- Email: [email protected]
Upconversion and concentration quenching in Er3+-doped TeO2–Na2O binary glasses
Journal of Non-Crystalline Solids 353 (2007) 1383–1387 www.elsevier.com/locate/jnoncrysol
Upconversion and concentration quenching in Er3+-doped TeO2–Na2O binary glasses Kaushal Kumar, S.B. Rai *, D.K. Rai Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India Available online 29 March 2007
Abstract The spectroscopic properties of Er3+-doped alkali tellurite TeO2–Na2O glasses are investigated. Infrared-to-visible upconversion emission bands are observed at 410, 525, 550 and 658 nm using 797 nm excitation wavelength. These bands are assigned to the 2H9/2 ! 4I15/2, 2 H11/2 ! 4I15/2, 4S3/2 ! 4I15/2 and 4F9/2 ! 4I15/2 transition, respectively. The power dependence study reveals that the 2H9/2 ! 4I15/2 transition involves a three-step process while the other upconversion transitions involve only two steps. An excitation with 532 nm wavelength, two upconversion bands are observed in the UV region at 380 and 404 nm in addition to bands in the visible region at 410, 475, 525, 550, 658 and 843 nm. These bands are ascribed to 4G11/2 ! 4I15/2, 2P3/2 ! 4I13/2, 2H9/2 ! 4I15/2, 2P3/2 ! 4I11/2, 2H11/2 ! 4I15/2, 4S3/2 ! 4I15/2, 4 F9/2 ! 4I15/2 and 4S3/2 ! 4I13/2 transition, respectively. Increasing Er3+ concentration leads to a rapid growth in the intensity of red emission relative to that for the green emission. An explanation for this observation has been suggested. The temperature dependence profile for the two thermally coupled levels (2H11/2, 4S3/2) shows that they can be used for measuring the temperature. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.70.Hj; 42.70.Ce Keywords: Optical properties; Luminescence; Optical spectroscopy; Upconversion; Tellurite Glass
1. Introduction With the development of efficient semicondutor lasers emitting in the near infrared region, the frequency upconversion of infrared-to-visible radiation in rare-earth doped materials is given much attention now a days for applications in areas like optical data transmission, display devices, sensors etc. [1–5]. Rare-earth doped glasses probably will have greater impact than the crystals because of their several superior properties such as easy preparation, large RE3+ doping, large inhomogeneous line broadening etc. The glasses based on tellurite as glass former have several advantages over other oxide glasses [6,7] and is a focus of attention. Er3+ is one of the most extensively studied rare-earth ions and the upconversion processes in this ion have been *
Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 368 468. E-mail address: [email protected] (S.B. Rai).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.09.068
investigated in a variety of host materials [6–23]. The host material should have low phonon energy to reduce the nonradiative loss due to the multiphonon relaxation and thus yield strong upconversion luminescence. So far, many efforts have been made to enhance the upconversion efficiencies in glasses through the addition of different glass modifiers. In this work, we report on the luminescence and upconversion properties of Er3+ in TeO2–Na2O binary glasses to investigate the effect of alkali oxide on the optical properties of Er3+. In addition, the effect of changing rareearth concentration having resonant excitation and increased input power has been investigated.
2. Experimental TeO2–Na2O binary glasses used in the present work were synthesized by the conventional melting and quenching method. The starting materials were analytical grade
1384
K. Kumar et al. / Journal of Non-Crystalline Solids 353 (2007) 1383–1387
99.9% pure TeO2, Na2CO3 and Er2O3. The composition was taken in mol% as ð100 y xÞTeO2 þ yNa2 CO3 þ xEr2 O3 ; where x ¼ 0:5; 1:0; 1:5 and 2:0 y ¼ 10; 15; 20; 25 and 30: Several pieces of glass were prepared for each composition. The prepared glass samples were cut in rectangular shape with diamond blade keeping their thicknesses (2 mm) almost the same. Fluorescence emission from these glasses was measured using a 0.5 m monochromator equipped with S-20 photo-multiplier tube as a detector. Upconversion measurements were made with two different excitation sources viz. Ti: Sapphire and Nd:YVO4 lasers emitting at 797 nm and 532 nm, respectively. For fluorescence intensity comparison among glasses with different rare-earth content, spectra were recorded under the same experimental conditions and at the same excitation power. Absorption 4 intensity for the 4S3/2 H15/2 transition was measured using a halogen lamp and monochromator assembly. The absorption coefficient for a particular transition was determined from the following formula: aðkÞ ¼ 2:203D0 ðkÞ=‘; D0 ¼ Log10 I=I 0 ; where ‘ is the sample thickness, I and I0 are the absorbed and total incident photon intensity, respectively. 3. Results As mentioned in the experimental section, glasses were prepared by with different concentrations for both the modifier and the rare-earth dopant. For glasses with varying the modifier proportion, the rare-earth content was kept constant. It was observed that the fluorescence intensity increased with the increase in modifier content up to
20 mol%. Increasing the modifier content further did not increase the fluorescence intensity and actually a decrease in the fluorescence intensity was observed when the modifier concentration was 30 mol%. This decrease is mainly related to a decrease in the glass transparency. In the next series of studies, the content of the modifier was kept fixed at 20 mol% and the rare-earth concentration was varied from 0.5 to 2.0 mol%. Measurements showed a concentration quenching at 1.5 mol% of Er3+. All the prepared glass samples showed good optical transparency and homogeneity. The absorption coefficient at different concentrations of 4 Er3+ ion was calculated for the 4S3/2 I15/2 transition. The graph between absorption coefficient versus concentration of Er3+ ions is shown in Fig. 1. A reasonably good linear increase with the concentration from 0.5 to 2.0 mol% is observed with a sign of saturation at higher concentration. 3.1. Excitation with 797 nm radiation 4 The 797 nm line is resonant with the 4I9/2 I15/2 transi3+ tion of Er . The observed fluorescence spectrum shows five emission bands centered at 410, 525, 550, 658 and 803 nm. Which are assigned to 2H9/2 ! 4I15/2, 2H11/2 ! 4I15/2, 4 S3/2 ! 4I15/2, 4F9/2 ! 4I15/2 and 4I9/2 ! 4I15/2 transition, respectively. The intense green emission at 550 nm is visible to eye even at a low pump power (<30 mW). It clearly indicates an efficient upconversion in this glass host. On the other hand, the emission at 410 nm appears only at the relatively high pump power (>80 mW). To get more insight into the upconversion mechanisms the dependence of upconversion intensity was measured as a function of the incident pump power P. In frequency upconversion process, the upconversion intensity Iup varies with the nth power of the pump power P (that is Iup / Pn), where n is the number of pump photons absorbed per visible photon emitted. For the 4S3/2 ! 4I15/2 and 4 F9/2 ! 4I15/2 transitions the values of n are found to be 1.75 and 1.65, respectively, which indicates that these transitions must originate by two photon absorption. The corresponding n value for the 2H9/2 ! 4I15/2 transition at 410 nm is 2.73, indicating it to involve three incident photons. The departure of n from integral values is a consequence of many factors e.g. absorption of the upconverted fluorescence, saturation effects, involvement of non-radiative decays, intermediate levels, etc. On increasing the Er3+ concentration, upconversion intensity of the two emission bands at 550 nm and 658 nm is seen to vary in different manner. Fig. 2 shows the relative change in the intensity of the red (658 nm) emission with the Er3+ ion concentration (from 0.5 to 2.0 mol%). In the figure, the plotted intensity is normalized relative to the intensity of the green emission.
3.2. Excitation with 532 nm radiation Fig. 1. Absorption coefficient versus Er3+ concentration graph for the 4 S3/2 4I15/2 transition.
Use of 532 nm radiation for excitation leads to the observation of seven fluorescence bands at 380, 404, 470,
K. Kumar et al. / Journal of Non-Crystalline Solids 353 (2007) 1383–1387
Fig. 2. Relative change in intensities for the green and red emissions with the increase of Er3+ concentration.
1385
(380 nm) and the violet (404 and 410 nm) bands. While the green (525 and 550 nm) and the red (658 nm) emissions involve only one photon excitation. The bands at 475 and 843 nm are too weak and it was not possible to study the power dependence. Considerations of relevant energy suggest that 843 nm band is due to one photon process whereas 475 nm band involves two incident photons. The incident photon energy (18 796 cm1) is slightly higher than the excitation energy for the 4S3/2 state (18 400 cm1), so the excitation of the 4S3/2 state involves phonon relaxation. Fluorescence due to the 2H11/2 ! 4I15/2 transition is observed since the 2H11/2 level is thermally coupled with the 4S3/2 level even at room temperature. Some of the ions decay non-radiatively from 4S3/2 level to the 4F9/2 level and give the red (658 nm) emission through 4F9/2 ! 4I15/2 transition. The infrared emission at 843 nm occurs due to a transition from the 4S3/2 level to the 4I13/2 level. Pathways for the observed fluorescence bands are represented schematically in the energy level diagram (Fig. 4). A change in slope of the LnI–LnP curve for red (658 nm) emission with the increase in Er3+ concentration is noted and is shown in Fig. 5. 4. Discussion
525, 550, 658 and 843 nm. The emission in the region 404– 410 nm is probably due to an overlapping of emission from the two different excited levels. The intense peak at 404 nm is ascribed to the 2P3/2 ! 4I13/2 transition while the weaker peak at 410 nm is ascribed to the 2H9/2 ! 4I15/2 transition. The band at 475 nm is assigned to the 2P3/2 ! 4I11/2 transition. The 2P3/2 ! 4I15/2 transition, which has a smaller transition probability, is not observed clearly except as a broad feature around 320 nm. It may also be due to the experimental limitations. The observed fluorescence spectrum is shown in Fig. 3. The log–log plots of the fluorescence intensity versus the input pump power for the various bands indicate involvement of two photons in the emission of ultraviolet
4.1. Upconversion mechanism and quenching behavior It is well known that on increasing the rare-earth concentration beyond some definite value the fluorescence intensity gets quenched for all the rare-earth ions, but the limit of ion concentration vary from ion to ion and also glass to glass. The concentration quenching of Er3+ ion in a simple TeO2 glass host has been observed at Er3+ concentration of 1.5 mol%. The quenching rate for the Er3+ ions is found somewhat slow and glass fluoresces strongly from a trace level (0.001 mol%) to a high value (8.0 mol%) of Er3+. Lifetime of the 4I11/2 level of Er3+ ion is quite large 13 ms [16] so that it can act as a reservoir for the upconversion process.
Fig. 3. Fluorescence spectrum of 1.0 mol% Er3+-doped TeO2–Na2O glass with 532 nm excitation (lines are drawn as guides for the eyes).
1386
K. Kumar et al. / Journal of Non-Crystalline Solids 353 (2007) 1383–1387
Fig. 4. Simplified energy level diagram of Er3+ ion and possible upconversion transition pathways for 532 nm excitation.
processes. Er3+ concentration above the 1.5 mol%, speed up the luminescence decay due to the increased non-radiative relaxations. To explain the observed intensity behavior for the above two concentration ranges, generally two excitation mechanisms are considered to play role in populating the various excited energy states. These are the excited state absorption (ESA) and energy transfer (ET) mechanisms. In the ESA process, an ion already in an excited state absorbs another incident photon and is promoted to a higher excited energy state. The excited ion then decays to the ground or any intermediate excited state emitting a photon of higher energy. In the ET process, excitation energy migrates from one ion to another ion in an isolated ion pair or in a cluster, raising the latter ion/ions into a higher energy excited state. The ET process is strongly dependent on separation between the rare-earth ions (i.e. concentration of rareearth) in glasses and varies as r6 for the electric dipole transition, where r is the separation between the two rare-earth ions. As the rare-earth concentration increases, the probability of ET increases and sometimes both the mechanisms become active. Er3+ ions excited by absorption of 797 nm radiation populate the 4I9/2 level and relax through multiphonon processes to the 4I11/2 and 4I13/2 levels. At low concentrations (<1.5 mol%), ions in these low excited states absorb photons from the incident beam and get promoted to higher excited levels 4S3/2/2H11/2. The ions excited to these levels decay to the ground state emitting the intense green radiation. Some of the excited ions in the 4S3/2 level decay non-radiatively to the 4F9/2 level from which they decay radiatively to the ground state giving the red fluorescence. From Fig. 2, it is noted that the red band grows in rapidly relative to the green band. It is also noted that quenching of the fluorescence starts at the concentrations >1.0 mol% of Er2O3 however, the red emission is not quenched as fast as the green emission. It seems that at low Er3+ concentrations, the 4F9/2 level is probably populated only via a nonradiative relaxation of the 4S3/2 level with moderate rate; however at higher concentrations of Er3+, energy transfer between two excited ions may also become important and the following process takes place: Er3þ ð4 S3=2 Þ þ Er3þ ð4 I9=2 Þ ! Er3þ ð4 F9=2 Þ þ Er3þ ð4 F9=2 Þ
Fig. 5. Slopes of the red emission band at different Er2O3 concentration on 532 nm excitation (lines are drawn as guides for the eyes). (For interpretation of the references to colour this figure legend, the reader is referred to the web version of this article.)
The intensities of different upconversion bands are found to vary with Er3+ ion concentration in different manner. For Er3+ concentration lower than 1.5 mol%, the luminescence can be interpreted very simply in terms of competition between radiative and non-radiative relaxations without taking account of possible energy transfer
It means that an ion in the 4S3/2 level decays to the 4F9/2 level and the excess excitation energy is taken up by another ion in the 4I9/2 level, which is raised to the 4F9/2 level. With inhomogeneous broadening and Stark splitting of the levels in question, there is sufficient energy coincidence between the two states of the above equation. The inter-ionic distance and experimental evidence of decrease in the slope between rare-earth concentration and incident photon intensity for the red emission suggests both the energy transfer processes. The inter ionic distance between two ˚ for 0.5 mol% of rare-earth Er3+ ions is estimated as 19 A ˚ doping and as 12 A for 2.0 mol% of rare-earth doping. So energy transfer for 0.5 mol% rare earth would be at least 10
K. Kumar et al. / Journal of Non-Crystalline Solids 353 (2007) 1383–1387
times smaller than for 2.0 mol%. The slopes for the red emission are found to be 1.69 and 1.53 for 0.5 mol% and 2.0 mol% rare-earth doped glasses, respectively. The slope value 1.53 suggests that two photons are involved in populating the 4F9/2 level, which can be explained by considering the above energy transfer process. Another evidence of energy transfer is that the change in value of n in relation Iup / Pn. In Fig. 5, slope (n) is found to decrease with the increase in Er3+ concentration for the red emission band (shown with 532 nm excitation). This decrease in the value of n occurs due to the energy transfer process. The violet emission also increases rapidly with the Er3+concentration, but since this emission appears even at low (0.5 mol%) Er3+ ion concentrations, it is concluded that 2H9/2 level is populated through excited state absorption (absorption of an incident photon by an ion) in the 4F9/2 level. With 532 nm excitation, again both ESA and ET mechanisms are responsible for the observed fluorescence. In the framework of ESA mechanism, an excited Er3+ ion in the 4 S3/2 level may absorb a second incident photon and is excited to the 2D5/2 level. The ions excited to this level decay non-radiatively to the lower lying 2P3/2, 4G11/2 and 2 H9/2 levels and then radiatively to the ground level. It is also possible that ions in the 4S3/2 level relax non-radiatively to the lower lying 4F9/2 and 4I9/2 levels and then absorb the second photon. On the other hand, two Er3+ ions both excited to the 4S3/2 level may share their energies in such a way that one is excited to the 2D5/2 level and the other goes to the ground level. 4.2. Effect of temperature on luminescence properties of Er3+ The dependence of fluorescence intensity on the temperature has been studied in the range 300–490 K. It is observed that the fluorescence intensities of the 380, 550 and 658 nm emissions decrease with an increase in temperature, the rate of decrease being smallest for the peak at 658 nm. One can conclude that relaxation rates for the corresponding upper state of the different transitions change with temperature in dissimilar fashion. The intensities of the two close lying fluorescence lines at 525 and 550 nm vary with temperature at different rates. The 525 nm intensity varies slowly with the temperature since this state is thermally coupled to the 4S3/2 level. Measuring the fluorescence intensity ratio of these two emitting states the temperature of the glass can be calculated using the formula [21,22]: N 2 I 2j g2 r2j x2j DE ðFIRÞ ¼ ¼ ¼ ; exp KT N 1 I 1j g1 r1j x1j where Ni is the number of excited ions, Iij is the fluorescence intensity from the two states, gi is the degeneracy of the states, rij is the emission cross section and xij is the transition probability. DE is the separation between the two states. A preliminary study on the Er3+-doped tellurite glass indicates that the system works well as an optical temperature sensor within the 300–500 K temperature range.
1387
5. Conclusions Er3+-doped tellurite glasses containing alkali oxide as modifier have been studied. Excitation of the Na2O– TeO2:Er3+ glass with 797 and 532 nm wavelengths results in emissions at various wavelengths ranging from NIR to UV region. The green emission has the maximum fluorescence intensity and is visible even at low NIR pump power (<30 mW), which shows an efficient upconversion in this glass host. The temperature dependence profile of the two thermally coupled levels shows that they can be used for estimating the temperature. Acknowledgements Authors are grateful to CSIR and DST New Delhi for financial assistance. One of the authors (Mr Kaushal Kumar) would like to thank M/S Laser Science Bombay and The Laser Spectroscopy Society of India for providing a Scholarship. References [1] B.P. Scott, F. Zhao, R.S.F. Chang, N. Djeu, Opt. Lett. 18 (1993) 113. [2] T. Hebert, R. Wannemacher, W. Lenth, R.M. Macfarlane, Appl. Phys. Lett. 57 (1990) 1727. [3] P. Xie, T.R. Gosnell, Opt. Lett. 20 (1995) 1014. [4] M. Yamada, H. Ono, T. Kanamori, S. Sudo, Y. Ohishi, Electron. Lett. 33 (1997) 710. [5] Y. Ohishi, A. Mori, M. Yamada, H. Ono, Y. Nishida, K. Oikawa, Opt. Lett. 23 (1998) 274. [6] M. Tsuda, K. Sousa, H. Inoue, S. Inoue, A. Makishima, J. Appl. Phys. 85 (1999) 29. [7] P.V. dos Santos, E.A. Gouveia, M.T. de Araujo, A.S. Liouveia-Neto, A.S.B. Sombra, J.A. Medeiros Neto, Appl. Phys. Lett. 74 (1999) 3607. [8] S.A. Pollack, D.B. Chang, R.A. McFarlane, H. Jenssen, J. Appl. Phys. 67 (1990) 648. [9] M.P. Hehlen, N.J. Cockroft, T.R. Gosnell, A.J. Bruce, Phys. Rev. B 56 (1997) 9302. [10] S.A. Pollack, D.B. Chang, J. Appl. Phys. 64 (1988) 2885. [11] K. Kumar, S.B. Rai, D.K. Rai, Solid State Commun. 139 (2006) 363. [12] V. Aruna, N. Sooraj Hussain, S. Buddhudu, Mater. Res. Bull. 33 (1998) 149. [13] S. Tanabe, K. Hirao, N. Soga, J. Non-Cryst. Solids 122 (1990) 79. [14] M.J. Weber, J.D. Myres, D.H. Bluchburn, J. Appl. Phys. 52 (1981) 2944. [15] J.S. Wang, E.M. Vogel, E. Snitzer, Opt. Mater. 3 (1993) 187. [16] G.S. Maciel, C.B. de Araujo, Y. Messaddeq, M.A. Aegerter, Phys. Rev. B 55 (1997) 6335. [17] F.E. Auzel, Proc. IEEE 61 (1973) 758. [18] M. Pollnau, D.R. Gamelin, S.R. Luthi, H.U. Gudel, M.P. Hehlen, Phys. Rev. B 61 (2001) 3357. [19] M.P. Hehlen, G. Frei, H.U. Gudel, Phys. Rev. B 60 (1994) 16265. [20] L. Petit, T. Cardinal, J.J. Videau, G. Le Flem, Y. Guyot, G. Boulon, M. Couzi, T. Buffeteau, J. Non-Cryst. Solids 298 (2002) 76. [21] S.A. Wade, S.F. Collins, G.W. Baxter, J. Appl. Phys. 94 (2003) 4743. [22] S.F. Collins, G.W. Baxter, S.A. Wade, T. Sun, K.T.V. Grattan, Z.Y. Zhang, A.W. Palmer, J. Appl. Phys. 84 (1998) 4649. [23] Z. Pan, S.H. Morgan, K. Dyer, A. Ueda, H. Liu, J. Appl. Phys. 79 (1996) 8906.
Upconversion and concentration quenching in Er3+-doped TeO2–Na2O binary glasses Kaushal Kumar, S.B. Rai *, D.K. Rai Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India Available online 29 March 2007
Abstract The spectroscopic properties of Er3+-doped alkali tellurite TeO2–Na2O glasses are investigated. Infrared-to-visible upconversion emission bands are observed at 410, 525, 550 and 658 nm using 797 nm excitation wavelength. These bands are assigned to the 2H9/2 ! 4I15/2, 2 H11/2 ! 4I15/2, 4S3/2 ! 4I15/2 and 4F9/2 ! 4I15/2 transition, respectively. The power dependence study reveals that the 2H9/2 ! 4I15/2 transition involves a three-step process while the other upconversion transitions involve only two steps. An excitation with 532 nm wavelength, two upconversion bands are observed in the UV region at 380 and 404 nm in addition to bands in the visible region at 410, 475, 525, 550, 658 and 843 nm. These bands are ascribed to 4G11/2 ! 4I15/2, 2P3/2 ! 4I13/2, 2H9/2 ! 4I15/2, 2P3/2 ! 4I11/2, 2H11/2 ! 4I15/2, 4S3/2 ! 4I15/2, 4 F9/2 ! 4I15/2 and 4S3/2 ! 4I13/2 transition, respectively. Increasing Er3+ concentration leads to a rapid growth in the intensity of red emission relative to that for the green emission. An explanation for this observation has been suggested. The temperature dependence profile for the two thermally coupled levels (2H11/2, 4S3/2) shows that they can be used for measuring the temperature. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.70.Hj; 42.70.Ce Keywords: Optical properties; Luminescence; Optical spectroscopy; Upconversion; Tellurite Glass
1. Introduction With the development of efficient semicondutor lasers emitting in the near infrared region, the frequency upconversion of infrared-to-visible radiation in rare-earth doped materials is given much attention now a days for applications in areas like optical data transmission, display devices, sensors etc. [1–5]. Rare-earth doped glasses probably will have greater impact than the crystals because of their several superior properties such as easy preparation, large RE3+ doping, large inhomogeneous line broadening etc. The glasses based on tellurite as glass former have several advantages over other oxide glasses [6,7] and is a focus of attention. Er3+ is one of the most extensively studied rare-earth ions and the upconversion processes in this ion have been *
Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 368 468. E-mail address: [email protected] (S.B. Rai).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.09.068
investigated in a variety of host materials [6–23]. The host material should have low phonon energy to reduce the nonradiative loss due to the multiphonon relaxation and thus yield strong upconversion luminescence. So far, many efforts have been made to enhance the upconversion efficiencies in glasses through the addition of different glass modifiers. In this work, we report on the luminescence and upconversion properties of Er3+ in TeO2–Na2O binary glasses to investigate the effect of alkali oxide on the optical properties of Er3+. In addition, the effect of changing rareearth concentration having resonant excitation and increased input power has been investigated.
2. Experimental TeO2–Na2O binary glasses used in the present work were synthesized by the conventional melting and quenching method. The starting materials were analytical grade
1384
K. Kumar et al. / Journal of Non-Crystalline Solids 353 (2007) 1383–1387
99.9% pure TeO2, Na2CO3 and Er2O3. The composition was taken in mol% as ð100 y xÞTeO2 þ yNa2 CO3 þ xEr2 O3 ; where x ¼ 0:5; 1:0; 1:5 and 2:0 y ¼ 10; 15; 20; 25 and 30: Several pieces of glass were prepared for each composition. The prepared glass samples were cut in rectangular shape with diamond blade keeping their thicknesses (2 mm) almost the same. Fluorescence emission from these glasses was measured using a 0.5 m monochromator equipped with S-20 photo-multiplier tube as a detector. Upconversion measurements were made with two different excitation sources viz. Ti: Sapphire and Nd:YVO4 lasers emitting at 797 nm and 532 nm, respectively. For fluorescence intensity comparison among glasses with different rare-earth content, spectra were recorded under the same experimental conditions and at the same excitation power. Absorption 4 intensity for the 4S3/2 H15/2 transition was measured using a halogen lamp and monochromator assembly. The absorption coefficient for a particular transition was determined from the following formula: aðkÞ ¼ 2:203D0 ðkÞ=‘; D0 ¼ Log10 I=I 0 ; where ‘ is the sample thickness, I and I0 are the absorbed and total incident photon intensity, respectively. 3. Results As mentioned in the experimental section, glasses were prepared by with different concentrations for both the modifier and the rare-earth dopant. For glasses with varying the modifier proportion, the rare-earth content was kept constant. It was observed that the fluorescence intensity increased with the increase in modifier content up to
20 mol%. Increasing the modifier content further did not increase the fluorescence intensity and actually a decrease in the fluorescence intensity was observed when the modifier concentration was 30 mol%. This decrease is mainly related to a decrease in the glass transparency. In the next series of studies, the content of the modifier was kept fixed at 20 mol% and the rare-earth concentration was varied from 0.5 to 2.0 mol%. Measurements showed a concentration quenching at 1.5 mol% of Er3+. All the prepared glass samples showed good optical transparency and homogeneity. The absorption coefficient at different concentrations of 4 Er3+ ion was calculated for the 4S3/2 I15/2 transition. The graph between absorption coefficient versus concentration of Er3+ ions is shown in Fig. 1. A reasonably good linear increase with the concentration from 0.5 to 2.0 mol% is observed with a sign of saturation at higher concentration. 3.1. Excitation with 797 nm radiation 4 The 797 nm line is resonant with the 4I9/2 I15/2 transi3+ tion of Er . The observed fluorescence spectrum shows five emission bands centered at 410, 525, 550, 658 and 803 nm. Which are assigned to 2H9/2 ! 4I15/2, 2H11/2 ! 4I15/2, 4 S3/2 ! 4I15/2, 4F9/2 ! 4I15/2 and 4I9/2 ! 4I15/2 transition, respectively. The intense green emission at 550 nm is visible to eye even at a low pump power (<30 mW). It clearly indicates an efficient upconversion in this glass host. On the other hand, the emission at 410 nm appears only at the relatively high pump power (>80 mW). To get more insight into the upconversion mechanisms the dependence of upconversion intensity was measured as a function of the incident pump power P. In frequency upconversion process, the upconversion intensity Iup varies with the nth power of the pump power P (that is Iup / Pn), where n is the number of pump photons absorbed per visible photon emitted. For the 4S3/2 ! 4I15/2 and 4 F9/2 ! 4I15/2 transitions the values of n are found to be 1.75 and 1.65, respectively, which indicates that these transitions must originate by two photon absorption. The corresponding n value for the 2H9/2 ! 4I15/2 transition at 410 nm is 2.73, indicating it to involve three incident photons. The departure of n from integral values is a consequence of many factors e.g. absorption of the upconverted fluorescence, saturation effects, involvement of non-radiative decays, intermediate levels, etc. On increasing the Er3+ concentration, upconversion intensity of the two emission bands at 550 nm and 658 nm is seen to vary in different manner. Fig. 2 shows the relative change in the intensity of the red (658 nm) emission with the Er3+ ion concentration (from 0.5 to 2.0 mol%). In the figure, the plotted intensity is normalized relative to the intensity of the green emission.
3.2. Excitation with 532 nm radiation Fig. 1. Absorption coefficient versus Er3+ concentration graph for the 4 S3/2 4I15/2 transition.
Use of 532 nm radiation for excitation leads to the observation of seven fluorescence bands at 380, 404, 470,
K. Kumar et al. / Journal of Non-Crystalline Solids 353 (2007) 1383–1387
Fig. 2. Relative change in intensities for the green and red emissions with the increase of Er3+ concentration.
1385
(380 nm) and the violet (404 and 410 nm) bands. While the green (525 and 550 nm) and the red (658 nm) emissions involve only one photon excitation. The bands at 475 and 843 nm are too weak and it was not possible to study the power dependence. Considerations of relevant energy suggest that 843 nm band is due to one photon process whereas 475 nm band involves two incident photons. The incident photon energy (18 796 cm1) is slightly higher than the excitation energy for the 4S3/2 state (18 400 cm1), so the excitation of the 4S3/2 state involves phonon relaxation. Fluorescence due to the 2H11/2 ! 4I15/2 transition is observed since the 2H11/2 level is thermally coupled with the 4S3/2 level even at room temperature. Some of the ions decay non-radiatively from 4S3/2 level to the 4F9/2 level and give the red (658 nm) emission through 4F9/2 ! 4I15/2 transition. The infrared emission at 843 nm occurs due to a transition from the 4S3/2 level to the 4I13/2 level. Pathways for the observed fluorescence bands are represented schematically in the energy level diagram (Fig. 4). A change in slope of the LnI–LnP curve for red (658 nm) emission with the increase in Er3+ concentration is noted and is shown in Fig. 5. 4. Discussion
525, 550, 658 and 843 nm. The emission in the region 404– 410 nm is probably due to an overlapping of emission from the two different excited levels. The intense peak at 404 nm is ascribed to the 2P3/2 ! 4I13/2 transition while the weaker peak at 410 nm is ascribed to the 2H9/2 ! 4I15/2 transition. The band at 475 nm is assigned to the 2P3/2 ! 4I11/2 transition. The 2P3/2 ! 4I15/2 transition, which has a smaller transition probability, is not observed clearly except as a broad feature around 320 nm. It may also be due to the experimental limitations. The observed fluorescence spectrum is shown in Fig. 3. The log–log plots of the fluorescence intensity versus the input pump power for the various bands indicate involvement of two photons in the emission of ultraviolet
4.1. Upconversion mechanism and quenching behavior It is well known that on increasing the rare-earth concentration beyond some definite value the fluorescence intensity gets quenched for all the rare-earth ions, but the limit of ion concentration vary from ion to ion and also glass to glass. The concentration quenching of Er3+ ion in a simple TeO2 glass host has been observed at Er3+ concentration of 1.5 mol%. The quenching rate for the Er3+ ions is found somewhat slow and glass fluoresces strongly from a trace level (0.001 mol%) to a high value (8.0 mol%) of Er3+. Lifetime of the 4I11/2 level of Er3+ ion is quite large 13 ms [16] so that it can act as a reservoir for the upconversion process.
Fig. 3. Fluorescence spectrum of 1.0 mol% Er3+-doped TeO2–Na2O glass with 532 nm excitation (lines are drawn as guides for the eyes).
1386
K. Kumar et al. / Journal of Non-Crystalline Solids 353 (2007) 1383–1387
Fig. 4. Simplified energy level diagram of Er3+ ion and possible upconversion transition pathways for 532 nm excitation.
processes. Er3+ concentration above the 1.5 mol%, speed up the luminescence decay due to the increased non-radiative relaxations. To explain the observed intensity behavior for the above two concentration ranges, generally two excitation mechanisms are considered to play role in populating the various excited energy states. These are the excited state absorption (ESA) and energy transfer (ET) mechanisms. In the ESA process, an ion already in an excited state absorbs another incident photon and is promoted to a higher excited energy state. The excited ion then decays to the ground or any intermediate excited state emitting a photon of higher energy. In the ET process, excitation energy migrates from one ion to another ion in an isolated ion pair or in a cluster, raising the latter ion/ions into a higher energy excited state. The ET process is strongly dependent on separation between the rare-earth ions (i.e. concentration of rareearth) in glasses and varies as r6 for the electric dipole transition, where r is the separation between the two rare-earth ions. As the rare-earth concentration increases, the probability of ET increases and sometimes both the mechanisms become active. Er3+ ions excited by absorption of 797 nm radiation populate the 4I9/2 level and relax through multiphonon processes to the 4I11/2 and 4I13/2 levels. At low concentrations (<1.5 mol%), ions in these low excited states absorb photons from the incident beam and get promoted to higher excited levels 4S3/2/2H11/2. The ions excited to these levels decay to the ground state emitting the intense green radiation. Some of the excited ions in the 4S3/2 level decay non-radiatively to the 4F9/2 level from which they decay radiatively to the ground state giving the red fluorescence. From Fig. 2, it is noted that the red band grows in rapidly relative to the green band. It is also noted that quenching of the fluorescence starts at the concentrations >1.0 mol% of Er2O3 however, the red emission is not quenched as fast as the green emission. It seems that at low Er3+ concentrations, the 4F9/2 level is probably populated only via a nonradiative relaxation of the 4S3/2 level with moderate rate; however at higher concentrations of Er3+, energy transfer between two excited ions may also become important and the following process takes place: Er3þ ð4 S3=2 Þ þ Er3þ ð4 I9=2 Þ ! Er3þ ð4 F9=2 Þ þ Er3þ ð4 F9=2 Þ
Fig. 5. Slopes of the red emission band at different Er2O3 concentration on 532 nm excitation (lines are drawn as guides for the eyes). (For interpretation of the references to colour this figure legend, the reader is referred to the web version of this article.)
The intensities of different upconversion bands are found to vary with Er3+ ion concentration in different manner. For Er3+ concentration lower than 1.5 mol%, the luminescence can be interpreted very simply in terms of competition between radiative and non-radiative relaxations without taking account of possible energy transfer
It means that an ion in the 4S3/2 level decays to the 4F9/2 level and the excess excitation energy is taken up by another ion in the 4I9/2 level, which is raised to the 4F9/2 level. With inhomogeneous broadening and Stark splitting of the levels in question, there is sufficient energy coincidence between the two states of the above equation. The inter-ionic distance and experimental evidence of decrease in the slope between rare-earth concentration and incident photon intensity for the red emission suggests both the energy transfer processes. The inter ionic distance between two ˚ for 0.5 mol% of rare-earth Er3+ ions is estimated as 19 A ˚ doping and as 12 A for 2.0 mol% of rare-earth doping. So energy transfer for 0.5 mol% rare earth would be at least 10
K. Kumar et al. / Journal of Non-Crystalline Solids 353 (2007) 1383–1387
times smaller than for 2.0 mol%. The slopes for the red emission are found to be 1.69 and 1.53 for 0.5 mol% and 2.0 mol% rare-earth doped glasses, respectively. The slope value 1.53 suggests that two photons are involved in populating the 4F9/2 level, which can be explained by considering the above energy transfer process. Another evidence of energy transfer is that the change in value of n in relation Iup / Pn. In Fig. 5, slope (n) is found to decrease with the increase in Er3+ concentration for the red emission band (shown with 532 nm excitation). This decrease in the value of n occurs due to the energy transfer process. The violet emission also increases rapidly with the Er3+concentration, but since this emission appears even at low (0.5 mol%) Er3+ ion concentrations, it is concluded that 2H9/2 level is populated through excited state absorption (absorption of an incident photon by an ion) in the 4F9/2 level. With 532 nm excitation, again both ESA and ET mechanisms are responsible for the observed fluorescence. In the framework of ESA mechanism, an excited Er3+ ion in the 4 S3/2 level may absorb a second incident photon and is excited to the 2D5/2 level. The ions excited to this level decay non-radiatively to the lower lying 2P3/2, 4G11/2 and 2 H9/2 levels and then radiatively to the ground level. It is also possible that ions in the 4S3/2 level relax non-radiatively to the lower lying 4F9/2 and 4I9/2 levels and then absorb the second photon. On the other hand, two Er3+ ions both excited to the 4S3/2 level may share their energies in such a way that one is excited to the 2D5/2 level and the other goes to the ground level. 4.2. Effect of temperature on luminescence properties of Er3+ The dependence of fluorescence intensity on the temperature has been studied in the range 300–490 K. It is observed that the fluorescence intensities of the 380, 550 and 658 nm emissions decrease with an increase in temperature, the rate of decrease being smallest for the peak at 658 nm. One can conclude that relaxation rates for the corresponding upper state of the different transitions change with temperature in dissimilar fashion. The intensities of the two close lying fluorescence lines at 525 and 550 nm vary with temperature at different rates. The 525 nm intensity varies slowly with the temperature since this state is thermally coupled to the 4S3/2 level. Measuring the fluorescence intensity ratio of these two emitting states the temperature of the glass can be calculated using the formula [21,22]: N 2 I 2j g2 r2j x2j DE ðFIRÞ ¼ ¼ ¼ ; exp KT N 1 I 1j g1 r1j x1j where Ni is the number of excited ions, Iij is the fluorescence intensity from the two states, gi is the degeneracy of the states, rij is the emission cross section and xij is the transition probability. DE is the separation between the two states. A preliminary study on the Er3+-doped tellurite glass indicates that the system works well as an optical temperature sensor within the 300–500 K temperature range.
1387
5. Conclusions Er3+-doped tellurite glasses containing alkali oxide as modifier have been studied. Excitation of the Na2O– TeO2:Er3+ glass with 797 and 532 nm wavelengths results in emissions at various wavelengths ranging from NIR to UV region. The green emission has the maximum fluorescence intensity and is visible even at low NIR pump power (<30 mW), which shows an efficient upconversion in this glass host. The temperature dependence profile of the two thermally coupled levels shows that they can be used for estimating the temperature. Acknowledgements Authors are grateful to CSIR and DST New Delhi for financial assistance. One of the authors (Mr Kaushal Kumar) would like to thank M/S Laser Science Bombay and The Laser Spectroscopy Society of India for providing a Scholarship. References [1] B.P. Scott, F. Zhao, R.S.F. Chang, N. Djeu, Opt. Lett. 18 (1993) 113. [2] T. Hebert, R. Wannemacher, W. Lenth, R.M. Macfarlane, Appl. Phys. Lett. 57 (1990) 1727. [3] P. Xie, T.R. Gosnell, Opt. Lett. 20 (1995) 1014. [4] M. Yamada, H. Ono, T. Kanamori, S. Sudo, Y. Ohishi, Electron. Lett. 33 (1997) 710. [5] Y. Ohishi, A. Mori, M. Yamada, H. Ono, Y. Nishida, K. Oikawa, Opt. Lett. 23 (1998) 274. [6] M. Tsuda, K. Sousa, H. Inoue, S. Inoue, A. Makishima, J. Appl. Phys. 85 (1999) 29. [7] P.V. dos Santos, E.A. Gouveia, M.T. de Araujo, A.S. Liouveia-Neto, A.S.B. Sombra, J.A. Medeiros Neto, Appl. Phys. Lett. 74 (1999) 3607. [8] S.A. Pollack, D.B. Chang, R.A. McFarlane, H. Jenssen, J. Appl. Phys. 67 (1990) 648. [9] M.P. Hehlen, N.J. Cockroft, T.R. Gosnell, A.J. Bruce, Phys. Rev. B 56 (1997) 9302. [10] S.A. Pollack, D.B. Chang, J. Appl. Phys. 64 (1988) 2885. [11] K. Kumar, S.B. Rai, D.K. Rai, Solid State Commun. 139 (2006) 363. [12] V. Aruna, N. Sooraj Hussain, S. Buddhudu, Mater. Res. Bull. 33 (1998) 149. [13] S. Tanabe, K. Hirao, N. Soga, J. Non-Cryst. Solids 122 (1990) 79. [14] M.J. Weber, J.D. Myres, D.H. Bluchburn, J. Appl. Phys. 52 (1981) 2944. [15] J.S. Wang, E.M. Vogel, E. Snitzer, Opt. Mater. 3 (1993) 187. [16] G.S. Maciel, C.B. de Araujo, Y. Messaddeq, M.A. Aegerter, Phys. Rev. B 55 (1997) 6335. [17] F.E. Auzel, Proc. IEEE 61 (1973) 758. [18] M. Pollnau, D.R. Gamelin, S.R. Luthi, H.U. Gudel, M.P. Hehlen, Phys. Rev. B 61 (2001) 3357. [19] M.P. Hehlen, G. Frei, H.U. Gudel, Phys. Rev. B 60 (1994) 16265. [20] L. Petit, T. Cardinal, J.J. Videau, G. Le Flem, Y. Guyot, G. Boulon, M. Couzi, T. Buffeteau, J. Non-Cryst. Solids 298 (2002) 76. [21] S.A. Wade, S.F. Collins, G.W. Baxter, J. Appl. Phys. 94 (2003) 4743. [22] S.F. Collins, G.W. Baxter, S.A. Wade, T. Sun, K.T.V. Grattan, Z.Y. Zhang, A.W. Palmer, J. Appl. Phys. 84 (1998) 4649. [23] Z. Pan, S.H. Morgan, K. Dyer, A. Ueda, H. Liu, J. Appl. Phys. 79 (1996) 8906.