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Optical properties and upconversion in Er3+ and Ho3+ doped in lithium tellurite glass
Progress in Crystal Growth and Characterization of Materials 52 (2006) 99e106 www.elsevier.com/locate/pcrysgrow
Optical properties and upconversion in Er3þ and Ho3þ doped in lithium tellurite glass Anant Kumar Singh a, S.B. Rai a,*, Anita Rai b a
Department of Physics, Laser and Spectroscopy Lab, Banaras Hindu University, Varanasi 221 005, Uttar Pradesh, India b Department of Chemistry, Jagatpur P.G. College, Varanasi, India
Abstract Ho3þ and Er3þ doped lithium tellurite glasses have been formed with different concentrations of rare earth and their optical properties are derived on the basis of their absorption and fluorescence spectra. Intense upconversion emission in ultraviolet/violet, green and red regions have been observed when these glasses are pumped with NIR radiation. A preliminary study indicates that they can be used as temperature sensors. Ó 2006 Elsevier Ltd. All rights reserved. PACS: -42.70 Ce Keywords: Upconversion; ESA; ET; Temperature sensor
1. Introduction Glasses have certain advantage over crystals as host material. Glasses can be made of different shapes and sizes easily and can also be doped with larger concentrations of the lasing atoms. Due to energy level broadening, the ions in a glass host can absorb larger incident energy compared to the same ion in a crystal, which lowers the threshold for lasing. Rare earth doped glasses beside their applications as lasing material are also used as on line amplifier. They are used as storage material, high energy particle detector, sensors, X-ray and g-ray * Corresponding author. Tel.: þ91 542 2307308; fax: þ91 542 368468. E-mail address: [email protected] (S.B. Rai). 0960-8974/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pcrysgrow.2006.03.014
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absorber, etc. Tellurite glasses are superior to many other oxide glasses because of their high chemical and thermal durability and mechanical strength. They are also non-hygroscopic. In the present paper we have studied the optical properties of Ho3þ and Er3þ doped in lithium tellurite glass. The optical properties of Ho3þ and Er3þ doped in different lattices have been studied earlier by large number of workers [1e10]. For example Reddy et al. [8] have observed green upconversion in Ho3þ:LaF3 whereas Hirao et al. [9] using sodium tellurite glass as host for Ho3þ reported green as well as red upconversions. Codoping Yb3þ with Ho3þ in glass hosts has been found to enhance the upconversion efficiency [11e13]. Similarly red and green upconverted fluorescences have also been observed in several glass lattices [11e15] doped with Er3þ. In this paper we report the observation of upconversion emission in red, green and blue/ UV regions from Ho3þ and Er3þ doped in LiTeO2 glass when pumped with NIR laser radiation. The effect of temperature, concentration of rare earth ion and that of input power on the fluorescence and upconverted emission intensity has been studied. The lifetime of the 5F4(5S2) level of Ho3þ and of 4S3/2 level in Er3þ has been measured. 2. Experimental procedure The chemical composition of Ho3þ and Er3þ doped lithium tellurite glasses is (80x) mol% TeO2 þ 20 mol% Li2CO3 þ x mol% Rare earth oxide with x ¼ 0.5, 1.0, 1.5, 2.0 and 2.5 mol%. The method used to develop the glass is very similar to that described in our earlier paper [16]. The absorption spectra of the glasses with 1 mol% of rare earth were recorded using a Cary 2390 UVevisibleeNIR spectrophotometer in the range 300e900 nm. The fluorescence spectra were recorded using 476.5 nm (100 mW) line of Arþ laser and 0.5 m Spex monochromater in the 500e700 nm region for different concentrations of the rare earth and also at different temperatures. For upconversion studies we have used NIR radiation from a TieSapphire laser. On focusing the NIR radiation in the glass green emission is seen [at 780 nm in Er3þ and at 812 nm in Ho3þ] even at very small pump power. On increasing the laser power, fluorescence in ultraviolet/violet and in red regions is also seen along with green. Fluorescence and upconversion have also been recorded for different concentrations of rare earth and at different temperatures. 3. Results and discussion 3.1. Studies on Ho3þ doped tellurite glass The absorption spectrum of Ho3þ (1 mol%) in tellurite host shows 12 bands at 330.0, 346.0, 358.0, 411.2, 449.6, 466.7, 481.0, 537.6, 644.0, 768.2, 873.1 and 893.2 nm very much similar to those observed in other glass hosts except a small shift in position or broadening. These peaks could be easily assigned to be due to 3F4 ) 3H4, 3L9 ) 3H4, 3K7 ) 3H4, 5G5 ) 3H4, 5 G6 ) 3H4, 3K8 ) 3H4, 5F3 ) 3H4, 5S2(5F4) ) 3H4, 5F5 ) 3H4, 5I4 ) 3H4, and 5I5 ) 3H4 transitions. The oscillator strength for different transitions were determined using the relation Fm ¼ 4:32 109
Z xðnÞdv
where x(n) is the molar absorptivity at frequency n. We have also calculated the oscillator strength using Judd Ofelt theory [17,18] and a good agreement is observed. We have calculated
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the branching ratio, transition probability, total transition probability and radiative lifetime of these levels. The Ho3þ doped tellurite glass on excitation with 476.5 nm line of Arþ gives two fluorescence peaks at 540 nm and at 650 nm due to 5S2(5F4) / 5I8 and 5F5 / 5I8 transitions, respectively. The peak in the green region has two components. While the first peak is very intense, the second one is weak. The fluorescence intensity is optimum for 1.5 mol% concentration. It shows a quenching for higher concentration of rare earth. We also monitored the fluorescence spectrum of the glass (1 mol% rare earth) at different temperatures from 298 K to 533 K. It is noted that the fluorescence intensity decreases as the temperature is raised. This can be understood to be due to increase in non-radiative relaxation at higher temperatures. The non-radiative relaxation WNR is given by WNR ¼
1 1 tm tR
where tm is the measured lifetime of the level and tR is the corresponding radiative lifetime. At room temperature this value is 1.5 104/s which increases with temperature. We measured the lifetime of 5S2(5F4) for present glass using 308 nm radiation from a XeCl laser at different temperatures. The measured lifetime at room temperature is 31 ms and is seen to decrease with the increase of temperature. It is observed that when we excite the Ho3þ doped glass with 812 nm (12312 cm1) radiation from a TieSapphire laser upconversion emission is seen in red, green and blue regions (see Fig. 1). These bands could easily be assigned to be due to 5F5 / 5I8, 5S2(5F4) / 5I8 and 5F3 / 5I8 transitions, respectively. The signal in green region is almost 50 times more intense than in the red and blue regions. The intensity variation of green upconversion signal with pump power is shown in Fig. 2. The slope of this plot is 1.7. A similar plot for red and blue upconversions yields values 1.9
Fig. 1. Upconversion emission in Ho3þ doped lithium tellurite glass on pumping with 812 nm radiation.
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Fig. 2. Variation of intensity with laser power (a) upconversion and (b) fluorescence.
and 1.8, respectively. These values indicate that two photons of the pump laser are involved in these upconversions. It is worth mentioning that blue upconversion has earlier been observed for Ho3þ mostly in crystalline hosts [19e21]. In the case of glasses use of high power pulsed lasers emitting in red regions has been required to get these upconversions (3). These upconversions may arise either by excited state absorption (ESA) or by energy transfer (ET). The low lying excited energy states in Ho3þ, lie at (13185 cm1) 5I4, (11185 cm1) 5I5, (8650 cm1) 5I6 and (5150 cm1) 5I7, respectively. The pump laser at 812 nm can populate the 5 I5 level through phonon-assisted excitation. Since the lifetime of 5I5 is <0.5 ms, the excited ion in 5I5 level rapidly relaxes to the longer lived levels 5I6 (t w 60 ms) and 5I7 (t w 3 ms). These metastable levels act as reservoirs. The excited level 5S2(5F4) lying at energy 18 500 cm1 cannot be populated through energy transfer amongst the ions lying in 5I6 or 5I7. Therefore the excited state absorption seems to be the most probable mechanism for these upconversions. The excited Ho3þ ion in the 5I6 level absorbs an additional pump photon to reach the 5F2 level. The ions from 5 F2 level relax via 5F3 level to populate the close lying 5S2(5F4) and 5F5 levels. Transitions from these levels viz 5F3 / 5I8 (blue fluorescence), 5S2(5F4) / 5I8 (green fluorescence) and 5F5 / 5I8 (red fluorescence) give upconversion transitions. We also studied the variation of the intensity of the green upconversion fluorescence with temperature of the glass. It is found that the upconversion emission intensity also decreases linearly with temperature as was observed for the normal fluorescence on 476.5 nm excitation.
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3.2. Studies on Er3þ doped tellurite glass The absorption spectrum of Er3þ doped tellurite glass shows nine bands in the 300e900 nm regions. These bands are easily assigned to be due to 2K15/2 ) 4I15/2 (360 nm), 4G11/2 ) 4I15/2 (375 nm), 4G9/2 ) 4I15/2 (416 nm), 4F5/2 ) 4I15/2, (445 nm), 4F7/2 ) 4I15/2 (495 nm), 2H11/2 ) 4I15/2 (520 nm), 4S3/2 ) 4I15/2 (543 nm), 4F9/2 ) 4I15/2 (653 nm) and 4I9/2 ) 4I15/2 (800 nm) transitions. The bands involving levels 4G11/2 and 2H11/2 are very intense compared to other bands. The oscillator strength, transition probability, total transition probability for different transitions and radiative lifetime for different levels of Er3þ in LiTeO2 glass have again been calculated in the same way. We have also recorded the fluorescence spectrum of this glass using 476.5 nm line of Arþ as excitation source. Two bands one in green and the other in red region are observed. The green band has a weak component (at 530 nm) and an intense one (at 551 nm) due to 2H11/2 / 4I15/2 and 4S3/2 / 4I15/2 transitions, respectively. The red band at 654 nm is again relatively weak and arises due to 4F9/2 / 4I15/2 transition. Er3þ doped in different crystal/glass hosts is known to give upconversion spectrum when pumped with NIR radiation. We observed that tellurite glass shows intense green emission at room temperature when pumped with 780 nm radiation from a TieSapphire laser. At higher pump power (>150 mW) red and ultraviolet upconverted fluorescence are also seen. A Stokes line at 798 nm also appears (see Fig. 3). The 780 nm radiation excites the 4I9/2 level. The appearance of Stokes fluorescence at 798 nm due to 4I9/2 ) 4I15/2 transition supports this. Ultraviolet fluorescence at 380 nm is due to 4G11/2 / 4I15/2 transition. Similarly the green fluorescence is due to 2H11/2 / 4I15/2 (530 nm), 2S3/2 / 4I15/2 (551 nm) and the red is due to 4F9/2 / 4I15/2 (654 nm). We also recorded these spectra at different pump powers. A plot of log I versus log P for different transitions is shown in Fig. 4. The dependence of the fluorescence signal on pump power yields a slope w2.96 for ultraviolet upconversion, 1.77 for 530 nm, 1.66 for 551 nm, 2.01 for 654 nm and 0.94 for the emission at 798 nm. This clearly establishes that the ultraviolet emission is due to threephoton absorption while green and the red fluorescence are due to two-photon absorption. The Stoke fluorescence is due to one photon process. Different processes may lead to population of high lying excited states of Er3þ. Since the upconversion emission appears even at very low pump power it cannot be due to simultaneous absorption of two or three photons. Another way of populating upper levels may be energy transfer (ET) amongst the excited Er3þ ions or through the absorption of photons by the
Fig. 3. Upconversions observed in the spectrum of Er3þ doped in lithium tellurite glass under 780 nm excitation.
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Fig. 4. A logarithmic plot of laser power versus emission intensity under 780 nm excitation.
ions present in excited state (ESA). The low lying excited states 4I9/2, 4I11/2 and 4I13/2 have energies 12 402 cm1, 10 141 cm1, 6524 cm1 and lifetimes w9 ms, w11 ms and w13 ms, respectively. All these levels are suitable for energy transfer as well as for ESA. The energy transfer amongst the ions in 4I13/2 level will not populate any level of our interest. However, energy transfer amongst the ions in 4I9/2 or 4I11/2 can excite the ions above 2H11/2(4S3/2) level but below 4G11/2 level. Ions in the levels above 2H11/2(4S3/2) relax to populate 2H11/2(4S3/2) level. Two things may happen here. I Most of the excited ions in 2H11/2(4S3/2) make a radiative transition to the ground state yielding green fluorescence. While some of these ions relaxed non-radiatively to the 4 F9/2 level are responsible for red fluorescence. Ions left in this state absorb a fresh incident photon (780 nm) and are excited to 4G11/2 level yielding the ultraviolet fluorescence. II A pair of excited ions each in 2H11/2(4S3/2) state shares their energy in such a way that one ion (from 4S3/2) is promoted to 4G11/2 and the other returns to a lower level to populate the 4 F9/2 level (or directly to the ground state). This 4F9/2 level is the origin of the red fluorescence. Channel (II) seems more probable as when ultraviolet fluorescence is observed red fluorescence also appears very intense. Another excitation may involve the ions in 4I13/2 level which absorb fresh incident photon and are promoted resonantly to the 2H11/2 level. Absorption of another photon by an ion in 2H11/2 will promote Er3þ to the 4G11/2 to give ultraviolet upconversions. Both channels may be contributing to the upconversions. It is possible to measure the temperature of an enclosure in which the glass is placed by monitoring the intensity ratio of the fluorescence arising from the two close lying levels of the rare earth ion present in the glass. Er3þ and Ho3þ both are suitable for such type of studies since they have two such close lying levels (2H11/2 and 4S3/2 in Er3þ) and (5F4 and 5S2 in Ho3þ).
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The fluorescence intensity ratio ðFIRÞ ¼
105
N2 I2j g2 s2j u2j DE ¼ ¼ exp KT N1 I1j g1 s1j u1j
where Ni is the number of ions, Iij is the fluorescence intensity from the two levels, gi is the degeneracy of the level, sij is the emission cross section and uij is the transition probability. DE is the separation between the two levels. A preliminary study on Er3þ in LiTeO2 indicates that the system works well within 300e500 K with an accuracy of 3e5%. More detailed study is in progress. 4. Conclusion Absorption and fluorescence spectra of Er3þ and Ho3þ doped into lithium tellurite glass have been studied at different concentrations of rare earth and at different temperatures of the glass. Concentration quenching and non-radiative relaxation at higher temperature have been observed. Intense upconversions in ultraviolet/blue, green and red regions have been observed when the glasses are excited with NIR radiation and their appearance is explained.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Y. Gao, Q.H. Nie, T.F. Xu, X. Shen, Spectrochim. Acta A 61 (2005) 1259. P.S. Peijzel, A. Meijerink, Chem. Phys. Lett. 401 (2005) 341. S. Kuck, I. Sokolska, Chem. Phys. Lett. 325 (2000) 257. M. Zambelli, A. Speghini, G. Ingletto, C. Locatelli, M. Bettinelli, F. Vetrone, J.C. Boyer, J.A. Capobianco, Opt. Mater. 25 (2004) 215. J.C. Boyer, F. Vetrone, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Appl. Phys. 93 (2003) 9460. J.C. Boyer, F. Vetrone, J.A. Capobianco, A. Speghini, M. Zambelli, M. Bettinelli, J. Lumin. 106 (2004) 263. M. Malinowski, M. Kaczkan, A. Wnuk, M. Szuflinska, J. Lumin. 106 (2004) 269. B.R. Reddy, S.K. Nash-Stevenson, P. Venkateswarlu, J. Opt. Soc. Am. B 11 (1994) 923. K. Hirao, S. Kishimoto, K. Tanaka, S. Tanabe, N. Soga, J. Non-Cryst. Solids 139 (1992) 151. R. Rolli, K. Gatterer, M. Wachtler, M. Bettinelli, A.S. peghini, D. Ajo, Spectrochim. Acta A 57 (2001) 2009. L. Esterowitz, A. Schnitzler, J. Noonan, J. Bahler, Appl. Opt. 7 (1968) 2053. L. Esterowitz, J. Noonan, J. Bahler, Appl. Phys. Lett. 10 (1967) 126. V.V. Ovsyankin, P.P. Feofilov, Opt. Spectrosc. 31 (1971) 510. L.F. Johnson, H.G. Guggenheim, Appl. Phys. Lett. 19 (1971) 44. X. Zhang, J. Jourt, G. Mary, J. Phys. Condens. Matter. 10 (1998) 493. S.B. Rai, A.K. Singh, S.K. Singh, Spectrochim. Acta A 59 (2003) 3221. B.R. Judd, Phys. Rev. 127 (1962) 750. G.S. Ofelt, J. Chem. Phys. 27 (1962) 511. E. Osiac, I. Sokolska, S. Kuck, Phys. Rev. B 65 (2002) 235119. G.A. Kumar, R. Riman, S.C. Chae, Y.N. Jang, I.K. Bae, H.S. Moon, J. Appl. Phys. 95 (2004) 3243. R. Lisiecki, G.D. Dzik, W.R. Romanowski, T. Lukasiewicz, J.Appl. Phys. 96 (2004) 6323.
Anant Kumar Singh has done his B.S. and M.S. from BHU, Varanasi, India. He is working for his Ph.D. from the same university collaborated with Purvanchal University, India. Currently he is working on rare earth / transition metal doped glassy network to search new material for the laser application. Now he has more than five research papers to his credit. Anita Rai is lecturer in the Department of Chemistry of a Post Graduate college. She did her M.S. and Ph.D. from Gorakhpur University, U.P. India. She has worked on organometallic compounds and on rare earth doped glasses. She has more than 15 research papers to her credit.
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S.B. Rai, Professor of Physics, Banaras Hindu University, India did his graduation from the same University. He has more then 35 years teaching / research experience and published more than 200 research papers in different area of atomic, molecular and laser physics. Presently, he is working on rare earth doped glasses suitable for UV / Visible and upconversion lasers.
Optical properties and upconversion in Er3þ and Ho3þ doped in lithium tellurite glass Anant Kumar Singh a, S.B. Rai a,*, Anita Rai b a
Department of Physics, Laser and Spectroscopy Lab, Banaras Hindu University, Varanasi 221 005, Uttar Pradesh, India b Department of Chemistry, Jagatpur P.G. College, Varanasi, India
Abstract Ho3þ and Er3þ doped lithium tellurite glasses have been formed with different concentrations of rare earth and their optical properties are derived on the basis of their absorption and fluorescence spectra. Intense upconversion emission in ultraviolet/violet, green and red regions have been observed when these glasses are pumped with NIR radiation. A preliminary study indicates that they can be used as temperature sensors. Ó 2006 Elsevier Ltd. All rights reserved. PACS: -42.70 Ce Keywords: Upconversion; ESA; ET; Temperature sensor
1. Introduction Glasses have certain advantage over crystals as host material. Glasses can be made of different shapes and sizes easily and can also be doped with larger concentrations of the lasing atoms. Due to energy level broadening, the ions in a glass host can absorb larger incident energy compared to the same ion in a crystal, which lowers the threshold for lasing. Rare earth doped glasses beside their applications as lasing material are also used as on line amplifier. They are used as storage material, high energy particle detector, sensors, X-ray and g-ray * Corresponding author. Tel.: þ91 542 2307308; fax: þ91 542 368468. E-mail address: [email protected] (S.B. Rai). 0960-8974/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pcrysgrow.2006.03.014
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absorber, etc. Tellurite glasses are superior to many other oxide glasses because of their high chemical and thermal durability and mechanical strength. They are also non-hygroscopic. In the present paper we have studied the optical properties of Ho3þ and Er3þ doped in lithium tellurite glass. The optical properties of Ho3þ and Er3þ doped in different lattices have been studied earlier by large number of workers [1e10]. For example Reddy et al. [8] have observed green upconversion in Ho3þ:LaF3 whereas Hirao et al. [9] using sodium tellurite glass as host for Ho3þ reported green as well as red upconversions. Codoping Yb3þ with Ho3þ in glass hosts has been found to enhance the upconversion efficiency [11e13]. Similarly red and green upconverted fluorescences have also been observed in several glass lattices [11e15] doped with Er3þ. In this paper we report the observation of upconversion emission in red, green and blue/ UV regions from Ho3þ and Er3þ doped in LiTeO2 glass when pumped with NIR laser radiation. The effect of temperature, concentration of rare earth ion and that of input power on the fluorescence and upconverted emission intensity has been studied. The lifetime of the 5F4(5S2) level of Ho3þ and of 4S3/2 level in Er3þ has been measured. 2. Experimental procedure The chemical composition of Ho3þ and Er3þ doped lithium tellurite glasses is (80x) mol% TeO2 þ 20 mol% Li2CO3 þ x mol% Rare earth oxide with x ¼ 0.5, 1.0, 1.5, 2.0 and 2.5 mol%. The method used to develop the glass is very similar to that described in our earlier paper [16]. The absorption spectra of the glasses with 1 mol% of rare earth were recorded using a Cary 2390 UVevisibleeNIR spectrophotometer in the range 300e900 nm. The fluorescence spectra were recorded using 476.5 nm (100 mW) line of Arþ laser and 0.5 m Spex monochromater in the 500e700 nm region for different concentrations of the rare earth and also at different temperatures. For upconversion studies we have used NIR radiation from a TieSapphire laser. On focusing the NIR radiation in the glass green emission is seen [at 780 nm in Er3þ and at 812 nm in Ho3þ] even at very small pump power. On increasing the laser power, fluorescence in ultraviolet/violet and in red regions is also seen along with green. Fluorescence and upconversion have also been recorded for different concentrations of rare earth and at different temperatures. 3. Results and discussion 3.1. Studies on Ho3þ doped tellurite glass The absorption spectrum of Ho3þ (1 mol%) in tellurite host shows 12 bands at 330.0, 346.0, 358.0, 411.2, 449.6, 466.7, 481.0, 537.6, 644.0, 768.2, 873.1 and 893.2 nm very much similar to those observed in other glass hosts except a small shift in position or broadening. These peaks could be easily assigned to be due to 3F4 ) 3H4, 3L9 ) 3H4, 3K7 ) 3H4, 5G5 ) 3H4, 5 G6 ) 3H4, 3K8 ) 3H4, 5F3 ) 3H4, 5S2(5F4) ) 3H4, 5F5 ) 3H4, 5I4 ) 3H4, and 5I5 ) 3H4 transitions. The oscillator strength for different transitions were determined using the relation Fm ¼ 4:32 109
Z xðnÞdv
where x(n) is the molar absorptivity at frequency n. We have also calculated the oscillator strength using Judd Ofelt theory [17,18] and a good agreement is observed. We have calculated
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the branching ratio, transition probability, total transition probability and radiative lifetime of these levels. The Ho3þ doped tellurite glass on excitation with 476.5 nm line of Arþ gives two fluorescence peaks at 540 nm and at 650 nm due to 5S2(5F4) / 5I8 and 5F5 / 5I8 transitions, respectively. The peak in the green region has two components. While the first peak is very intense, the second one is weak. The fluorescence intensity is optimum for 1.5 mol% concentration. It shows a quenching for higher concentration of rare earth. We also monitored the fluorescence spectrum of the glass (1 mol% rare earth) at different temperatures from 298 K to 533 K. It is noted that the fluorescence intensity decreases as the temperature is raised. This can be understood to be due to increase in non-radiative relaxation at higher temperatures. The non-radiative relaxation WNR is given by WNR ¼
1 1 tm tR
where tm is the measured lifetime of the level and tR is the corresponding radiative lifetime. At room temperature this value is 1.5 104/s which increases with temperature. We measured the lifetime of 5S2(5F4) for present glass using 308 nm radiation from a XeCl laser at different temperatures. The measured lifetime at room temperature is 31 ms and is seen to decrease with the increase of temperature. It is observed that when we excite the Ho3þ doped glass with 812 nm (12312 cm1) radiation from a TieSapphire laser upconversion emission is seen in red, green and blue regions (see Fig. 1). These bands could easily be assigned to be due to 5F5 / 5I8, 5S2(5F4) / 5I8 and 5F3 / 5I8 transitions, respectively. The signal in green region is almost 50 times more intense than in the red and blue regions. The intensity variation of green upconversion signal with pump power is shown in Fig. 2. The slope of this plot is 1.7. A similar plot for red and blue upconversions yields values 1.9
Fig. 1. Upconversion emission in Ho3þ doped lithium tellurite glass on pumping with 812 nm radiation.
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Fig. 2. Variation of intensity with laser power (a) upconversion and (b) fluorescence.
and 1.8, respectively. These values indicate that two photons of the pump laser are involved in these upconversions. It is worth mentioning that blue upconversion has earlier been observed for Ho3þ mostly in crystalline hosts [19e21]. In the case of glasses use of high power pulsed lasers emitting in red regions has been required to get these upconversions (3). These upconversions may arise either by excited state absorption (ESA) or by energy transfer (ET). The low lying excited energy states in Ho3þ, lie at (13185 cm1) 5I4, (11185 cm1) 5I5, (8650 cm1) 5I6 and (5150 cm1) 5I7, respectively. The pump laser at 812 nm can populate the 5 I5 level through phonon-assisted excitation. Since the lifetime of 5I5 is <0.5 ms, the excited ion in 5I5 level rapidly relaxes to the longer lived levels 5I6 (t w 60 ms) and 5I7 (t w 3 ms). These metastable levels act as reservoirs. The excited level 5S2(5F4) lying at energy 18 500 cm1 cannot be populated through energy transfer amongst the ions lying in 5I6 or 5I7. Therefore the excited state absorption seems to be the most probable mechanism for these upconversions. The excited Ho3þ ion in the 5I6 level absorbs an additional pump photon to reach the 5F2 level. The ions from 5 F2 level relax via 5F3 level to populate the close lying 5S2(5F4) and 5F5 levels. Transitions from these levels viz 5F3 / 5I8 (blue fluorescence), 5S2(5F4) / 5I8 (green fluorescence) and 5F5 / 5I8 (red fluorescence) give upconversion transitions. We also studied the variation of the intensity of the green upconversion fluorescence with temperature of the glass. It is found that the upconversion emission intensity also decreases linearly with temperature as was observed for the normal fluorescence on 476.5 nm excitation.
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3.2. Studies on Er3þ doped tellurite glass The absorption spectrum of Er3þ doped tellurite glass shows nine bands in the 300e900 nm regions. These bands are easily assigned to be due to 2K15/2 ) 4I15/2 (360 nm), 4G11/2 ) 4I15/2 (375 nm), 4G9/2 ) 4I15/2 (416 nm), 4F5/2 ) 4I15/2, (445 nm), 4F7/2 ) 4I15/2 (495 nm), 2H11/2 ) 4I15/2 (520 nm), 4S3/2 ) 4I15/2 (543 nm), 4F9/2 ) 4I15/2 (653 nm) and 4I9/2 ) 4I15/2 (800 nm) transitions. The bands involving levels 4G11/2 and 2H11/2 are very intense compared to other bands. The oscillator strength, transition probability, total transition probability for different transitions and radiative lifetime for different levels of Er3þ in LiTeO2 glass have again been calculated in the same way. We have also recorded the fluorescence spectrum of this glass using 476.5 nm line of Arþ as excitation source. Two bands one in green and the other in red region are observed. The green band has a weak component (at 530 nm) and an intense one (at 551 nm) due to 2H11/2 / 4I15/2 and 4S3/2 / 4I15/2 transitions, respectively. The red band at 654 nm is again relatively weak and arises due to 4F9/2 / 4I15/2 transition. Er3þ doped in different crystal/glass hosts is known to give upconversion spectrum when pumped with NIR radiation. We observed that tellurite glass shows intense green emission at room temperature when pumped with 780 nm radiation from a TieSapphire laser. At higher pump power (>150 mW) red and ultraviolet upconverted fluorescence are also seen. A Stokes line at 798 nm also appears (see Fig. 3). The 780 nm radiation excites the 4I9/2 level. The appearance of Stokes fluorescence at 798 nm due to 4I9/2 ) 4I15/2 transition supports this. Ultraviolet fluorescence at 380 nm is due to 4G11/2 / 4I15/2 transition. Similarly the green fluorescence is due to 2H11/2 / 4I15/2 (530 nm), 2S3/2 / 4I15/2 (551 nm) and the red is due to 4F9/2 / 4I15/2 (654 nm). We also recorded these spectra at different pump powers. A plot of log I versus log P for different transitions is shown in Fig. 4. The dependence of the fluorescence signal on pump power yields a slope w2.96 for ultraviolet upconversion, 1.77 for 530 nm, 1.66 for 551 nm, 2.01 for 654 nm and 0.94 for the emission at 798 nm. This clearly establishes that the ultraviolet emission is due to threephoton absorption while green and the red fluorescence are due to two-photon absorption. The Stoke fluorescence is due to one photon process. Different processes may lead to population of high lying excited states of Er3þ. Since the upconversion emission appears even at very low pump power it cannot be due to simultaneous absorption of two or three photons. Another way of populating upper levels may be energy transfer (ET) amongst the excited Er3þ ions or through the absorption of photons by the
Fig. 3. Upconversions observed in the spectrum of Er3þ doped in lithium tellurite glass under 780 nm excitation.
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Fig. 4. A logarithmic plot of laser power versus emission intensity under 780 nm excitation.
ions present in excited state (ESA). The low lying excited states 4I9/2, 4I11/2 and 4I13/2 have energies 12 402 cm1, 10 141 cm1, 6524 cm1 and lifetimes w9 ms, w11 ms and w13 ms, respectively. All these levels are suitable for energy transfer as well as for ESA. The energy transfer amongst the ions in 4I13/2 level will not populate any level of our interest. However, energy transfer amongst the ions in 4I9/2 or 4I11/2 can excite the ions above 2H11/2(4S3/2) level but below 4G11/2 level. Ions in the levels above 2H11/2(4S3/2) relax to populate 2H11/2(4S3/2) level. Two things may happen here. I Most of the excited ions in 2H11/2(4S3/2) make a radiative transition to the ground state yielding green fluorescence. While some of these ions relaxed non-radiatively to the 4 F9/2 level are responsible for red fluorescence. Ions left in this state absorb a fresh incident photon (780 nm) and are excited to 4G11/2 level yielding the ultraviolet fluorescence. II A pair of excited ions each in 2H11/2(4S3/2) state shares their energy in such a way that one ion (from 4S3/2) is promoted to 4G11/2 and the other returns to a lower level to populate the 4 F9/2 level (or directly to the ground state). This 4F9/2 level is the origin of the red fluorescence. Channel (II) seems more probable as when ultraviolet fluorescence is observed red fluorescence also appears very intense. Another excitation may involve the ions in 4I13/2 level which absorb fresh incident photon and are promoted resonantly to the 2H11/2 level. Absorption of another photon by an ion in 2H11/2 will promote Er3þ to the 4G11/2 to give ultraviolet upconversions. Both channels may be contributing to the upconversions. It is possible to measure the temperature of an enclosure in which the glass is placed by monitoring the intensity ratio of the fluorescence arising from the two close lying levels of the rare earth ion present in the glass. Er3þ and Ho3þ both are suitable for such type of studies since they have two such close lying levels (2H11/2 and 4S3/2 in Er3þ) and (5F4 and 5S2 in Ho3þ).
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The fluorescence intensity ratio ðFIRÞ ¼
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N2 I2j g2 s2j u2j DE ¼ ¼ exp KT N1 I1j g1 s1j u1j
where Ni is the number of ions, Iij is the fluorescence intensity from the two levels, gi is the degeneracy of the level, sij is the emission cross section and uij is the transition probability. DE is the separation between the two levels. A preliminary study on Er3þ in LiTeO2 indicates that the system works well within 300e500 K with an accuracy of 3e5%. More detailed study is in progress. 4. Conclusion Absorption and fluorescence spectra of Er3þ and Ho3þ doped into lithium tellurite glass have been studied at different concentrations of rare earth and at different temperatures of the glass. Concentration quenching and non-radiative relaxation at higher temperature have been observed. Intense upconversions in ultraviolet/blue, green and red regions have been observed when the glasses are excited with NIR radiation and their appearance is explained.
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Anant Kumar Singh has done his B.S. and M.S. from BHU, Varanasi, India. He is working for his Ph.D. from the same university collaborated with Purvanchal University, India. Currently he is working on rare earth / transition metal doped glassy network to search new material for the laser application. Now he has more than five research papers to his credit. Anita Rai is lecturer in the Department of Chemistry of a Post Graduate college. She did her M.S. and Ph.D. from Gorakhpur University, U.P. India. She has worked on organometallic compounds and on rare earth doped glasses. She has more than 15 research papers to her credit.
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S.B. Rai, Professor of Physics, Banaras Hindu University, India did his graduation from the same University. He has more then 35 years teaching / research experience and published more than 200 research papers in different area of atomic, molecular and laser physics. Presently, he is working on rare earth doped glasses suitable for UV / Visible and upconversion lasers.