Solid State Communications 140 (2006) 335–339 www.elsevier.com/locate/ssc
Ho3+ sensitized upconverted emission from Er3+ doped TeO2 glass system Anant Kumar Singh a,∗ , S.B. Rai a , Anita Rai b a Department of Physics, Laser and Spectroscopy Laboratory, BHU, Varanasi – 221005, India b Department of Chemistry, Jagatpur P.G. College, Varanasi, India
Received 15 June 2006; received in revised form 11 August 2006; accepted 2 September 2006 by D.D. Sarma Available online 20 September 2006
Abstract The energy transfer between two similar rare earth ions (Ho3+ ↔ Er3+ ) doped in lithium tellurite glass has been studied. The change in fluorescence emission and the lifetime of the donor and acceptor involved in the energy transfer has been measured by varying the concentrations of Ho3+ (donor), keeping the Er3+ (acceptor) concentration fixed. It is found that the intensity of Er3+ bands increases on increasing the Ho3+ concentration. The lifetime of the Er3+ levels involved in transition also increases simultaneously. The probability of energy transfer and its quantum efficiency has also been studied. c 2006 Elsevier Ltd. All rights reserved.
PACS: 42.65 Pc; 78.20 Bh; 42.70 Hj; 42.55 Rz; 42.70 Ce Keywords: D. Quantum efficiency; D. Probability of energy transfer; D. Lifetime; D. Upconversion
1. Introduction The presence of two rare-earth ions in the same host material may lead to an energy transfer from the excited ion (donor) to the unexcited ion (acceptor). As a result of this, the intensity of acceptor emission increases. This effect has been found useful for improving the efficiency of phosphors and lasers. Reisfeld [1] and, Boureet and Fang [2] have discussed this situation, and have defined a non-radiative energy transfer efficiency term η, which can be related to the change in the donor excitation decay time, or change in the fluorescence intensity when the acceptor ion is also present as: Id τd η =1− =1− τdo Ido where τd (τdo ) is the radiative lifetime of the donor in the presence (absence) of the acceptor, while Id (Ido ) are the corresponding fluorescence intensities. Khandpal and Tripathi [3] have measured the lifetime of different levels of Eu3+ ion in Calibo glass in the presence and ∗ Corresponding author. Tel.: +91 0542 2307308; fax: +91 0542 2369889.
E-mail addresses:
[email protected] (A.K. Singh),
[email protected] (S.B. Rai). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.09.005
absence of Ho3+ ions. Their results imply that the donor– acceptor energy transfer is mediated through an electric dipole–dipole interaction. In 1993, Wang and Ohwaki [4] reported efficient near infrared-to-green upconversion in a transparent oxyfluoride glass ceramic containing YbF3 and ErF3 . Similar investigations of energy transfer and upconversions have been reported in several cases by several workers [5–20]. A mechanism for non-radiative energy transfer from donor to acceptor was first suggested by Forster [21] and later extended by Dexter [22] and Fong and Diestler [23]. In sodium borate glass codoped with Tb3+ and Er3+ , Joshi et al. [24] found that at low acceptor (Er3+ ) concentrations, the energy transfer is diffusion limited, while at higher acceptor concentrations it involves electric dipole–dipole interaction. Nakazava and Shionoya [25] studied energy transfer between Tb3+ and several other rare earth ions including Er3+ in calcium phosphate glass, and the energy transfer mechanism was shown to involve an electric quadrupolar interaction. The energy transfer from Tb3+ to Er3+ in a phosphate glass was also studied by Van Uitert [26], and a dipole–dipole transfer mechanism was reported. The energy transfer from Er3+ to Ho3+ in codoped glass on excitation of Er3+ with 980 nm laser radiation has been studied by Johnson et al. [27].
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Fig. 1a. Upconversion spectrum of [A] 0.5 mol% Ho3+ in TeO2 glass under 889.6 nm excitation [B] 1.5 mol% of Er3+ at different concentrations of Ho3+ in TeO2 glass under same excitation wavelength.
Inokuti and Hirayama [28] proposed a mathematical model for this, which also shed light on the mechanism involved in energy transfer. The near infrared-to-green and blue upconversion induced by the energy transfer from Yb3+ ions to Ln3+ ions (Er3+ , Ho3+ , Tm3+ ) has been investigated extensively in various host materials [29–38], and the first upconversion laser operating through energy transfer was also reported by Johnson and Guggenheim [16]. Keeping in view the above results, Ho3+ and Er3+ codoped in a tellurite glass has been prepared, and the upconversion fluorescence and the energy transfer from Ho3+ to Er3+ has been studied in the present work. This combination is interesting since it has several close-lying levels in which energy transfer is very likely. 2. Experimental The fluorescence spectra of the 3 types of tellurite glasses, namely one doped with 0.5 mol% of Ho3+ , another doped with 0.5 mol% of Er3+ and the third doped with both Ho3+ and Er3+ ions [with different concentrations] has been studied. The doped glasses were prepared by the usual melt and quenching method. A large number of codoped glasses with varying content of Ho3+ and a fixed concentration of Er3+ were prepared, and their fluorescence studied. The 889.6 nm radiation from a Ti-Sapphire laser (260 mW power) and the 532 nm [power 600 mW] radiation from a NdYAG laser have been used to excite the samples. The fluorescence was collected perpendicularly to the direction of the incident laser beam, and was monitored after disperson with a 0.5 m Spex monochromator. The measurements were carried out at different temperatures and at different input laser powers. The lifetimes of the levels have been measured in different experimental conditions.
Fig. 1b. Upconversion spectrum of Ho3+ doped TeO2 glass at different concentrations.
3. Analysis and discussion The absorption spectrum of the Ho3+ in tellurite glass has already been reported in our earlier paper [37]. There are 9 bands in the absorption spectrum of Ho3+ . Similarly there are 12 bands in the absorption spectrum of Er3+ -doped tellurite glass [38]. The bands observed in the two cases are assigned with the help of the energy level positions reported by Dieke [39]. The emission spectra of the three different glasses on excitation with 889.6 nm radiation are shown in Figs. 1a and 1b. As expected, the codoped glass shows emission bands due to Er3+ and Ho3+ both. The peaks at 380, 530, 551, and 660 nm are due to Er3+ , while those at 485, 545, 645 nm are due to Ho3+ . It is observed that the intensity of the fluorescence bands due to Ho3+ in the codoped glass increases more slowly [for green as well as for red both, but the blue one does not appear any more even at higher Ho3+ concentration] than it does for the glass containing only Ho3+ , perhaps due to energy transfer. On the other hand, the intensity of the fluorescence bands due to Er3+ in the codoped glass is larger than in the glass with Er3+ alone. Thus the intensity of the Er3+ peaks is seen to increase with an increase in Ho3+ concentration. It is suggested that some of the excited Ho3+ ions in 5 F4 (5 S2 ) state transfered their excitation energy to the unexcited Er3+ ions and raised them to the 4 S3/2 level. An energy level diagram representing this energy transfer is shown in Fig. 2. The lifetime of the 5 S2 and 5 F5 levels of Ho3+ and 4 S3/2 and 4 F9/2 levels of Er3+ have been measured. The values are 0.135 ms and 0.103 ms for the 5 S2 and 5 F5 levels of Ho3+ respectively. Similarly the two values are 0.570 ms and 0.304 ms for 4 S3/2 and 4 F9/2 levels of Er3+ respectively [see Table 1]. Furthermore, the energy of the Er3+ , 4 S3/2 level is relatively lower, and the difference in the excitation energies of
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A.K. Singh et al. / Solid State Communications 140 (2006) 335–339 Table 1 Lifetimes of 5 F5 and 5 S2 levels of Ho3+ and, 4 F9/2 and 4 S3/2 levels of Er3+ measured individually and in codoped glasses Concentration in mol%
Lifetime of 5 F5 level of Ho3+ (ms)
Lifetime of 5 S2 level of Ho3+ (ms)
Lifetime of 4 F9/2 level of Er3+ (ms)
Lifetime of 4 S3/2 level of Er3+ (ms)
0.2 0.5 1.0 1.3 1.5 0.2(Ho3+ ) + 1.5(Er3+ ) 0.5(Ho3+ ) + 1.5(Er3+ ) 1.0(Ho3+ ) + 1.5(Er3+ ) 1.2(Ho3+ ) + 1.5(Er3+ ) 1.5(Ho3+ ) + 1.5(Er3+ ) 1.7(Ho3+ ) + 1.5(Er3+ )
0.087 0.093 0.109 0.116 0.118 0.115 0.111 0.108 0.101 0.091 0.079
0.131 0.135 0.141 0.147 0.147 0.139 0.127 0.113 0.099 0.086 0.080
0.278 0.289 0.301 0.310 0.314 0.319 0.326 0.336 0.341 0.359 0.372
0.566 0.574 0.592 0.597 0.598 0.607 0.609 0.609 0.609 0.607 0.606
calculated, using the formula [40] Interatomic separation = (Total rare earth ion concentration)−1/3 ˚ 16.1 A, ˚ 15.0 A ˚ and 12.3 A ˚ respectively, and found to be 17.8 A, which is much larger than the average interatomic radial separation. Therefore, in this case the process of energy transfer must be due to a resonant multipolar interaction between the ions [41]. From the study of the fluorescence spectra of several mixed glasses with varying concentrations of Ho3+ (0.2, 0.5, 1.0, 1.2, 1.5, 1.6, 1.7 mol%) and a fixed concentration (∼1.5 mol%) of Er3+ , it is observed that on increasing the concentration of Ho3+ , the fluorescence intensity of Er3+ bands increase. The rate of increase is different for different bands, and one concludes that the saturation limit is different for different transitions. Thus the Er3+ fluorescence involving the transition 4F 4 3+ 9/2 → I15/2 (660 nm) saturates only at 1.7 mol% of Ho , 4 4 and the green emission at 551 nm due to S3/2 → I15/2 saturates at 1.2 mol% of Ho3+ . Similarly, the upconverted emission due to 4 G11/2 → 4 I15/2 transition of Er3+ in the UV region increases more slowly with the increasing concentration of Ho3+ and saturates only at 1.9 mol% of Ho3+ . At higher concentrations of Ho3+ , the resultant intensity of both Ho3+ and Er3+ bands decrease due to self quenching [42]. 4. Dynamics of Ho–Er energy transfer Fig. 2. Energy level scheme presenting mechanism of energy transfer from Ho3+ to Er3+ doped in TeO2 on exciting with 889.6 nm laser radiation. 5S 2
state of Ho3+ and the 4 S3/2 state of Er3+ is only 150 cm−1 . Similarly, the corresponding difference between the 5 F5 state of Ho3+ and the 4 F9/2 state of Er3+ is only 70 cm−1 . These small differences in energy suggest that mixing of these levels through Stark splitting and through thermalization will take place. The average interatomic separation between the two ions (Ho3+ :Er3+ ) for 0.2 mol%, 0.5 mol% and 1.5 mol% of Ho3+ and 1.5 mol% of Er3+ (fixed) in codoped glasses have also been
The enhancement in intensity for the red and the ultraviolet emissions of Er3+ on the addition of Ho3+ is obviously due to energy transfer from Ho3+ to Er3+ . The relevant energy levels for Ho3+ and Er3+ are shown in Fig. 2. The 889.6 nm radiation (11 145 cm−1 ) excites the 5 I5 level of Ho3+ ion resonantly. These ions relax non-radiatively to the relatively longer lived 5 I and 5 I levels. An ion in any of these (5 I , 5 I and 5 I ) 7 7 6 6 5 levels can absorb another laser photon and get promoted to the 5 F5, 5 F3 and 3 K8 levels respectively. Ions in the 5 F3 and 3 K levels relax to the lower levels 5 F , 5 F (5 S ) and 5 F , 8 3 4 2 5 which finally results in emission in the blue, green and red regions, respectively. Similarly the ions in 5 I6 , after absorption
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Fig. 3. Logarithmic plot of fluorescence decay curve arising due to 5 F5 → 5 I8 transition of Ho3+ at various concentrations and codoped with Er3+ .
Fig. 4. Plot of η0 /η versus C[Er + Ho]S/3 give straight line for S = 6 showing dipole–dipole interaction.
of incident photons, populate the 5 F4 (5 S2 ) level and that of 5 I7 , 5 F level of Ho3+ . 5 On the other hand, in the Er3+ -doped tellurite glass, the 4I 11/2 level gets populated on excitation with 889.6 nm radiation. Absorption of another incident photon by Er3+ ions in the 4 I11/2 level promotes the Er3+ ions to the 4 F5/2 level, which relax to the 2 H11/2 and 4 S3/2 metastable levels. These are the levels responsible for the strong green fluorescence in Er3+ - doped glasses. In fact, these two levels are also important for the other fluorescence lines observed in Er3+ -doped glasses. Er3+ ions in the 4 S3/2 level may relax non-radiatively to the 4F 9/2 level to give red fluorescence through a transition to the ground level. Some of the ions present in the 4 F9/2 level may absorb another incident photon and reach the 4 G11/2 level. The 4G 11/2 level may also be populated by absorption of incident photons by ions already in the 2 H11/2 /4 S3/2 levels. A transition from the 4 G11/2 level to the ground state emits UV fluorescence at 380 nm. When Er3+ and Ho3+ are present together in the glass, the incident radiation excites both types of ions, and the fluorescence seen is due to both. The fluorescence emission in the red and the green regions of the spectrum for the two types of ions overlap strongly, but the UV fluorescence of Er3+ is seen separately. The weak blue fluorescence observed in Ho3+ -doped glasses is not seen in the codoped glass. Since the 5 F5 level of Ho3+ has an excitation energy only 70 cm−1 more than the excitation energy for the 4 F9/2 level of Er3+ at room temperature (thermal energy kT at room temperature >70 cm−1 ), the two levels will be in thermal equilibrium. Moreover, 4 F9/2 is long lived and a transfer of energy from Ho3+ (5 F5 ) to Er3+ (4 F9/2 ) cannot be ruled out. This results in the additional excitation of the 4 F9/2 level of Er3+ . The increase in population of the 4 F9/2 level of Er3+ naturally enhances the fluorescence intensity of any band which involves this level. An increase in the concentration of Ho3+ in the codoped sample results in a greater energy transfer from the Ho3+ (5 F5 ) to the Er3+ (4 F9/2 ) state. As a result of this, the fluorescence intensity of the red fluorescence of Er3+ increases till saturation is reached.
As mentioned earlier, the 4 F9/2 level is populated not only through the energy transfer from 5 F5 of the Ho3+ ions, but also by a relaxation of the Er3+ ions in 4 S3/2 (2 H11/2 ). Ho3+ ions excited to the 5 S2 /5 F4 state can transfer their excitation energy to Er3+ ions to promote them to the 4 S3/2 (2 H11/2 ) levels, as the two excitation energies are nearly the same. This additional mechanism for the creation of Er3+ ions in the 4 S3/2 (2 H11/2 ) level is not seen to increase the intensity of the green fluorescence, since the signal is already nearly saturated. However these ions (Er3+ in 4 S3/2 , 2 H11/2 ) relax to the 4 F9/2 level and thereby enhance the red fluorescence. The ways of excitation of the 4 G11/2 level of Er3+ are through the states 4 F9/2 and 4 S3/2 . Hence, any increase in the population of these levels would add to the intensity of the emission from the 4 G11/2 level. The enhancement in UV fluorescence in the Er3+ in presence of Ho3+ is thus well understood. Since the excitation of 4 G11/2 occurs in a complex way, it is obvious that the saturation of the fluorescence intensity of the UV emission should occur at a higher concentration of Ho3+ . The lifetimes of the Er3+ and Ho3+ levels are also seen to vary gradually as one adds more and more Ho3+ [see Fig. 3; Table 1]. This is also in agreement with the above explanation. Energy transfer between Er3+ ions themselves can be neglected because self-quenching in Er3+ doped tellurite glass takes place at higher concentrations of Er3+ . Though Ho3+ fluorescence is seen to be quenched at much lower concentrations (0.25 mol%), this however would no longer be true in the codoped glass, since the Er3+ ions in the vicinity provide another way of energy removal. A graph plotted of ηη0 versus [C(Er) + C(Ho)]S/3 gives a straight line for S = 6 [Fig. 4]. η0 is the fluorescence efficiency τexp of the donor, given by the relation η0 = τRad , τexp , τRad being the measured lifetime and the radiative lifetime respectively of the donor Ho3+ , and η is the quantum efficiency of energy transfer as defined earlier. The quantum efficiencies obtained in the two cases are given in Table 2. This clearly indicates the interaction to be of a dipole–dipole type [28]. The probability of energy transfer was calculated using two different methods.
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A.K. Singh et al. / Solid State Communications 140 (2006) 335–339 Table 2 Quantum efficiency and probability of energy transfer for (Ho3+ ↔ Er3+ ) at 1.5 mol% Er3+ Concentration of Ho3+ (mol%)
0.2 0.5 1.0 1.3 1.5
Quantum efficiency of energy transfer (η)
Probability of energy transfer (×103 ) using Eq. (1) (s−1 )
Probability of energy transfer (×103 ) using Eq. (2) (s−1 )
5F 4
5S 2
5F 4
5S
5F 4
5S 2
0.19 0.24 0.30 0.39 0.53
0.17 0.22 0.29 0.33 0.38
73.90 89.19 102.50 137.41 199.73
51.77 67.50 83.20 113.91 181.32
57.23 85.90 99.02 132.81 194.30
32.75 52.33 71.29 104.61 178.38
I. If one assumes that the excited Ho3+ ions in the 5 S2 and 5 F4 levels transfer their energy to the Er3+ ions via multipolar interaction, the energy transfer probability is given by the formula 1 1 K1 = 5 − 5 (1) 5 τ ( S2 , F4 )Ho–Er τ ( S2 , 5 F4 )Ho where τ in the first term is the lifetime of the 5 S2 (5 F4 ) level of Ho3+ in the presence of Er3+ , and in the second term it is in the absence of Er3+ . These values are given in Table 2. II. The energy transfer probability can also be calculated using the relative intensities of fluorescence in the presence and absence of acceptor. 1 Ido −1 (2) P= τ Id where Ido and Id are the intensities of donor fluorescence in the absence and presence of the acceptor respectively, and τ0 is the lifetime of the donor. The two values are in good agreement to each other. Acknowledgements Authors are grateful to DST and CSIR New Delhi for financial support and also to Prof. D.K. Rai, Physics Department, BHU, Varanasi for valuable suggestions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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