Luminescence quantum efficiency at 1.5 μm of Er3+-doped tellurite glass determined by thermal lens spectroscopy

Optical Materials xxx (2013) xxx–xxx

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Luminescence quantum efficiency at 1.5 lm of Er3+-doped tellurite glass determined by thermal lens spectroscopy M.S. Figueiredo a, F.A. Santos a, K. Yukimitu a, J.C.S. Moraes a, J.R. Silva b, M.L. Baesso b, L.A.O. Nunes c, L.H.C. Andrade d, S.M. Lima d,⇑ a

Departamento de Física e Química, Universidade Estadual Paulista, Av. Brasil, 56, 15385-000 Ilha Solteira, SP, Brazil Departamento de Física, Universidade Estadual de Maringá, Av. Colombo, 5790, 87020-900 Maringá, PR, Brazil Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, São Carlos, SP, Brazil d Grupo de Espectroscopia Óptica e Fototérmica, Universidade Estadual de Mato Grosso do Sul, Cidade Universitária, CP 351, Dourados, MS, Brazil b c

a r t i c l e

i n f o

Article history: Received 15 May 2013 Received in revised form 19 June 2013 Accepted 21 June 2013 Available online xxxx Keywords: Tellurite glass Erbium Luminescence quantum efficiency Thermal lens spectroscopy

a b s t r a c t Erbium doped tellurite glasses (TeO2 + Li2O + TiO2) were prepared by conventional melt-quenching method to study the influence of the Er3+ concentration on the luminescence quantum efficiency (g) at 1.5 lm. Absorption and luminescence data were used to characterize the samples, and the g parameter was measured using the well-known thermal lens spectroscopy. For low Er3+ concentration, the measured values are around 76%, and the concentration behavior of g shows Er–Er and Er–OH interactions, which agreed with the measured lifetime values. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Rare earth doped TeO2 based glasses are promising materials for applications in optical devices as optical fibers, solid state lasers, waveguides, optical amplifiers, optical modulators and frequency doubling [1–6]. This matrix presents useful and advantageous optical properties when compared to other glasses, such as high linear and non-linear refractive index, low phonon energy (750 cm1), wide-ranging optical window transparency, 0.4 up to 6 lm, and higher rare earth solubility compared to oxide glasses [7]. In addition, this glass shows good thermal and chemical stabilities as well as low melting point temperature [8]. Among the rare earth elements used for doping oxide hosts, the Er3+ ion has been widely studied because it presents broad emission bands, high emission cross-section and long lifetime in the 1.5 lm emission band [9–11]. These characteristics allow its application as active medium in solid-state lasers and optical amplifier at 1.5 lm, what is of interest for telecommunication purposes. Thus, it is desirable to determine the luminescence quantum efficiency (g) of the samples, mainly because it is an optical parameter that establishes the relationship between the absorbed and emitted energies [12–14]. Despite the fact that several methods have been developed to investigate g, the precise determination of its

⇑ Corresponding author. Tel.: +55 6739022653; fax: +55 6739022652. E-mail address: [email protected] (S.M. Lima).

absolute value, especially for solid samples, has been shown to be a difficult challenge [15]. The spatial and spectral distributions of the emitted light combined with different processes as reabsorption, reemission, polarization, refractive index change, and energy transfer between the active fluorescent centers are limitations of the currently employed methods, and may be the main reason for the controversial results found for this parameter in the literature. Besides, specifically to the emission at 1.5 lm of Er3+, the presence of OH in the glass can contribute significantly to the luminescence decay [16–18]. The evaluation of the fraction of radiative and nonradiative processes in fluorescent materials has shown to be an advantageous alternative route to determine the luminescence quantum efficiency of solids. The determination of the absorbed energy which is converted to heat, i.e., the nonradiative processes, is complementary to pure optical procedures, like conventional luminescence and emission lifetime methods. The detection of nonradiative processes is the fundamental base of photothermal methods, and among them, the Thermal Lens (TL) spectroscopy has been shown to present good accuracy in the determination of g of different luminescent materials, including solid and liquid samples [19–22]. Thus, the aim of this work is to investigate the Er3+ doping influence on g values around 1.5 lm of the Te–Li–Ti (TLT) matrix using the TL spectroscopy. The experimental setup was developed to excite the samples at 976 nm, and the obtained results were compared to those measured with conventional spectroscopies, such absorption, luminescence and lifetime.

0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.06.041

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M.S. Figueiredo et al. / Optical Materials xxx (2013) xxx–xxx

Tellurite glasses with nominal composition (100x)  (0.8TeO2 + 0.1Li2O + 0.1TiO2) + xEr2O3, in which x = 0.05, 0.2, 0.3, 0.4, 0.5, 1, 2 and 4 mol% (TLT:xEr2O3) were prepared by the conventional melt-quenching method. Grade reagents of the Sigma–Aldrich (>99.99%) were weighted, mixed in agate mortar and melted in air atmosphere in a platinum crucible at 900 °C by 30 min. The melt was poured in a stainless-steel mold pre-heated near of glass transition temperature (Tg). Subsequently, after the thermal relaxation, the mold with the glass was returned to the furnace for annealing, in order to reduce mechanical stress caused by thermal shock. The annealing occurred during 5 h at a same temperature Tg, and then the furnace was cooled down slowly to room temperature. Finally, the produced glasses were cut and polished to reach 1.4 mm in thickness. Ground state absorption spectra were obtained by using a Perkin Elmer Lambda 900 spectrophotometer in the range 420– 1700 nm. The upconversion emission in the visible and the infrared luminescence were both measured after excitation at 976 nm with a diode laser. The signal was detected after the luminescence being dispersed by a Jobin–Yvon IHR320 monochromator, what was performed using a CCD and a cooled InGaAs detector. The luminescence lifetime measurements of the 4I13/2 level were obtained using an optical parametrical oscillator (OPO) (Surelite SLOP/Continumm) pumped by the third harmonic from a Nd:YAG laser (Surelite SLII-10/Continumm, 355 nm, 5 ns, 10 Hz). The TL setup was a two beam mode-mismatched configuration having a Ti:Sapphire laser tuned at 976 nm as the excitation source and a HeNe laser at 632.8 nm to probe the thermal effect. The experimental curves were fitted using the theoretical model proposed by Shen et al. [23]. The experimental geometrical parameters were m = 30 and V = 19. More details about the experimental setup can be seen in [24].

3. Results and discussion Fig. 1(a) shows the absorption coefficient spectrum in the UV– Vis–NIR regions of the 1 mol% of Er2O3 doped TLT glass. All the absorption peaks are indicated in the figure, corresponding to the

(b)

-1

I15/2 →

20

I15/2 → I11/2

6

4

λ = 976nm

2

4

1.0 3

2

0.8

1

0.1

0

1

2

4

6

8

10

12

14

16

20

3

-1

Energy (× 10 cm )

F3/2

I11/2 4

4

I9/2

S3/2 4

I11/2

6

4

4 2

I13/2

500

1000

Wavelength (nm)

1500

1400

1500

1600

0.2

4

0

0

0.4

8

3 4

4

I13/2

F9/2

F7/2 4

4

10

4

F5/2

3

Er2O3 concentration (×10 ions/cm )

18

Er2O3 concentration (×10 ions/cm )

10

0.6

10 20

0

1532 nm

4

4

976 nm

2

30

4

4

Absorption coefficient (cm-1)

Absorption Coefficient (cm )

8

(a) H11/2

40

transitions from the ground state 4I15/2 to the excited states of Er3+ ion. The area of the absorption band due to 4I15/2 ? 4I11/2 transition was calculated for different Er3+ concentrations and a linear dependence can be seen in the inset (part (a)). Near infrared luminescence spectroscopy was done by exciting the samples at 976 nm (4I15/2 ? 4I11/2 transition). Fig. 1(b) shows the luminescence spectrum obtained for the 1 mol% of Er2O3 doped TLT glass. The observed broadband luminescence is due to the transition from 4I13/2 level to the ground state 4I15/2, which corresponds to the most important Er3+ emission. The average emission wavelength < kem> = (1541 ± 5) nm was calculated from the luminescence spectrum, which is independent on the Er3+ concentration. The inset on part (b) displays the emission area normalized by the absorption coefficient at 976 nm, as a function of the Er3+ concentration. The amount of Er2O3 added in the tellurite glass matrix influences this NIR emission intensity: it is noted that initially it is constant as a function of concentration between (0.9– 2.2)  1020 ions/cm3 (0.2 and 0.5 mol%) and subsequently it decreases for higher Er2O3 concentration. This decrease is the main point of our discussion. It should be related to Er3+–Er3+ energy transfer which can be easily seen through the high absorption coefficient at 976 nm and long lifetime (units of ms) of the excited level (4I11/2). As a consequence of this interaction, the upconversion effect can occurs, resulting in visible emission from the Er3+ ion. In order to verify this assumption, the upconversion spectra were done exciting at 976 nm, what can be seen in Fig. 2 for different Er3+ concentration. The upconversion effect is not observed for low concentration of Er2O3, and it increases reaching a maximum in the intensity for higher Er3+ concentration. This result is consistent with that shown in Fig. 1(b). In addition to the Er–Er interaction, it is known that the 1.5 lm emission intensity can be affected by interaction from Er3+ ion and OH impurity presents into the host, which was observed by the IR absorption spectrum around 3400 cm1 (not shown). This interaction can also contribute to increase the non-radiative decay and consequently to decrease the lifetime and the luminescence quantum efficiency. This OH influence was well explored at the literature and the energy transfer from Er3+ to OH was determined in different systems, including tellurite glass [16–18]. It was verified that the energy transfer between Er3+ and OH groups quenching center is less intensive for low Er concentration [17]. So, lifetime

Normalized luminescence (arb. units)

2. Materials and methods

Integrated intensity (arb. units)

2

I15/2

1700

0.0

1800

Wavelength (nm)

Fig. 1. (a) Absorption coefficient spectrum of the 1 mol% of Er2O3 doped TLT glass. The inset shows the linear dependence of the absorption coefficient at 976 nm (4I15/2 ? 4I11/ (b) Normalized luminescence spectrum of the 1 mol% of Er2O3 doped TLT glass under 976 nm excitation wavelength (4I15/2 ? 4I11/2). In the inset, the integrated luminescence is plotted as a function of the Er3+ concentration.

2).

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M.S. Figueiredo et al. / Optical Materials xxx (2013) xxx–xxx

F7/2 H11/2 4 S3/2

976 nm

15

0.8

0.6

525 nm 545 nm

3

-1

Energy (× 10 cm )

2

I11/2

0.4

976 nm

10

4

5

0.2 4

0

Normalized intens. (arb. units)

1.0

4

20

I15/2

ol % )

0.0 2.0

500

520

540

560

580

3

2

xE rO

0.1 0.025

(M

0.5 0.3

600

Wavelength (nm) Fig. 2. Upconversion emission spectra of the tellurite glasses as function of the Er3+ concentration under 976 nm excitation.

and luminescence quantum efficiency were independently determined for our system to better understand the Er3+ concentration quenching behavior. Fig. 3 shows the emission decay curves from the 4I13/2 ? 4I15/2 transition by means of the excitation at the 4I11/2 level. It can be noted a simple exponential behavior to the decay, observed for all samples. For low Er3+concentration, the lifetime is around 4.28 ± 0.01 ms and it decreases with the increase of rare earth doping, resulting in a lifetime of 0.84 ± 0.01 ms for the sample with 4 mol% of Er2O3. The obtained values for the lifetime (s) are similar to that found in the literature for Er3+-doped tellurite glasses [16,25]. The lifetime dependence with Er3+ concentration will be shown and discussed together with the TL results. We used the TL method to quantify the total absorbed energy that is converted into heat after the samples were excited at kexc = 976 nm. This technique allowed to determine the luminescence quantum efficiency for the 1.5 lm emission as a function of the Er3+ concentration. In this procedure, characteristic TL transient signals is obtained for different excitation powers, as shown in Fig. 4(a) for the 0.05 mol% of Er2O3 doped TLT glass. The curves behavior was similar for all samples. Using Shen’s theoretical model [23], the experimental curves were fitted to determine the

1

TLT: xEr2O3

λexc = 976 nm

x = 0.05 x = 0.2 x = 0.3 x = 0.4 x = 1.0 x = 2.0 x = 4.0

1.5 μm emission

λobs = 1530 nm

0.1

amplitude of the TL signal, h which is proportional to the excitation laser power (Pe), given by the expression [26]:

h ¼ CPe aLeff u

in which a (cm ) is the optical absorption coefficient, Leff = (1  eaL)/a (cm) is the effective thickness of the sample, L (cm) is the thickness of the sample, / is the fraction of absorbed energy converted into heat and C = (Kkp)1ds/dT, with K (W/Kcm) being the thermal conductivity, ds/dT (K1) the temperature coefficient of the optical path length change at the probe beam wavelength kp (cm). It can be noted that C is a constant dependent mainly on the characteristics of the host matrix, by K and ds/dT, so that we can suppose that the ion concentration do not affect its value for each sample. Besides, there are no luminescence from the host matrix and consequently / = 1 can be used in Eq. (1) for the undoped samples. So, C was previously determined for the undoped TLT glass in a TL experimental setup by using a HeNe laser as probe beam at kp = 632.8 nm, and the obtained value is equal to 34 W1 [27]. This value was used to determine / for the doped samples. According to Eq. (1), there is a linear dependency of h with the excitation power, what can be observed in Fig. 4(b) for four different Er3+ concentrations of the doped in TLT samples. It is possible to note the occurrence of different angular coefficients for each sample, which is related to the product between a Leff and /. So, fitting the experimental data by a linear curve, the angular coefficients were determined. Since a Leff can be determined by the absorption spectrum (shown in Table 1), the fraction of absorbed energy converted into non-radiative relaxation (phonons) for the doped samples could be determined by normalizing the angular coefficient value by C. The / values are also listed in Table 1. The luminescence quantum efficiency to the 1.5 lm emission was determined for all samples by the relationship [13]:

u¼1g

0.01

0

2

4

6

8

10

12

14

16

18

20

Time (ms) Fig. 3. Emission decay time at 1.53 lm of the Er3+ doped tellurite glasses excited with a modulated diode laser at 976 nm.

ð1Þ 1

hkem i kexc

ð2Þ

Fig. 5 shows the obtained values of g determined by TL method and lifetime values at the 4I13/2 level. For low Er3+ concentration, g = 0.76 ± 0.06 and this value is approximately constant for concentrations up to 0.3 mol% of Er2O3. This value is in good agreement with the value determined by Pilla et al. in 70TeO2–19WO3–7Na2O–4Nb2O5 tellurite glass doped with 1.19  1020 ions/cm3, using TL spectroscopy with excitation in the visible

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M.S. Figueiredo et al. / Optical Materials xxx (2013) xxx–xxx

(a)

250mW

TLT: 0.05Er 2O3

1.10

(b) 0.0

TLT: xEr2O3

225mW 200mW

1.08 -0.1

175mW

x = 0.05

-0.2

x = 0.3 1.04

Experimental data Theoretical fit

1.02

x = 4.0

θ (rad)

I(t)/I(0)

150mW 1.06

-0.3

λ exc = 976nm

x = 0.5

1.00

-0.4 0

100

200

300

400

500

0.00

0.05

0.10

Time (ms)

0.15

0.20

0.25

0.30

Pe (W)

Fig. 4. (a) Thermal lens transient signal for 0.05 mol% Er2O3 doped TLT glass with kexc = 976 nm and kp = 632.8 nm. (b) Thermal lens phase shift versus the excitation power for different Er2O3 concentrations.

Table 1 Results obtained by thermal lens spectrometry for different Er2O3 concentrations. Er2O3 (mol%)

N (1020 ions/cm3)

h/P (±0.05 W1)

a Leff (±0.005)

u (±1%)

0 0.05 0.20 0.30 0.40 0.50 1.00 2.00 4.00

– 0.23 0.90 1.35 1.80 2.24 4.47 8.86 17.44

0.67 1.24 2.03 2.03 3.06 3.58 7.46 9.18 17.34

0.019 0.069 0.100 0.106 0.140 0.151 0.264 0.355 0.590

100 53 60 56 64 70 94 76 86

4. Conclusion

1.0

Quantum efficiency, η

5 4

0.6 3 0.4 2 0.2

0.0

0.1

1

Lifetime, τ (ms)

η τ

0.8

In conclusion, the luminescence quantum efficiency at 1.5 lm (I13/2 ? 4I15/2) for Er2O3 doped TLT glasses were determined by thermal lens spectrometry. The results were shown to be higher for low Er3+ concentration (76% for 0.05 mol% of Er3+-doped TLT glass), reaching the reduced value of 0.22 for the 4.0 mol% of Er3+. This reduction was attributed to the Er3+–Er3+ ions and also to the Er3+–OH interactions.

1

Acknowledgments

0

We gratefully acknowledge the financial support of the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP) and Fundação Araucária research agencies.

Er2O3 concentration (Mol%) Fig. 5. Er2O3 concentration behavior of the fluorescence quantum efficiency and lifetime in tellurite glasses.

region [28]. A possible explanation because the g value is different from 100% for low concentration of Er3+ is that our glass is not free of OH. As mentioned before, the presence of OH can strongly affect the IR luminescence [16–18]. For samples with higher concentration, g value exhibits a decrease to g  0.22, indicating interaction between Er3+ ions, as evidenced by the luminescence and upconversion spectra. In Fig. 5 is also shown the s behavior, which indicates good agreement to g value determined by TL method.

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