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Yb3+ doped tellurite glasses
Optics Communications 284 (2011) 4584–4587
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Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Effect of ZnO as modifier on up and downconversion properties of Ho 3+/Yb 3+ doped tellurite glasses C. Joshi a, K. Kumar b, S.B. Rai a,⁎ a b
Laser & Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi-221005, India Nanotechnology Application Centre University of Allahabad, Allahabad-India
a r t i c l e
i n f o
Article history: Received 23 November 2010 Received in revised form 5 March 2011 Accepted 20 May 2011 Available online 13 June 2011 Keywords: Optical absorption Photoluminescence Upconversion Power dependence
a b s t r a c t Optical absorption and photoluminescent properties of Ho3+/Yb3+ co-doped tellurite and zinc tellurite glasses are investigated. The effect of zinc oxide as a modifier on the luminescence properties of above mentioned samples has been explored. Two intense upconversion emission bands centered at 546 (5F4 + 5S2 → 5I8) and 660 nm (5F5 → 5I8) are observed on excitation with 976 nm diode laser. Zinc oxide acts as a quencher for 976 nm excited upconversion emission. The up and downconversion emission spectra are recorded with 532 nm excitation source also. In this case zinc oxide improves the up and downconversion emissions. A large enhancement in upconversion intensity has been observed when Ho3+ ion is co-doped with Yb3+ ion. The dependence of upconversion intensities on excitation power and on temperature has also been studied. The power dependence study shows a quadratic dependence of the fluorescence intensity on the excitation power while a decrement in emission intensity of all the transitions at different rates with increase in temperature is observed in temperature dependence study. The possible mechanisms are also discussed in order to understand the upconversion and energy transfer processes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Conventional oxides such as B2O3, SiO2 and P2O5 can form glasses either alone or when mixed with non-glass-forming oxides [1,2]. These glasses doped with rare earth ions though have many applications in areas like optical communication, laser technology, display devices, etc., however, they are not suitable for observing upconversion emission [3] and references therein since they posses high phonon frequency. Tellurium oxide based glasses are of technological interest because of their low phonon frequency (600–800 cm−1), low melting points and non-hygroscopic properties [4]. Besides this, several other properties such as their high refractive index, anomalous partial dispersion in the visible region, high third-order non-linear susceptibility, good host for the doping of rare earth ions, good transmittance in the near infrared region [5–7], etc. of tellurite based glasses made them superior compared to other oxide glasses. In many cases these properties are combined with good chemical and crystallization properties [4] to make this as one of the ideal hosts. Ho3+ ion has been extensively investigated owing to laser action in infrared, visible, and ultraviolet (UV) regions [8–11]. It is also useful for medical applications involving coagulative cutting and tissue welding since liquid water has strong absorption in this wavelength region [12,13]. In addition, green upconversion emission of Ho3+ is suitable for
⁎ Corresponding author. Tel.: + 91 542 230 7308, fax: + 91 542 2369889. E-mail address: [email protected] (S.B. Rai). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.05.053
diode pumping. It can also be utilized as a green short-wavelengthemitting solid-state laser [14]. Above all, Ho3+ ion gives emission in UV, blue, green, red and IR, almost every wavelength region of the visible spectrum which is beneficial for the realization of upconversion lasers and white light luminescence [15]. The Ho3+/Yb 3+ combination has been extensively investigated in many low phonon host glasses/crystals [16–19]. Yb3+ ion acts as a sensitizer for Ho3+ ion and, therefore, emission of Ho3+ ion enhances many times in presence of Yb3+ ion. Use of different glass modifiers is one way to enhance the upconversion emission efficiency of rare earth ions [20–22]. Kumar et al. [20,21] have studied the effect of Li2O, Na2O, K2O, PbO and PbF2 modifiers in tellurite glass on the upconversion intensity of Er 3+ ions and found optimum emission for glass modified with PbF2. Several authors have studied upconversion emission of singly Ho 3+ ion doped in zinc modified tellurite glasses [13,23–27]. The effect of zinc oxide on the luminescence intensity of singly doped Ho 3+ and doubly doped Ho 3+-Yb 3+ systems, however, has not been studied in detail. Ho 3+-Yb 3+ combination is a promising candidate for achieving strong green upconversion. It would be, therefore, interesting to observe the effect of zinc oxide on the upconversion emission intensity of Ho 3+ and Ho 3+-Yb 3+ doped tellurite glasses and to compare its emission in presence of other modifiers. In this paper, Ho3+ and Ho3+-Yb 3+ are doped in Li2CO3 and ZnO modified tellurite glasses and effect of these modifiers on the upconversion intensity has been studied. There are two reasons for choosing these compositions. First is to see the effect of zinc oxide on the luminescence properties of Ho3+/Yb 3+ doped tellurite glasses with 976
C. Joshi et al. / Optics Communications 284 (2011) 4584–4587
and 532 nm laser excitation. The wavelength 976 nm excites Yb 3+ but not Ho3+. On the other hand 532 nm excites only Ho3+ not Yb 3+. Second is to study the energy transfer from Yb3+ to Ho3+ ions in such systems. 2. Experimental Glass samples were prepared using reagent grade raw materials according to the following compositions in mol%: 74TeO2 þ 25ZnO þ 1Ho2 O3
½TZH
72TeO2 þ 25ZnO þ 1Ho2 O3 þ 2Yb2 O3
½TZHY
72TeO2 þ 25Li2 CO3 þ 1Ho2 O3 þ 2Yb2O3
½TLHY
Conventional melt-quench method has been opted for synthesis of the samples. In a typical process, appropriate weight ratios of required powder materials were well-mixed in an agate mortar. Well mixed powder was then taken into platinum crucible and heated to melt at 900 °C for 30 min in an electric furnace. The molten mass was then poured into a preheated brass plate at 200 °C and then immediately covered by another plate. The sample was then left to cool down to room temperature gradually. The samples thus obtained were cut into proper shape and size and polished carefully for optical measurements. The absorption spectra of the samples were measured at room temperature using a JASCO V-670 absorption spectrophotometer in the range of 350–1200 nm. The emission spectra were recorded by exciting the samples using 976 and 532 nm laser wavelengths from diode and Nd-YAG lasers, respectively. An iHR320 (Horiba Jobin Yvon) spectrometer was used to disperse and detect the signal.
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absorption from ground state 5I8 of Ho3+ ion to its various excited states G5, 5G6, 5S2 + 5F4, 5F5 and 5I6, respectively. The absorption band observed at 976 nm is associated with Yb3+ ion with transition from ground state 2F7/2 to excited state 2F5/2. From the spectra one can see that the absorption band at 976 nm due to Yb 3+ ion appears with larger intensity for TLHY glass sample than TZHY sample. Inset in Fig. 1 shows the absorption of 532 nm radiation by these two samples. It is worth mentioning that the variation in absorption intensity in two hosts causes a systematic change in up and downconversion emission intensity which will be discussed in the next section. 5
3.2. Upconversion emission spectra Fig. 2(a) compares upconversion emission spectra of Ho3+/Yb 3+ codoped samples excited with 976 nm diode laser. To compare the emission intensity, spectra were recorded with fixed external parameter such as slit width, integration time, excitation power, etc. In such a condition, integrated area of emission peaks could be compared. The upconversion bands are observed at 492 (shown in inset (a) of Fig. 2(a)), 546, 660 and 754 nm which arise due to the 5F3 → 5I8, 5F4 + 5S2 → 5I8, 5 F5 → 5I8, and 5S2 → 5I7 transitions of Ho3+ ion, respectively. The Yb3+
3. Results and discussion 3.1. Absorption spectra Fig. 1 shows the absorption spectra of Ho3+ and Yb 3+ co-doped samples in different hosts in 350–1200 nm region. The absorption spectra of all the glass samples were recorded exactly in the same condition keeping sample thickness constant so their absorption intensity referring the effect of host can be compared. The absorption bands are observed at 416, 454, 541, 646 and 1155 nm due to the
Fig. 1. Absorption spectra of tellurite glass samples; TLHY; TZHY. Inset shows the absorption of 532 nm radiation by these samples in the wavelength range 520–560 nm.
Fig. 2. A comparison of upconversion emission spectra of glass samples TLHY; TZHY with 976 nm laser excitation. Inset (a) shows 492 nm (5F3 → 5I8) transition of Ho3+ ion in 470–500 nm wavelength range. Inset (b) shows an enhancement in upconversion emission intensity of Ho3+ ions in presence of Yb3+ ions (a). Emission spectra of glass samples TLHY; TZHY with 532 nm laser excitation. Inset shows the upconversion emission spectra of these glass samples in 375–500 nm wavelength range. The 5S2 → 5I8 transition lie very close to laser line 532 nm and could not be scanned (b).
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ion acts as sensitizer and its presence enhances the emission of Ho3+ ion several times. Inset (b) in Fig. 2(a) compares emission intensity between the TZHY and TZH samples on 976 nm excitation and one can clearly see an enhancement of about 15 times in emission intensity on introduction of Yb 3+ ions. Fig. 2(a) also indicates that upconversion emission intensity is higher for TLHY sample. On replacement of Li2CO3 with ZnO [TZHY glass], the emission intensity is reduced considerably for all wavelengths. This variation in emission intensity is in agreement with absorption spectra shown in Fig. 1. The absorption cross section of Yb 3+ ions is higher for 976 nm radiation for sample TLHY, and therefore it absorbs more 976 nm photons and transfers its energy to Ho3+ ions, which results in higher emission intensity. Fig. 2(b) shows the up and downconversion emission spectra of same glass samples excited with 532 nm laser wavelength. It is interesting to note that the emission spectra shows relatively higher fluorescence for sample TZHY compared to sample TLHY. It can be explained on the basis of absorption spectra (shown in Fig. 1) again. In this case, the absorption cross section of Ho3+ ion is higher for TZHY sample for 532 nm radiation than for sample TLHY (given in inset of Fig. 1), therefore, Ho3+ ions absorb more 532 nm photons for TZHY sample than for sample TLHY resulting in relatively higher emission intensity while Yb3+ ion plays no role in this case. The upconversion emission bands are observed at 392, 423 and 492 nm wavelength positions and two downconversion emission bands at 661 and 754 nm wavelengths. The upconversion bands observed at 392, 423 and 492 nm positions arise due to the 5G4 → 5I8, 5G5 → 5I8 and 5F3 → 5I8 transitions of Ho3+ ion, respectively (inset of Fig. 2(b)). The peak at 546 nm in green region also appears very intense but in the present case a red filter was used to cut off the excitation wavelength which suppresses 546 nm emission of Ho3+ ion. That is why the region from 510 to 600 nm is not considered here. From Fig. 2(b) we conclude that emission intensity follows the trend TZHYN TLHY, however, in case of 976 nm excitation, it is as TLHYN TZHY. The conclusion is that ZnO enhances the emission intensity on direct excitation with 532 nm but not in the case when NIR (976 nm) excitation is used.
3.3. Power dependence The upconversion emission intensity, IUP, is proportional to the nth power of the NIR excitation intensity (INIR) according to the IUP α I nNIR relation [28]. Here n is the number of NIR photons absorbed per visible photon emitted. The power dependence study of the upconversion emission bands was carried out for TLHY glass sample for both the excitation wavelengths 976 and 532 nm. A log–log plot between the excitation power and emission intensity is shown in Fig. 3. Slopes of the power dependence spectra recorded by the 976 nm diode laser are shown in Fig. 3(a) and (b). Slope of log–log plot for 492 nm is found to be 2.92 (see Fig. 3(a)) which confirms that this level is populated by threephoton absorption process. Values of n for 546, 660 and 754 nm emission wavelengths were found to be 2.07, 2.24 and 1.52, respectively (shown in Fig. 3(b)). The power dependence of upconversion emission has also been studied for 532 nm laser excitation and slope is shown in Fig. 3(c). The slope (n) for 392, 423 and 492 nm wavelengths were found to be 2.38, 1.91 and 1.9, respectively. We observed that when the incident power was low (up to 200 mW), the intensities of these bands increased almost linearly. When the pump power further increased (from 200 mW to 1300 mW), the intensity of these bands raised very rapidly showing slope of almost ≈ 2. When the incident power increased, further, saturation was observed for all the bands. From this, it is clear that these upconversion emissions show a quadratic dependence of the fluorescence intensity on excitation power as observed in Ho 3+/Yb 3+ co-doped other glasses [29,30]. These results indicate that the green and the red emitting states are populated by two-photon absorption process.
Fig. 3. Variation of emission intensity (I) with incident power (P) for 492 nm band on excitation with 976 nm diode laser (a). Variation of emission intensity (I) versus incident power (P) for the bands at 546, 660 and 754 nm position for the glass sample TLHY on excitation with 976 nm diode laser (b). Variation of emission intensity (I) versus incident power (P) for the bands at 392, 423 and 492 nm position for the glass sample TLHY on excitation with 532 nm diode laser (c).
3.4. Temperature dependence The temperature dependence of upconversion emission has also been studied in order to see the effect of temperature on the emission intensity. Graph between temperature and emission intensity corresponding to 546, 660 and 754 nm emission bands for TLHY
C. Joshi et al. / Optics Communications 284 (2011) 4584–4587
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multiphonon relaxation process. From 5F3 state, a small number of Ho3+ ions return to ground state emitting 492 nm blue wavelength via 5 F3 → 5I8 transition. A major part of Ho3+ ions in 5S2 state returns to 5I8 state producing a strong 546 nm green emission via 5F4 + 5S2 → 5I8 radiative transition. A small part of Ho3+ ions in 5S2 state also depopulate to the 5I7 metastable state by emitting a weak band at 754 nm via 5S2 → 5I7 radiative transition. On 532 nm excitation, the Ho 3+ ions are excited to 5F4 + 5S2 states. The multiphonon relaxation of Ho 3+ ions populates 5F5 level; Ho 3+ ions in this level reabsorb the 532 nm radiation and populate some high lying energy level. A non-radiative relaxation from this level populates 5G4, 5G5, 5F3 levels, etc. The excited ions from these levels return to ground state via 5G4 → 5I8, 5G5 → 5I8 and 5F3 → 5I8 transitions by emitting 392, 423 and 492 nm wavelengths, respectively. 4. Conclusions Fig. 4. Variation of emission intensity (I) with temperature (T) for the bands at 546, 660 and 754 nm position for the glass sample TLHY.
sample is shown in Fig. 4. It is clear from the graph that as the temperature increases, emission intensity of all the bands decreases linearly but with different rates. The intensity of red emission decreases faster than intensity of green emission. From this, we can say that the red emission is more sensitive to temperature than green emission. This decrease in emission intensity with an increase in temperature results due to the increase in non-radiative transitions. 3.5. Energy levels of Ho 3+/Yb 3+ and energy transfer mechanisms The upconversion mechanism in Ho3+/Yb3+ co-doped tellurite glass samples under 976 nm excitation is proposed in Fig. 5. In the case of direct upconversion sensitization, 5I6 level plays an important role. On 976 nm laser excitation, large numbers of Yb3+ ions are initially excited to 2F5/2 state from the ground 2F7/2 state due to large absorption cross section of Yb3+ ions at 976 nm. The excited Yb3+ ions transfer their energy to the nearby Ho3+ ions in ground state via ET process, thereby exciting them to 5I6 metastable state. The mismatch of energy between the 2F5/2 level of Yb3+ and 5I6 level of Ho3+ is compensated by emission of two phonons. The Ho3+ ions in the 5I6 excited state reabsorb the incident photons through energy transfer upconversion (ETU) process and promoted to the 5S2 (5F4) state. The non-radiative relaxation from this level populates the 5F5 level. This level plays an important role. Besides a radiative emission from 5F5 level (5F5 → 5I8) emitting at 660 nm, some of the ions from this level are promoted to 5G4 level. The Ho 3+ ions from 5G4 state return to 5F3 state by non-radiative
Upconversion emission has been studied in Ho 3+/Yb 3+ co-doped TeO2 glass systems with different modifiers using 976 and 532 nm excitation sources. Effect of ZnO as modifier has been investigated on the up and downconversion emission intensity and has been compared with Li2CO3 modified glass. Though no improvement in upconversion emission intensity on 976 nm excitation has been observed when Li2CO3 modifier is replaced with ZnO modifier, however, ZnO modified host shows enhanced emission intensity when 532 nm was used as excitation source. Incident power dependent emission studies show a quadratic dependence of fluorescence intensity on excitation power. Temperature dependent emission studies show a variation in sensitivity at different emission wavelengths and accordingly their suitability for temperature sensor. Acknowledgements One of the authors, Chetan Joshi, is grateful to UGC, New Delhi, India for the financial assistance in the form of meritorious fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Fig. 5. Simplified energy level diagram of Ho3+/Yb3+ ions and possible transition pathways under 976 nm excitation.
[29] [30]
A.R. West, Solid state chemistry and its applications, Wiley, New York, 1984. Y. He, D.E. Day, Glass Technol. 33 (1992) 214. H. Yung, P.Y. Shih, H.S. Liu, T.S. Chin, J. Am. Ceram. Soc. 80 (1997) 2213. S. Rada, E. Culea, V. Rus, M. Pica, M. Culea, J. Mater. Sci. 43 (2008) 3713. O.V. Mazurin, M.V. Streltsina, T.P. Shvaiko- Shvaikovskaya, Handbook of Glass Data, Part B, Elsevier, Amsterdam, 1985. J.S. Wang, E.M. Vogel, E. Snitzer, Opt. Mater. 3 (1994) 187. H. Buerger, W. Vogel, V. Kozhukharov, Infrared Phys. 25 (1985) 395. H.P. Ho, W.W. Wong, S.Y. Wu, Opt. Eng. (Bellingham) 42 (2003) 2349. J.C. Boyer, F. Vetrone, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Appl. Phys. 93 (2003) 9460. V. Lavin, F. Lahoz, I.R. Martin, U.R. Rodriguez-Mendoza, J.M. Caceres, Opt. Mater. 27 (2005) 1754. L. Feng, J. Wang, Q. Tang, L.F. Liang, H.B. Liang, Q. Su, J. Lumin. 124 (2007) 187. M. Marano, G. Galzerano, S. Taccheo, P. Laporta, E. Sani, A. Toncelli, M. Tonelli, Opt. Mater. 24 (2003) 327. T. Rothacher, W. Luthy, P. Weber, Opt. Commun. 155 (1998) 68. J.Y. Allain, M. Monerie, H. Poignant, Elec. Lett. 26 (1990) 261. H. Gong, D. Yang, X. Zhao, E.Y.B. Pun, H. Lin, Opt. Mater. 32 (2010) 554. X. Li, Q. Nie, S. Dai, T. Xu, X. Shen, X. Zhang, J. Phys. Chem. Solids 68 (2007) 1566. X. Li, Q. Nie, S. Dai, T. Xu, L. Lu, X. Zhang, J. Alloys Compd. 454 (2008) 510. B. Zhou, E. Yue, B. Pun, H. Lin, D. Yang, L. Huang, J. Appl. Phys. 106 (2009) 103105. I.R. Martín, V.D. Rodriguez, V. Lavin, U.R. Rodriguez-Mendoza, J. Alloys Compd. 275 (1998) 345. K. Kumar, S.B. Rai, D.K. Rai, Spectrochim. Acta Part A 66 (2007) 1052. K. Kumar, S.B. Rai, D.K. Rai, Solid State Commun. 139 (2006) 363. F. Zeng, G. Ren, X. Qiu, Q. Yang, J. Chen, J. Non- Cryst. Solids 354 (2008) 3428. A.K. Singh, S.B. Rai, V.B. Singh, J. Alloys Compd. 403 (2005) 97. B. Karmakar, J. Solid State Chem. 178 (2005) 2663. L.M. Fortes, L.F. Santos, M.C. Goncalves, R.M. Almeida, J. Non- Cryst. Solids 324 (2003) 150. L. Zhang, L. Wen, H. Sun, J. Zhang, L. Hu, J. Alloys Compd. 391 (2005) 156. S. Xu, H. Ma, D. Fang, Z. Zhang, Z. Jiang, J. Lumin. 117 (2006) 135. G.Y. Chen, G. Somesfalean, Y. Liu, Z.G. Zhang, Q. Sun, F.P. Wang, Phys. Rev. B 195204 (75) (2007) 1. J. Zhang, Y. Kawamoto, S.-X. Dai, Chin. Phys. B 13 (2004) 1156. A.S. Gouveia-Neto, E.B. da Costa, L.A. Bueno, S.J.L. Ribeiro, J. Lumin. 110 (2004) 79.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Effect of ZnO as modifier on up and downconversion properties of Ho 3+/Yb 3+ doped tellurite glasses C. Joshi a, K. Kumar b, S.B. Rai a,⁎ a b
Laser & Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi-221005, India Nanotechnology Application Centre University of Allahabad, Allahabad-India
a r t i c l e
i n f o
Article history: Received 23 November 2010 Received in revised form 5 March 2011 Accepted 20 May 2011 Available online 13 June 2011 Keywords: Optical absorption Photoluminescence Upconversion Power dependence
a b s t r a c t Optical absorption and photoluminescent properties of Ho3+/Yb3+ co-doped tellurite and zinc tellurite glasses are investigated. The effect of zinc oxide as a modifier on the luminescence properties of above mentioned samples has been explored. Two intense upconversion emission bands centered at 546 (5F4 + 5S2 → 5I8) and 660 nm (5F5 → 5I8) are observed on excitation with 976 nm diode laser. Zinc oxide acts as a quencher for 976 nm excited upconversion emission. The up and downconversion emission spectra are recorded with 532 nm excitation source also. In this case zinc oxide improves the up and downconversion emissions. A large enhancement in upconversion intensity has been observed when Ho3+ ion is co-doped with Yb3+ ion. The dependence of upconversion intensities on excitation power and on temperature has also been studied. The power dependence study shows a quadratic dependence of the fluorescence intensity on the excitation power while a decrement in emission intensity of all the transitions at different rates with increase in temperature is observed in temperature dependence study. The possible mechanisms are also discussed in order to understand the upconversion and energy transfer processes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Conventional oxides such as B2O3, SiO2 and P2O5 can form glasses either alone or when mixed with non-glass-forming oxides [1,2]. These glasses doped with rare earth ions though have many applications in areas like optical communication, laser technology, display devices, etc., however, they are not suitable for observing upconversion emission [3] and references therein since they posses high phonon frequency. Tellurium oxide based glasses are of technological interest because of their low phonon frequency (600–800 cm−1), low melting points and non-hygroscopic properties [4]. Besides this, several other properties such as their high refractive index, anomalous partial dispersion in the visible region, high third-order non-linear susceptibility, good host for the doping of rare earth ions, good transmittance in the near infrared region [5–7], etc. of tellurite based glasses made them superior compared to other oxide glasses. In many cases these properties are combined with good chemical and crystallization properties [4] to make this as one of the ideal hosts. Ho3+ ion has been extensively investigated owing to laser action in infrared, visible, and ultraviolet (UV) regions [8–11]. It is also useful for medical applications involving coagulative cutting and tissue welding since liquid water has strong absorption in this wavelength region [12,13]. In addition, green upconversion emission of Ho3+ is suitable for
⁎ Corresponding author. Tel.: + 91 542 230 7308, fax: + 91 542 2369889. E-mail address: [email protected] (S.B. Rai). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.05.053
diode pumping. It can also be utilized as a green short-wavelengthemitting solid-state laser [14]. Above all, Ho3+ ion gives emission in UV, blue, green, red and IR, almost every wavelength region of the visible spectrum which is beneficial for the realization of upconversion lasers and white light luminescence [15]. The Ho3+/Yb 3+ combination has been extensively investigated in many low phonon host glasses/crystals [16–19]. Yb3+ ion acts as a sensitizer for Ho3+ ion and, therefore, emission of Ho3+ ion enhances many times in presence of Yb3+ ion. Use of different glass modifiers is one way to enhance the upconversion emission efficiency of rare earth ions [20–22]. Kumar et al. [20,21] have studied the effect of Li2O, Na2O, K2O, PbO and PbF2 modifiers in tellurite glass on the upconversion intensity of Er 3+ ions and found optimum emission for glass modified with PbF2. Several authors have studied upconversion emission of singly Ho 3+ ion doped in zinc modified tellurite glasses [13,23–27]. The effect of zinc oxide on the luminescence intensity of singly doped Ho 3+ and doubly doped Ho 3+-Yb 3+ systems, however, has not been studied in detail. Ho 3+-Yb 3+ combination is a promising candidate for achieving strong green upconversion. It would be, therefore, interesting to observe the effect of zinc oxide on the upconversion emission intensity of Ho 3+ and Ho 3+-Yb 3+ doped tellurite glasses and to compare its emission in presence of other modifiers. In this paper, Ho3+ and Ho3+-Yb 3+ are doped in Li2CO3 and ZnO modified tellurite glasses and effect of these modifiers on the upconversion intensity has been studied. There are two reasons for choosing these compositions. First is to see the effect of zinc oxide on the luminescence properties of Ho3+/Yb 3+ doped tellurite glasses with 976
C. Joshi et al. / Optics Communications 284 (2011) 4584–4587
and 532 nm laser excitation. The wavelength 976 nm excites Yb 3+ but not Ho3+. On the other hand 532 nm excites only Ho3+ not Yb 3+. Second is to study the energy transfer from Yb3+ to Ho3+ ions in such systems. 2. Experimental Glass samples were prepared using reagent grade raw materials according to the following compositions in mol%: 74TeO2 þ 25ZnO þ 1Ho2 O3
½TZH
72TeO2 þ 25ZnO þ 1Ho2 O3 þ 2Yb2 O3
½TZHY
72TeO2 þ 25Li2 CO3 þ 1Ho2 O3 þ 2Yb2O3
½TLHY
Conventional melt-quench method has been opted for synthesis of the samples. In a typical process, appropriate weight ratios of required powder materials were well-mixed in an agate mortar. Well mixed powder was then taken into platinum crucible and heated to melt at 900 °C for 30 min in an electric furnace. The molten mass was then poured into a preheated brass plate at 200 °C and then immediately covered by another plate. The sample was then left to cool down to room temperature gradually. The samples thus obtained were cut into proper shape and size and polished carefully for optical measurements. The absorption spectra of the samples were measured at room temperature using a JASCO V-670 absorption spectrophotometer in the range of 350–1200 nm. The emission spectra were recorded by exciting the samples using 976 and 532 nm laser wavelengths from diode and Nd-YAG lasers, respectively. An iHR320 (Horiba Jobin Yvon) spectrometer was used to disperse and detect the signal.
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absorption from ground state 5I8 of Ho3+ ion to its various excited states G5, 5G6, 5S2 + 5F4, 5F5 and 5I6, respectively. The absorption band observed at 976 nm is associated with Yb3+ ion with transition from ground state 2F7/2 to excited state 2F5/2. From the spectra one can see that the absorption band at 976 nm due to Yb 3+ ion appears with larger intensity for TLHY glass sample than TZHY sample. Inset in Fig. 1 shows the absorption of 532 nm radiation by these two samples. It is worth mentioning that the variation in absorption intensity in two hosts causes a systematic change in up and downconversion emission intensity which will be discussed in the next section. 5
3.2. Upconversion emission spectra Fig. 2(a) compares upconversion emission spectra of Ho3+/Yb 3+ codoped samples excited with 976 nm diode laser. To compare the emission intensity, spectra were recorded with fixed external parameter such as slit width, integration time, excitation power, etc. In such a condition, integrated area of emission peaks could be compared. The upconversion bands are observed at 492 (shown in inset (a) of Fig. 2(a)), 546, 660 and 754 nm which arise due to the 5F3 → 5I8, 5F4 + 5S2 → 5I8, 5 F5 → 5I8, and 5S2 → 5I7 transitions of Ho3+ ion, respectively. The Yb3+
3. Results and discussion 3.1. Absorption spectra Fig. 1 shows the absorption spectra of Ho3+ and Yb 3+ co-doped samples in different hosts in 350–1200 nm region. The absorption spectra of all the glass samples were recorded exactly in the same condition keeping sample thickness constant so their absorption intensity referring the effect of host can be compared. The absorption bands are observed at 416, 454, 541, 646 and 1155 nm due to the
Fig. 1. Absorption spectra of tellurite glass samples; TLHY; TZHY. Inset shows the absorption of 532 nm radiation by these samples in the wavelength range 520–560 nm.
Fig. 2. A comparison of upconversion emission spectra of glass samples TLHY; TZHY with 976 nm laser excitation. Inset (a) shows 492 nm (5F3 → 5I8) transition of Ho3+ ion in 470–500 nm wavelength range. Inset (b) shows an enhancement in upconversion emission intensity of Ho3+ ions in presence of Yb3+ ions (a). Emission spectra of glass samples TLHY; TZHY with 532 nm laser excitation. Inset shows the upconversion emission spectra of these glass samples in 375–500 nm wavelength range. The 5S2 → 5I8 transition lie very close to laser line 532 nm and could not be scanned (b).
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ion acts as sensitizer and its presence enhances the emission of Ho3+ ion several times. Inset (b) in Fig. 2(a) compares emission intensity between the TZHY and TZH samples on 976 nm excitation and one can clearly see an enhancement of about 15 times in emission intensity on introduction of Yb 3+ ions. Fig. 2(a) also indicates that upconversion emission intensity is higher for TLHY sample. On replacement of Li2CO3 with ZnO [TZHY glass], the emission intensity is reduced considerably for all wavelengths. This variation in emission intensity is in agreement with absorption spectra shown in Fig. 1. The absorption cross section of Yb 3+ ions is higher for 976 nm radiation for sample TLHY, and therefore it absorbs more 976 nm photons and transfers its energy to Ho3+ ions, which results in higher emission intensity. Fig. 2(b) shows the up and downconversion emission spectra of same glass samples excited with 532 nm laser wavelength. It is interesting to note that the emission spectra shows relatively higher fluorescence for sample TZHY compared to sample TLHY. It can be explained on the basis of absorption spectra (shown in Fig. 1) again. In this case, the absorption cross section of Ho3+ ion is higher for TZHY sample for 532 nm radiation than for sample TLHY (given in inset of Fig. 1), therefore, Ho3+ ions absorb more 532 nm photons for TZHY sample than for sample TLHY resulting in relatively higher emission intensity while Yb3+ ion plays no role in this case. The upconversion emission bands are observed at 392, 423 and 492 nm wavelength positions and two downconversion emission bands at 661 and 754 nm wavelengths. The upconversion bands observed at 392, 423 and 492 nm positions arise due to the 5G4 → 5I8, 5G5 → 5I8 and 5F3 → 5I8 transitions of Ho3+ ion, respectively (inset of Fig. 2(b)). The peak at 546 nm in green region also appears very intense but in the present case a red filter was used to cut off the excitation wavelength which suppresses 546 nm emission of Ho3+ ion. That is why the region from 510 to 600 nm is not considered here. From Fig. 2(b) we conclude that emission intensity follows the trend TZHYN TLHY, however, in case of 976 nm excitation, it is as TLHYN TZHY. The conclusion is that ZnO enhances the emission intensity on direct excitation with 532 nm but not in the case when NIR (976 nm) excitation is used.
3.3. Power dependence The upconversion emission intensity, IUP, is proportional to the nth power of the NIR excitation intensity (INIR) according to the IUP α I nNIR relation [28]. Here n is the number of NIR photons absorbed per visible photon emitted. The power dependence study of the upconversion emission bands was carried out for TLHY glass sample for both the excitation wavelengths 976 and 532 nm. A log–log plot between the excitation power and emission intensity is shown in Fig. 3. Slopes of the power dependence spectra recorded by the 976 nm diode laser are shown in Fig. 3(a) and (b). Slope of log–log plot for 492 nm is found to be 2.92 (see Fig. 3(a)) which confirms that this level is populated by threephoton absorption process. Values of n for 546, 660 and 754 nm emission wavelengths were found to be 2.07, 2.24 and 1.52, respectively (shown in Fig. 3(b)). The power dependence of upconversion emission has also been studied for 532 nm laser excitation and slope is shown in Fig. 3(c). The slope (n) for 392, 423 and 492 nm wavelengths were found to be 2.38, 1.91 and 1.9, respectively. We observed that when the incident power was low (up to 200 mW), the intensities of these bands increased almost linearly. When the pump power further increased (from 200 mW to 1300 mW), the intensity of these bands raised very rapidly showing slope of almost ≈ 2. When the incident power increased, further, saturation was observed for all the bands. From this, it is clear that these upconversion emissions show a quadratic dependence of the fluorescence intensity on excitation power as observed in Ho 3+/Yb 3+ co-doped other glasses [29,30]. These results indicate that the green and the red emitting states are populated by two-photon absorption process.
Fig. 3. Variation of emission intensity (I) with incident power (P) for 492 nm band on excitation with 976 nm diode laser (a). Variation of emission intensity (I) versus incident power (P) for the bands at 546, 660 and 754 nm position for the glass sample TLHY on excitation with 976 nm diode laser (b). Variation of emission intensity (I) versus incident power (P) for the bands at 392, 423 and 492 nm position for the glass sample TLHY on excitation with 532 nm diode laser (c).
3.4. Temperature dependence The temperature dependence of upconversion emission has also been studied in order to see the effect of temperature on the emission intensity. Graph between temperature and emission intensity corresponding to 546, 660 and 754 nm emission bands for TLHY
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multiphonon relaxation process. From 5F3 state, a small number of Ho3+ ions return to ground state emitting 492 nm blue wavelength via 5 F3 → 5I8 transition. A major part of Ho3+ ions in 5S2 state returns to 5I8 state producing a strong 546 nm green emission via 5F4 + 5S2 → 5I8 radiative transition. A small part of Ho3+ ions in 5S2 state also depopulate to the 5I7 metastable state by emitting a weak band at 754 nm via 5S2 → 5I7 radiative transition. On 532 nm excitation, the Ho 3+ ions are excited to 5F4 + 5S2 states. The multiphonon relaxation of Ho 3+ ions populates 5F5 level; Ho 3+ ions in this level reabsorb the 532 nm radiation and populate some high lying energy level. A non-radiative relaxation from this level populates 5G4, 5G5, 5F3 levels, etc. The excited ions from these levels return to ground state via 5G4 → 5I8, 5G5 → 5I8 and 5F3 → 5I8 transitions by emitting 392, 423 and 492 nm wavelengths, respectively. 4. Conclusions Fig. 4. Variation of emission intensity (I) with temperature (T) for the bands at 546, 660 and 754 nm position for the glass sample TLHY.
sample is shown in Fig. 4. It is clear from the graph that as the temperature increases, emission intensity of all the bands decreases linearly but with different rates. The intensity of red emission decreases faster than intensity of green emission. From this, we can say that the red emission is more sensitive to temperature than green emission. This decrease in emission intensity with an increase in temperature results due to the increase in non-radiative transitions. 3.5. Energy levels of Ho 3+/Yb 3+ and energy transfer mechanisms The upconversion mechanism in Ho3+/Yb3+ co-doped tellurite glass samples under 976 nm excitation is proposed in Fig. 5. In the case of direct upconversion sensitization, 5I6 level plays an important role. On 976 nm laser excitation, large numbers of Yb3+ ions are initially excited to 2F5/2 state from the ground 2F7/2 state due to large absorption cross section of Yb3+ ions at 976 nm. The excited Yb3+ ions transfer their energy to the nearby Ho3+ ions in ground state via ET process, thereby exciting them to 5I6 metastable state. The mismatch of energy between the 2F5/2 level of Yb3+ and 5I6 level of Ho3+ is compensated by emission of two phonons. The Ho3+ ions in the 5I6 excited state reabsorb the incident photons through energy transfer upconversion (ETU) process and promoted to the 5S2 (5F4) state. The non-radiative relaxation from this level populates the 5F5 level. This level plays an important role. Besides a radiative emission from 5F5 level (5F5 → 5I8) emitting at 660 nm, some of the ions from this level are promoted to 5G4 level. The Ho 3+ ions from 5G4 state return to 5F3 state by non-radiative
Upconversion emission has been studied in Ho 3+/Yb 3+ co-doped TeO2 glass systems with different modifiers using 976 and 532 nm excitation sources. Effect of ZnO as modifier has been investigated on the up and downconversion emission intensity and has been compared with Li2CO3 modified glass. Though no improvement in upconversion emission intensity on 976 nm excitation has been observed when Li2CO3 modifier is replaced with ZnO modifier, however, ZnO modified host shows enhanced emission intensity when 532 nm was used as excitation source. Incident power dependent emission studies show a quadratic dependence of fluorescence intensity on excitation power. Temperature dependent emission studies show a variation in sensitivity at different emission wavelengths and accordingly their suitability for temperature sensor. Acknowledgements One of the authors, Chetan Joshi, is grateful to UGC, New Delhi, India for the financial assistance in the form of meritorious fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Fig. 5. Simplified energy level diagram of Ho3+/Yb3+ ions and possible transition pathways under 976 nm excitation.
[29] [30]
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