- Email: [email protected]
Silver to erbium energy transfer in phosphate glasses
Journal of Non-Crystalline Solids 353 (2007) 498–501 www.elsevier.com/locate/jnoncrysol
Silver to erbium energy transfer in phosphate glasses M. Mattarelli a,*, M. Montagna a, E. Moser a, K.C. Vishnubhatla a, C. Armellini b, A. Chiasera b, M. Ferrari b, G. Speranza c, M. Brenci d, G. Nunzi Conti d, G.C. Righini d b
a Dipartimento di Fisica, Universita` di Trento, CSMFO group, Via Sommarive 14, 38050 Trento, Italy CNR-IFN, Istituto di Fotonica e Nanotecnologie, CSMFO group, Via Sommarive 14, 38050 Trento, Italy c ITC-IRST, via Sommarive 18, 38050 Trento, Italy d CNR-IFAC Istituto di Fisica Applicata, ‘‘Nello Carrara’’, Optoelectronics and Photonics Department, Via Madonna del Piano 10, 50019 Sesto Fiorentino (Firenze), Italy
Available online 6 February 2007
Abstract Silver and erbium co-doped sodium metaphosphate glasses were prepared by standard melt-quench technique. The samples underwent heat or 355 nm laser radiation exposure treatments in order to promote silver clustering. Absorption and photoluminescence spectra allowed distinguishing several active species related to silver, whose concentration in the samples depends on the treatment. A broad emission of silver related species has been detected in the visible region of the spectrum. The excitation spectra of the band and of the 1.5 lm luminescence of Er demonstrated energy transfer from Ag aggregates to Er. Differently from silicate glasses, in phosphates the silver to erbium energy transfer is very weak. Ó 2007 Elsevier B.V. All rights reserved. PACS: 81.40.Tv; 78.67.n; 78.60.b; 42.70.Ce Keywords: Optical properties; Luminescence; Optical spectroscopy; Oxide glasses; Phosphates; Rare-earths in glasses
1. Introduction The increase of the intensity of rare earth photoluminescence (PL) in glasses after the addition of metal (usually silver) ions is interesting from the point of view of the basic physical mechanisms and of the possible applications in photonic devices. After the first work of Malta et al. [1] and Hayakawa et al. [2], which considered an enhancement of the local field at the rare earth site due to the interaction with surface plasmon excitations of metallic nanoclusters, the intensity increase is now widely attributed to energy transfer from small metal aggregates, even if with some doubts about the actual species involved [3,4].
*
Corresponding author. Tel.: +39 (0)461 881543; fax: +39 (0)461 881696. E-mail address: [email protected] (M. Mattarelli). 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.10.017
The matrices where this phenomenon has been observed were silicate glasses. On the contrary, silver exchanged sodium phosphate glass, a matrix broadly used in application because of the high solubility of rare earths and its advantageous mechanical properties, has shown no energy transfer and therefore no significant increase of Er3+ luminescence at 1.5 lm [4]. This has motivated us to a further and deeper study into the effect of the matrix on such energy transfer processes. 2. Experimental Powders of AgO2, HPO3, NaPO3 and ErCl3 were mixed and melted in alumina crucibles at 700 °C in air for 4 h with a resulting nominal composition of 10(AgPO3)– 89(NaPO3)–1ErO3/2. By pouring the melt on a metal mould of cylindrical shape, four glass pieces were obtained
M. Mattarelli et al. / Journal of Non-Crystalline Solids 353 (2007) 498–501
with height of 2.5 mm and diameter of 15 mm. After annealing at 200 °C for 2 h to remove the stress, the samples were optically polished. A sample of 99(NaPO3)– 1ErO3/2 composition, hereafter NaPh, was obtained with the same procedure and kept as a reference. The Ag-doped samples underwent different heat treatments at 300 and 400 °C in air for 30 min to promote the formation of silver clusters (and hereafter they are called AgPh3 and AgPh4, respectively, while the untreated one is called AgPh2). During the heat treatments some silver migrated toward the surface; a brief polishing was required to restore the optical quality of the sample. After the annealing at 200 °C, an Agdoped sample (AgPhLT), that did not undergo any further thermal treatment, was laser treated with a 10 Hz pulsed Nd-YAG laser operating at 355 nm, with 6 ns pulse and energy of 29 mJ. The laser was focused on 0.3 cm2 for 20 min. Optical absorption spectra were obtained in the 0.2– 3.2 lm range using a double beam spectrometer with a resolution of 1 nm. Photoluminescence measurements in the visible and infrared were obtained upon several excitation wavelengths. The set-up for the detection of the infrared signal consisted in a 320 mm single-grating monochromator with a resolution of 2 nm connected to an InGaAs photodiode, while for the visible range it consisted in a double monochromator with a resolution of 0.5 nm connected to a photon counting system. Excitation spectra were excited by a Xe lamp coupled to a single-grating monochromator, in a spectral range extending from 320 to 750 nm, with a resolution of 5 nm. The signal was collected at several wavelengths in visible and IR ranges. 3. Results The absorption spectra of the samples are shown in Fig. 1. The NaPh sample has a wide transparency window
down to the NUV. Many sharp lines of the Er3+ appear in the spectrum. In the samples containing silver, the intense absorption due to the 4d10 ! 4d95s1 transition of Ag+ ions shifts the energy gap. The samples are too thick for showing the absorption band shape, whose maximum should be at 250 nm [5]. The sample annealed at 400 °C becomes slightly opaque: a beginning devitrification process is probably active, since NaPO3 has a low glass transition, around 300 °C [6]. The laser treatment also produces red-shifts of the absorption that can be ascribed to a clustering of Ag+ ions. This has been evidenced in the inset, where the differential absorption coefficient of AgPhLT, before and after the laser treatment, has been reported. The broad band centered at about 370 nm, reported in the inset of Fig. 1, has been observed also in AgPO3 crystals and assigned to Ag+–Ag+ dimers [7]. Fig. 2 shows the excitation spectra collected at 1532 nm, on the maximum of the 4I13/2 ! 4I15/2 erbium emission. In addition to the erbium excitation sharp bands, a weak broad band centered at around 400 nm appears in the silver co-doped samples. It is important to note that no luminescence, other than that of erbium, is detected at around 1.5 lm after excitation at any frequency. As shown in the inset, for an excitation at 420 nm in the weak broad band and therefore not resonant with Er3+ excitations, the luminescence intensity goes to zero on the two sides of the Er3+ luminescence band. Furthermore, the shape of the luminescence band was the same for the different excitations. Fig. 3 shows the PL spectra of the silver doped glasses before and after the laser treatment. After excitation at 514.5 nm, on the high energy tail of the 4I15/2 ! 2 H11/2Er3+ transition, a broad band emission appears together with the 2H11/2 ! 4I15/2 and 4S3/2 ! 4I15/2 lines of Er3+ luminescence. The intensity of the band is very weak both in the untreated and in thermally treated samples. It increases by a factor of about 50 after the laser 15
20
NaPh AgPh2 AgPh3 AgPhLT AgPh4
Absorption Coefficient (cm-1)
G11/2
2
0
10
350 4
450
4
F3/2- F5/2
2
G9/2
550
2
4
0.6 0.4 0.2
10
0.0 1400
5
1500
1600
1700
x 10
4
F7/2
2
H9/2
0 400
λexc= 420 nm
0.8
H11/2- S3/2
0 350
1.0
NaPh AgPh2 AgPh3 AgPhLT
4
PL intensity (arb. units)
4
499
450
500
550
Wavelength (nm) Fig. 1. Room temperature absorption spectra of the phosphate samples. The inset shows the difference between the spectra after and before the laser treatment.
350
400
450
500
Excitation wavelength (nm) Fig. 2. Excitation spectra obtained collecting the erbium luminescence at 1.53 lm. The spectra have been multiplied by a factor 10 for wavelengths higher than 390 nm. The inset shows the PL spectrum of the 4I13/2 ! 4I15/2 erbium transition upon 420 nm excitation by Xe lamp.
500
M. Mattarelli et al. / Journal of Non-Crystalline Solids 353 (2007) 498–501
PL intensity (arb. units)
4
λexc= λexc= λexc= λexc=
4. Discussion
514 nm (AgPh2) 514 nm (AgPhLT) 476 nm 458 nm
3
2
1
x 10
0 450
500
550
600
Wavelength (nm) Fig. 3. PL spectra of sample AgPh2 upon 514.5 nm laser excitation and of the laser treated sample AgPhLT upon 458, 476 and 514.5 nm laser excitation.
treatment, while position and shape are scarcely effected. By exciting the AgPhLT sample at 476 nm and 458 nm, the band progressively blue-shifts, indicating the presence of different centers, which absorb and emit at different frequencies [5]. Fig. 4 shows the excitation spectrum (kcoll = 515 nm), together with the PL spectrum (kexc = 350 nm) and the absorption spectrum for sample AgPhLT. The PL band is further shifted to shorter wavelengths, with respect to those of Fig. 3, obtained by excitation in the visible. The excitation at 350 nm is obtained by a Xe lamp, and therefore, the excited volume is not confined near the surface like with laser excitation. The PL has to travel a longer path inside the sample to reach the surface. The dips in the PL band in correspondence with the erbium absorption lines, the strongest one being around 520 nm (2H11/2 level), are a clear evidence of radiative energy transfer from silver aggregate to Er3+ ions, i.e. reabsorption of the luminescence.
14000
PL (λ exc = 350 nm) Abs Exc (λ coll = 515 nm)
Intensity (arb. units)
12000 10000 8000 6000 4000 2000 0 350
400
450
500
550
600
In silver-doped silicate glasses it is possible to promote the formation of silver aggregates by heat treatments in air at temperatures above 450 °C [8]. Instead, in metaphosphate glasses the heat treatments seem to have smaller effects. The AgPh3 sample, annealed at 300 °C, shows a very weak luminescence band from silver aggregates, of intensity comparable with the one of AgPh2 (Fig. 3). When heated at higher temperatures, the glass tends to devitrificate before showing important silver clustering. On the other hand, the laser treatment at 355 nm is more effective and the luminescence of silver aggregates increases of about two order of magnitude. This is probably due to a selective excitation of the silver ions, which induces the silver clustering without affecting the glassy structure. The shape of observed luminescence, in particular the frequency position of its maximum, shifts with the excitation frequency. This is a clear indication that different centers are co-existent, probably Ag+–Ag+ and Ag+–Ag0 dimers, trimers and other small aggregates, as already observed in silicate glasses [5]. The increase of the broad band PL intensity in the laser treated sample can be associated to the increase of the Er3+ PL upon non-resonant excitation. However, the energy transfer process is weaker than in silicate [4]. The presence of dips in correspondence to Er absorption in the PL bands of silver aggregates clearly indicates that radiative transfer plays some role. This effect mainly depends on the geometry of the samples. Massive samples, as those of the present work, should show larger reabsorption effects than the thinner glasses used in previous studies [4]. On the contrary, the non-radiative transfer depends on the microscopic structure and spatial distribution of acceptor and donor species. In this respect, the property of good solution with large separation among the doping species, which make phosphate glasses favored for high doping level active materials [9], is a drawback. In fact, metaphosphate glasses are constituted of long polyphosphate chains, which allow the clustering in pair of ions that are coordinated to a small number of oxygen ions (3 for Ag+, 5 for Na+), but prevent it for high coordination number ions such as rare earth (7) [10]. This physical constrain suggests that in phosphate glasses thermal annealing produces few Ag small aggregates, which should act as donors in the process of energy transfer to erbium ions [3,4]. Laser annealing is more efficient in producing them, but energy transfer from the Ag aggregates to Er ions is less effective than in silicate glasses, probably because of a larger mean distance between donors and acceptors induced by the phosphate structure. A very weak energy transfer is observed and an important part of it is probably due to radiative reabsorption.
650
Wavelength (nm) Fig. 4. Excitation spectrum collected at 515 nm, PL spectrum upon 350 nm excitation by a Xe lamp and absorption spectra of the laser treated sample AgPhLT.
5. Conclusions Laser annealing allowed to produce luminescent silver aggregates in a phosphate glass. Energy transfer from
M. Mattarelli et al. / Journal of Non-Crystalline Solids 353 (2007) 498–501
these centers to Er3+ ions is less efficient than in silicate glasses. Acknowledgments The authors acknowledge the financial support by MIUR-PRIN 2004 and ITPAR projects. References [1] O.L. Malta, P.A. Santa-Cruz, G.F. de Sa´, F. Auzel, J. Lumin. 33 (1985) 261. [2] T. Hayakawa, S.T. Selvan, M. Nogami, Appl. Phys. Lett. 74 (1999) 1513. [3] C. Strohho¨fer, A. Polman, Appl. Phys. Lett. 81 (2002) 1414.
501
[4] H. Portales, M. Mattarelli, M. Montagna, A. Chiasera, M. Ferrari, A. Martucci, P. Mazzoldi, S. Pelli, G.C. Righini, J. Non-Cryst. Solids 351 (2005) 1738. [5] M.A. Villegas, J.M. Fernandez Navarro, S.E. Paje, J. Llopis, Phys. Chem. Glasses 37 (1996) 248. [6] P.E. Hart, M.G. Mesko, J.E. Shelby, J. Non-Cryst. Solids 263&264 (2000) 305. [7] I. Belharouak, H. Aouad, M. Mesnaoui, M. Maazaz, C. Parent, B. Tanguy, P. Gravereau, G. Le Flem, J. Solid State Chem. 145 (1999) 97. [8] E. Borsella, E. Cattaruzza, G. De Marchi, F. Gonella, G. Mattei, P. Mazzoldi, A. Quaranta, G. Battaglin, R. Polloni, J. Non-Cryst. Solids 245 (1999) 122. [9] S. Jiang, T. Luo, B. Hwang, F. Smekatala, K. Seneschal, J. Lucas, N. Peyghambarian, J. Non-Cryst. Solids 263&264 (2000) 364. [10] R.K. Brow, J. Non-Cryst. Solids 263&264 (2000) 1.
Silver to erbium energy transfer in phosphate glasses M. Mattarelli a,*, M. Montagna a, E. Moser a, K.C. Vishnubhatla a, C. Armellini b, A. Chiasera b, M. Ferrari b, G. Speranza c, M. Brenci d, G. Nunzi Conti d, G.C. Righini d b
a Dipartimento di Fisica, Universita` di Trento, CSMFO group, Via Sommarive 14, 38050 Trento, Italy CNR-IFN, Istituto di Fotonica e Nanotecnologie, CSMFO group, Via Sommarive 14, 38050 Trento, Italy c ITC-IRST, via Sommarive 18, 38050 Trento, Italy d CNR-IFAC Istituto di Fisica Applicata, ‘‘Nello Carrara’’, Optoelectronics and Photonics Department, Via Madonna del Piano 10, 50019 Sesto Fiorentino (Firenze), Italy
Available online 6 February 2007
Abstract Silver and erbium co-doped sodium metaphosphate glasses were prepared by standard melt-quench technique. The samples underwent heat or 355 nm laser radiation exposure treatments in order to promote silver clustering. Absorption and photoluminescence spectra allowed distinguishing several active species related to silver, whose concentration in the samples depends on the treatment. A broad emission of silver related species has been detected in the visible region of the spectrum. The excitation spectra of the band and of the 1.5 lm luminescence of Er demonstrated energy transfer from Ag aggregates to Er. Differently from silicate glasses, in phosphates the silver to erbium energy transfer is very weak. Ó 2007 Elsevier B.V. All rights reserved. PACS: 81.40.Tv; 78.67.n; 78.60.b; 42.70.Ce Keywords: Optical properties; Luminescence; Optical spectroscopy; Oxide glasses; Phosphates; Rare-earths in glasses
1. Introduction The increase of the intensity of rare earth photoluminescence (PL) in glasses after the addition of metal (usually silver) ions is interesting from the point of view of the basic physical mechanisms and of the possible applications in photonic devices. After the first work of Malta et al. [1] and Hayakawa et al. [2], which considered an enhancement of the local field at the rare earth site due to the interaction with surface plasmon excitations of metallic nanoclusters, the intensity increase is now widely attributed to energy transfer from small metal aggregates, even if with some doubts about the actual species involved [3,4].
*
Corresponding author. Tel.: +39 (0)461 881543; fax: +39 (0)461 881696. E-mail address: [email protected] (M. Mattarelli). 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.10.017
The matrices where this phenomenon has been observed were silicate glasses. On the contrary, silver exchanged sodium phosphate glass, a matrix broadly used in application because of the high solubility of rare earths and its advantageous mechanical properties, has shown no energy transfer and therefore no significant increase of Er3+ luminescence at 1.5 lm [4]. This has motivated us to a further and deeper study into the effect of the matrix on such energy transfer processes. 2. Experimental Powders of AgO2, HPO3, NaPO3 and ErCl3 were mixed and melted in alumina crucibles at 700 °C in air for 4 h with a resulting nominal composition of 10(AgPO3)– 89(NaPO3)–1ErO3/2. By pouring the melt on a metal mould of cylindrical shape, four glass pieces were obtained
M. Mattarelli et al. / Journal of Non-Crystalline Solids 353 (2007) 498–501
with height of 2.5 mm and diameter of 15 mm. After annealing at 200 °C for 2 h to remove the stress, the samples were optically polished. A sample of 99(NaPO3)– 1ErO3/2 composition, hereafter NaPh, was obtained with the same procedure and kept as a reference. The Ag-doped samples underwent different heat treatments at 300 and 400 °C in air for 30 min to promote the formation of silver clusters (and hereafter they are called AgPh3 and AgPh4, respectively, while the untreated one is called AgPh2). During the heat treatments some silver migrated toward the surface; a brief polishing was required to restore the optical quality of the sample. After the annealing at 200 °C, an Agdoped sample (AgPhLT), that did not undergo any further thermal treatment, was laser treated with a 10 Hz pulsed Nd-YAG laser operating at 355 nm, with 6 ns pulse and energy of 29 mJ. The laser was focused on 0.3 cm2 for 20 min. Optical absorption spectra were obtained in the 0.2– 3.2 lm range using a double beam spectrometer with a resolution of 1 nm. Photoluminescence measurements in the visible and infrared were obtained upon several excitation wavelengths. The set-up for the detection of the infrared signal consisted in a 320 mm single-grating monochromator with a resolution of 2 nm connected to an InGaAs photodiode, while for the visible range it consisted in a double monochromator with a resolution of 0.5 nm connected to a photon counting system. Excitation spectra were excited by a Xe lamp coupled to a single-grating monochromator, in a spectral range extending from 320 to 750 nm, with a resolution of 5 nm. The signal was collected at several wavelengths in visible and IR ranges. 3. Results The absorption spectra of the samples are shown in Fig. 1. The NaPh sample has a wide transparency window
down to the NUV. Many sharp lines of the Er3+ appear in the spectrum. In the samples containing silver, the intense absorption due to the 4d10 ! 4d95s1 transition of Ag+ ions shifts the energy gap. The samples are too thick for showing the absorption band shape, whose maximum should be at 250 nm [5]. The sample annealed at 400 °C becomes slightly opaque: a beginning devitrification process is probably active, since NaPO3 has a low glass transition, around 300 °C [6]. The laser treatment also produces red-shifts of the absorption that can be ascribed to a clustering of Ag+ ions. This has been evidenced in the inset, where the differential absorption coefficient of AgPhLT, before and after the laser treatment, has been reported. The broad band centered at about 370 nm, reported in the inset of Fig. 1, has been observed also in AgPO3 crystals and assigned to Ag+–Ag+ dimers [7]. Fig. 2 shows the excitation spectra collected at 1532 nm, on the maximum of the 4I13/2 ! 4I15/2 erbium emission. In addition to the erbium excitation sharp bands, a weak broad band centered at around 400 nm appears in the silver co-doped samples. It is important to note that no luminescence, other than that of erbium, is detected at around 1.5 lm after excitation at any frequency. As shown in the inset, for an excitation at 420 nm in the weak broad band and therefore not resonant with Er3+ excitations, the luminescence intensity goes to zero on the two sides of the Er3+ luminescence band. Furthermore, the shape of the luminescence band was the same for the different excitations. Fig. 3 shows the PL spectra of the silver doped glasses before and after the laser treatment. After excitation at 514.5 nm, on the high energy tail of the 4I15/2 ! 2 H11/2Er3+ transition, a broad band emission appears together with the 2H11/2 ! 4I15/2 and 4S3/2 ! 4I15/2 lines of Er3+ luminescence. The intensity of the band is very weak both in the untreated and in thermally treated samples. It increases by a factor of about 50 after the laser 15
20
NaPh AgPh2 AgPh3 AgPhLT AgPh4
Absorption Coefficient (cm-1)
G11/2
2
0
10
350 4
450
4
F3/2- F5/2
2
G9/2
550
2
4
0.6 0.4 0.2
10
0.0 1400
5
1500
1600
1700
x 10
4
F7/2
2
H9/2
0 400
λexc= 420 nm
0.8
H11/2- S3/2
0 350
1.0
NaPh AgPh2 AgPh3 AgPhLT
4
PL intensity (arb. units)
4
499
450
500
550
Wavelength (nm) Fig. 1. Room temperature absorption spectra of the phosphate samples. The inset shows the difference between the spectra after and before the laser treatment.
350
400
450
500
Excitation wavelength (nm) Fig. 2. Excitation spectra obtained collecting the erbium luminescence at 1.53 lm. The spectra have been multiplied by a factor 10 for wavelengths higher than 390 nm. The inset shows the PL spectrum of the 4I13/2 ! 4I15/2 erbium transition upon 420 nm excitation by Xe lamp.
500
M. Mattarelli et al. / Journal of Non-Crystalline Solids 353 (2007) 498–501
PL intensity (arb. units)
4
λexc= λexc= λexc= λexc=
4. Discussion
514 nm (AgPh2) 514 nm (AgPhLT) 476 nm 458 nm
3
2
1
x 10
0 450
500
550
600
Wavelength (nm) Fig. 3. PL spectra of sample AgPh2 upon 514.5 nm laser excitation and of the laser treated sample AgPhLT upon 458, 476 and 514.5 nm laser excitation.
treatment, while position and shape are scarcely effected. By exciting the AgPhLT sample at 476 nm and 458 nm, the band progressively blue-shifts, indicating the presence of different centers, which absorb and emit at different frequencies [5]. Fig. 4 shows the excitation spectrum (kcoll = 515 nm), together with the PL spectrum (kexc = 350 nm) and the absorption spectrum for sample AgPhLT. The PL band is further shifted to shorter wavelengths, with respect to those of Fig. 3, obtained by excitation in the visible. The excitation at 350 nm is obtained by a Xe lamp, and therefore, the excited volume is not confined near the surface like with laser excitation. The PL has to travel a longer path inside the sample to reach the surface. The dips in the PL band in correspondence with the erbium absorption lines, the strongest one being around 520 nm (2H11/2 level), are a clear evidence of radiative energy transfer from silver aggregate to Er3+ ions, i.e. reabsorption of the luminescence.
14000
PL (λ exc = 350 nm) Abs Exc (λ coll = 515 nm)
Intensity (arb. units)
12000 10000 8000 6000 4000 2000 0 350
400
450
500
550
600
In silver-doped silicate glasses it is possible to promote the formation of silver aggregates by heat treatments in air at temperatures above 450 °C [8]. Instead, in metaphosphate glasses the heat treatments seem to have smaller effects. The AgPh3 sample, annealed at 300 °C, shows a very weak luminescence band from silver aggregates, of intensity comparable with the one of AgPh2 (Fig. 3). When heated at higher temperatures, the glass tends to devitrificate before showing important silver clustering. On the other hand, the laser treatment at 355 nm is more effective and the luminescence of silver aggregates increases of about two order of magnitude. This is probably due to a selective excitation of the silver ions, which induces the silver clustering without affecting the glassy structure. The shape of observed luminescence, in particular the frequency position of its maximum, shifts with the excitation frequency. This is a clear indication that different centers are co-existent, probably Ag+–Ag+ and Ag+–Ag0 dimers, trimers and other small aggregates, as already observed in silicate glasses [5]. The increase of the broad band PL intensity in the laser treated sample can be associated to the increase of the Er3+ PL upon non-resonant excitation. However, the energy transfer process is weaker than in silicate [4]. The presence of dips in correspondence to Er absorption in the PL bands of silver aggregates clearly indicates that radiative transfer plays some role. This effect mainly depends on the geometry of the samples. Massive samples, as those of the present work, should show larger reabsorption effects than the thinner glasses used in previous studies [4]. On the contrary, the non-radiative transfer depends on the microscopic structure and spatial distribution of acceptor and donor species. In this respect, the property of good solution with large separation among the doping species, which make phosphate glasses favored for high doping level active materials [9], is a drawback. In fact, metaphosphate glasses are constituted of long polyphosphate chains, which allow the clustering in pair of ions that are coordinated to a small number of oxygen ions (3 for Ag+, 5 for Na+), but prevent it for high coordination number ions such as rare earth (7) [10]. This physical constrain suggests that in phosphate glasses thermal annealing produces few Ag small aggregates, which should act as donors in the process of energy transfer to erbium ions [3,4]. Laser annealing is more efficient in producing them, but energy transfer from the Ag aggregates to Er ions is less effective than in silicate glasses, probably because of a larger mean distance between donors and acceptors induced by the phosphate structure. A very weak energy transfer is observed and an important part of it is probably due to radiative reabsorption.
650
Wavelength (nm) Fig. 4. Excitation spectrum collected at 515 nm, PL spectrum upon 350 nm excitation by a Xe lamp and absorption spectra of the laser treated sample AgPhLT.
5. Conclusions Laser annealing allowed to produce luminescent silver aggregates in a phosphate glass. Energy transfer from
M. Mattarelli et al. / Journal of Non-Crystalline Solids 353 (2007) 498–501
these centers to Er3+ ions is less efficient than in silicate glasses. Acknowledgments The authors acknowledge the financial support by MIUR-PRIN 2004 and ITPAR projects. References [1] O.L. Malta, P.A. Santa-Cruz, G.F. de Sa´, F. Auzel, J. Lumin. 33 (1985) 261. [2] T. Hayakawa, S.T. Selvan, M. Nogami, Appl. Phys. Lett. 74 (1999) 1513. [3] C. Strohho¨fer, A. Polman, Appl. Phys. Lett. 81 (2002) 1414.
501
[4] H. Portales, M. Mattarelli, M. Montagna, A. Chiasera, M. Ferrari, A. Martucci, P. Mazzoldi, S. Pelli, G.C. Righini, J. Non-Cryst. Solids 351 (2005) 1738. [5] M.A. Villegas, J.M. Fernandez Navarro, S.E. Paje, J. Llopis, Phys. Chem. Glasses 37 (1996) 248. [6] P.E. Hart, M.G. Mesko, J.E. Shelby, J. Non-Cryst. Solids 263&264 (2000) 305. [7] I. Belharouak, H. Aouad, M. Mesnaoui, M. Maazaz, C. Parent, B. Tanguy, P. Gravereau, G. Le Flem, J. Solid State Chem. 145 (1999) 97. [8] E. Borsella, E. Cattaruzza, G. De Marchi, F. Gonella, G. Mattei, P. Mazzoldi, A. Quaranta, G. Battaglin, R. Polloni, J. Non-Cryst. Solids 245 (1999) 122. [9] S. Jiang, T. Luo, B. Hwang, F. Smekatala, K. Seneschal, J. Lucas, N. Peyghambarian, J. Non-Cryst. Solids 263&264 (2000) 364. [10] R.K. Brow, J. Non-Cryst. Solids 263&264 (2000) 1.