Yb3+ co-doped tellurite glasses

Yb3+ co-doped tellurite glasses

Spectrochimica Acta Part A 61 (2005) 1259–1262 Study of luminescence properties of novel Er3+ single-doped and Er3+/Yb3+ co-doped tellurite glasses Y...

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Spectrochimica Acta Part A 61 (2005) 1259–1262

Study of luminescence properties of novel Er3+ single-doped and Er3+/Yb3+ co-doped tellurite glasses Yuan Gao∗ , Qiu-Hua Nie, Tie-Feng Xu, Xiang Shen College of Information Science and Engineering, Ningbo University, Ningbo 315211, China Received 16 May 2004; accepted 28 June 2004

Abstract The novel Er3+ single-doped and Er3+ /Yb3+ co-doped tellurite glasses were prepared. The effect of Yb2 O3 concentration on absorption spectra, emission spectra and upconversion spectra of glasses were measured and investigated. The emission intensity, fluorescence full width at half maximum (FWHM) and upconversion luminescence of Er3+ go up with the increasing concentration of Yb3+ ions. The maximum FWHM of 4 I13/2 → 4 I15/2 transition of Er3+ is approximate 77 nm for 1.41 × 1021 ions/cm3 concentration of Yb3+ -doped glass. The visible upconversion emissions at about 532, 546 and 659 nm, corresponding to the 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions of Er3+ , respectively, were simultaneously observed under the excitation at 970 nm. Subsequently, the possible upconversion mechanisms and important role of Yb3+ on the green and red emissions were discussed and compared. The results demonstrate that this kind of tellurite glass may be a potentially useful material for developing potential amplifiers and upconversion optical devices. © 2004 Elsevier B.V. All rights reserved. Keywords: Tellurite glasses; Rare earth ions; Luminescence properties; Upconversion

1. Introduction Recently, there has been a renaissance in the study of rare earth doped materials for photonic applications, e.g. phosphors, display monitors, X-ray imaging, scintillators, lasers, upconversion and amplifiers for fiber optical communications [1]. Rare earth ions, especially erbium, have played an important role in the development of broadband erbium-doped fiber amplifiers (EDFA) at the third communication window (1.5 um) [1] and frequency upconversion for their potential laser applications in many fields during the past few decades [2]. Therefore, the choice of host material is very important in the development of more efficient optical devices based on Er3+ -doped glasses. Among the numerous host glasses, tellurite glasses combine the attributes of wide transmission region, good corrosion resistance, low phonon energy, high refractive indices and are capable of incorporating large concentrations of rare ∗

Corresponding author. Tel.: + 86 574 87600947; fax: +86 574 87600319. E-mail addresses: [email protected], [email protected].com.cn (Y. Gao). 1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.06.050

earth ions into the matrix [3]. Several studies have reported on the property investigations of Er3+ -doped tellurite glasses for EDFA and upconversion lasers [3–6]. But till now, no studies have been found concerning the glasses of Er3+ single-doped and Er3+ /Yb3+ co-doped TeO2 –Li2 O–B2 O3 –GeO2 system. In this paper, the new kind of Er3+ single-doped and 3+ Er /Yb3+ co-doped tellurite glasses were prepared. We focused on the analysis of the absorption spectra, emission spectra and upconversion spectra of Er3+ in these studied tellurite oxide glasses, based on TeO2 , Li2 O, B2 O3 , GeO2 , Er2 O3 and Yb2 O3 , which have not been studied. The results confirm that this novel type of tellurite glass is a promising candidate host material for Er3+ -doped broadband amplifiers and upconversion lasers.

2. Experimental Rare earth doped 70TeO2 –5Li2 O–10B2 O3 –15GeO2 glasses were prepared from reagent grade commercial oxides of TeO2 , Li2 O, B2 O3 , GeO2 , Er2 O3 and Yb2 O3 with more than 99.99% purity. Each sample was doped with 1 wt.%

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Table 1 Er3+ and Yb3+ concentrations in glass samples Glass

NEr (1020 ions/cm3 )

NYb (1020 ions/cm3 )

TLBG1 TLBG2 TLBG3 TLBG4 TLBG5

1.55 1.57 1.58 1.58 1.59

0 3.04 6.13 10.83 14.1

Er2 O3 in the batch. The purpose of adding Yb2 O3 is to enhance the pumping efficiency of 970 nm laser diode (LD). The concentrations of Er3+ and Yb3+ in glass samples are listed in Table 1. They are named by TLBG1, TLBG2, TLBG3, TLBG4 and TLBG5 in order. About 20 g batches of starting materials were fully mixed and then melted in a platinum crucibles at 800–900 ◦ C in an electronic furnace for ∼40 min. 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 15 × 15 × 5 mm3 in order to meet the requirements for optical measurements. The absorption spectra of glasses were recorded with a Perkin-Elmer Lambda 900 UV–vis–NIR spectrophotometer over the spectral range of 300–1700 nm. The emission spectra and upconversion spectra were measured with a TRIAX550 spectrofluorimeter on excitation at 970 nm. All the measurements were taken at room temperature.

3. Results and discussion 3.1. Solubility of Er2 O3 and Yb2 O3 The variations of absorption intensity with rare earth ions concentrations were measured as a convenient way of examining the solubility [7]. Fig. 1 depicts the relationship of integral absorption intensity of the 2 F7/2 → 2 F5/2 transition of Yb3+ as a function of doping concentration. It shows good linearity except for a slight deviation in the glass with 6.13 × 1020 ions/cm3 of Yb3+ concentration. Hence, it is confirmed that Yb3+ has a good solubility in this kind of tellurite glass.

Fig. 1. The compositional dependence of integrated absorption intensity of Yb3+ (2 F7/2 → 2 F5/2 ) in TLBG glasses.

Fig. 2. The absorption spectra of TLBG glasses.

Furthermore, we can deduce that Er3+ ions should have good solubility owing to the similar nature of rare earth ions. 3.2. Absorption spectra Fig. 2 shows the absorption spectra of glass samples with various Yb3+ content. The optical absorption bands are assigned according to transitions from the ground states 4 I15/2 of Er3+ and 2 F7/2 of Yb3+ to excited states. It is clear that the positions of absorption peaks scarcely change with the increasing Yb3+ content when the Er3+ concentration almost does not change. Due to the strong absorption of glass hosts at the ultraviolet range, the absorption bands at wavelength shorter than 400 nm cannot be distinguished. In addition, the cut off bands are less than 400 nm, indicating that the energy transfer (ET) from Er3+ to the band gap of glass matrix is negligible [8]. Evidently, the absorption band around 980 nm has a strong optical intensity and which obviously increases with an increasing of Yb3+ content. Compared to Er3+ , the absorption of Yb3+ at this band is predominant with increasing Yb3+ content. This is associated to an efficient ET from Yb3+ to Er3+ : 2 F5/2 (Yb3+ ) + 4 I15/2 (Er3+ ) → 2 F7/2 (Yb3+ ) + 4 I11/2 (Er3+ ) in Er3+ /Yb3+ co-doped glasses. That is to say, addition of Yb2 O3 to glass can enhance the pumping efficiency of 970 nm LD though the ET between Yb3+ to Er3+ . 3.3. Emission spectra and FWHM The 4 I13/2 → 4 I15/2 transition of Er3+ fluorescence emission centered at about 1.5 um was observed in TLBG glasses under the 970 nm LD excitation. As shown in Fig. 3, the emission intensity of glass samples increases monotonically with the increasing Yb3+ content gradually. The reason is the function of ET from Yb3+ to Er3+ , which act as an indirect pumping of Er3+ ions. The contribution to the 4 I13/2 population from the ET will be much greater than that from the ground state absorption (GSA) process of the Er3+ ions. As a result, with the increasing Yb3+ content, the ability of Er3+ to absorb pumping energy becomes higher and higher and thus the emission intensity corresponding to the 4 I13/2 → 4 I15/2 transition gets stronger and

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Fig. 3. The emission spectra of Er3+ -doped and Er3+ /Yb3+ co-doped TLBG glasses.

stronger. So, adding Yb3+ to improve the pumping efficiency of Er3+ excited at 970 nm LD is available in these studied glasses. As is known, the bandwidth of traditional silica-based EDFA is limited to 35 nm that cannot satisfy the requirement of information capacity [9]. It is important to find other Er3+ -doped glass hosts to produce broader bandwidth than silica-based EDFA. Fig. 4 illustrates the normalized emission spectra of TLBG1, TLBG3 and TLBG5 glasses. As can be seen from Fig. 4, neither the line shape nor the peak position changes with Yb3+ concentration increase. The FWHM value of Er3+ about 1.5 um increases from 71 to 77 nm with the increasing Yb3+ concentration from 0 to 14.1 × 1020 ions/cm3 while the Er3+ concentration change slightly. The broad emission bandwidth (71–77 nm) is mainly due to the greater variation of evironment and the coordination number surrounding Er3+ ions [9]. It is also reported that B2 O3 has two forms: [BO3 ] trigonal and [BO4 ] tetrahedral, respectively, when B2 O3 is introduced into the glass composition [10]. GeO2 can form two different sites with [GeO4 ] tetrahedral and [GeO6 ] octahedral units [11] and the structural units of TeO2 glasses are [TeO4 ] trigonal bipyramid and [TeO3 ] trigonal pyramid [10]. Therefore, these studied glasses produce a complex network structure with greater variety of dopant sites of glass forming constituents, B2 O3 , GeO2 and TeO2 . Further work on structural investigations is needed to explain the phenomenon. The broad bandwidth indicates that this series of tellurite glasses can be used as a candidate host material for potential broadband optical amplifier.

Fig. 4. The normalized emission spectra of TLBG glasses.

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Fig. 5. Upconversion luminescence spectra of TLBG glasses.

3.4. Upconversion luminescence and mechanisms analysis Fig. 5 illustrates the upconversion luminescence spectra of Er3+ in TLBG1, TLBG3 and TLBG5 glasses with different concentrations of Yb3+ . Three emission bands centered at 532, 546 and 659 nm are simultaneously observed and assigned to the transitions of 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 , respectively. A significant increase in the overall intensity is observed with an increasing concentration of Yb3+ , indicating Yb3+ ions play an important role in the upconversion emissions. Clearly, the red emission intensity at 659 nm is relatively stronger than the green bands at 532 and 546 nm under the excitation of 970 nm LD. For TLBG1 glass, the upconversion intensity is relatively weak, this is because the band at about 980 nm of Er3+ has a weak ability to absorb the pump light, and therefore it has a very low pumping efficiency. Consequently, the population accumulations of all levels of Er3+ are difficult and lead to the three weak emissions. On the contrary, in TLBG3 and TLBG5 glasses that containing Yb3+ ions, through the ET between Yb3+ and Er3+ to enhance the pumping efficiency, the emission intensity increases gradually with the increasing Yb2 O3 content. From the above results, we can deduce that the upconversion process is Yb3+ concentration dependent in these TLBG glasses. The upconversion processes and mechanisms include the GSA, excited state absorption (ESA) and ET, but among of them the ESA and ET are most important. According to the simplified energy levels graph of Er3+ and Yb3+ illustrated in Fig. 6 [12–14], the possible upconversion mechanisms for three emission bands are discussed as follows: First, the 4I 3+ is directly excited by 970 nm LD and 11/2 level of Er by ET process from the 2 F5/2 level of Yb3+ : 2 F5/2 (Yb3+ ) + 4 I15/2 (Er3+ ) → 2 F7/2 (Yb3+ ) + 4 I11/2 (Er3+ ). Since Yb3+ has a much larger absorption cross-section than that of Er3+ in the 970 nm, the GSA of Er3+ from 4 I15/2 → 4 I11/2 is not the main reason for the population of the 4 I11/2 level of Er3+ . The ET process mainly contributes to the population accumulation on the 4 I11/2 . The high-populated the 4 I11/2 level is supposed to serve as the intermediate state responsible for the upconversion processes. Second, some portions of Er3+ in the 4 I11/2 level relax rapidly to 4 I13/2 through the

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4. Conclusions

Fig. 6. Simplified energy level diagram of Er3+ /Yb3+ and possible transition mechanisms in TLBG glasses.

non-radiative process, consequently, the light around 1.5 um produced through the radiative process. Other portions of Er3+ in the 4 I11/2 level may have the two processes: the ET: 4I 3+ 4 3+ 4 3+ 4 3+ 11/2 (Er ) + I11/2 (Er ) → F7/2 (Er ) + I15/2 (Er ) 4 3+ 4 3+ and the ESA: I11/2 (Er ) + a phonon → F7/2 (Er ). The two processes are important to populate the 4 F7/2 level. The populated 4 F7/2 level then relaxes rapidly and non-radiatively to the next lower levels 2 H11/2 and 4 S3/2 resulting from the small energy gap between them. Third, some populated Er3+ in the 2 H11/2 level relax radiatively to the ground 4 I15/2 level, which produces the green light centered at 532 nm. Others Er3+ in the 2 H11/2 level relax to the lower level 4 S3/2 through the fast thermal equilibrium process [15,16]. The Er3+ in the 4S 4 3/2 level transit to the ground I15/2 level owing to the radiative relaxation, which produces the other green light centered at 546 nm. For the population of the 4 F9/2 level, the reasons may be: the non-radiative process from the 4 S3/2 to 4 F9/2 level, the ESA: 4 I13/2 (Er3+ ) + a phonon → 4 F9/2 (Er3+ ) and the ET between Er3+ : 4 I13/2 + 4 I11/2 → 4 I15/2 + 4 F9/2 . Most of Er3+ ions in 4 F9/2 level relax radiatively to the ground state 4 I15/2 level and thus produces the red emission centered at 659 nm. The detailed studies on the upconversion mechanisms reveal that the long-lived 4 I11/2 level plays an important role in the upconversion processes for both green and red emissions. From Fig. 5, it is also important to point out that with the increasing Yb3+ content, the increasing rate of emission bands are not same, the ratio of green light (546 nm) to red light (659 nm) is about 0.44, 0.51, 0.86, respectively, which indicate that this green light increases faster than red light. This phenomenon can be interpreted as follows: Yb3+ ions have stronger ability to absorb pumping light than that of Er3+ ions and the higher ET from Yb3+ to Er3+ with the increasing Yb3+ content. Therefore, the population of Er3+ in the 4 I11/2 level increase, thus, attributing to the non-radiative and thermal equilibrium process, the Er3+ in the 2 H11/2 , 4 S3/2 and 4 F9/2 levels should increase also. Subsequently, the three emission bands all increase. And the increasing population accumulation rate of the 4 S3/2 level is faster than that of the 4F 9/2 level, which results in the relative faster increasing of green light (546 nm) than red light (659 nm) under the 970 nm LD pumping.

We have studied the luminescence properties of Er3+ single-doped and Er3+ /Yb3+ co-doped TeO2 –Li2 O–B2 O3 –GeO2 glasses. Various Yb2 O3 content were used to investigate the effect of Yb2 O3 on the spectroscopic properties of these studied glasses. It was found that Er3+ ions and Yb3+ ions have a good solubility in these glasses. The values of FWHM ranging from 71 to 77 nm with the increasing Yb2 O3 content were obtained. The upconversion bands at 532, 546 and 659 nm, corresponding to the 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 and 4 F9/2 → 4I 3+ 15/2 transitions of Er , respectively, were simultaneously observed at room temperature. The possible upconversion processes and mechanisms were mainly to involve the excited state absorption and the energy transfer. These properties make these novel tellurite glasses promising host material for broadband amplifiers and developing upconversion optical devices.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 60272034), Natural Science Foundation of the Zhejiang Province of China (No. 601011) and Key Doctor Foundation of the Ningbo Science & Technology Bureau of China (No. 02J20101-01).

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