Study of luminescence properties of Er3+-ions in new tellurite glasses

Study of luminescence properties of Er3+-ions in new tellurite glasses

Optical Materials 26 (2004) 267–270 www.elsevier.com/locate/optmat Study of luminescence properties of Er3þ -ions in new tellurite glasses R. El-Mall...

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Optical Materials 26 (2004) 267–270 www.elsevier.com/locate/optmat

Study of luminescence properties of Er3þ -ions in new tellurite glasses R. El-Mallawany a,b, Amitava Patra a,c,*, Christopher S. Friend a, Rakesh Kapoor a, Paras N. Prasad a a

Institute for Lasers, Photonics and Biophotonics, University at Buffalo, State University of New York, Buffalo, NY 14260, USA b Physics Department, Faculty of Science, Qatar University, P.O. Box 2713, Doha, Qatar c Sol-Gel Division, Central Glass and Ceramic Research Institute, Jadavpur, Kolkata 700 032, India Received 17 September 2003; received in revised form 16 December 2003; accepted 5 January 2004 Available online 3 March 2004

Abstract The effects of concentration of Er3þ ions in tellurite glasses on the decay time, spectral width and weighted spectral peak are studied. A broad emission spectrum of width 121 nm peaked at wavelength of 1.5 lm is obtained in 2.5 mol% Er doped glasses. The maximum decay time of 4 I13=2 fi 4 I15=2 transition (at wavelength 1.5 lm) is about 4.5 ms for 1.0 mol% Er doped glasses. Strong green and red upconversion emission at emission 550 nm (4 S3=2 fi 4 I15=2 ) and 670 nm (4 F9=2 fi 4 I15=2 ) are observed from Er3þ -doped tellurite glasses upon continuous wave excitation at 975 nm. It is found that the upconversion efficiency goes up with the increasing concentration of the Er3þ ions.  2004 Elsevier B.V. All rights reserved. Keywords: Tellurite glass; Rare-earth; Luminescence; Upconversion

1. Introduction In the new millennium, 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-optic communications [1–9]. Rare-earth ions, especially erbium, have played an important role in the development of broadband erbium-doped fiber amplifiers (EDFA) in optical communication technology during the past few decades. It is known that erbium-doped glass has attracted much interest because of the 4 I13=2 fi 4 I15=2 transition in Er3þ at a wavelength around 1.5 lm, coinciding with the low-loss window of standard optical communications fiber. For a practical standpoint, the flatness of the gain is also critically important because the light intensity for different channels would be varied by multistep amplifications. It is reported so far that the values of FWHM of 1.5 lm *

Corresponding author. Address: Sol-Gel Division, Central Glass and Ceramic Research Institute, Jadavpur, Kolkata 700 032, India. Fax: +91-33-2473-0957. E-mail address: [email protected] (A. Patra). 0925-3467/$ - see front matter  2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.01.002

are 44, 65 and 85 nm for Al/P silica [10], fluozirconate [11,12] and tellurite glass host [13] respectively. Therefore, tellurite oxide glass host has been attractive from the fundamental point of view to the importance of practical applications [14–18]. Among oxide glasses, tellurite glasses have several advantages such as a wide transmission window (typically, 0.4–5.0 lm), high linear and nonlinear refractive indices, good mechanical stability and lowest cut-off phonon energy. For the WDM system in the C-band, Er3þ in tellurites showed the highest emission cross-section over the entire range of emission wavelength. Therefore, luminescence study of Er ions in a new tellurite glass composition is important. No studies have been found concerning the glass of TeO2 –WO3 –TiO2 –Er2 O3 system. In this communication, we report primary results of luminescence and upconversion properties of a new tellurite glass composition doped with Er3þ ions.

2. Experimental Erbium titanium tungsten tellurite glasses in the systems: 80TeO2 –5TiO2 –(15)x)WO3 –xEr2 O3 , (x ¼ 5:0,

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2.5, 1.0, 0.01 mol%; 1 mol% ¼ 1.6 · 1020 cm3 ) were prepared by mixing specified weights of tellurium oxide (TeO2 ; 99:999% purity, Aldrich), erbium oxide (Er2 O3 99:99% purity, Aldrich), titanium oxide (TiO2 99:999% purity, Aldrich) and tungsten oxide (WO3 99:995% purity, Aldrich). The powder mixture was introduced in a gold crucible and heated in a melting furnace at temperatures in the range of 930–950 C for 30 min. The melt was cast at room temperature in a stainless steel mold. Subsequently, the sample was transferred to an annealing furnace and kept for 1 h at 300 C. Then the furnace was switched off and the glass sample was allowed to cool. The glasses were homogenous. All glasses prepared were transparent and had greenish to pink color with increasing the concentration of Er2 O3 %. The glasses were polishing by a lapping machine to make 1 mm bulk glass. The samples were irradiated with a diode laser tuned to 975 nm. Fluorescence spectra for different compositions were recorded with the help of an IR spectrometer. For life time measurement, the diode laser was chopped with a mechanical chopper at 30 Hz. The overall response time of our detection system was <100 ls. The decay curves were recorded with the help of a digital oscilloscope (HP Infinium) and the life times were estimated by fitting an exponential function to the recorded curves. The up conversion fluorescence spectra were recorded by a CCD-coupled spectrometer.

3. Results and discussion Table 1 gives the composition of the glass sample investigated in the present work. Fig. 1 shows the emission spectra of Er3þ doped tellurite glasses in different concentrations. The emission peak at a wavelength around 1550 nm is due to 4 I13=2 fi 4 I15=2 transition of Er3þ . It shows that with increasing the concentration of erbium ions, the peak at 1550 nm shifts towards the higher wavelength from 1541 to 1554 nm. The red shift of the emission band indicates the increase in covalent bonding between the Er3þ ions and associated ligands; the more covalent the bond, the less electron-electron interaction. Reason for this appears to be due to orbital expansion effects within the RE ions brought about the electron donation by the oxygen ligands. This effect is known as the nephelauxetic effect. It

can also be seen that the values of FWHM ranging from 93 to 121 nm for 0.01 to 2.5 mol%Er2 O3 . It is reported that the FWHM value of tungsten-tellurite glasses is 85 nm [15]. To our knowledge, this is the highest value in the literature. The distinct broadening of the fluorescence band in highly concentrated sample suggests greater variation of environment and coordination numbers surrounding the rare-earth ions. It is known that the main reason for the broadening is the local crystal field symmetry at the rare-earth ion site. The inhomogeneous broadening is mainly due to the local structure and the coordination number of the Er3þ ions. It is also reported that WO3 can form two different sites with [WO4 ] tetrahedral and [WO6 ] octahedral units and the structural units of TeO2 glasses are TeO4 trigonal bipyramid and TeO3 trigonal pyramid [19,20]. Therefore, tungsten-tellurite glasses produce a complex network structure with greater variety of dopant sites of glass forming constituents, WO3 and TeO2 . In the studied glasses, the addition of Ti4þ component could also help to modify the structure of glass network and produces a complex network structure with greater variety of dopant sites. Further work on structural investigations is definitely needed to explain the phenomenon. This result clearly suggests that the local crystal field generated in this new composition helps to broadband Er3þ emission at 1550 nm (Table 1). A typical recorded decay curve is shown in Fig. 2. Variation of measured life times of 4 I13=2 fi 4 I15=2 transition for different composition is shown in Table 1. In low Er-ions content glasses, there is a less probability of nonradiative relaxation by ion–ion interaction. Therefore, in the case of higher concentration(>1.0 mol%) of Er3þ ions, the cross relaxation process occurs followed by nonradiative decay of the two-ions to the ground state (Fig. 3). A typical energy level diagram for the upconverted emission from a sample doped with Er3þ ions under infrared excitation is shown in Fig. 4. The upconverted fluorescence spectra of the Er3þ in tellurite glasses in different concentrations of Er3þ are shown in Fig. 5. A significant increase in the overall intensity is observed with an increasing concentration of Er3þ ions. The relative increase in intensity of red emission with respect to the intensity of green emission was observed with an increasing concentration of erbium (>1.0 mol%). The

Table 1 Spectral width, weighted peak and decay time of 4 I13=2 fi 4 I15=2 transition of Er3þ in tellurite–titanium–tungsten glasses Glass composition (mol%)

Weighted peak (nm)

FWHM (nm)

Decay time (ms)

80TeO2 –5TiO2 –14.99WO3 –0.01Er2 O3 80TeO2 –5TiO2 –14WO3 –Er2 O3 80TeO2 –5TiO2 –12.5WO3 –2.5Er2 O3 80TeO2 –5TiO2 –10WO3 –5Er2 O3 40TeO2 –40WO3 –20Li2 O [14] Al/silica [17]

1541 1548 1553 1554 1500 1500

93 111 121 120 85 45

4.0 ± 0.6 4.5 ± 0.7 2.30 ± 0.08 1.00 ± 0.01 5 10

R. El-Mallawany et al. / Optical Materials 26 (2004) 267–270 Tellurium Glass

Tellurium Glass 1.0

0.01 Er 1.0 Er 2.5 Er 5.0 Er

FWHM (nm)

0.6

0.4

0.2

0.0

1450

1500

1550

1600

1650

124 122 120 118 116 114 112 110 108 106 104 102 100 98 96 94 92 90

8

6

fwhm lifetime

2

0 0

1700

2

4

6

Sample (%Er)

Wavelength (nm)

Fig. 1. Er3þ doped spectra of tellurite–titanium–tungsten glass for different concentration of Er3þ ions.

4

Emission Lifetime (ms)

Emission (a.u.)

0.8

1400

269

Fig. 3. Variation of spectral width and decay time of 4 I13=2 fi 4 I15=2 transition with concentration of Er3þ ions.

4

20x10 3 cm—1-

F7/2 2 H 11/2 S3/2

4

550 nm

15

CR

4

F9/2

4

I

670 nm

9/2

4

I 11/2

10

4

I 13/2

975 nm 5

CR 1550 nm

Fig. 2. A typical decay curve of 4 I13=2 level of Er3þ in tellurite–titanium–tungsten glass.

predominant mechanisms [6–9] of up conversion in these materials are excited state absorption (ESA) and energy transfer up conversion (ETU). In the excited state absorption (ESA) process, the diode-laser wavelength (975 nm) matches the absorption transition between the ground state, 4 I15=2 , and the excited level 4 I11=2 . After first-level excitation, the same laser pumps the excited atom from the 4 I11=2 to the 4 F7=2 level. In case of energy transfer up conversion (ETU), where two excited (4 I11=2 ) Er3þ ions interact with each other and one ion is de excited to 4 I15=2 and the other is excited to 4 F7=2 . This mechanism can be described as: 2 4 I11=2 ! 4 I15=2 þ 4 F7=2 The 4 F7=2 (Er3þ ) state decays nonradiatively to the S3=2 / 2 H11=2 and 4 F9=2 (Er3þ ) levels [21–23]. The green emission (550 nm) is observed from the 4 S3=2 fi 4 I15=2 transition while the 4 F9=2 fi 4 I15=2 transition produces 4

4

I 15/2

Fig. 4. The energy level diagram for Er3þ ions, infrared excitation of 975 nm used.

red emission (675 nm). It is quite clear from the result that the upconversion process is concentration dependent. At higher concentrations, one may postulate substantial energy transfer can occur, since the energy transfer rate is strongly dependent on the distance between the ions involved. At low dopant concentration, ions are usually randomly distributed in the host and the 4 S3=2 /2 H11=2 levels decay mostly radiatively to 4 I15=2 . Therefore, the green emission has a higher intensity. It is also reported [8] that at a higher concentration (>1.6 · 1020 cm3 ), the luminescent lifetime of 4 S3=2 / 2 H11=2 levels is shortened as a result of the cross-relaxation (CR) processes take place between (2 H11=2 fi 4 I9=2 ) and (4 I15=2 fi 4 I13=2 ) transitions as shown in Fig. 4. The increase in concentration of Er3þ ions should greatly promote the ETU from one excited (4 I11=2 ) Er3þ to

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Acknowledgements This work was supported by National Science Foundation through contract # 22662-1019521 and Enhanced Center for Advanced Technology through contract # 55415-00-01A. One of the authors (A.P) thanks Dr. H.S. Maiti, Director of CGCRI for his interest.

References Fig. 5. Up converted fluorescence emission spectra of different concentrations (mol%) of Er3þ in tellurite glasses.

another excited Er3þ (4 I11=2 ) and finally excited to 4 F7=2 level. As a result, the 4 F7=2 level (Er3þ ) decays nonradiatively to the 4 F9=2 (Er3þ ) levels and bright red emission due to 4 F9=2 fi 4 I15=2 transition of Er3þ is observed. Therefore, the red upconversion efficiency goes up with the increasing concentration of the Er3þ ions. This observation clearly indicates that the upconversion in these samples is due to an ETU process at higher concentration of Er3þ ions.

4. Conclusion We obtained a broad emission spectrum of width 121 nm peaked at wavelength of 1550 nm. The maximum observed decay time of 4 I13=2 fi 4 I15=2 transition at wavelength 1550 nm is about 4.5 ms. Therefore, tellurite–titanium–tungsten glasses are proposed to be good candidate for broadband EDFA. The values of FWHM ranging from 93 to 121 nm for 0.01 to 2.5 mol%Er2 O3 were observed. A significant increase in the overall intensity of upconversion fluorescence is observed with an increasing concentration of Er3þ ions. The relative increase in intensity of red emission with respect to the intensity of green emission was observed with an increasing concentration of erbium. It can be concluded from our data that Er3þ -doped TeO2 –WO3 –TiO2 glasses are promising photonic materials for the infrared amplifiers as well as the green and red up conversion emission.

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