Energy transfer and 1.3 lm emission in Pr±Yb codoped tellurite glasses

Transcription

1 Journal of Non-Crystalline Solids 274 (2000) 55±61 Energy transfer and 1.3 lm emission in Pr±Yb codoped tellurite glasses Setsuhisa Tanabe *, Taku Kouda, Teiichi Hanada Faculty of Integrated Studies, Kyoto University, Kyoto , Japan Abstract To seek the possibility of an e cient 1.3 lm ampli er in oxide materials, the concentration and temperature dependence of uorescence properties of the Pr 3 : 1 G 4 3 H 5 at 1.3 lm and Yb 3 : 2 F 5=2 2 F 7=2 at 1 lm were investigated in codoped tellurite glasses with compositions of 75TeO 2 ±20ZnO±5Na 2 O±xPr 2 O 3 ±yyb 2 O 3. By 0.97 lm pumping, the 1.3 lm emission of Pr 3 was sensitized by the Yb Pr energy transfer. The lifetime obtained by 1.02 lm emission of Yb 3 at room temperature was concentration quenched with the presence of only 0.1 mol% Pr 2 O 3. The energy transfer e ciency from Yb to Pr in a codoped glass was found to be more than 90% even in the glass of YbŠ= PrŠ > 100. With increasing temperature, the Yb-lifetime in singly doped glass increased and that in codoped glass decreased. These results can be explained by assuming rapid energy migration between Yb ions and promotion of Yb 3 : 2 F 5=2! Pr 3 : 1 G 4 transfer at higher temperature. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction The energy transfer in rare earth codoped glasses have been studied in various combinations because sensitization phenomena of speci c luminescence are attractive from viewpoints of both application and basic interests [1±3]. The Yb 3 ion is one of the best known lanthanide elements as a sensitizer for visible upconversion emissions in Ho 3,Er 3,orTm 3 codoped glasses and crystals by 1 lm laser pumping. The absorption band located around 0.98 lm has a cross section which enables e cient pumping by high power III±V diode lasers now available, such as those for pumping erbium-doped ber ampli ers (EDFA). Also it is interesting to observe the sensitization of * Corresponding author. Tel.: ; fax: address: tanabe@chem.h.kyoto-u.ac.jp (S. Tanabe). Pr 3 : 1.3 lm emission, because the initial 1 G 4 level is located at 1.02 lm with a cross section which makes the pumping e ciency of Pr-doped ber ampli ers (PDFA) very low in various glass hosts [4,5]. Development of an e cient 1.3 lm-ampli er still attracts interest [6±8], since the worldõs landbased ber networks are composed mostly of simple single-mode bers operating at 1.3 lm. In this study, luminescence of Pr 3 and Yb 3 ions were investigated in tellurite glasses with various doping levels. A tellurite glass with composition of 75TeO 2 ±20ZnO±5Na 2 O [9] was selected as a host because it has chemical durability, thermal stability, and rare earth solubility, which reduces clustering and thus concentration quenching. The spectral features and lifetime of Pr 3 and Yb 3 ions were investigated for the glasses with various doping levels to determine the energy transfer e ciency. Moreover, temperature dependence of lifetimes in Yb singly and Yb±Pr codoped /00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S (00)

2 56 S. Tanabe et al. / Journal of Non-Crystalline Solids 274 (2000) 55±61 Fig. 1. Sensitization mechanisms of Pr:1.3 lm emission by Yb Pr energy transfer. glasses were also investigated to understand the relaxation and energy transfer mechanisms in these glasses (see Fig. 1). 2. Experimental Tellurite glasses with the composition of 75TeO 2 ±20ZnO±5Na 2 O±xPr 2 O 3 ±yyb 2 O 3 were prepared with various (x, y), where x and y are the Pr 2 O 3 - and Yb 2 O 3 -content in mol%, respectively. Chemicals of TeO 2 (99.999% purity (5N), Cerac), ZnO (4N), Na 2 CO 3 (4N), Yb 2 O 3 (Mitsuwa Chem., 3N) and Pr 6 O 11 (3N) were used as starting materials and a mixed batch was melted for 30 min in a gold crucible at 850±930 C depending on the rare earth content. The liquid was poured into a stainless mold at 300 C and the obtained glass was annealed at 330 C, which is near the glass transition temperature determined by di erential thermal analysis. The batch of composition with x y < 6:0 was vitri ed without crystallization and thus a glass with optical quality was obtained. The glass was cut and polished into about 5 5 2mm 3 size. The density of obtained glass was measured by Archimedean method with a silica glass as a standard. Fluorescence spectra in the range of 800±1600 nm was measured with a computer-controlled monochromator (Nikon, G-250) and InGaAsphotodiode (Electro-Optical System, IGA-010-H) by pumping with a diode laser (LD) (SDL P1). CW light of 970 nm from the LD was chopped with a chopper and a lock-in-ampli er (Shimadzu, AT-100AM) was used. For the lifetime measurement, a 100 MHz digital oscilloscope (LeCroy, LS-140) was used to monitor the uorescence decay as the time evolution of the output from the InGaAs-photodiode (f ˆ 160 khz). The same laser diode was used and the pulse modulation was done with an LD modulation box (Yamaki, KLJT-3PTM) by voltage output from a synthesized function generator (Toa Electronics, FS-2201) to obtain the pulse light of 970 nm with 5 ls width. For measurement of temperature dependence of uorescence characteristics, a He-cycling cryostat (Iwatani Plantec, TCU4) was used to vary the temperature of the glass sample from 15 to 300 K. For thermal conduction and temperature accuracy, samples less than 2 mm-thick were attached to a copper holder, which is kept at a lower pressure than atmosphere and surrounded by silica windows. The lifetime was determined by least-square tting of exponential functions to digital decay data from the oscilloscope. The error in the lifetime data is less than 20 ls (see Fig. 4). 3. Results 3.1. Absorption cross section Fig. 2 shows the absorption spectra of Pr 3 or Yb 3 doped tellurite samples, where the ordinate is normalized as the cross section calculated using the density and molecular weight. The Yb 3 : 2 F 2 5=2 F 7=2 transition has a large absorption cross section around 0.98 lm, whereas the Pr 3 : 1 G 3 4 H 4 band has a much smaller cross section around 1.02 lm. The cross section of Yb 3 at 978 nm is about 250 times larger than that of Pr Concentration dependence of uorescence properties Fig. 3 shows the Yb-content dependence of intensity ratio of Pr:1.3 lm to Yb:1 lm emission in the x ˆ 0:1 samples (0.1 mol% ± Pr 2 O 3 ). The ratio increases monotonically with increasing Ybcontent, y. The apparent maximum at 5 mol% may

3 S. Tanabe et al. / Journal of Non-Crystalline Solids 274 (2000) 55±61 57 transfer e ciency, g ET in the y ˆ 5 samples are plotted in Fig. 4. The g ET was calculated by Tanabe et al. [10] g ET ˆ 1 s f x =s 0 : 1 For the x ˆ 0 sample, the lifetime is 830 ls. However, it decreases to less than 100 ls with only 0.03 mol% Pr 2 O 3 doping, where the [Yb]/[Pr] ratio is about 167 and the g ET becomes 90%. The g ET becomes more than 95% at x ˆ 0:10. Fig. 5 shows the y-dependence of the lifetimes of Yb 3 : 2 F 5=2 and Pr 3 : 1 G 4 levels in the x ˆ 0:1 samples. The s f of the Yb 3 decreases with increasing y. Both s f are similar in the same composition for all the concentrations. Fig. 2. Absorption spectra of 75TeO 2 ±20ZnO±5Na 2 O±1Ln 2 O 3 glass (Ln ˆ Pr or Yb). The ordinate is normalized as the cross section per one ion Temperature dependence of uorescence properties Temperature variations of the Pr 3 : 1.3 lm emission spectra in a (0.1, 3.5) sample are shown in Fig. 6. Compared with the peak intensity at lower energy side, the emission band in the tellurite sample at 300 K has su cient intensity at 1.31 lm, for application at the telecommunication wavelength. Temperature dependence of the mean Fig. 3. Yb 2 O 3 -content dependence of intensity ratio in 75TeO 2 ±20ZnO±5Na 2 O±0.1Pr 2 O 3 ±yyb 2 O 3 glasses. The errors are less than the size of the data symbols. be due to partial crystallization of the y ˆ 6 sample. The Pr-concentration dependence of the lifetime of Yb 3 : 2 F 5=2 level and the calculated energy Fig. 4. Pr 2 O 3 concentration dependence of Yb 3 : 2 F 5=2 lifetime and the energy transfer e ciency in codoped glasses (y ˆ 5:0). The lines are drawn as a guide for the eye. The errors are less than the sizes of the data symbols.

4 58 S. Tanabe et al. / Journal of Non-Crystalline Solids 274 (2000) 55±61 Fig. 5. Yb 2 O 3 -content dependence of lifetime of Yb 3 and Pr 3 in 75TeO 2 ±20ZnO±5Na 2 O±0.1Pr 2 O 3 ±yyb 2 O 3 glasses. The errors are less than the size of the data symbols. Fig. 7. Temperature dependence of mean wavelength of emission bands in a codoped glass (0.1, 3.5). The errors are less than the size of the data symbols. Fig. 8. Temperature dependence of uorescence intensities of two ions in a codoped glass (0.1, 3.5). The errors are less than the size of the data symbols. Fig. 6. Temperature variations of uorescence spectra around 1.3 lm in 75TeO 2 ±20ZnO±5Na 2 O±0.1Pr 2 O 3 ±3.5Yb 2 O 3 glass. wavelength, k m, of the Pr:1.3 lm band and Yb:1 lm band is shown in Fig. 7. The k m was obtained by calculating a center of gravity. At 300 K the Pr 3 emission band is around 1.33 lm and shifts to longer wavelength with decreasing temperature. Fig. 8 shows the temperature dependence of the integrated intensity of these emission bands. The intensity of the Yb:1 lm-band decreased, whereas that of the Pr:1.3 lm band slightly increased with increasing temperature.

5 S. Tanabe et al. / Journal of Non-Crystalline Solids 274 (2000) 55±61 59 Fig. 9. Temperature dependence of lifetime of Yb 3 : 2 F 5=2 and the energy transfer e ciency in 75TeO 2 ±20ZnO±5Na 2 O± xpr 2 O 3 ±3.5Yb 2 O 3 glasses (x ˆ 0 and 0.1). The errors are less than the size of the data symbols. To investigate the temperature dependence of sensitization in codoped samples (y ˆ 3:5), the lifetime of the Yb 3 : 2 F 5=2 in a Yb-singly doped sample (x ˆ 0) and in a codoped glass (x ˆ 0:1) were measured and plotted in Fig. 9. The s f x ˆ 0:1 is much shorter than the s f x ˆ 0 and decreases with increasing temperature. The increase of the Yb lifetime in a singly doped sample was discussed elsewhere [11]. Calculated g ET Yb! Pr is also plotted in the same gure. It is seen that the g ET increaes at less than 100 K and almost saturates around 95% with temperature above 150 K. 4. Discussion 4.1. Evidence of sensitization of 1.3 lm The uorescence spectra shape was almost independent of concentration. The wavelength of the 1.3 lm band at room temperature in Fig. 6 is better for an ampli er than that of chalcogenide glasses, which have a maximum at longer wavelengths [12,13]. As can be seen from the absorption spectra in Fig. 2, the energy level of Yb: 2 F 5=2 is a little bit larger than that of the Pr: 1 G 4 and the cross section of the former is much larger than that of the latter. These situation makes the Yb 3 ion a very e ective sensitizer for the Pr 3 : 1 G 4 level which has a lower pumping e ciency, which makes the performance of PDFA worse than EDFA working at 1.55 lm. Since high-power laser diodes for pumping EDFA are now available for the wavelength of 0.98 lm, the absorption spectrum of Yb 3 can be desirable from a practical point of view. For the samples of 0.1 mol% Pr 2 O 3, the intensity of 1.3 lm emission by 0.97 lm pumping increased with increasing Yb 2 O 3 content as shown in Fig. 3. At constant Yb content, the s f decreased with addition of only 0.03 mol% Pr 2 O 3 as shown in Fig. 4. Accordingly, the g ET increases reaching 91% at x ˆ 0:03 and becomes more than 95% at x ˆ 0:1. With constant Pr content, the decrease of the Yb lifetime in Fig. 5 can also be ascribed to the increasing energy transfer e ciency with increasing Yb 2 O 3 content. Usually in most hosts, the lifetime of the Pr 3 : 1 G 4 level is much shorter than that of the Yb 3 : 2 F 5=2, owing to its small energy gap of about 3000 cm 1 [8]. In Yb-singly doped samples, the assumption of 100% quantum e ciency is reasonable at least in tellurites where the phonon energy is 750 cm 1 and thus 12 phonons are required to bridge the energy gap of the Yb 3 : 2 F 5=2, almost 10 4 cm 1. Based on the similarity of the apparent Pr-lifetime to that of Yb in the same samples with any concentrations in Fig. 5, we suggest that the rate-limiting process is the Yb! Pr energy transfer, because the relaxation of the 1 G 4 level should also be as fast in tellurite glasses Temperature dependence of spectra and energy transfer rate The wavelength shift of the Pr:1.3 lm band with temperature in Fig. 6 we attribute to thermalization of the 1 G 4 level. The increased intensity of the Pr-emission with increasing temperature in Fig. 8 is probably due to the promotion of the phononassisted energy transfer. It is clear that the

6 60 S. Tanabe et al. / Journal of Non-Crystalline Solids 274 (2000) 55±61 increasing temperature can improve the spectral properties of the present codoped samples as an ampli er at 1.31 lm. Compared with the decrease of the Yb:1 lm intensity with increasing temperature in Fig. 8, the increase of the Pr:1.3 lm is not remarkable. This can be due to the promoted multiphonon decay rate from the 1 G 4 level at higher temperature. In Fig. 9, the s f x ˆ 0 in a Yb-singly doped samples increases with temperature and the s f x ˆ 0:1 in a codoped sample decreased. Based on increasing lifetime with temperature in the Ybsingly doped sample, we suggest that the energy migration between Yb 3 ions is promoted with temperature, owing to increased spectral overlap by thermalization of electronic levels. The quantum e ciency of the 2 F 7=2 state should be almost unity without multiphonon relaxation as mentioned in Section 4.1. Another possibility is that the radiative decay rate may decrease with temperature, due to the change of site symmetry of Yb 3 in the tellurite samples with a relatively larger thermal expansion coe cient. The g ET by Eq. (1) increases with temperature and becomes invariant at temperatures >150 K. We suggest that the increase of the relative ratio of 1.3 lm band to 1 lm band with Yb-content in Figs. 3 and 9 is due to the Yb to Yb then to Pr energy transfer as shown in Fig. 10. E cient energy transfer from Yb 3 : 2 F 5=2 to Pr 3 : 1 G 4 in highly Yb-doped samples (y ˆ 5) is supported by the x-dependence of Yb-lifetime and g ET Yb! Pr in Fig. 4, where even 0.03 mol% Pr 2 O 3 quenches the Yb emission to less than one tenth (g ET > 90%). This concentration ratio corresponds to the [Yb]/[Pr] ratio ˆ Conclusion We have shown that the Pr:1.3 lm emission in tellurite glasses is sensitized by the energy transfer from Yb 3, which is promoted by the higher Yb concentrations and temperatures. The lifetime of the Yb 3 : 2 F 5=2 was concentration quenched by only 0.1 mol% Pr 2 O 3, while it was insensitive with the Yb 2 O 3 concentration in singly doped glasses. The energy transfer e ciency from Yb to Pr in a codoped glass was found to be more than 90% even at [Pr]/[Yb] < The Yb-lifetime in singly doped glass increased and that in Pr-codoped glass decreased with increasing temperature. We explain these results by considering energy migration between Yb 3 ions and promotion of the Yb: 2 F 5=2! Pr: 1 G 4 transfer at larger Yb-concentration and higher temperature. Acknowledgements The author thanks Professor E. Snitzer and Dr J.S. Wang for their valuable suggestions. This work was partially supported by the Support Center for Advanced Telecommunications Technology Research in Ministry of Postal and Telecommunication. References Fig. 10. Energy transfer and uorescence mechanisms in Pr±Yb codoped system. [1] F.E. Auzel, Proc. IEEE 61 (1973) 6. [2] H. Kuroda, S. Shionoya, T. Kushida, J. Phys. Soc. Jpn. 33 (1) (1972) 125. [3] S. Tanabe, K. Suzuki, N. Soga, T. Hanada, J. Opt. Soc. Am. B 11 (1994) 933. [4] Y. Ohishi, T. Kanamori, S. Takahashi, E. Snitzer, G.H. Sigel, Opt. Lett. 16 (1991) [5] E. Ishikawa, H. Tawarayama, K. Ito, H. Aoki, H. Yanagita, H. Toratani, Technical Report of IECIE 1996, OPE [6] D.W. Hewak, B.N. Samson, J.A. Mederios Neto, R.I. Laming, D.N. Payne, Electron. Lett. 30 (12) (1994) 968.

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