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Local devitrification on an oxyfluoride glass doped with Ho3+ ions under Argon laser irradiation
Optical Materials 31 (2009) 1373–1375
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Local devitrification on an oxyfluoride glass doped with Ho3+ ions under Argon laser irradiation S. González-Pérez *, I.R. Martín, P. Haro-González Departamento de Física Fundamental y Experimental, Electrónica y Sistemas, Universidad de La Laguna, E-38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain
a r t i c l e
i n f o
Article history: Available online 24 January 2009 PACS: 42.70. a 42.62.Fi 81.07.Bc Keywords: Laser heating Glass ceramic Ho3+
a b s t r a c t Devitrification in a holmium-doped oxyfluoride glass has been obtained under laser irradiation. The emissions from 5S2 (5F4) and 5F5 levels have been studied in order to analyze structure changes by the irradiation of the laser beam. The emission spectra before and after the irradiation with 2.5 W reveal that the structure of the glass sample has been modified permanently due to the laser action. Time-resolved spectra have been analyzed in the wavelength range 720–780 nm in the glass before and after irradiation, and in glass ceramic bulk samples. Lifetime measurements as a function of the distance to the centre of the damaged zone of the irradiated glass have also been carried out, and the results confirm that the devitrification was achieved successfully. Moreover, from the analyzes of these results it can be concluded that approximately 70% of the Ho3+ ions are located in the nanocrystals and less than 30% in the glassy phase. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction It is well known that the luminescence properties of RE ions depend strongly on their local environment, and therefore differences in the spectroscopic properties of the impurities are expected if they are placed in a glassy or in a crystalline surrounding of a glass ceramic sample [1]. In the last few years, research about the modification of the structure of the selected points in glasses by laser irradiation has been carried out [2]. The use of intensive laser pulses focused into the bulk of a transparent material is a direct novel technique to induce the changes on the irradiated area. Until now oxyfluoride glass ceramics have been basically obtained by heat treating rare earth-doped oxyfluoride glasses using a furnace, but using this technique it can also be obtained at selected points in glasses using a high power laser irradiation [3]. In this work, a local area of an oxyfluoride glass doped with 2.5 mol% of Ho3+ has been irradiated using an Argon laser obtaining glass ceramic properties on the irradiated area. The emission fluorescence spectra from 5S2 (5F4) and 5F5 levels of Ho3+ ions have been measured inside and outside the locally damaged zone of the glass. Time-resolved emission spectra in a glass ceramic bulk have been measured to be compared with the results obtained from the glass before and after the irradiation by the laser beam. Fluorescence lifetimes obtained under excitation at 532 nm at RT have also been * Corresponding author. Address: Universidad de La Laguna, Facultad de Física, Departamento de física fundamental y experimental, electrónica y sistemas, 38206, La Laguna, S/C de Tenerife, Spain. Tel.: +34 922 318651; fax: +34 922 318228. E-mail address: [email protected] (S. González-Pérez). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.10.015
analyzed with the position on the sample from the centre of the damaged zone to 1600 lm outside the irradiated area. 2. Experimental The samples used in this study were prepared with the following composition in mol%: 30SiO2, 15Al2O3, 29CdF2, 22PbF2, 1.5YF3, and 2.5HoF3, the final glass was obtained by melting at 1050 °C for 2 h and finally casting the melt into a slab on a stainless steel plate at room temperature. One of the glass samples obtained was heated at 470 °C for 36 h to be converted into a transparent glass ceramic that will be used to be compared with the local glass ceramic obtained in the glass sample using the laser treatment. The local heating of the glass was performed with a continuous Ar laser irradiation increasing the average laser power from 144 mW to 2500 mW. The laser beam was focused onto the sample with a focal length lens of 5 cm and a focused waist of about 45 lm. Timeresolved measurements were performed by exciting the samples with a Q-switched Nd-YAG excitation at the wavelength range 720–780 nm. Measurements with the distance were carried out by using a (x–y–z) micro-positioner. The corresponding fluorescence was detected through a TRIAX monochromator with a photomultiplier, and the signal was acquired by a digital oscilloscope. 3. Results and discussion A local area of an oxyfluoride glass doped with 2.5 mol% of Ho3+ has been heated using an Argon laser in a multiline mode in order
1374
S. González-Pérez et al. / Optical Materials 31 (2009) 1373–1375
to obtain glass ceramic properties at the irradiated local zone. The fluorescence emission spectra were obtained exciting the 5I8 ? 5S2 (5F4) energy levels of Ho3+ ions at low power (see Fig. 1) before and after the irradiation at 2500 mW power intensity at RT, and the results are shown in Fig 2. The emission peaks obtained at 660, 750 and 970 nm are clearly identified with the transitions of Ho3+ ions: 5 F5 ? 5I8, 5S2 (5F4) ? 5I7, and 5F5 ? 5I7, respectively. It is interesting to note the changes observed in the total (about a factor 10) and relative intensities of these emissions. Locally resolved fluorescence decay curves of the thermalized 5 S2 (5F4) levels inside and outside the locally damaged zone of the glass have been measured under excitation at 532 nm at RT (see Fig. 3). Both behaviours are completely different as can be observed. The results show a typical glass ceramic and glass behaviour inside and outside the damaged zone, respectively. Inside the damage zone the decay curve is the result of the contribution of two different kinds of centres, the fluoride nanocrystals (fast initial decay) and the glassy phase (slower tail). The value obtained inside the damaged zone for the fast initial decay constant is about 0.2 ls followed by a much slower tail with a decay constant around 1.2 ls approximately. The value obtained for the fast decay constant is similar to the lifetime observed in the glass ceramic bulk
sample. These results confirm the hypothesis that Ho3+ ions localized in the nanocrystals formed by the laser action having a strong interaction (energy transfer processes) due to the smaller distances between them and give place to the observed fast initials decays of the 5S2 (5F4) levels, while the slower part of the decay is due to Ho3+ ions in the glassy phase. To define clearly the changes obtained when the glass sample is irradiated at high power density, that the glass sample subject to a heating process using a furnace, i.e. a glass ceramic sample, has also been analyzed. Time-resolved fluorescence emission spectra of 5S2 (5F4) ? 5I7 energy levels of Ho3+ ions have been carried out in the wavelength range from 720 to 780 nm in the 2.5 mol% of Ho3+ glass and 2.5 mol% of Ho3+ glass ceramic bulk, both at t = 0.2 ls. Moreover, it has also been performed inside the damaged zone of the glass after irradiation at t = 0.2 ls and t = 2 ls to be compared with the bulk measurements, and the results are shown in Fig. 4. Changes in the emission bands between glass and glass ceramic bulk samples prove that a new environment has been formed around the RE impurities in the glass ceramic sample as it is normally achieved when the structure changes due to the heating process [4]. Comparing both, glass ceramic spectrum is more resolved than the glass one, and as it can be observed in Fig 4b resembles that of the devitrificated point by the laser
20 5
5
S2 , F4
5
I4
5
I5
10 5
I6
970 nm 750 nm 5
Intensity (a.u.)
F5
3
-1
Energy (x10 cm )
5
15
I7
5
514nm 5
I8
0
0
3+
Ho
1
2
3
t (μs)
Fig. 1. Energy level diagram of Ho3+ ions.
Fig. 3. Lifetime fluorescence spectra obtained inside (full circles) and outside (open circles) glass ceramic structure created on the spot by laser damage.
1.5
(b)
Intensity (a.u.)
Intensity (a.u.)
1.0 0.5 0.0 1.5
(a)
1.0 0.5 0.0 730
600
800
1000
λ (nm) Fig. 2. Emission spectra realized before (thick line – magnified 20 times) and after (thin line) laser irradiation at 2500 mW at RT.
740
750
760
770
λ (nm) Fig. 4. (a) Time-resolved emission spectra of the glass bulk (dashed line) and glass ceramic bulk (solid line) samples at t = 0.2 ls. (b) Time-resolved emission spectra inside the irradiated area at t = 0.2 ls (solid line) and t = 2 ls (dashed line).
S. González-Pérez et al. / Optical Materials 31 (2009) 1373–1375
1375
4. Conclusions
Preexponential Factor (%)
70
60
50
40
30
0
500
1000
1500
Position (μm) Fig. 5. Contribution of Ho3+ ions located at the nanocrystals (squares) or at the glassy phase (circles) of the irradiated zone of the glass with the distance to the centre of the damaged zone.
irradiation at t = 0.2 ls. Moreover, it can be seen that the spectrum inside the locally damaged zone at t = 2 ls looks almost the same as the spectra of the glass bulk sample. This indicates that at longer times the emission comes from a small proportion of ions that reside in the glassy phase. These results demonstrate that the locally irradiated zone has been devitrificated successfully. The weight of the contributions of each of the centres (fluoride nanocrystals and glassy phase) has been analyzed using the preexponential factor of the fast and slow decay contribution to the total curve as a function of the distance to the centre of the damaged zone, and the result is shown in Fig. 5. The fast decay excitation has a maximum at the centre where the devitrification made by the laser damage is maxima, whereas the slow decay excitation curve has the minimum value at the same point due to the diminishing of the glassy environment. The ratio of Ho3+ ions inside the crystalline phase in this devitrificated point can be estimated from these results. Therefore, it can be concluded that more than 70% of the Ho3+ ions are located in the nanocrystals and less than 30% in the glassy phase. These values are in good agreement with the previous results obtained in similar oxyfluoride glass ceramics made by heating process using a furnace [5,6].
Fluoride nanocrystals have grown in a 2.5 mol% Ho3+ doped oxyfluoride glass after pumping with an Ar laser at 2500 mW. This devitrification process has been monitored by analyzing the emission around 750 nm that originated from the 5S2 (5F4) thermalized levels inside and outside the damaged zone. Moreover, time-resolved spectra have been measured in the wavelength range 720–780 nm in the bulk samples of glass (before and after irradiation) and glass ceramic. The lifetimes with the distance to the centre of the damaged zone of the irradiated glass have also been studied. Results confirm that the devitrification was successfully achieved and therefore the nanocrystals have been created by the laser action, confirming that the transition from glass to glass ceramic structure has been completed. These decay curves are the result of the contribution of two different kinds of centres, the fluoride nanocrystals and the glassy phase of the glass ceramic sample created due to the irradiation. And from the analysis of these contributions it can be concluded that approximately 70% of the Ho3+ ions are located in the nanocrystals and about 30% in the glassy phase. Acknowledgements This work was supported by ‘Comisión Interministerial de Ciencia y Tecnología’ (under projects MAT 2004-06868 and MAT 2007-63319) and also to contracts of ‘Tecnólogos’ by ‘Consejería de Industria Comercio y Nuevas Tecnologías del Gobierno de Canarias’. References [1] H. Hashima, A. Konishi, Y. Tanigami, D. Shibata, Y. Kawamoto, Opt. Comp. Mater. 5350 (2004) 212. [2] P. Yang, G.R. Burns, J. Guo, T.S. Luk, G.A. Vawter, J. Appl. Phys. 95 (2004) 5280. [3] V.K. Tikhomirov, A.B. Seddon, J. Koch, D. Wandt, B.N. Chichkov, Rap. Res. Lett. 202 (2003) 2295. [4] F. Lahoz, I.R. Martín, J.M. Calvilla-Quintero, Appl. Phys. Lett. 86 (2005) 051106. [5] M. Abril, J. Méndez-Ramos, I.R. Martín, U.R. Rodríguez-Mendoza, V. Lavín, A. Delgado-Torres, V.D. Rodríguez, P. Núñez, A.D. Lozano-Gorrín, J. Appl. Phys. 95 (2004) 5271. [6] F. Lahoz, I.R. Martín, J. Méndez-Ramos, P. Núñez, J. Chem. Phys. 120 (2004) 6180.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Local devitrification on an oxyfluoride glass doped with Ho3+ ions under Argon laser irradiation S. González-Pérez *, I.R. Martín, P. Haro-González Departamento de Física Fundamental y Experimental, Electrónica y Sistemas, Universidad de La Laguna, E-38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain
a r t i c l e
i n f o
Article history: Available online 24 January 2009 PACS: 42.70. a 42.62.Fi 81.07.Bc Keywords: Laser heating Glass ceramic Ho3+
a b s t r a c t Devitrification in a holmium-doped oxyfluoride glass has been obtained under laser irradiation. The emissions from 5S2 (5F4) and 5F5 levels have been studied in order to analyze structure changes by the irradiation of the laser beam. The emission spectra before and after the irradiation with 2.5 W reveal that the structure of the glass sample has been modified permanently due to the laser action. Time-resolved spectra have been analyzed in the wavelength range 720–780 nm in the glass before and after irradiation, and in glass ceramic bulk samples. Lifetime measurements as a function of the distance to the centre of the damaged zone of the irradiated glass have also been carried out, and the results confirm that the devitrification was achieved successfully. Moreover, from the analyzes of these results it can be concluded that approximately 70% of the Ho3+ ions are located in the nanocrystals and less than 30% in the glassy phase. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction It is well known that the luminescence properties of RE ions depend strongly on their local environment, and therefore differences in the spectroscopic properties of the impurities are expected if they are placed in a glassy or in a crystalline surrounding of a glass ceramic sample [1]. In the last few years, research about the modification of the structure of the selected points in glasses by laser irradiation has been carried out [2]. The use of intensive laser pulses focused into the bulk of a transparent material is a direct novel technique to induce the changes on the irradiated area. Until now oxyfluoride glass ceramics have been basically obtained by heat treating rare earth-doped oxyfluoride glasses using a furnace, but using this technique it can also be obtained at selected points in glasses using a high power laser irradiation [3]. In this work, a local area of an oxyfluoride glass doped with 2.5 mol% of Ho3+ has been irradiated using an Argon laser obtaining glass ceramic properties on the irradiated area. The emission fluorescence spectra from 5S2 (5F4) and 5F5 levels of Ho3+ ions have been measured inside and outside the locally damaged zone of the glass. Time-resolved emission spectra in a glass ceramic bulk have been measured to be compared with the results obtained from the glass before and after the irradiation by the laser beam. Fluorescence lifetimes obtained under excitation at 532 nm at RT have also been * Corresponding author. Address: Universidad de La Laguna, Facultad de Física, Departamento de física fundamental y experimental, electrónica y sistemas, 38206, La Laguna, S/C de Tenerife, Spain. Tel.: +34 922 318651; fax: +34 922 318228. E-mail address: [email protected] (S. González-Pérez). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.10.015
analyzed with the position on the sample from the centre of the damaged zone to 1600 lm outside the irradiated area. 2. Experimental The samples used in this study were prepared with the following composition in mol%: 30SiO2, 15Al2O3, 29CdF2, 22PbF2, 1.5YF3, and 2.5HoF3, the final glass was obtained by melting at 1050 °C for 2 h and finally casting the melt into a slab on a stainless steel plate at room temperature. One of the glass samples obtained was heated at 470 °C for 36 h to be converted into a transparent glass ceramic that will be used to be compared with the local glass ceramic obtained in the glass sample using the laser treatment. The local heating of the glass was performed with a continuous Ar laser irradiation increasing the average laser power from 144 mW to 2500 mW. The laser beam was focused onto the sample with a focal length lens of 5 cm and a focused waist of about 45 lm. Timeresolved measurements were performed by exciting the samples with a Q-switched Nd-YAG excitation at the wavelength range 720–780 nm. Measurements with the distance were carried out by using a (x–y–z) micro-positioner. The corresponding fluorescence was detected through a TRIAX monochromator with a photomultiplier, and the signal was acquired by a digital oscilloscope. 3. Results and discussion A local area of an oxyfluoride glass doped with 2.5 mol% of Ho3+ has been heated using an Argon laser in a multiline mode in order
1374
S. González-Pérez et al. / Optical Materials 31 (2009) 1373–1375
to obtain glass ceramic properties at the irradiated local zone. The fluorescence emission spectra were obtained exciting the 5I8 ? 5S2 (5F4) energy levels of Ho3+ ions at low power (see Fig. 1) before and after the irradiation at 2500 mW power intensity at RT, and the results are shown in Fig 2. The emission peaks obtained at 660, 750 and 970 nm are clearly identified with the transitions of Ho3+ ions: 5 F5 ? 5I8, 5S2 (5F4) ? 5I7, and 5F5 ? 5I7, respectively. It is interesting to note the changes observed in the total (about a factor 10) and relative intensities of these emissions. Locally resolved fluorescence decay curves of the thermalized 5 S2 (5F4) levels inside and outside the locally damaged zone of the glass have been measured under excitation at 532 nm at RT (see Fig. 3). Both behaviours are completely different as can be observed. The results show a typical glass ceramic and glass behaviour inside and outside the damaged zone, respectively. Inside the damage zone the decay curve is the result of the contribution of two different kinds of centres, the fluoride nanocrystals (fast initial decay) and the glassy phase (slower tail). The value obtained inside the damaged zone for the fast initial decay constant is about 0.2 ls followed by a much slower tail with a decay constant around 1.2 ls approximately. The value obtained for the fast decay constant is similar to the lifetime observed in the glass ceramic bulk
sample. These results confirm the hypothesis that Ho3+ ions localized in the nanocrystals formed by the laser action having a strong interaction (energy transfer processes) due to the smaller distances between them and give place to the observed fast initials decays of the 5S2 (5F4) levels, while the slower part of the decay is due to Ho3+ ions in the glassy phase. To define clearly the changes obtained when the glass sample is irradiated at high power density, that the glass sample subject to a heating process using a furnace, i.e. a glass ceramic sample, has also been analyzed. Time-resolved fluorescence emission spectra of 5S2 (5F4) ? 5I7 energy levels of Ho3+ ions have been carried out in the wavelength range from 720 to 780 nm in the 2.5 mol% of Ho3+ glass and 2.5 mol% of Ho3+ glass ceramic bulk, both at t = 0.2 ls. Moreover, it has also been performed inside the damaged zone of the glass after irradiation at t = 0.2 ls and t = 2 ls to be compared with the bulk measurements, and the results are shown in Fig. 4. Changes in the emission bands between glass and glass ceramic bulk samples prove that a new environment has been formed around the RE impurities in the glass ceramic sample as it is normally achieved when the structure changes due to the heating process [4]. Comparing both, glass ceramic spectrum is more resolved than the glass one, and as it can be observed in Fig 4b resembles that of the devitrificated point by the laser
20 5
5
S2 , F4
5
I4
5
I5
10 5
I6
970 nm 750 nm 5
Intensity (a.u.)
F5
3
-1
Energy (x10 cm )
5
15
I7
5
514nm 5
I8
0
0
3+
Ho
1
2
3
t (μs)
Fig. 1. Energy level diagram of Ho3+ ions.
Fig. 3. Lifetime fluorescence spectra obtained inside (full circles) and outside (open circles) glass ceramic structure created on the spot by laser damage.
1.5
(b)
Intensity (a.u.)
Intensity (a.u.)
1.0 0.5 0.0 1.5
(a)
1.0 0.5 0.0 730
600
800
1000
λ (nm) Fig. 2. Emission spectra realized before (thick line – magnified 20 times) and after (thin line) laser irradiation at 2500 mW at RT.
740
750
760
770
λ (nm) Fig. 4. (a) Time-resolved emission spectra of the glass bulk (dashed line) and glass ceramic bulk (solid line) samples at t = 0.2 ls. (b) Time-resolved emission spectra inside the irradiated area at t = 0.2 ls (solid line) and t = 2 ls (dashed line).
S. González-Pérez et al. / Optical Materials 31 (2009) 1373–1375
1375
4. Conclusions
Preexponential Factor (%)
70
60
50
40
30
0
500
1000
1500
Position (μm) Fig. 5. Contribution of Ho3+ ions located at the nanocrystals (squares) or at the glassy phase (circles) of the irradiated zone of the glass with the distance to the centre of the damaged zone.
irradiation at t = 0.2 ls. Moreover, it can be seen that the spectrum inside the locally damaged zone at t = 2 ls looks almost the same as the spectra of the glass bulk sample. This indicates that at longer times the emission comes from a small proportion of ions that reside in the glassy phase. These results demonstrate that the locally irradiated zone has been devitrificated successfully. The weight of the contributions of each of the centres (fluoride nanocrystals and glassy phase) has been analyzed using the preexponential factor of the fast and slow decay contribution to the total curve as a function of the distance to the centre of the damaged zone, and the result is shown in Fig. 5. The fast decay excitation has a maximum at the centre where the devitrification made by the laser damage is maxima, whereas the slow decay excitation curve has the minimum value at the same point due to the diminishing of the glassy environment. The ratio of Ho3+ ions inside the crystalline phase in this devitrificated point can be estimated from these results. Therefore, it can be concluded that more than 70% of the Ho3+ ions are located in the nanocrystals and less than 30% in the glassy phase. These values are in good agreement with the previous results obtained in similar oxyfluoride glass ceramics made by heating process using a furnace [5,6].
Fluoride nanocrystals have grown in a 2.5 mol% Ho3+ doped oxyfluoride glass after pumping with an Ar laser at 2500 mW. This devitrification process has been monitored by analyzing the emission around 750 nm that originated from the 5S2 (5F4) thermalized levels inside and outside the damaged zone. Moreover, time-resolved spectra have been measured in the wavelength range 720–780 nm in the bulk samples of glass (before and after irradiation) and glass ceramic. The lifetimes with the distance to the centre of the damaged zone of the irradiated glass have also been studied. Results confirm that the devitrification was successfully achieved and therefore the nanocrystals have been created by the laser action, confirming that the transition from glass to glass ceramic structure has been completed. These decay curves are the result of the contribution of two different kinds of centres, the fluoride nanocrystals and the glassy phase of the glass ceramic sample created due to the irradiation. And from the analysis of these contributions it can be concluded that approximately 70% of the Ho3+ ions are located in the nanocrystals and about 30% in the glassy phase. Acknowledgements This work was supported by ‘Comisión Interministerial de Ciencia y Tecnología’ (under projects MAT 2004-06868 and MAT 2007-63319) and also to contracts of ‘Tecnólogos’ by ‘Consejería de Industria Comercio y Nuevas Tecnologías del Gobierno de Canarias’. References [1] H. Hashima, A. Konishi, Y. Tanigami, D. Shibata, Y. Kawamoto, Opt. Comp. Mater. 5350 (2004) 212. [2] P. Yang, G.R. Burns, J. Guo, T.S. Luk, G.A. Vawter, J. Appl. Phys. 95 (2004) 5280. [3] V.K. Tikhomirov, A.B. Seddon, J. Koch, D. Wandt, B.N. Chichkov, Rap. Res. Lett. 202 (2003) 2295. [4] F. Lahoz, I.R. Martín, J.M. Calvilla-Quintero, Appl. Phys. Lett. 86 (2005) 051106. [5] M. Abril, J. Méndez-Ramos, I.R. Martín, U.R. Rodríguez-Mendoza, V. Lavín, A. Delgado-Torres, V.D. Rodríguez, P. Núñez, A.D. Lozano-Gorrín, J. Appl. Phys. 95 (2004) 5271. [6] F. Lahoz, I.R. Martín, J. Méndez-Ramos, P. Núñez, J. Chem. Phys. 120 (2004) 6180.