer critical interaction distance between terbium and europium. The temporal evolution of the Tb3+ emission, under short-time excitation, was analyzed following the Inokuti-Hirayama model through the following exponential function [15]:

3/ S ⎞ ⎛ t 4π ⎛ 3 ⎞ 3⎛ t ⎞ ⎜ I (t ) = I 0 exp − o − Γ⎜1 − ⎟ ρ Eu RC ⎜⎜ o ⎟⎟ ⎟ , ⎜ τ Tb 3 ⎝ S ⎠ ⎝ τ Tb ⎠ ⎟⎠ ⎝

(1)

o is the decay time of Tb3+ in absence of where I 0 is the initial intensity at t = 0, τ Tb

Eu3+ ions, ρEu is the Eu3+ density (1.6 × 1020 ions/cm3), S = 6, 8 or 10 corresponds to the transfer mechanism of electric dipole-dipole, dipole-quadrupole or quadrupolequadrupole character, respectively, and the gamma function Γ(1 − 3 / S ) is

π for S =

6, 1.43 for S = 8, and 1.30 for S = 10. Thus, the temporal decay of the terbium 5D4 → 7

F5 emission in the ZN5Tb2Eu glass excited at 344 nm was fitted to Eq. (1) taking RC

and S as adjustable parameters. The best fit with τ do = 3.2 ms (decay time measured for the terbium 5D4 level emission in the ZN5Tb glass) is attained with S = 8 and RC = 5.9 Å (see Fig. 9). Therefore, an electric dipole-quadrupole interaction could be the dominant mechanism in the energy transfer from Tb3+ to Eu3+. On the other hand, the Tb3+-Eu3+ average interaction distance assuming a random ion distribution can be roughly estimated from the densities of Tb3+ ( ρ Tb ≈ 4.0 × 1020 ions/cm3) and Eu3+ ( ρ Eu ≈ 1.6 × 1020 ions/cm3) through the following relation [16]: 1/ 3

Drandom

⎛ ⎞ 3 ⎟⎟ . = 2⎜⎜ ⎝ 4π ( ρ Tb + ρ Eu ) ⎠

(2)

Thus, the value estimated for Drandom (≈ 15 Å) is larger than the critical interaction distance for energy transfer assuming an electric dipole-quadrupole interaction. Therefore, it can be concluded that in the ZN5Tb2Eu glass the Tb3+ → Eu3+ energy transfer could be occurring in Tb3+-Eu3+ clusters, which can favor a short-range interaction mechanism. The η energy transfer efficiency from Tb3+ to Eu3+ was estimated from Tb3+ o emission decay time data in presence ( τ Tb ) and absence ( τ Tb ) of Eu3+ through the o . Using the average lifetimes of 1.2±0.1 ms and 3.2±0.1 known relation, η = 1 − τ Tb τ Tb o , respectively, η resulted to be 0.62±0.07. ms obtained for τ Tb and τ Tb

4. Conclusions

Optical and spectroscopic properties of zinc-sodium-aluminosilicate glasses singly doped with Eu3+ and codoped with Eu3+ and Tb3+ were investigated. Reddish-orange light emission, with x = 0.64 and y = 0.36 CIE1931 chromaticity coordinates, is

obtained in the 2.0% Eu(PO3)3 singly doped glass excited at 393 nm. Such chromaticity coordinates are close to those (0.67,0.33) standard of the National Television System Committee for the red phosphor. Reddish-orange light emission, with (0.61,0.37) CIE1931 chromaticity coordinates, is obtained in the 5.0% Tb(PO3)3-2.0% Eu(PO3)3 codoped glass upon Tb3+ excitation at 344 nm. This excitation wavelength fits the requirements of AlGaN based LEDs. The reddish-orange luminescence is generated mainly by 5D0 → 7F1 and 5D0 →7F2 emissions of Eu3+, europium being sensitized by terbium through a non-radiative energy transfer. The non-radiative nature of the Tb3+ → Eu3+ energy transfer is inferred from the increase in the decay rate of the Tb3+ emission with respect to that in the Tb3+-singly doped glass. From an analysis of the Tb3+ emission decay curves it is inferred that the Tb3+ → Eu3+ energy transfer might take place between Tb3+ and Eu3+ clusters through a (electric dipole-quadrupole) short-range interaction mechanism. The efficiency of such energy transfer estimated from Tb3+ emission decay time data is 62%, indicating that the 5.0% Tb(PO3)3-2.0% Eu(PO3)3 codoped zinc-sodium-aluminosilicate glass could be attractive as conversion phosphor for LED applications. Moreover, in both the singly doped and codoped glasses multimode optical waveguides were successfully produced by diluted silver–sodium exchange, and characterized at various wavelengths. The process parameters thus obtained would allow one to optimize the design of the guided-wave structures for the development of integrated optical devices for specific applications. Acknowledgements

This work was supported by the CONACYT-CNR bilateral agreement under Project Contract 173855. We thank Roberto Calzolai (IFAC-CNR) and Erica Viviani (Univ. Verona) for expert technical assistance. References

[1] A. Pinos, S. Marcinkevičius, M. S. Shur, J. Appl. Phys. 109 (2011) 103108. [2] U. Caldiño, A. Speghini, M. Bettinelli, J. Phys.: Condens. Matter 18 (2006) 3499. [3] U. Caldiño, E. Álvarez, A. Speghini, M. Bettinelli, J. Lumin. 132 (2012) 2077. [4] G.C. Righini, M. Ferrari, Riv. Nuovo Cimento 28 (2005) 1. [5] A. Tervonen, S. Honkanen, B.R. West, Opt. Engin. 50 (2011) 071107. [6] U. Caldiño, A. Speghini, E. Álvarez, S. Berneschi, M. Bettinelli, M. Brenci, G.C. Righini, Opt. Mater. 33 (2011) 1892. [7] U. Caldiño, A. Speghini, S. Berneschi, M. Bettinelli, M. Brenci, S. Pelli, G.C. Righini, Opt. Mater. 34 (2012) 1067.

[8] G. Jose, G. Sorbello, S. Taccheo, E. Cianci, V. Foglietti, P. Laporta, J. Non-Cryst. Solids 322 (2003) 256. [9] U. Caldiño, E. Álvarez, A. Speghini, M. Bettinelli, J. Lumin. 135 (2013) 216. [10] G. L. Yip, J. Albert, Opt. Lett. 10 (1985) 151. [11] G. Blasse, B.C. Grabmaier, Luminescence Materials, Springer-Verlag, Berlin, 1994 (Chapters 4-5). [12] T. Nishida, T. Ban, N. Kobayashi, Phys. Stat. Sol. (a) 200 (2003) 106. [13] Y. Liu, G.D. Qian, Z.Y. Wang, M.Q. Wang, Appl. Phys. Lett. 86 (2005) 071907. [14] Z.Q. Wang, Y. Yang, Y.J. Cui, Z.Y. Wang, G.D. Qian, J. Alloy Compd. 510 (2012) L5. [15] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978. [16] R. Martínez-Martínez, M. García, A. Speghini, M. Bettinelli, C. Falcony, U. Caldiño, J. Phys.: Condens. Matter 20 (2008) 395205. Figure Captions Fig. 1. Reconstructed Gaussian index profile of the waveguide produced in the ZN2Eu

glass by silver ion-exchange. Dots indicate the modal effective indices experimentally measured. Fig. 2. Emission spectrum of ZN2Eu glass excited at 393 nm. Fig. 3. Energy level scheme for Tb3+ → Eu3+ energy transfer and cross-relaxation

transitions of Eu3+. Fig. 4. Excitation spectrum of ZN2Eu glass monitored at 610 nm. Fig. 5. Decay curve of europium emission (λem= 610 nm) recorded for the ZN2Eu glass

excited at 392 nm, which is fitted to a single-exponential function (solid line). Fig. 6. Emission spectrum of ZN5Tb2Eu glass excited at 344 nm. Fig. 7. Excitation spectrum of ZN5Tb2Eu glass monitored at 704 nm. Fig. 8. Overlap region between Tb3+ emissions (solid line) and Eu3+ absorptions (dotted

line). The Eu3+ absorption spectrum was taken from the ZN2Eu excitation spectrum displayed in Fig. 4. Fig. 9. Decay curves of the terbium 5D4→7F5 emission (λem= 542 nm) recorded for the

ZN5Tb and ZN5Tb2Eu glasses excited at 376 and 344 nm, respectively. Dotted line, thick solid line and thin solid line are the best fits to Eq. (1) for electric (dd) dipoledipole,

(dq)

respectively.

dipole-quadrupole

and

(qq)

quadrupole-quadrupole

interactions,

►Reddish-orange light emission can be generated from Tb3+ and Eu3+ codoped zincsodium-aluminosilicate glasses excited at 344 nm. ►The Eu3+ is sensitized by Tb3+ through a non-radiative energy transfer. ►Highly multimode waveguides can be fabricated by diluted silver-sodium exchange. ►This type of AlGaN LEDs pumped glass phosphors might be useful for generation of reddish-orange light.

Figure 1

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Figure 9