Yb3+ tri-doped tellurite glass

Journal of Alloys and Compounds 580 (2013) 310–315

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Energy transfer induced photoluminescence improvement in Er3+/Ce3+/Yb3+ tri-doped tellurite glass W. Stambouli a,⇑, H. Elhouichet a,b, C. Barthou c, M. Férid a a

Laboratoire des Matériaux Minéraux et leurs Applications, Centre National de Recherches en Sciences des Matériaux, Technopole de Borj-Cedria, B.P. 95 Hammam-Lif 2050, Tunisia Département de Physique, Faculté des Sciences de Tunis, Université de Tunis-ElManar, ElManar 2092, Tunisia c Institut des Nanosciences de Paris, Université P. et M. Curie, Centre Nationale de la Recherche Scientifique, UMR-7588, 4 place Jussieu, Boîte Courrier 840, 75252 Paris Cedex 05, France b

a r t i c l e

i n f o

Article history: Received 2 April 2013 Received in revised form 14 June 2013 Accepted 18 June 2013 Available online 27 June 2013 Keywords: Energy transfer Photoluminescence Tellurite glass Tri-doped Up-conversion

a b s t r a c t Tellurite glasses doped with Er3+, Er3+/Ce3+ and Er3+/Ce3+/Yb3+, have been elaborated from the conventional melt-quenching method. It was found that both the photoluminescence (PL) intensity and the PL lifetime relative to the 4I13/2 ? 4I15/2 transition of Er3+ were found to increase with Ce3+ co-doping and Yb3+ tri-doping. We show that an efficient energy transfer can occur from Ce3+ and Yb3+ to Er3+. Efficient green (533 nm, 546 nm) emission spectra, associated to the 2H11/2 ? 4I15/2, and 4S3/2 ? 4I15/2 transitions of Er3+ respectively, were observed. By adding Ce3+ ions, the intensities of green up-conversion emissions were found to decrease hardly do to energy transfer rate of Er3+:4I11/2 ? Ce3+:2F5/2. Band diagram energy is proposed to explain the up-converted PL, under 980 nm excitation, in mono-doped, codoped and tri-doped glasses. The results suggest that Er3+/Yb3+/Ce3+ tri-doped tellurite glass may be a potential material for developing optical amplifiers and up-conversion optical devices. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the rare earth (RE) doped optical materials have been extensively investigated due to their potential applications in many fields, such as color display, optical data storage, sensor, laser, and optical amplifier for communication [1,2]. Er3+-doped fiber amplifier (EDFA) is one of the key devices for use in 1.5 lm wavelength optical communication window. Among these materials, particular attention has been devoted to the study of tellurite glasses. Tellurite glass is a particularly attractive host because it exhibit large transmittance window, from the visible to the infrared region, have low cutoff phonon energy 700 cm1, present high refractive index 2.0 and show high chemical stability [3]. As the sensitizer, Yb3+ ions play an important role to improve pumping rate excited at 980 nm by efficient energy transfers between Yb3+ and Er3+.Thus, Er3+/Yb3+ co-doped systems have been widely researched in the past few years [2,4–6]. However, serious up-conversion emission appears and decreases the efficiency of 1.54 lm emission. In order to enhance the near infrared (NIR) 1.54 lm emission intensity, population feeding rate from the 4I11/2 to the 4 I13/2 level of Er3+, should be enhanced to reduce the lifetime of the 4I11/2 level [4,5]. One possible way is by resonant energy transfer processes. The influences of Ce3+, Eu3+, or Tb3+ addition on the

⇑ Corresponding author. Tel.: +216 23689406. E-mail address: [email protected] (W. Stambouli). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.06.115

luminescence performance of Er3+ doped host materials have been reported [7–9]. These results indicated that Ce3+, Eu3+, or Tb3+ ions may increase non-radiative relaxation of 4I11/2 ? 4I13/2 transition of Er3+, but Eu3+ and Tb3+ ions also shortened the lifetime of the Er3+:4I13/2 level, which is not beneficial to the NIR emission. In this paper, Er3+ single doped, Er3+/Ce3+ co-doped and Er3+/ Ce3+/Yb3+ tri-doped glasses were prepared. The effect of Ce3+ and Yb3+ on the infrared and visible upconversion emission of Er3+ was systematically investigated and energy transfer properties among Er3+, Yb3+ and Ce3+ are also analyzed. 2. Experimental details Glasses system: 60.5TeO2–30ZnO–5Nb2O5–4Na2CO3–0.5Er2O3 (mono-doped glass), 60 TeO2–30ZnO–5Nb2O5–4Na2CO3–0.5Er2O3–0.5CeO2 (co-doped glass) and 59TeO2–30ZnO–5Nb2O5–4Na2CO3–0.5Er2O3–0.5CeO2–1Yb2O3 (tri-doped glass) in mole%, was prepared by conventional melt-quenching method. All the starting chemical constituents are more than 99.9% purity. Calculated quantities of the chemical components were mixed in an agate mortar and melted in an electric furnace at 850 °C for 1 h platinum crucibles so that a homogeneously mixed melt was obtained. Immediately after the quench, the glass (as-prepared sample) was annealed at 250 °C (for 1 h) and then slowly cooled until ambient temperature. The annealing process was made with the objective of minimizing the internal mechanical stress and obtaining glasses with good mechanical stability. The obtained glasses were cut and polished carefully in order to meet the requirements for optical measurements. Absorption spectra of these samples were measured in the range 300–1800 nm using a Varian 5000 spectrophotometer. The optical excitation in the near IR range (k  980 nm) was obtained with a cw tunable diode laser with output power up to 1 W. For decay analysis, the diode laser was pulsed with a frequency of 5 Hz, a

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W. Stambouli et al. / Journal of Alloys and Compounds 580 (2013) 310–315 width of 100 ms, a rise time of 6 ls and a decay time of 4 ls. The IR emitted light from the sample, collected by an optical fiber, was analyzed on the same side as the excitation with a TRIAX 320 spectrophotometer and a near IR array detector Hamamatsu (256 pixels). This setup has a resolution of 1 nm/point for a slit 650 lm. The decays were analyzed with a InGaAs detector and a scope Nicolet 400 with a time constant of the order of 50 ls. Refractive indexes of all the samples were measured using the UVISEL SPME ellipsometer. All the optical measurements were performed at room temperature.

3. Results and discussion

of the electron, c is the velocity of the light in the vacuum, h is the Planck constant, k is the mean wavelength of the absorption band, N0 is the Er3+ ion concentration per unit volume, n the meaR sured refractive index and aðkÞdk is the integrated absorption coefficient as a function of k. The values of Smeas obtained by the numerical integration of the absorption line shapes were used to obtain the phenomenological Judd–Ofelt parameters Xt by fitting the experimental value with the theoretical expression derived by Judd [14] and Ofelt [15]:

3.1. Absorption spectra and Judd–Ofelt analysis

Scal ðJ ! J 0 Þ ¼

X

Xt jhðS; LÞJjjU ðtÞ jjðS0 ; L0 ÞJ0 ij2

ð2Þ

t¼2;4;6

The absorption spectra of the Er3+ mono-doped, Er3+/Ce3+ codoped and Er3+/Ce3+/Yb3+ tri-doped tellurite glasses are shown in Fig. 1. The spectrum of Er3+ singly doped glass contains eleven peaks centered at 380, 407, 444, 452, 490, 521, 545, 653, 799, 976 and 1532 nm which are assigned to the transitions from ground 4I15/2 to the various excited, 2G11/2, 2H9/2, 4F3/2, 4F5/2, 4F7/2, 2 H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2 levels, respectively. The spectrum of co-doped glass reveals only seven peaks in the spectral region 500–1600 nm. It is clear from Fig. 1, that with the addition of Ce3+ ions, the UV-absorption edge shifts towards the longer wavelength due to the broad absorption band of Ce3+ ion corresponding to the interconfiguration 4f1 ? 4f05d1 transition [10,11]. Hence, the absorption bands below 500 nm are obscured. For Er3+/Ce3+/Yb3+ tri-doped glasses, we did not include the 4I11/2 of Er3+ in our analysis since the intense absorption of the 2F5/2 of Yb3+ is observed at the same wavelength region (900–1000 nm) as shown in Fig. 1. Furthermore, since the absorption cross-section of Yb3+ is about 10 times larger than that of Er3+ [12], it can be considered that only Yb3+ contributes to this absorption region. The data from these absorption spectra can be used to predict the radiative transition probabilities, the branching ratio and the radiative lifetime of different transitions, in particular from 4I13/2 to the ground state (1.53 lm). The measured absorption line strength (Smeas) for the induced electric dipole transition of each band was determined experimentally from the area under the absorption band and can be expressed in terms of absorption coefficient by the equation [13]

Smeas ðJ ! J 0 Þ ¼

 Z 3chð2J þ 1Þ 9n aðkÞdk 8p3 ke2 N0 ðn2 þ 2Þ

ð1Þ

where J and J0 represent the total angular momentum quantum numbers of the initial and final levels, respectively, e is the charge

4G 11/2 2H 9/2 3. 4F3/2 4 4. F5/2 5. 4F7/2 6. 2H11/2 7. 4S3/2 8. 4F9/2 9. 4I9/2 4 10. I11/2 11. 4I13/2

1.

2.

Absorbance

60 4I 15/2

40 4 3+ 2 3+ I11/2(Er )+ F5/2(Yb )

20

1

DSrms ¼ ½ðq  pÞ1 

X

1

ðSmes  Scal Þ2 2

ð3Þ

Here q is the number of analyzed spectral bands and p the number of parameters sought, which in this case is three. The calculated values of DSrms and the J–O intensity parameters for mono-doped, codoped and tri-doped glasses are listed in Table 1. As can be seen from Table 1, X2 decreases as the concentration of rare earth increases and TeO2 reduces suggesting a strong dependence with the environment around the active ion [17]. The X2 parameter is closely related to the hypersensitive transitions, i.e., the larger the line strength of the hypersensitive transition is the larger will be the value of X2 [17,18]. It is well known that the hypersensitivity is related to the covalency parameter through nephelauxetic effect and it is attributed to the increasing polarizability of the ligands around the rare earth ions. Higher ligand polarizability results in a larger overlap between rare earth ions and ligands orbital, i.e., higher degree of covalency between rare earth ions and the ligands. Thus, the increment of the rare earth ions concentration and reduction of tellurite content decreases the X2 and the degree of covalency. This result is contrary to reported results where an increment of the X2 value was observed with the ionic concentration [19]. This difference suggests that the environment effect is dominated by Te ion. That is understood considering that having larger field strength is responsible for a higher polarizability of oxygen at the rare earth site. The value of X2 is also affected by the asymmetry of the rare earth sites that is reflected in the crystal field parameter.

(c)

Table 1 Judd–Ofelt intensity parameters of mono-doped, co-doped and tri-doped tellurite glasses.

(b)

2

0

Here X2, X4, and X6 are the Judd–Ofelt parameters and kUtk is the doubly reduced matrix element of the unit tensor operator of rank t = 2, 4 and 6, which are calculated from the intermediate coupling approximation between states characterized by the quantum numbers (S, L, J) and (S0 , L0 , J0 ). The reduced matrix elements are virtually independent of the ligand species surrounding the rare earth (RE3+) ions and thus approximately unchanged from host to host. We have used the values of the reduced matrix elements for the chosen Er3+ bands calculated by Carnall et al. [16]. When three absorption manifolds overlapped, the squared matrix element was taken to be the sum of the corresponding squared matrix elements. Judd–Ofelt intensity parameters Xt were derived from the electric-dipole contributions of the experimental oscillator strengths using a leastsquare adjusting approach. The root-mean-square deviation DSrms between the measured (Smeas) and the calculated (Scal) oscillator strengths can be calculated based on the following relationship:

3 4 56 7

400

(a) 8

9

11

10

800

1200

1600

Wavelength (nm) Fig. 1. Absorption spectra of (a) mono-doped, (b) co-doped and (c) tri-doped tellurite glasses.

Glasses

X2 (1020 cm2)

X4 (1020 cm2)

X6 (1020 cm2)

Dsrms (1020)

Monodoped Co-doped Tri-doped

4.97

1.25

1.42

0.30

3.94 1.88

1.07 1.98

1.97 1.39

0.23 0.19

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W. Stambouli et al. / Journal of Alloys and Compounds 580 (2013) 310–315

The lower values of X2 result from lower rigidity of the host matrix and may be due to the low asymmetry and rigidity of the rare earth sites. Using the Xt parameters the radiative transition probability, A (J ? J0 ) from different upper states to the corresponding lower manifold states can be evaluated from the relation [20]:

64p4 e2 3

3hð2J þ 1Þk

2

nðn2 þ 2Þ Sed þ n3 Smd 9

12000

# ð4Þ

Here, Sed and Smd represent the predicted fluorescence line strength for the induced electric and magnetic dipole transition respectively. Sed, is calculated using Eq. (2) and presents a host dependence trough the Xt parameters. Smd can be calculated with the expression

λexc = 980 nm

10000

Intensity (a.u.)

AðJ ! J 0 Þ ¼ Aed þ Amd ¼

"

14000

8000

(c)

6000 4000

(b)

2000

(a)

0 2

h Smd ðJ ! J Þ ¼ jhðS; LÞJjjL þ 2SjjðS0 ; L0 ÞJ 0 ij2 16p2 m2 c2 0

1400

ð5Þ

Amd ¼

n0

A0md

ð7Þ

sr ¼

1 ¼P 0 0 AðJ ! J Þ J

1650

1700

Fig. 2 depicts the NIR emission spectra of tellurite glasses monodoped with 0.5 mol% Er3+, co-doped with 0.5Er3+/0.5Ce3+ and tri-doped with 0.5Er3+/0.5Ce3+/1Yb3+. It is seen that these spectra appear similar with fluorescence lines occurring at the same wavelength (1.53 lm) which were studied in the same conditions, but the emission intensity of the 4I13/2 ? 4I15/2 transition increased with addition of Ce3+ and Yb3+. For example, the intensity of the near infrared emission in co-doped sample is increased about two times and about four times in tri-doped sample than that of simple doped 0.5% Er3+. We can conclude that the Ce3+ and Yb3+content play an important role in the dynamic of the Er3+ emission. The emission spectra were used to estimate all the stimulated emission parameters. The stimulated emission cross-section re (kp) for 4I13/2 ? 4I15/2 transition was evaluated from the measured emission line shape using the expression [22]

re ðkp Þ ¼

The fluorescence branching ratio characterize the possibility to attain stimulated emission from any specific transition and shows a host dependence trough the Xt parameters and the numerical values are also shown in Table 2. The total radiative transition probability AT also permit to calculate the radiative lifetime sr for an excited state J by using the expression

A1 T

1600

3.2. Infrared fluorescence and cross-section at 1.53 lm

ð6Þ

AðJ ! J 0 Þ AðJ ! J 0 Þ ¼P AT AðJ ! J 0 Þ

1550

Fig. 2. PL spectra relatives to 4I13/2 ? 4I15/2 transition, under 980 nm excitation wavelength, for (a) 0.5 mol% Er3+, (b) 0.5/0.5 Er3+/Ce3+ and (c) 0.5/0.5/1 Er3+/Ce3+/ Yb3+ doped tellurite glasses.

where n and n0 are the refractive indices of erbium doped tellurite glasses and LaF3, respectively. The values of the radiative transition probability from the upper manifold states, 4I13/2, 4I11/2, and 4I9/2, to their corresponding lower-lying manifold states are shown in Table 2. It can be observed a decreasing tendency in every transition as the glass was co-doped by Er3+ and Ce3+ rare earth. The introduction of Yb3+ in co-doped glass increases the radiative transition probability indicating that the fluorescence dynamics of the ion emission could change. Adding the radiative transition probability of every transition (running over all final state J0 ) results in the total radiative transition probability AT that is related with the fluorescence branching ratio b by the expression:

bðJ ! J 0 Þ ¼

1500

λ (nm)

where jhðS; LÞJjjL þ 2SjjðS0 ; L0 ÞJ 0 ij are the reduced matrix elements of the operator L + 2S. In this work, values of Amd were calculated using the values for LaF3 ðA0md Þ and corrected for the refractive index difference [21]. The relationship is:

 n 3

1450

k4p A 8pcn2 Dkeff

ð9Þ

where kp is the peak emission, n is the refractive index, A is the total spontaneous transition probability, and Dkeff is the effective line width given by [23,24]

R

Dkeff ¼

IðkÞdk Imax

ð10Þ

where I(k) is the emission intensity at the wavelength k and Imax the intensity at the peak wavelength k. The calculated values of the stimulated emission parameters as a function of the ionic concentration are listed in Table 3. As expected, the full width at half maximum (FWHM), the effective line width (Dkeff) and the emission cross-section increases with the increment of the cerium, being the maximum by adding cerium and ytterbium rare earth. It is

ð8Þ

Because the total radiative transition probability depends of Xt parameters the radiative decay time is inversely proportional to a linear combination of these Judd–Ofelt parameters.

Table 2 Calculated radiative parameters of, Er3+ mono-doped, Er3+/Ce3+ co-doped and Er3+/Ce3+/Yb3+ tri-doped samples. Glasses

Transition 4

I13/2 ? 4I15/2

Atot (s Mono-doped Co-doped Tri-doped

1

345.02 238.52 292.08

)

4

I11/2 ? 4I15/2 1

b

s (ms)

Atot(s

1 1 1

2.40 4.19 3.42

369 253.33 254.17

)

I9/2 ? 4I15/2

4

b

s (ms)

Atot(s1)

b

s (ms)

0.86 0.85 0.84

2.33 3.37 3.36

251 127.13 328.08

0.73 0.55 0.77

2.90 4.31 2.34

313

W. Stambouli et al. / Journal of Alloys and Compounds 580 (2013) 310–315 Table 3 Comparison of FWHM, effective bandwidths (Dkeff,), stimulated emission crosssections (re) and re  FWHM for the 4I13/2 ? 4I15/2 transition of our samples with the other hosts. Glasses Mono-doped (TW) Co-doped (TW) Tri-doped (TW) TZNY1 [16] SBS1 [17] Phosphate [18] TKNNZG4 [19]

FWHM (nm)

Dkeff (nm)

re

re  FWHM

54

72

8.3

448.2

56 69 56 88 55 46

72 80 73 – – –

8.5 9.4 8.8 6.8 8.0 7.6

476.0 648.6 491 598.4 440 349.6

21

(10

2

cm )

(10

28

3

cm )

Table 4 Measured lifetimes time (smea), Radiative decay time (sr), and quantum efficiency (g) of 4I13/2 ? 4I15/2 transition for mono-doped, co-doped and tri-doped glass samples. Glasses

smea (ms)

sr (ms)

g = smea/sr (%)

Mono-doped Co-doped Tri-doped

1.40 3.52 3.56

2.39 4.19 3.42

58 84 104

3.3. Lifetime and quantum efficiency

TW: this work.

important to find Er3+-doped glass hosts to produce broader bandwidth than that of tellurite based EDFA. The broadening of the luminescence band in this glass is mainly due to variation of local structure and coordination numbers surrounding Er3+ ions site. Generally, inhomogeneous broadening is dominant for band- width broadening of Er3+ in glasses caused by variations of ligand fields of Er3+ from site to site [25]. Dkeff changes significantly in tri-doped sample, suggesting that the introduction of Yb2O3 concentration effectively affects differences in the ligand field of Er3+ from site to site and thus leads to larger inhomogeneous broadening of the emission spectra. The larger value of Dkeff could be interesting for wavelength-division multiplexing applications. Emission cross-section re and FWHM are important parameters in optical amplifiers achieving broad band and high gain amplification. Band width properties of the optical amplifier can be evaluated from the product FWHM  re. The larger the product, the better the properties of amplifier [26]. It is clear from Table 3 that adding 0.5% Ce3+ and 1% Yb3+ ions to tellurite glass enhances the value of FWHM  re for tellurite doped 0.5% Er3+. We can conclude that tri-doped tellurite glass plays a key role in optical amplifiers. A comparison of the important optical parameters is given in Table 3. The value of FWHM  re for 4I13/2 ? 4I15/2 transition is found to be higher than those of TZNY1 [26], SBS1 [27], phosphate [28] and TKNNZG4 [29].

Besides the stimulated emission cross-section and effective width of emission band, lifetime of 4I13/2 of Er3+ is also an important parameter for broadband optical amplifiers. The luminescence decay curves of the 4I13/2 ? 4I15/2 transition in the mono-doped, codoped and tri-doped glasses, under 980 nm excitation wavelength, have been measured at RT to evaluate the quantum efficiency. As can be seen in Fig. 3, all the decay profiles are well fitted to a single exponential function:

It ¼ I0 et=s

ð11Þ

where It is the actual luminescence intensity, I0 is the luminescence intensity at the start of the decay process, t is the time and s is the decay time. The resulting PL decay times are collected in Table 4. As can be seen, the decay times support the increase in luminescence intensity of 4I13/2 ? 4I15/2 transition in tellurite glasses. It is obvious that the decay time increases with increase of rare earth concentration. The quantum efficiency (g) is calculated from the radiative lifetime sr, obtained from the J–O theory, and the measured experimental lifetime smea, as given by

gð%Þ ¼

smea sr

ð12Þ

The results show a significant increase of quantum efficiency after adding 0.5 mol% Ce3+ for co-doped glass and 0.5Ce3+/1Yb3+ for tri-doped glass. The quantum efficiency (g) of tri-doped sample is larger than TZNB0 [25], TZNY1 [26] and 3 mol% Er3+ concentration for tellurite glass [30], 2.5Yb3+/0.5Er3+ [17] and 3Yb3+/0.5Er3+ [17] co-doped phosphate glass. 3.4. Up-conversion luminescence and mechanisms analysis

mono-doped co-doped tri-doped

Normalized PL intensity (a.u)

1

0.04979

(c) (b)

(a)

0.00248

Fig. 4 shows the up-conversion emission of Er3+ mono-doped, Er3+/Ce3+ co-doped and Er3+/Ce3+/Yb3+ tri-doped tellurite glasses under 980 nm excitation. The strong green emissions at 533 and 546 nm are ascribed to the 2H11/2 ? 4I15/2 and 4S3/2 ? 4I15/2 transitions, respectively. As can be seen from Fig. 4, the up-conversion luminescence dramatically decreased in the presence of Ce3+ ions. For example, the emission intensity of the 546 nm band in codoped sample decreased by a factor of 150 compared to mono-doped sample. We can notice that by adding 1 mol% Yb3+ to co-doped simple, the up-conversion luminescence increases by a factor of 26. The up-conversion emission for the two emission bands could be related to a two-photon absorption process. In fact, the intensity of up-conversion emission, Iup, is proportional to the mth power of the excitation intensity [31], IExc, i.e.

Iup / ðIEXC Þm

0

7

14

t (ms) Fig. 3. Room temperature PL decay curves of 4I13/2 ? 4I15/2 transition, under excitation wavelength k = 980 nm, for mono-doped (a), co-doped (b) and tri-doped (c) glass samples.

ð13Þ

where m denotes the number of photons involved in the up-conversion process. The experimental data of up-conversion emission intensity were fitted to straight lines with a slope of m. As can be seen in the inset of the Fig. 4, the values of m were estimated approximately to be 2.26 for the 2H11/2 ? 4I15/2 transition and 2.16 for the 4S3/2 ? 4I15/2 transition, indicating that a two-photon process was involved in the emission mechanisms in mono-doped

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W. Stambouli et al. / Journal of Alloys and Compounds 580 (2013) 310–315

mono-doped co-doped tri-doped

4 S3/2

30 2 H11/2

4

I15/2

100000 λ em= 546 nm, slope 2.16

10000 1000 λem=533 nm,

slope 2.26

100 10 10

100

Power (mW)

10

x5 x10

0 520

560

600

λ (nm) Fig. 4. Visible up-conversion spectrum of Er3+ mono-doped, Er3+/Ce3+ co-doped and Er3+/Ce3+/Yb3+ tri-doped glass samples under 980 nm excitation. Inset: integrated green up-conversion PL as a function of pump power P for mono-doped glass.

tellurite glass. For co-doped and tri-doped samples, the up-conversion emission is also governed with a two-photon process. According to the energy matching condition and the quadratic dependence on the 980 nm pump power, the possible IR and up-conversion luminescence mechanisms are discussed based on the simplified energy levels of Er3+, Ce3+ and Yb3+ depicted in Fig. 5. For monodoped glass, when the 4I11/2 level is excited by the 980 nm photons, part of the excitation energy at the 4I11/2 level relaxes non-radiatively to the 4I13/2. The radiative transition via 4I13/2 ? 4I15/2 leads to the characteristic 1530 nm emission. Along with this, the excited state absorption (ESA) from 4I11/2 can populate the 4F7/2 level: an Er3+ ion cannot obtain enough energy from a single pumping photon to reach the 4F7/2 level. Therefore, Er3+ ions must have obtained additional energy from absorption of a second pumping photon. The 4 F7/2 level decays non-radiatively to 2H11/2 + 4S3/2 and the transition to 4I15/2 gives the green emission. In addition to this, the possible cross relaxation (CR) through Er3+ (4I11/2) + Er3+ (4I11/2) ? Er3+ (4F7/ 3+ 4 ( I15/2) can also increase the population of the 4F7/2 level. 2) + Er As be expected, the introduction of Ce3+ ions significantly decreases the up-conversion emission. The reason may be ascribe to the shortening lifetime of the 4I11/2 level in Er3+/Ce3+ co-doped glass, since the probability of up-conversion excitation decreases if the lifetime of the intermediate 4I11/2 level decreases [32]. Moreover, the energy gap between 4I11/2 and 4I13/2 levels is about 3600 cm1 in Er3+doped tellurite-based glass [10]. But Ce3+ ion possess only two

4

4. Conclusion The absorption, emission and up-conversion spectra of Er3+ mono-doped, Er3+/Ce3+ co-doped and Er3+/Ce3+/Yb3+ tri-doped tellurite glasses were measured and investigated. The Judd–Ofelt intensity parameters Xt (t = 2, 4, 6), spontaneous emission probability, radiative lifetime and branching ratios of several Er3+ transitions were calculated from the absorption spectra based on the Judd–Ofelt theory. The addition of Ce3+ is effective on the decrease of the up-conversion fluorescence intensity of Er3+ owing to the phonon-assisted energy transfer: Er3+:4I11/2 + Ce3+:2F5/ 3+ 4 3+ 2 3+ ? Er : I + Ce : F . The addition of Yb has the effect of 13/2 7/2 2 4 increasing the population of I11/2 level by ET process from the 2 F5/2 level of Yb3+:2F5/2 (Yb3+) + 4I15/2 (Er3+) ? 2F7/2 (Yb3+) + 4I11/2

F5/2

4

4

15 4 4

10 4

F9/2

I9/2

I11/2

533 nm

Energie / cm-1

F7/2 H11/2 4 S3/2

546 nm

2

20

ET CR

20

F5/2 and 2F7/2 levels with the energy gap of 2200 cm1. So, there exists an energy mismatch of about 1400 cm1 and this could be bridged by two phonons at most for the tellurite glass with maximum phonon energy of about 750 cm1 [33,34]. This results in the phonon assisted energy transfer across the channels Er3+:4I11/ 3+ 2 3+ 4 3+ 2 2 + Ce : F5/2 ? Er : I13/2 + Ce : F7/2. As a result, the nonradiative 3+ 4 4 transition rate of Er : I11/2 ? I13/2 is improved further, the population accumulation of 4I13/2 level should get more densely, a much weaker up-conversion and stronger 4I13/2 ? 4I15/2 fluorescence emission can be anticipated. For tri-doped tellurite glass, It is seen from Fig. 5 that there are mainly three up-conversion mechanism occurring: the energy transfer (ET) between Yb3+ and Er3+ ions; the excited state absorption (ESA) from the 4I11/2 level of Er3+ and cross relaxation (CR) between Er3+ ions such as (4I11/2, 4I11/ 4 4 2) ? ( I15/2, F7/2). One can notice that, the green up-conversion emission depend on the excited state 4I11/2. The addition of Yb3+ has the effect of increasing the population of 4I11/2 level and thus increased the intensity of up-conversion luminescence compared to co-doped sample: the 4I11/2 level of Er3+ is directly excited by 980 nm and by ET process from the 2F5/2 level of Yb3+:2F5/ 3+ 4 3+ 2 3+ 4 3+ 3+ has a 2(Yb ) + I15/2(Er ) ? F7/2(Yb ) + I11/2(Er ). Since Yb 3+ much larger absorption cross-section than Er in the 980 nm, the ground state absorption of Er3+ from 4I15/2 ? 4I11/2 is not the main reason for the population of the 4I11/2 level of Er3+; the ET process mainly contributes to the population accumulation on the 4I11/2 level. The highly populated 4I11/2 level is supposed to serve as the intermediate state responsible for the up-conversion processes. The presence of Ce3+ in tri-doped tellurite glass will be affected the energy transfer ET between Yb3+ and Er3+, excited state absorption ESA of Er3+ and cross relaxation CR.

ESA

Intensity (a.u.)

40

4 I15/2

Integrated PL intensity (a.u.).

2

50

2

I13/2

Pump

5

0

F5/2

F7/2

ET

2

F5/2

4

Ce3+

ET

1.53

2

2

I15/2

Er3+

F7/2

Yb3+

Fig. 5. Simplified energy levels diagram of Ce3+, Er3+, and Yb3+ ions and transitions for observable emission in the tellurite glasses.

W. Stambouli et al. / Journal of Alloys and Compounds 580 (2013) 310–315

(Er3+). The study clearly show that the Ce3+ and Yb3+ co-doping into Er3+-doped tellurite glasses effectively enhanced the 1.53 lm fluorescence characteristics. Compared with other glass hosts, the obtained data reveal that tri-doped tellurite glass will be preferable for efficient 980 nm pumped Er3+-doped optical amplifiers and lasers.

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