Ho3+ co-doped tellurite glasses

Journal of Alloys and Compounds 450 (2008) 306–309

Investigation of energy transfer and frequency upconversion in Er3+/Ho3+ co-doped tellurite glasses Xudong Zhang a,∗ , Tiefeng Xu a , Shixun Dai a , Qiuhua Nie a , Xiang Shen a , Longjun Lu a , Xianghua Zhang b a

Faculty of Information Science and Engineering, The State Key Laboratory Base of Novel Functional Materials and Preparation Science, Ningbo University, Zhejiang 315211, PR China b UMR-CNRS 6512 “Verres et C´ eramiques”, Institut de Chimie de Rennes, Universit´e de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France Received 31 July 2006; received in revised form 19 October 2006; accepted 25 October 2006 Available online 28 November 2006

Abstract The energy-transfer mechanisms and frequency upconversion emissions in 0.5Er3+ /xHo3+ co-doped tellurite glasses by exciting at 980 nm have been investigated. Three intense upconversion luminescence emissions are observed at around 525, 548, and 660 nm, which correspond to Er3+ :2 H11/2 → 4 I15/2 , Er3+ :4 S3/2 → 4 I15/2 + Ho3+ :5 S2 (5 F4 ) → 5 I8 , and Er3+ :4 F9/2 → 4 I15/2 + Ho3+ :5 F5 → 5 I8 transitions, respectively. The upconversion emissions reach the maximum values when Ho2 O3 is 0.5 mol%, and the intensities of the green and red light emissions were about 4.5 and 6 times stronger than those un-doped Ho2 O3 , respectively. The possible upconversion mechanisms and energy transfer between Er3+ and Ho3+ were also estimated and evaluated. All the three emissions are based on two photon absorption processes. © 2006 Elsevier B.V. All rights reserved. Keywords: Energy transfer; Upconversion; Luminescence; Er3+ /Ho3+ co-doped

1. Introduction The frequency conversion of infrared-to-visible light by rareearth ions doped glasses has been extensively investigated owing to the potential applications in optical data storage, color displays, infrared sensors and biomedical diagnostics [1–3]. In recent years, blue, green or red upconversion emissions luminescence have been obtained by doping glasses with Tm3+ , Er3+ , Ho3+ , Nd3+ , etc., both at low and room temperature [4–7], respectively. It is well known that energy transfer was one of the most important upconversion mechanisms and had widely used to achieve more efficienct laser action by co-doping some sensitizer ions into laser materials [8–10]. According to previous studies [11,12], upconversion emission luminescence can be easily obtained from Er3+ , or Ho3+ doped glass system, and also have the same green (∼548 nm) and red (∼660 nm) upconversion emission bands. In our previous work [13], high efficient upconversion emissions were found in Er3+ /Ho3+ /Yb3+

∗

Corresponding author. Tel.: +86 574 87600358; fax: +86 574 87600946. E-mail addresses: [email protected], [email protected] (X. Zhang).

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.10.097

co-doped tellurite glasses, but the energy transfer between Er3+ and Ho3+ was not analyzed in detail. And until now, the energy transfer between Er3+ and Ho3+ was rarely investigated, only Johnson et al reported the energy transfer and upconversion luminescence of Er3+ /Ho3+ co-doped in crystals [14]. In this work, we have presented an investigation on the upconversion luminescence of Ho3+ , Er3+ co-doped tellurite glasses under 980 nm laser excitation. And then the effect of energy transfer between Er3+ and Ho3+ on the upconversion luminescence in this system was analyzed in detail. 2. Experimental Glasses composition employed were (70–x) TeO2 –25ZnO–5La2 O3 – 0.5Er2 O3 –xHo2 O3 (x = 0, 0.2, 0.5, 0.8, 1, 1.5, 2) in mol%. The starting materials were reagent grade TeO2 , ZnO, and La2 O3 , Ho2 O3 and Er2 O3 with more than 99.99% purity. About 15 g batches of starting materials were fully mixed and then melted in the Pt crucibles at 850 ◦ C in an electronic furnace. After completely melting, the glass liquids were poured into a stainless mold and then annealed to room temperature. The obtained glasses were cut and polished carefully to 10 mm × 10 mm × 1.2 mm in order to meet the requirements for optical measurements. Absorption spectrum was recorded on a PERKINELMER 900UV/VIS/NIR spectrophotometer. The upconversion spectra were measured with a TRIAX550 spectrophotometer on excitation at 980 nm laser

X. Zhang et al. / Journal of Alloys and Compounds 450 (2008) 306–309

Fig. 1. Absorption spectra of 0.5Er3+ /0.5Ho3+ co-doped tellurite glass.

diode (LD). In order to compare the luminescence intensity, the position and power (125 mW) of the pumping beam and the width (1 mm) of the slit were fixed under the same condition; and the samples were set at the same place in the experimental setup. The fluorescence lifetime of Er3+ :4 I13/2 level was measured with light pulses of 980 LD. The pulse modulation was performed to obtain a light pulse with 5 ␮s widths. The decay traces were recorded on a digital oscilloscope and fitted by single exponential functions to obtain the decay rates. All the measurements were taken at room temperature.

3. Results and discussion Fig. 1 shows typical absorption spectra of 0.5Er3+ / 0.5Ho3+ co-doped tellurite glass in the visible and near-infrared region. Each assignment corresponds to the excited level of Er3+ and Ho3+ ions. It is obvious that Er3+ :4 I15/2 → 4 F9/2 and 4 3+ 5 5 4I 15/2 → S3/2 transition overlap that of Ho : I8 → I5 and 5 I → 5 S (5 F ) transition, respectively [13]. Therefore, it is 8 2 4 expected that erbium and holmium ions co-doped in tellurite glasses should have stronger upconversion emissions due to the large overlap of Er3+ and Ho3+ ions in the visible region. The room temperature upconversion luminescence spectrum in the range of 500–700 nm for 0.5Er3+ doped and 0.5Er3+ / 0.5Ho3+ co-doped tellurite glasses under 980 nm excitation is shown in Fig. 2. Obviously, the green (525 and 548 nm) and red (660 nm) light emissions are simultaneously observed. The green (525 nm) emission is originated from the 2 H11/2 → 4 I15/2 transition of Er3+ ion. The green (548 nm) and red (660 nm) are attributed to the Er3+ :4 S3/2 → 4 I15/2 + Ho3+ :5 S2 (5 F4 ) → 5 I8 , and Er3+ :4 F9/2 → 4 I15/2 + Ho3+ :5 F5 → 5 I8 transitions, respectively[5,6]. It is important to mention at this point that the upconversion luminescence in 0.5Er3+ /0.5Ho3+ co-doped tellurite glass is intense enough to be seen by naked eye at excitation power as low as 60 mW. Fig. 3 shows the dependences of upconversion emission intensities upon the Ho2 O3 concentration in the 0.5Er3+ /xHo3+ co-doped tellurite glasses in which the Ho2 O3 concentration ranges from 0 to 2 mol%. Obviously, the intensities of the upconversion emissions first increase steeply with

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Fig. 2. Upconversion luminescence spectra of 0.5Er3+ doped and 0.5Er3+ / 0.5Ho3+ co-doped tellurite glasses under 980 nm excitation.

the Ho2 O3 addition, exhibit the maximum values at approximately 0.5 mol% Ho2 O3 , and then the 525 and 548 nm emission bands decrease steeply while the 660 nm decreases slightly with further Ho2 O3 addition. On the other hand, the 660 nm red light emission intensity ratio all increases with increasing Ho2 O3 concentration and reaches the maximum value 31.5% at Ho2 O3 = 2 mol%. It is worth noting that the intensities of green and red light emission bands at 0.5 mol%Ho2 O3 are about 4.5 and 6 times stronger than those un-doped Ho2 O3 , respectively. In frequency upconversion process, the upconversion emission intensity Iup increases in proportion to the nth power of n , where n infrared (IR) excitation intensity IIR , that is, Iup ∝ IIR is the number of IR photons absorbed per visible photon emitted [15]. A plot of log Iup versus log IIR yield a straight line with slope n. Fig. 4 shows such a plot for the 525, 548 and 660 nm emission in the 0.5Er3+ /0.5Ho3+ co-doped tellurite glass under 980 nm excitation. Values of 1.44, 1.63 and 1.54 were obtained for n corresponding to the 525, 548 and 660 nm emis-

Fig. 3. Dependence of upconversion emission intensities upon Ho2 O3 concentration in the tellurite glasses.

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X. Zhang et al. / Journal of Alloys and Compounds 450 (2008) 306–309

Fig. 5. Schematic upconversion mechanism under 980 nm excitation proposed for Er3+ /Ho3+ co-doped tellurite glasses. Fig. 4. Dependence of upconversion emission intensity on excitation power under 980 nm excitation for 0.5Er3+ /0.5Ho3+ co-doped tellurite glass.

sion bands, respectively. All the upconversion emissions show a nearly quadratic dependence of the luminescence intensity for these transitions on the excitation intensity as observed previously in Er3+ , or Ho3+ doped glasses [11,12]. The results indicate that a two-photon absorption process predominantly populates the green and red light emission bands. In order to discuss the upconversion luminescence mechanisms in Er3+ /xHo3+ co-doped tellurite glasses, the energy-transfer mechanisms between Er3+ ions and Ho3+ ions should be examined at first. It is well known that a criterion for resonance of energy transfer is according to overlap integral between the fluorescence spectrum of a donor ion and the absorption spectrum of an acceptor ion [16]. From Figs. 1 and 2, it can be seen that Er3+ and Ho3+ ions have the same absorption and emission bands at 548 and 660 nm, thus the resonance of energy transfer can occur in the visible region and that of energy transfer is not influence on the upconversion emission [13]. However, in the near-infrared region, it can be concluded that instead of the no-phonon-involved energy transfer, it is considered that phonon-assisted energy transfer may play an important role in the energy transfer between Er3+ and Ho3+ . According to Miyakawa and Dexter, the probability of phonon-assisted energy-transfer rate [WPAT (E)] is obtained from [17]: WPAT (E) = WPAT (0)exp(−βE)

(1)

where E is an energy gap between the energy level of an energy donor and that of an energy acceptor, WPAT (0) the transfer rate when E = 0, and β is a function of effective phonon energy and electron–phonon coupling strength. According to the energy matching and quadratic dependence on excitation power, the possible upconversion mechanisms for the emissions are discussed based on the simplified energy levels of Er3+ and Ho3+ presented in Fig. 5. From Fig. 5, it can be found that the Ho3+ ions have about 1563 cm−1 energy gaps under 980 nm excitation. Therefore, it is considered that the Er3+ is main contribution to the upconversion luminescence

and energy transfer between Er3+ and Ho3+ [14]. In the first step, the 4 I11/2 level of Er3+ is directly excited by 980 nm LD (GSA) and some of Er3+ ions decay to the 4 I13/2 level. At the same time Ho3+ ions are excited to the 5 I6 level and the energy excess (∼1563 cm−1 ) is given to the matrix, and also some of Ho3+ ions can be decay to the 5 I7 level. The second step involves the excitation processes based on the long-lived 4 I11/2 level of Er3+ and 5 I6 level of Ho3+ as follows: Cross-relaxation (CR),4 I11/2 (Er3+ ) + 4 I11/2 (Er3+ ) → 4 F7/2 (Er3+ ) + 4 I15/2 (Er3+ ); excited state absorption (ESA), 4 I11/2 (Er3+ ) + a photon → 4 F7/2 (Er3+ ), 5 I6 (Ho3+ ) + a photon →5 S2 (5 F4 )(Ho3+ ); energy transfer (ET), 4 I11/2 (Er3+ ) + 5 I6 (Ho3+ ) → 4 I15/2 (Er3+ ) + 5 S2 (5 F4 ) (Ho3+ ). On the other hand, the 4 I13/2 level of Er3+ and 5 I7 level of Ho3+ are also excited to the 4 F9/2 level and 5 F5 level through ESA or ET process, respectively. The populated Er3+ :4 F7/2 level then relaxes rapidly and nonradiatively to the next lower levels 2 H11/2 and 4 S3/2 resulting from the small energy gap between them. It is considered that the 2 H11/2 level is populated from 4 S3/2 by a fast thermal equilibrium between the levels [15]. The above processes then produces the two 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 green emissions centered at 525 and 548 nm, respectively, and the excited Er3+ ions at 4 F9/2 level generate the 660 nm emission. And meanwhile, the excited Ho3+ ions at 5 S2 (5 F4 ) and 5 F5 levels also generate 548 and 660 nm emission, respectively. From Fig. 3, it can be seen that the upconversion luminescence in Er3+ /Ho3+ co-doped tellurite glasses are enhanced with an increase of Ho3+ from 0 to 0.5 mol%, and then decrease with further Ho2 O3 addition. It is well known that the probability of energy-transfer due to multipolar interactions increases when the distance R between the sensitizer and the acceptor decreases [18]. On the contrary, the luminescence will also be quenched when the distance R exceeds the critical distance R0 by energy-transfer processes at the high concentration [19]. Thus one conclude that the decrease of upconversion luminescence intensity are due to the self quenching and cross relaxation of Ho3+ in their high concentration regions, which is consistent with the earlier findings in the Nd3+ /Ho3+ co-doped system [20].

X. Zhang et al. / Journal of Alloys and Compounds 450 (2008) 306–309

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Er3+ :4 S3/2 → 4 I15/2 + Ho3+ :5 S2 (5 F4 ) → 5 I8 , and Er3+ :4 F9/2 → 3+ 5 5 15/2 + Ho : F5 → I8 transitions, respectively, were observed 3+ 3+ in 0.5Er /xHo co-doping tellurite glasses. It is found that the upconversion luminescence intensities depend remarkable on the Ho2 O3 concentration and the 660 nm red light emission intensity ratio all increases with addition Ho2 O3 . This indicates that energies can be transferred from Er3+ to Ho3+ ions and the upconversion emission of Er3+ ions can be enhanced by codoped a suitable amount of Ho3+ ions under 980 nm excitation.

4I

Acknowledgments The work was supported by the Natural Science Foundation of Zhejiang Province, china (Grant No. Y104498), the Foundation of Science and Technology department of Zhejiang province (Grant Nos. 2005C31014 and 2006C21082). Fig. 6. The fluorescence decay curve for Er3+ :4 I13/2 → 4 I15/2 transition in 0.5Er3+ doped tellurite glass and the insert shows the dependence of lifetime of 4I 3+ 13/2 level and 1.5 ␮m emission intensity of Er ions upon Ho2 O3 concentration in 0.5Er3+ /xHo3+ co-doped tellurite glasses.

In addition, it is found that the 660 nm red light emission intensity ratio all increases with increasing Ho2 O3 concentration, and it is not consistent with above discussions. Eq. (1) implies that the smaller the energy pap (E) is, the faster the energy-transfer occurs. From Fig. 5, it can found that the energy between the Er3+ :4 I13/2 → 4 I15/2 transition (6527 cm−1 ) and the Ho3+ : 5 I6 → 5 F5 transition (6847 cm−1 ) is quite small, the energy-transfer Ho3+ :5 I6 + Er3+ :4 I13/2 → Ho3+ :5 F5 + Er3+ :4 I15/2 can easily occur. The fluorescence decay curve for Er3+ :4 I13/2 → 4 I15/2 transition in 0.5Er3+ doped tellurite glass is shown in Fig. 6. The inset shows the dependence of Er3+ : 4 I13/2 level lifetime and 1.5 ␮m emission intensity upon Ho2 O3 concentration in the tellurite glasses. From Fig. 6, it is found that the lifetime of Er3+ :4 I13/2 level and emission intensity steeply decreases with an addition Ho2 O3 . The result unambiguously demonstrates that the increase of 660nm red light emission intensity ratio is attributed to energy -transfer between the Er3+ :4 I13/2 → 4 I15/2 transition and the Ho3+ :5 I6 → 5 F5 transition. 4. Conclusions The efficient upconversion luminescence at around 525, 548, and 660 nm, which correspond to the Er3+ :2 H11/2 → 4 I15/2 ,

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