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Role of Yb3+ and Tm3+ ions in upconversion emission of Tb3+ under 798 and 980 nm laser excitations in Tb3+–Tm3+–Yb3+ doped tellurite glass
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Optics Communications 281 (2008) 3547–3552 www.elsevier.com/locate/optcom
Role of Yb3+ and Tm3+ ions in upconversion emission of Tb3+ under 798 and 980 nm laser excitations in Tb3+–Tm3+–Yb3+ doped tellurite glass Neeraj Kumar Giri, Anant Kumar Singh, D.K. Rai, S.B. Rai * Department of Physics, Laser and Spectroscopy Laboratory, B.H.U., Varanasi 221 005, India Received 28 May 2007; received in revised form 6 March 2008; accepted 9 March 2008
Abstract Upconversion emission and energy transfer processes in singly, doubly and triply doped tellurite glasses have been studied under 798 and 980 nm laser excitations. Emissions have been observed at 482, 544, 584, 655 nm and at 477, 655, 698, 800 nm corresponding to Tb3+: 5D4 ? 7F6, 7F5, 7F4, 7FJ (J = 0, 1, 2, 3) and Tm3+: 1G4 ? 3H6, 1G4 ? 3F4, 3F3 ? 3H6, 3H4 ? 3H6 transitions, respectively. Among Tm3+, Yb3+and Tb3+ ions only Tm3+ has a ground state absorption at 798 nm excitation due to 3H4 3H6 transition. For 980 nm excitation only Yb3+ can absorb the incident radiation. However, for both types of excitations, emission from all the three ions Tb, Yb and Tm has been observed. Possible mechanisms are proposed as follows: under 798 nm excitation Tm3+ ions are excited which excite Yb3+ ions through energy transfer. Finally ‘‘cooperative energy transfer” from a pair of Yb3+ ions to Tm3+ and Tb3+ ions takes place. Under 980 nm excitation Yb3+ ions absorb the incident energy and excite Tm3+ and Tb3+ ions via cooperative energy transfer. Variation of emission intensity with the ion concentrations of Yb3+, Tm3+ and Tb3+ has been studied. The lifetime of the 1G4 level has also been measured. Ó 2008 Elsevier B.V. All rights reserved. PACS: 74.25.Gz; 42.70.Hj; 73.61.Jc; 76.30.Kg; 42.70.Ce Keywords: Cooperative energy transfer; Upconversion; Fluorescence; TeO2; Energy transfer; Lifetime
1. Introduction It is well known that glasses/crystals doped with certain triply ionized rare earth ions can convert near infrared radiation into visible or UV-light through the process called upconversion [1,2]. Rare earth doped glasses have shown potential for applications in various aspects of photonics [3–6]. Tellurite glasses have good optical quality and are stable against atmospheric moisture [7]. They have a wider transmission range (0.35–5 lm) compared
*
Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 2369889. E-mail address: [email protected] (S.B. Rai).
0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.03.016
to silicate glasses. They have good stability against corrosion resistance and in some sense are superior to fluoride glasses [8]. In addition, they exhibit the lowest average vibrational energy (about 780 cm1) among oxide glass formers and have a low process temperature [7,9]. Tellurite glasses also possess a high refractive index (2.0) which increases the local field correction at the doped lanthanide ion site leading to an enhancement of the radiative decay and to a lowering of the non-radiative relaxation rates. Lanthanide ions exhibit high solubility in these glasses allowing the incorporation of high concentration. These glasses are non-hygroscopic as well as highly stable against crystallization [7]. Because of the above merits, tellurite based glasses have the potential as optical amplifiers for telecommunication windows at
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1.3 and 1.5 lm, frequency upconverters, colour displays, high density optical data reading and storage, biomedical diagnostics, infrared laser viewers and indicators [7–12]. It has been seen in a number of cases that addition of a suitable second dopant to the glass (or crystalline) host often results in increased intensity of the desired emission. Even entirely new emission lines have also been observed. The energy transfer mechanism is considered as responsible for this observation and greater efficiency of phosphors and lasing has been achieved in many cases [13–16]. In the present paper we report the results of our studies on a tellurite glass, which has been doped with three different rare earth ions. Trivalent ytterbium ion (Yb3+) has 2F as the ground state. The doublet splitting of this state is approximately 10,000 cm1 with the 2F5/2 component lying above the ground 2F7/2 state. The radiative lifetime of the 2F5/2 being 0.28 ms, Yb3+ ion can act as a donor or alternately as an energy transfer bridging ion between a donor and an acceptor ion. For energy transfer between Nd3+ and Tb3+ (or Tm3+) ions, Yb3+ ion is known to act as an energy transfer bridging ion after pumping by NIR radiation [17–20]. In fact Courrol et al. [21] reported an enhancement of blue upconversion emission in YLiF4 crystal doped with Yb3+, Tm3+ and Nd3+ through this process. We have now investigated a similar process in a tellurite glass doped with Tm3+, Tb3+ and Yb3+ ions. In order to understand the role of Yb3+ as an excitation energy donor or as an energy transfer bridging ion, the singly and doubly doped glasses have also been investigated. Two different near infrared radiations, one at 798 nm and the other at 980 nm have been used for excitation. It is found that a TeO2 glass doped with Yb3+ and Tb3+ does not give any emission from Tb3+ ion in the visible region under 798 nm excitation, but the addition of Tm3+ to the glass causes such an emission to appear. Also a TeO2 glass doped only with Tb3+ does not result in any visible emission under 980 nm excitation but the addition of Yb3+ gives intense bands due to Tb3+. 2. Experimental Glasses were prepared from TeO2, Li2CO3 and different rare earth oxides (Alfa Aesar, 99.9%).We used the following compositions [22,23]: ð71-x-y-zÞTeO2 þ 24Li2 CO3 þ xTm2 O3 þ yYb2 O3 þ zTb4 O7 Various values of the fractions x, y and z, e.g. x = 0, 0.1, 0.2 and 0.5; y = 0, 2.0 and 4.0 and z = 0, 0.1, 0.2, 0.5, 1.0 and 1.5 mol% were used. The ingredients in proper proportion were mixed well and melted in a platinum crucible at 800–850 °C in an electric furnace. The melt was then quenched by pouring it on a rectangular iron cast kept at 250 °C. It was then cooled slowly to room temperature to get a properly annealed glass of good optical quality. Several pieces of glass for each composition were prepared for further measurements. The glasses were cleaned and pol-
ished in order to obtain samples suitable for optical measurements. A Ti–Sapphire laser pumped by the second harmonic (532 nm) radiation from an Nd:YVO4 laser was used as a source of 798 nm radiation and a diode laser was used for 980 nm, respectively. The fluorescence was collected at right angle to the incident beam and was dispersed using a iHR-320 spectrometer (resolution 0.06 nm) attached with M1424 model photomultiplier tube. The dependence of the upconversion signal on input laser power was also investigated. Lifetime of the 1G4 level of Tm3+ has been measured in differently doped glasses. 3. Results and discussion 3.1. Excitation with 798 nm laser radiation The 798 nm laser radiation is in resonance with the 3 H4 H6 absorption of Tm3+ ion. On excitation with this radiation a Tm3+ doped TeO2 glass is seen to emit a weak upconverted emission at 477 nm and a relatively stronger broad red emission at 698 nm (see Fig. 1A). These upconversion emissions have been earlier explained as involving two-step excitation of Tm3+ ion to the 1G4 state [23]. An increase in the concentration of Tm3+ beyond 0.5 mol% is seen to decrease the fluorescence intensity. On the other hand, a TeO2 glass doped with Yb3+ alone shows no fluorescence on excitation by 798 nm radiation. When both Yb3+ and Tm3+ ions are present in the glass, on excitation with 798 nm blue emission gains in intensity while the intensity of the red emission (698 nm) is reduced. In addition, another red emission at 655 nm is seen (Fig. 1B). It seems likely that the incident 798 nm radiation excites Tm3+ ions to the 3H4 level which then transfer the excitation energy to Yb3+ ions exciting these to the 2F5/2 level. The excess energy (2000 cm1) is most likely taken up by the glass host. We presume that the energy transfer mechanism is efficient enough to excite a substantial number of Yb3+ ions. At the concentration of Yb3+ used in our work (4.0 mol%) several pairs of excited Yb3+ ions lose their total excitation energy in a cooperative manner in either of the two ways which are as follows: (1) Both ions of the pair can simultaneously lose their energy emitting a single photon of energy equal to the combined excitation energy (20,964 cm1) or (2) these together excite a Tm3+ ion to its 1G4 level which decays radiatively. Since both mechanisms release photons of almost the same energy (see Fig. 2), the blue emission is reinforced. The 1G4 level of Tm3+ can also decay radiatively to the 3F4 level, and hence an additional emission at 655 nm is also seen when both Tm3+ and Yb3+ are present in the glass (this peak was not seen when Yb3+ was not present since in that situation the population in 1G4 level was contributed in a small extent only through the two-step excitation process). The additional peak at 655 nm is relatively sharp and has a larger intensity than the 698 nm broad peak. As the concentration of Yb3+ ions in the glass is increased a large number of excited Yb3+ ion pairs can be formed since 3
N.K. Giri et al. / Optics Communications 281 (2008) 3547–3552
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Fig. 1. Upconversion spectrum of Tm3+, Yb3+, Tb3+ doped in TeO2 glass under 798 nm excitation.
Fig. 2. Energy level scheme presenting different excitation mechanisms responsible for observed upconversion emissions.
the collective excitation energy of a pair of Yb3+ ions in the 2 F5/2 state is almost resonant with the excitation energy needed for raising a ground state Tm3+ ion to the 1G4 level. The intensity of different transitions from 1G4 level of Tm3+ would increase with the increase of Yb3+ ion’s concentration. This is indeed observed. The dependence of
fluorescence intensity of Tm3+ on Yb3+ concentration is discussed in our previous work [23]. A tellurite glass containing only Tb3+ ions gives no visible fluorescence on 798 nm excitation. If Tm3+ ions are also present (with Tb3+) in the glass, one observes the upconverted emission only from Tm3+ (Fig. 1C). Thus
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addition of Tm3+ to a glass containing Tb3+ does not influence the Tb3+ ions. However, when Tm3+, Tb3+ and Yb3+ all three are present together in the glass, one sees emission from all three ions. The upconversion fluorescence spectrum of a glass containing 0.5 mol% Tm2O3 + 4 mol% Yb2O3 + 0.5 mol% Tb4O7 under 798 nm excitation is shown in Fig. 1D and emissions at 477, 482, 522, 544, 584, 655 and 698 nm are seen. The emissions at 482, 544, 584 and 655 nm are ascribed to Tb3+ from the transitions 5 D4 ? 7F6, 5D4 ? 7F5, 5D4 ? 7F4, and 5D4 ? 7FJ (J = 0, 1, 2, 3), respectively. The peaks at 477, 655 and 698 nm are ascribed to Tm3+ involving the 1G4 ? 3H6, 1 G4 ? 3F4, 3F3 ? 3H6 transitions, respectively. The peak at 655 nm thus involves the superposition of transitions due to both Tm3+ and Tb3+. Similarly, the peak near 480 nm (477 and 482 nm) is due to the superposition of emission due to a cooperative decay of an excited ion pair of Yb3+, 1G4 ? 3H6 transition of Tm3+ (see the earlier paragraph) as well as due to 5D4 ? 7F6 transition of Tb3+. The visible upconversion emissions ascribed to Tb3+ thus must involve excitation through energy transfer from Tm3+– Yb3+ to Tb3+. Our Tb4O7 sample has traces of impurity of Er3+ and in the presence of Yb3+ + Tb3+ bands at 522 and 544 nm due to Er3+ are also seen. The band at 544 nm has contribution due to both Er3+ and Tb3+. In order to understand the appearance of Tb3+ upconversion luminescence in Tm3+–Yb3+–Tb3+ codoped glass, one must consider at the outset the case when only Tm3+ and Tb3+ ions are present. As mentioned earlier in this case red and blue emissions due to Tm3+ are seen (no emission from Tb3+). However, if Yb3+ is also present in it, intense blue, green, yellow, and red emissions are observed (see Fig. 1D). It appears that through energy transfer from Tm3+, excited Yb3+ ion pairs are created. These Yb3+ ion pairs in turn cooperatively transfer their excitation energy to Tm3+ exciting it to 1G4 level as well as to Tb3+ ions to excite them to 5D4 level. It is this excited 5D4 level which yields the green fluorescence at 544 nm while the excited Tm3+ ions in the 1G4 state are responsible for the 655 and 477 nm emissions. If the Yb and Tm ions concentrations are kept fixed and Tb concentration is increased, we observe a slight decrease in the intensities of the emissions from Tm3+ ions especially in its red emission (655 nm). The intensities of the Tb3+ emissions from its 5 D4 state are increased till Tb3+ concentration reaches 0.5 mol% above which the intensities show a decrease. For Tb3+ concentrations greater than 1.5 mol%, Tb3+ emissions completely disappear. An increase in the concentration of Yb3+ (keeping Tb3+ and Tm3+ concentrations fixed) increases the intensity of emission from Tm3+ and Tb3+ as long as Yb3+ concentration remains up to 4 mol% beyond which it is quenched. Dependence of the intensity of the blue upconversion emission upon the excitation power has also been examined, and a quadratic power dependence is seen (Fig. 3). This indicates that two IR pump photons are needed in the blue upconversion emission. Of the two possible ways
Fig. 3. ln I versus ln P plot for 1G4 ? 3H6 transition of Tm3+.
of excitation of 1G4 level of Tm3+, cooperative energy transfer from a pair of excited Yb3+ ions to a single Tm3+ ion seems more likely as in this case there is almost complete energy matching. In fact, twice the energy of the Yb3+: 2F5/2 ? 2F7/2 transition is equal to the energy 3 corresponding to the Tm3+: 1G4 H6 transition. A similar two photon dependence has been observed for the green emission (5D4 ? 7F5 transition) of Tb3+ also. It is worthwhile to note that the excitation energy for the 5D4 level of Tb3+ also matches with the energy of a pair of excited Yb3+ ions in the 2F5/2 state. 3.2. Excitation with 980 nm laser radiation The 980 nm radiation can resonantly excite the 2F5/2 level of Yb3+ from its ground state. It is interesting to note that a tellurite glass containing Yb3+ ions alone gives intense blue emission at 480 nm (Fig. 4A). This emission has been explained as being due to cooperative emission from a pair of excited Yb3+ ions. When Yb3+ is present along with either Tm3+ or Tb3+, the doped glass gives blue and red emissions or blue, green and red emissions. This has been explained as being again due to energy transfer from a pair of excited Yb3+ ions to a Tm3+ or a Tb3+ ion [23–25]. The intensities of the emitted bands does depend on the concentrations of the various ions. The glass containing all three ions viz. Tm3+, Yb3+and Tb3+ when excited with 980 nm radiation, emissions centered at 477, 522, 544 and 655 nm (Fig. 4E) are seen. Emission at 544 nm is ascribed to Tb3+: 5D4 ? 7F5 and Er3+: 2H11/2 ? 4I15/2 transitions. Emission at 522 nm corresponds to 4S3/2 ? 4I15/2 transition of erbium. Since all three ions, Tm3+, Tb3+ and Yb3+ give upconverted emission in the blue region at wavelengths close to 480 nm it is likely that the blue emission observed in the present case comprises a superposition of Tm3+: 1G4 ? 3H6, of Tb: 5 D4 ? 7F6 along with cooperative emission from a pair
N.K. Giri et al. / Optics Communications 281 (2008) 3547–3552
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Fig. 4. Upconversion spectrum of Tm3+, Yb3+, Tb3+ doped in TeO2 glass under 980 nm excitation.
of excited Yb3+ (2F5/2 ? 2F7/2). Similarly the red emission at 655 nm involves the superposition of the transitions, Tm3+: 1G4 ? 3F4 and Tb3+: 5D4 ? 7FJ (J = 0, 1, 2, 3), as even in the absence of Tm3+ when a Tb–Yb codoped sample is excited with the 980 nm radiation one observes an emission at 655 nm (see Fig. 4C). The lifetime of the 1G4 level of Tm3+ as a function of Tb3+ concentrations keeping Yb3+ and Tm3+concentrations fixed at 4.0 mol% and 0.5 mol%, respectively, has been measured
exciting at 798 and 980 nm radiations. Time vs. ln (intensity) plot for lifetime of 1G4 level in tellurite glass containing 0.5 mol% Tm2O3 + 4 mol% Yb2O3 + 0.5 mol% Tb4O7 with 798 and 980 nm is shown in Fig. 5. A comparison of the lifetime of the 1G4 level of Tm3+ under two different excitations reveals that the lifetime of the 1G4 level of Tm3+ under 798 nm excitation is appreciably larger than under 980 nm excitation. This is perhaps due to the fact that for 798 nm excitation two steps (viz. Tm3+(3H4) ? Yb3+(2F5/2) followed by 2Yb3+(2F5/2) ? Tm3+(1G4)) are required to populate this level where as only one step (Yb3+(2F5/ 3+ 2 3+ 1 2) + Yb ( F5/2) ? Tm ( G4)) is required under 980 nm excitation. Also a decrease in the lifetime of the 1G4 level of Tm3+ (at a fixed concentration of Tm3+) on increasing the concentration of Tb3+ is seen. This is probably due to the increased depopulation of the 1G4 level by Tb3+ ions (transfer of excitation energy from Tm3+ to Tb3+ 5D4 level). This also agrees with the observed decrease in the fluorescence intensity of transitions involving 1G4 level of Tm3+ when Tb3+ concentration is increased (see Table 1).
Table 1 Lifetime of 1G4 level of Tm3+ on excitation with different laser lines Excitation wavelength (nm) Fig. 5. Fluorescence decay of 1G4 ? 3H6 transition of Tm3+ for Tm3+ (0.5 mol%) + Tb3+ (0.5 mol%) + Yb3+ (4.0 mol%) doped in TeO2 glass for (A) 798 nm excitation (B) 980 nm excitation.
798 980
Lifetime (ms) of 1G4 level of Tm3+ in different conditions Tm = 0.5, Yb = 4.0, Tb = 0.5 mol%
Tm = 0.5, Yb = 4.0, Tb = 1.0 mol%
Tm = 0.5, Yb = 4.0, Tb = 1.5 mol%
0.94 0.57
0.80 0.44
0.80 0.44
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4. Conclusion Upconversion emission properties of a tellurite glass doped simultaneously with Tm3+, Yb3+and Tb3+ ions has been examined under 798 and 980 nm excitation. It is found that under 798 nm excitation upconversion in Tb3+ from the 5D4 level can be explained as involving Tm3+ as an energy donor while Yb3+ acts as an intermediate. For 980 nm excitation, Yb3+ ions directly absorb the pump radiation and pairs of Yb3+ ions cooperatively transfer the excitation energy to Tm3+ and Tb3+ resulting in different upconversion emissions from both Tm3+ and Tb3+. Lifetime of the 1G4 level of Tm3+has also been studied under 798 and 980 nm excitations and the observed changes are explained. Acknowledgements Authors are grateful to CSIR and DST New Delhi for financial assistance. One of us (D.K.R.) gratefully acknowledges the Grant of Emeritus Fellowship by AICTE, New Delhi. References [1] R. Balda, A.J. Garcia-Adeva, J. Fernandez, J.M.F. Navarro, J. Opt. Soc. Am. B 21 (2004) 744. [2] R. Balda, M. Sanz, A. Mendiroz, J. Fernandez, L.S. Griscom, J.L. Adam, Phys. Rev. B 64 (2001) 144101. [3] D. Lande, S.S. Orlov, A. Akella, L. Hesselink, R.R. Neurgaonkar, Opt. Lett. 22 (1997) 1722. [4] P. Xie, T.R. Gosnell, Opt. Lett. 20 (1995) 1014. [5] S.F. Collins, G.W. Baxter, S.A. Wade, T. Sun, K.T.V. Grattan, Z.Y. Zhang, A.M. Palmer, J. Appl. Phys. 84 (1998) 4649.
[6] E. Downing, L. Hesselink, J. Ralston, R. MacFarlane, Science 273 (1996) 1185. [7] H.T. Amorim, M.V.D. Vermelho, A.S. Gouveia-Neto, F.C. Cassanjees, S.J.L. Ribeiro, Y. Messaddeq, J. Alloys Compd. 346 (2002) 282. [8] E.R. Taylor, N.N. Li, N.P. Sessions, H. Buerger, J. Appl. Phys. 92 (2002) 112. [9] R. Rolli, M. Montagana, S. Chaussedent, A. Monteil, V.K. Tikhomirov, M. Ferrari, Opt. Mater. 21 (2003) 743. [10] S. Blanchandin, P. Thomas, P. Marchet, J.C. Champarnand-Mesjard, B. Frit, J. Alloys Compd. 347 (2002) 206. [11] P.V. dos Santos, M.V.D. Vermelho, E.A. Gouveia, M.T. de Araujo, A.S. Gouveia-Neto, F.C. Cassanjees, S.J.L. Ribeiro, Y. Messaddeq, J. Alloys Compd. 344 (2002) 304. [12] P. Charton, P. Armand, J. Non-Cryst. Solids 316 (2003) 189. [13] Y. Mita, M. Togashi, M. Yamamoto, J. Lumin. 87–89 (2000) 1026. [14] D.F. de Sousa, J.A. Sampaio, L.A.O. Nunes, M.L. Baesso, A.C. Bento, L.C.M. Miranda, Phys. Rev. B 62 (2000) 3176. [15] X. Zhang, J. Yuan, X. Liu, J.P. Jourat, G. Mary, J. Appl. Phys. 82 (1997) 3987. [16] A. Rapaport, J. Milliez, M. Bass, A. Cassanho, H. Jenssen, J. Display Technol. 2 (2006) 68. [17] J. Qiu, M. Shojiya, R. Kanno, Y. Kawamoto, M. Takahashi, J. Phys.: Condens. Matter 10 (1998) 11095. [18] J. Qiu, M. Shojiya, Y. Kawamoto, J. Appl. Phys. 86 (1999) 909. [19] J. Qiu, Y. Kawamoto, J. Appl. Phys. 91 (2002) 954. [20] J. Qiu, Y. Kawamoto, J. Zhang, J. Appl. Phys. 92 (2002) 5163. [21] Lilia C. Courrol, I.M. Ranieri, L.V. Gomes Tarelho, S.L. Baldochi, L. Gomes, N.D. Vieira Junior, J. Appl. Phys. 98 (2005) 113504. [22] Neeraj Kumar Giri, A.K. Singh, S.B. Rai, Spectrochim. Acta A 68 (2007) 117. [23] Neeraj Kumar Giri, A.K. Singh, S.B. Rai, J. Appl. Phys. 101 (2007) 033102. [24] I.R. Martin, A.C. Yanes, J.M. Ramos, M.E. Torres, V.D. Rodriguez, J. Appl. Phys. 89 (2001) 2520. [25] M.V.D. Vermelho, P.V. dos Santos, M.T. de Araujo, A.S. GouveiaNeto, F.C. Cassanjes, S.J.L. Ribeiro, Y. Messaddeq, J. Lumin. 102– 103 (2003) 762.
Optics Communications 281 (2008) 3547–3552 www.elsevier.com/locate/optcom
Role of Yb3+ and Tm3+ ions in upconversion emission of Tb3+ under 798 and 980 nm laser excitations in Tb3+–Tm3+–Yb3+ doped tellurite glass Neeraj Kumar Giri, Anant Kumar Singh, D.K. Rai, S.B. Rai * Department of Physics, Laser and Spectroscopy Laboratory, B.H.U., Varanasi 221 005, India Received 28 May 2007; received in revised form 6 March 2008; accepted 9 March 2008
Abstract Upconversion emission and energy transfer processes in singly, doubly and triply doped tellurite glasses have been studied under 798 and 980 nm laser excitations. Emissions have been observed at 482, 544, 584, 655 nm and at 477, 655, 698, 800 nm corresponding to Tb3+: 5D4 ? 7F6, 7F5, 7F4, 7FJ (J = 0, 1, 2, 3) and Tm3+: 1G4 ? 3H6, 1G4 ? 3F4, 3F3 ? 3H6, 3H4 ? 3H6 transitions, respectively. Among Tm3+, Yb3+and Tb3+ ions only Tm3+ has a ground state absorption at 798 nm excitation due to 3H4 3H6 transition. For 980 nm excitation only Yb3+ can absorb the incident radiation. However, for both types of excitations, emission from all the three ions Tb, Yb and Tm has been observed. Possible mechanisms are proposed as follows: under 798 nm excitation Tm3+ ions are excited which excite Yb3+ ions through energy transfer. Finally ‘‘cooperative energy transfer” from a pair of Yb3+ ions to Tm3+ and Tb3+ ions takes place. Under 980 nm excitation Yb3+ ions absorb the incident energy and excite Tm3+ and Tb3+ ions via cooperative energy transfer. Variation of emission intensity with the ion concentrations of Yb3+, Tm3+ and Tb3+ has been studied. The lifetime of the 1G4 level has also been measured. Ó 2008 Elsevier B.V. All rights reserved. PACS: 74.25.Gz; 42.70.Hj; 73.61.Jc; 76.30.Kg; 42.70.Ce Keywords: Cooperative energy transfer; Upconversion; Fluorescence; TeO2; Energy transfer; Lifetime
1. Introduction It is well known that glasses/crystals doped with certain triply ionized rare earth ions can convert near infrared radiation into visible or UV-light through the process called upconversion [1,2]. Rare earth doped glasses have shown potential for applications in various aspects of photonics [3–6]. Tellurite glasses have good optical quality and are stable against atmospheric moisture [7]. They have a wider transmission range (0.35–5 lm) compared
*
Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 2369889. E-mail address: [email protected] (S.B. Rai).
0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.03.016
to silicate glasses. They have good stability against corrosion resistance and in some sense are superior to fluoride glasses [8]. In addition, they exhibit the lowest average vibrational energy (about 780 cm1) among oxide glass formers and have a low process temperature [7,9]. Tellurite glasses also possess a high refractive index (2.0) which increases the local field correction at the doped lanthanide ion site leading to an enhancement of the radiative decay and to a lowering of the non-radiative relaxation rates. Lanthanide ions exhibit high solubility in these glasses allowing the incorporation of high concentration. These glasses are non-hygroscopic as well as highly stable against crystallization [7]. Because of the above merits, tellurite based glasses have the potential as optical amplifiers for telecommunication windows at
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1.3 and 1.5 lm, frequency upconverters, colour displays, high density optical data reading and storage, biomedical diagnostics, infrared laser viewers and indicators [7–12]. It has been seen in a number of cases that addition of a suitable second dopant to the glass (or crystalline) host often results in increased intensity of the desired emission. Even entirely new emission lines have also been observed. The energy transfer mechanism is considered as responsible for this observation and greater efficiency of phosphors and lasing has been achieved in many cases [13–16]. In the present paper we report the results of our studies on a tellurite glass, which has been doped with three different rare earth ions. Trivalent ytterbium ion (Yb3+) has 2F as the ground state. The doublet splitting of this state is approximately 10,000 cm1 with the 2F5/2 component lying above the ground 2F7/2 state. The radiative lifetime of the 2F5/2 being 0.28 ms, Yb3+ ion can act as a donor or alternately as an energy transfer bridging ion between a donor and an acceptor ion. For energy transfer between Nd3+ and Tb3+ (or Tm3+) ions, Yb3+ ion is known to act as an energy transfer bridging ion after pumping by NIR radiation [17–20]. In fact Courrol et al. [21] reported an enhancement of blue upconversion emission in YLiF4 crystal doped with Yb3+, Tm3+ and Nd3+ through this process. We have now investigated a similar process in a tellurite glass doped with Tm3+, Tb3+ and Yb3+ ions. In order to understand the role of Yb3+ as an excitation energy donor or as an energy transfer bridging ion, the singly and doubly doped glasses have also been investigated. Two different near infrared radiations, one at 798 nm and the other at 980 nm have been used for excitation. It is found that a TeO2 glass doped with Yb3+ and Tb3+ does not give any emission from Tb3+ ion in the visible region under 798 nm excitation, but the addition of Tm3+ to the glass causes such an emission to appear. Also a TeO2 glass doped only with Tb3+ does not result in any visible emission under 980 nm excitation but the addition of Yb3+ gives intense bands due to Tb3+. 2. Experimental Glasses were prepared from TeO2, Li2CO3 and different rare earth oxides (Alfa Aesar, 99.9%).We used the following compositions [22,23]: ð71-x-y-zÞTeO2 þ 24Li2 CO3 þ xTm2 O3 þ yYb2 O3 þ zTb4 O7 Various values of the fractions x, y and z, e.g. x = 0, 0.1, 0.2 and 0.5; y = 0, 2.0 and 4.0 and z = 0, 0.1, 0.2, 0.5, 1.0 and 1.5 mol% were used. The ingredients in proper proportion were mixed well and melted in a platinum crucible at 800–850 °C in an electric furnace. The melt was then quenched by pouring it on a rectangular iron cast kept at 250 °C. It was then cooled slowly to room temperature to get a properly annealed glass of good optical quality. Several pieces of glass for each composition were prepared for further measurements. The glasses were cleaned and pol-
ished in order to obtain samples suitable for optical measurements. A Ti–Sapphire laser pumped by the second harmonic (532 nm) radiation from an Nd:YVO4 laser was used as a source of 798 nm radiation and a diode laser was used for 980 nm, respectively. The fluorescence was collected at right angle to the incident beam and was dispersed using a iHR-320 spectrometer (resolution 0.06 nm) attached with M1424 model photomultiplier tube. The dependence of the upconversion signal on input laser power was also investigated. Lifetime of the 1G4 level of Tm3+ has been measured in differently doped glasses. 3. Results and discussion 3.1. Excitation with 798 nm laser radiation The 798 nm laser radiation is in resonance with the 3 H4 H6 absorption of Tm3+ ion. On excitation with this radiation a Tm3+ doped TeO2 glass is seen to emit a weak upconverted emission at 477 nm and a relatively stronger broad red emission at 698 nm (see Fig. 1A). These upconversion emissions have been earlier explained as involving two-step excitation of Tm3+ ion to the 1G4 state [23]. An increase in the concentration of Tm3+ beyond 0.5 mol% is seen to decrease the fluorescence intensity. On the other hand, a TeO2 glass doped with Yb3+ alone shows no fluorescence on excitation by 798 nm radiation. When both Yb3+ and Tm3+ ions are present in the glass, on excitation with 798 nm blue emission gains in intensity while the intensity of the red emission (698 nm) is reduced. In addition, another red emission at 655 nm is seen (Fig. 1B). It seems likely that the incident 798 nm radiation excites Tm3+ ions to the 3H4 level which then transfer the excitation energy to Yb3+ ions exciting these to the 2F5/2 level. The excess energy (2000 cm1) is most likely taken up by the glass host. We presume that the energy transfer mechanism is efficient enough to excite a substantial number of Yb3+ ions. At the concentration of Yb3+ used in our work (4.0 mol%) several pairs of excited Yb3+ ions lose their total excitation energy in a cooperative manner in either of the two ways which are as follows: (1) Both ions of the pair can simultaneously lose their energy emitting a single photon of energy equal to the combined excitation energy (20,964 cm1) or (2) these together excite a Tm3+ ion to its 1G4 level which decays radiatively. Since both mechanisms release photons of almost the same energy (see Fig. 2), the blue emission is reinforced. The 1G4 level of Tm3+ can also decay radiatively to the 3F4 level, and hence an additional emission at 655 nm is also seen when both Tm3+ and Yb3+ are present in the glass (this peak was not seen when Yb3+ was not present since in that situation the population in 1G4 level was contributed in a small extent only through the two-step excitation process). The additional peak at 655 nm is relatively sharp and has a larger intensity than the 698 nm broad peak. As the concentration of Yb3+ ions in the glass is increased a large number of excited Yb3+ ion pairs can be formed since 3
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Fig. 1. Upconversion spectrum of Tm3+, Yb3+, Tb3+ doped in TeO2 glass under 798 nm excitation.
Fig. 2. Energy level scheme presenting different excitation mechanisms responsible for observed upconversion emissions.
the collective excitation energy of a pair of Yb3+ ions in the 2 F5/2 state is almost resonant with the excitation energy needed for raising a ground state Tm3+ ion to the 1G4 level. The intensity of different transitions from 1G4 level of Tm3+ would increase with the increase of Yb3+ ion’s concentration. This is indeed observed. The dependence of
fluorescence intensity of Tm3+ on Yb3+ concentration is discussed in our previous work [23]. A tellurite glass containing only Tb3+ ions gives no visible fluorescence on 798 nm excitation. If Tm3+ ions are also present (with Tb3+) in the glass, one observes the upconverted emission only from Tm3+ (Fig. 1C). Thus
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addition of Tm3+ to a glass containing Tb3+ does not influence the Tb3+ ions. However, when Tm3+, Tb3+ and Yb3+ all three are present together in the glass, one sees emission from all three ions. The upconversion fluorescence spectrum of a glass containing 0.5 mol% Tm2O3 + 4 mol% Yb2O3 + 0.5 mol% Tb4O7 under 798 nm excitation is shown in Fig. 1D and emissions at 477, 482, 522, 544, 584, 655 and 698 nm are seen. The emissions at 482, 544, 584 and 655 nm are ascribed to Tb3+ from the transitions 5 D4 ? 7F6, 5D4 ? 7F5, 5D4 ? 7F4, and 5D4 ? 7FJ (J = 0, 1, 2, 3), respectively. The peaks at 477, 655 and 698 nm are ascribed to Tm3+ involving the 1G4 ? 3H6, 1 G4 ? 3F4, 3F3 ? 3H6 transitions, respectively. The peak at 655 nm thus involves the superposition of transitions due to both Tm3+ and Tb3+. Similarly, the peak near 480 nm (477 and 482 nm) is due to the superposition of emission due to a cooperative decay of an excited ion pair of Yb3+, 1G4 ? 3H6 transition of Tm3+ (see the earlier paragraph) as well as due to 5D4 ? 7F6 transition of Tb3+. The visible upconversion emissions ascribed to Tb3+ thus must involve excitation through energy transfer from Tm3+– Yb3+ to Tb3+. Our Tb4O7 sample has traces of impurity of Er3+ and in the presence of Yb3+ + Tb3+ bands at 522 and 544 nm due to Er3+ are also seen. The band at 544 nm has contribution due to both Er3+ and Tb3+. In order to understand the appearance of Tb3+ upconversion luminescence in Tm3+–Yb3+–Tb3+ codoped glass, one must consider at the outset the case when only Tm3+ and Tb3+ ions are present. As mentioned earlier in this case red and blue emissions due to Tm3+ are seen (no emission from Tb3+). However, if Yb3+ is also present in it, intense blue, green, yellow, and red emissions are observed (see Fig. 1D). It appears that through energy transfer from Tm3+, excited Yb3+ ion pairs are created. These Yb3+ ion pairs in turn cooperatively transfer their excitation energy to Tm3+ exciting it to 1G4 level as well as to Tb3+ ions to excite them to 5D4 level. It is this excited 5D4 level which yields the green fluorescence at 544 nm while the excited Tm3+ ions in the 1G4 state are responsible for the 655 and 477 nm emissions. If the Yb and Tm ions concentrations are kept fixed and Tb concentration is increased, we observe a slight decrease in the intensities of the emissions from Tm3+ ions especially in its red emission (655 nm). The intensities of the Tb3+ emissions from its 5 D4 state are increased till Tb3+ concentration reaches 0.5 mol% above which the intensities show a decrease. For Tb3+ concentrations greater than 1.5 mol%, Tb3+ emissions completely disappear. An increase in the concentration of Yb3+ (keeping Tb3+ and Tm3+ concentrations fixed) increases the intensity of emission from Tm3+ and Tb3+ as long as Yb3+ concentration remains up to 4 mol% beyond which it is quenched. Dependence of the intensity of the blue upconversion emission upon the excitation power has also been examined, and a quadratic power dependence is seen (Fig. 3). This indicates that two IR pump photons are needed in the blue upconversion emission. Of the two possible ways
Fig. 3. ln I versus ln P plot for 1G4 ? 3H6 transition of Tm3+.
of excitation of 1G4 level of Tm3+, cooperative energy transfer from a pair of excited Yb3+ ions to a single Tm3+ ion seems more likely as in this case there is almost complete energy matching. In fact, twice the energy of the Yb3+: 2F5/2 ? 2F7/2 transition is equal to the energy 3 corresponding to the Tm3+: 1G4 H6 transition. A similar two photon dependence has been observed for the green emission (5D4 ? 7F5 transition) of Tb3+ also. It is worthwhile to note that the excitation energy for the 5D4 level of Tb3+ also matches with the energy of a pair of excited Yb3+ ions in the 2F5/2 state. 3.2. Excitation with 980 nm laser radiation The 980 nm radiation can resonantly excite the 2F5/2 level of Yb3+ from its ground state. It is interesting to note that a tellurite glass containing Yb3+ ions alone gives intense blue emission at 480 nm (Fig. 4A). This emission has been explained as being due to cooperative emission from a pair of excited Yb3+ ions. When Yb3+ is present along with either Tm3+ or Tb3+, the doped glass gives blue and red emissions or blue, green and red emissions. This has been explained as being again due to energy transfer from a pair of excited Yb3+ ions to a Tm3+ or a Tb3+ ion [23–25]. The intensities of the emitted bands does depend on the concentrations of the various ions. The glass containing all three ions viz. Tm3+, Yb3+and Tb3+ when excited with 980 nm radiation, emissions centered at 477, 522, 544 and 655 nm (Fig. 4E) are seen. Emission at 544 nm is ascribed to Tb3+: 5D4 ? 7F5 and Er3+: 2H11/2 ? 4I15/2 transitions. Emission at 522 nm corresponds to 4S3/2 ? 4I15/2 transition of erbium. Since all three ions, Tm3+, Tb3+ and Yb3+ give upconverted emission in the blue region at wavelengths close to 480 nm it is likely that the blue emission observed in the present case comprises a superposition of Tm3+: 1G4 ? 3H6, of Tb: 5 D4 ? 7F6 along with cooperative emission from a pair
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Fig. 4. Upconversion spectrum of Tm3+, Yb3+, Tb3+ doped in TeO2 glass under 980 nm excitation.
of excited Yb3+ (2F5/2 ? 2F7/2). Similarly the red emission at 655 nm involves the superposition of the transitions, Tm3+: 1G4 ? 3F4 and Tb3+: 5D4 ? 7FJ (J = 0, 1, 2, 3), as even in the absence of Tm3+ when a Tb–Yb codoped sample is excited with the 980 nm radiation one observes an emission at 655 nm (see Fig. 4C). The lifetime of the 1G4 level of Tm3+ as a function of Tb3+ concentrations keeping Yb3+ and Tm3+concentrations fixed at 4.0 mol% and 0.5 mol%, respectively, has been measured
exciting at 798 and 980 nm radiations. Time vs. ln (intensity) plot for lifetime of 1G4 level in tellurite glass containing 0.5 mol% Tm2O3 + 4 mol% Yb2O3 + 0.5 mol% Tb4O7 with 798 and 980 nm is shown in Fig. 5. A comparison of the lifetime of the 1G4 level of Tm3+ under two different excitations reveals that the lifetime of the 1G4 level of Tm3+ under 798 nm excitation is appreciably larger than under 980 nm excitation. This is perhaps due to the fact that for 798 nm excitation two steps (viz. Tm3+(3H4) ? Yb3+(2F5/2) followed by 2Yb3+(2F5/2) ? Tm3+(1G4)) are required to populate this level where as only one step (Yb3+(2F5/ 3+ 2 3+ 1 2) + Yb ( F5/2) ? Tm ( G4)) is required under 980 nm excitation. Also a decrease in the lifetime of the 1G4 level of Tm3+ (at a fixed concentration of Tm3+) on increasing the concentration of Tb3+ is seen. This is probably due to the increased depopulation of the 1G4 level by Tb3+ ions (transfer of excitation energy from Tm3+ to Tb3+ 5D4 level). This also agrees with the observed decrease in the fluorescence intensity of transitions involving 1G4 level of Tm3+ when Tb3+ concentration is increased (see Table 1).
Table 1 Lifetime of 1G4 level of Tm3+ on excitation with different laser lines Excitation wavelength (nm) Fig. 5. Fluorescence decay of 1G4 ? 3H6 transition of Tm3+ for Tm3+ (0.5 mol%) + Tb3+ (0.5 mol%) + Yb3+ (4.0 mol%) doped in TeO2 glass for (A) 798 nm excitation (B) 980 nm excitation.
798 980
Lifetime (ms) of 1G4 level of Tm3+ in different conditions Tm = 0.5, Yb = 4.0, Tb = 0.5 mol%
Tm = 0.5, Yb = 4.0, Tb = 1.0 mol%
Tm = 0.5, Yb = 4.0, Tb = 1.5 mol%
0.94 0.57
0.80 0.44
0.80 0.44
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4. Conclusion Upconversion emission properties of a tellurite glass doped simultaneously with Tm3+, Yb3+and Tb3+ ions has been examined under 798 and 980 nm excitation. It is found that under 798 nm excitation upconversion in Tb3+ from the 5D4 level can be explained as involving Tm3+ as an energy donor while Yb3+ acts as an intermediate. For 980 nm excitation, Yb3+ ions directly absorb the pump radiation and pairs of Yb3+ ions cooperatively transfer the excitation energy to Tm3+ and Tb3+ resulting in different upconversion emissions from both Tm3+ and Tb3+. Lifetime of the 1G4 level of Tm3+has also been studied under 798 and 980 nm excitations and the observed changes are explained. Acknowledgements Authors are grateful to CSIR and DST New Delhi for financial assistance. One of us (D.K.R.) gratefully acknowledges the Grant of Emeritus Fellowship by AICTE, New Delhi. References [1] R. Balda, A.J. Garcia-Adeva, J. Fernandez, J.M.F. Navarro, J. Opt. Soc. Am. B 21 (2004) 744. [2] R. Balda, M. Sanz, A. Mendiroz, J. Fernandez, L.S. Griscom, J.L. Adam, Phys. Rev. B 64 (2001) 144101. [3] D. Lande, S.S. Orlov, A. Akella, L. Hesselink, R.R. Neurgaonkar, Opt. Lett. 22 (1997) 1722. [4] P. Xie, T.R. Gosnell, Opt. Lett. 20 (1995) 1014. [5] S.F. Collins, G.W. Baxter, S.A. Wade, T. Sun, K.T.V. Grattan, Z.Y. Zhang, A.M. Palmer, J. Appl. Phys. 84 (1998) 4649.
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