Spectroscopic investigation of zinc tellurite glasses doped with Yb3 + and Er3 + ions

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 165 (2016) 183–190

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Spectroscopic investigation of zinc tellurite glasses doped with Yb3 + and Er3 + ions Gökhan Bilir ⁎, Ayfer Kaya, Hatun Cinkaya, Gönül Eryürek Department of Physics Engineering, Istanbul Technical University, 34469 Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 16 September 2015 Received in revised form 8 April 2016 Accepted 17 April 2016 Available online 21 April 2016 Keywords: Zinc tellurite glasses Emission properties Color quality parameters

a b s t r a c t This paper presents a detailed spectroscopic investigation of zinc tellurite glasses with the compositions (0.80 − x − y) TeO2 + (0.20) ZnO + xEr2O3 + yYb2O3 (x = 0, y = 0; x = 0.004, y = 0; x = 0, y = 0.05 and x = 0.004, y = 0.05 per moles). The samples were synthesized by the conventional melt quenching method. The optical absorption and emission measurements were conducted at room temperature to determine the spectral properties of lanthanides doped zinc tellurite glasses and, to study the energy transfer processes between dopant lanthanide ions. The band gap energies for both direct and indirect possible transitions and the Urbach energies were measured from the absorption spectra. The absorption spectra of the samples were analyzed by using the Judd-Ofelt approach. The effect of the ytterbium ions on the emission properties of erbium ions was investigated and the energy transfer processes between dopant ions were studied by measuring the up-conversion emission properties of the materials. The color quality parameters of obtained visible up-conversion emission were also determined as well as possibility of using the Er3+ glasses as erbium doped fiber amplifiers at 1.55 μm in infrared emission region. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Lanthanide (Ln) ions doped materials have a very wide application area in photonics technology such as solid state lasers, optical amplifiers, etc. which require high emission quantum yields. Their selection varies depending on the particular application. The priority target in the production and development of such a device is to increase its quantum yield in the wavelength range of interest. The factor that directly affects the quantum yield is the energy loss due to nonradiative interactions [1–5]. In particular, among these materials, tellurite glasses with different network modifiers have attracted considerable attention because of having low vibrational frequencies (~780 cm−1). Furthermore, some excellent properties of tellurite glasses, such as high refractive index (2.1 to 2.3), transparency in a wide spectral region (0.3–7 μm), good corrosion resistance, high dielectric constant, thermal and chemical stability and low melting point (~750 °C), making them the center of interest for fiber amplifiers and 3D-display devices. They are capable of incorporating large concentrations of lanthanide ions without inducing crystallization [3–7]. In last several decades, the erbium ion (Er3+) played an important role as an active ion in optical amplification in telecommunications because of its excellent emission properties around 1.55 μm. Even if erbium doped fiber amplifiers (EDFA) widely used and is the main

⁎ Corresponding author. E-mail address: [email protected] (G. Bilir).

http://dx.doi.org/10.1016/j.saa.2016.04.042 1386-1425/© 2016 Elsevier B.V. All rights reserved.

technology, the demand for the more bandwidth continues to grow with the recent developments [7–10]. Tellurite glasses also exhibit large stimulated emission cross sections and broad emission bandwidth at 1.5 μm emission of Er3+ ions which is desirable feature for the EDFA applications [11,12]. High pumping rates are required to obtain population inversion in three level system of Er3+ for optical amplification at 1.5 μm. At this point Ytterbium (Yb3+) sensitized Er3+ emission starts to play an important role to obtain high gain bandwidth because of the large spectral overlap between 2F5/2 → 2F7/2 emission of Yb3+ and 4 I15/2 → 4I11/2 absorption band of Er3+ which results in an efficient energy transfer. It can also significantly improve the up-conversion emission properties of the materials. Extensive studies have also been made on the conversion of the low energy photons into high energy radiation in rare earth ions (REI) doped materials. Since, tellurite glasses have the lowest phonon energy and the larger refractive index among the oxide glasses, these hosts are promising for REI up-conversion luminescence. The obtained upconversion emission can also be characterized by CIE (Commission internationale de l'éclairage) color space that uses luminance parameter and two coordinates which specify the point on the chromaticity diagram [13–16]. Two other parameters, Correlated Color Temperature (CCT) and Color Rendering Index (CRI), are used for determining quality of the visible emission. In this paper, we presented the role of the Yb3+ co-dopant on the emission properties of the Er3+ in zinc tellurite glasses. We have also studied the energy transfer and the energy up-conversion processes between dopant ions.

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2. Experimental The zinc tellurite glasses with the compositions (0.80 − x − y) TeO2 + (0.20) ZnO + xEr2O3 + yYb2O3 (x = 0, y = 0; x = 0.004, y = 0; x = 0, y = 0.05 and x = 0.004, y = 0.05 per moles) were synthesized by the conventional melt quenching method. The compositions of the glasses are summarized in Table 1. Glasses were batched from powders of TeO2, ZnO, Er2O3, and Yb2O3 with a purity of at least 99.99%. The appropriate amounts of the powders in molar ratio were weighed using a high precision scale and were mixed in an agate mortar. Then, the mixed powders were melted in a platinum crucible in an electric furnace under air environment at 850–1000 °C for 1 h and quenched on a preheated stainless steel blocks. The as-cast glasses were annealed at 200 °C below their glass transition temperature [17] for 2 h to relieve internal stresses. Finally, the glass samples were cut and polished well to form parallel surfaces to carry out the optical measurements. The densities of the glasses were measured by employing Archimedes' principle with the usage of a pycnometer with an ultra-pure water as an immersion liquid. The absorption spectra were measured using a Varian Cary 5000 UV/ VIS/NIR spectrophotometer between 350 and 2000 nm. The un-doped glasses having the same thicknesses with the doped glasses were used as a reference to reduce the effect of reflection from surfaces of the samples. The emission spectra were collected using an Apollo Instruments diode laser (Model No: S30-808-6) with 805.2 nm wavelength and a 980 nm diode laser with 3 mW output as an excitation source, a Princeton Instruments SP2500i model monochromator and Acton series ID441-C Model InGaAs and SI440 Silicon detectors for the detection of the luminescence in the infrared and visible region, respectively. All measurements were carried out at room temperature. The CCT, the CRI and the CIE-1931 coordinates of the CIE931 of the obtained up-conversion emissions from samples were measured with an illuminance meter (AsenseTek Lighting Passport) to determine the quality of the emitted visible light. 3. Results and discussion 3.1. Absorption spectra, band gap calculations and Judd Ofelt analysis Absorption spectra of all glasses with corresponding transitions are given in Fig. 1. The peak positions of the absorption bands of each spectrum are consistent with those observed for different glass hosts. Optical band gaps of the glasses for the direct and indirect transitions were determined according to the Davis–Mott theory [18] and tabulated in Table 2. The Urbach [19] widths were also determined and given in Table 2. According to the Davis and Mott theory [18], the absorption coefficient obeys the relation below;
n αhν ¼ B hν−Eopt

ð1Þ

where α is the absorption coefficient, B is a constant, Eopt is the energy of the optical band gap and hν is the photon energy. This relation works very well for the linear part of the exponential tail of the absorption spectrum, which is used to extrapolate the x-intercept to determine optical band gap of the materials. The superscript n can have values of 1/2 and 2 for direct and indirect optical gaps, respectively. Table 1 Glass compositions, densities and ion concentrations for synthesized glass samples. Glass ID

TZ TZE TZY TZEY

Glass compositions (mol%) TeO2

ZnO

Er2O3

Yb2O3

80.0 79.6 75.0 74.6

20.0 20.0 20.0 20.0

– 0.4 – 0.4

– – 5.0 5.0

Density (g/cm3)

Ion concentration (1020 ions/cm3)

5.35 5.38 5.55 5.55

1.43 16.62 Er3+: 1.12; Yb3+: 15.91

Fig. 1. Absorption spectra of the studied glass systems, recorded at room temperature.

The absorption coefficient also shows exponential dependence on photon energy near the higher energy edge, which follows Urbach rule [19]   hν αðνÞ ¼ α0 exp ΔE

ð2Þ

A slight increasing tendency (blue shift) of the band gap values were observed with increasing dopant ions contents. The obtained band gap values of recent systems are quite close to that of the tellurite based glasses reported in the literature [5,20]. The differences could be due to the extrapolation method used in the estimation of the band gap energies. Spectroscopic intensity parameters Ω(2), Ω(4), and Ω(6) of the Er3+ were calculated using Judd Ofelt theory [21,22] in order to investigate the effect of the glass matrix on the radiative transition probabilities and the lifetimes of the energy levels. Experimental values of the oscillator strengths, fexp, were determined using Eq. (3). These values were then used to determine the J-O intensity parameters by a least squares fit using Eq. (4). The measured and calculated oscillator strengths, the Judd Ofelt intensity parameters, the spectroscopic quality factors (Ω(4)/ Ω(6)) with some physical parameters are tabulated in Table 3. As seen in this table, the calculated oscillator strengths from the J-O theory agree quite well with the experimental ones. The values of the quality of the calculations are also given in Table 3. It has been found that the intensity parameters follow the trend as Ω(2) N Ω(4) N Ω(6)   ! ! f exp ¼ 4:32  10−9 ∫ε ν d ν

ð3Þ

! where εð ν Þ is the molar extinction coefficient at ν (cm−1)


fcal J→ J

0




2 2 X
  n þ2 8π2 mc  ΩðtÞjhðS; LÞJ jU ðt Þ S0 ; L0 J 0 ¼  9n 3hð2Jþ1Þλ t¼2;4;6 ð4Þ

Table 2 Band gap values both for direct and indirect transitions and Urbach widths. Glass ID

Direct transition Eopt (eV)

Indirect transition Eopt (eV)

Urbach width ΔE (meV)

TZ TZE TZY TZEY

3.23 3.24 3.34 3.30

2.95 2.94 3.08 3.11

81.5 95.6 106.7 100.7

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Table 3 Measured and calculated line strengths, Judd-Ofelt parameters and spectroscopic quality factors for synthesized glasses. TZEY (Er3+)

TZE Transition

λ (nm)

4

489 1.26 522 5.69 544 0.34 654 2.82 801 0.81 979 1.41 1533 7.51 3.8 × 10−6 0.6 × 10−6 15.6 1.43 6.49 × 10−20 3.14 × 10−20 1.56 × 10−20 2.01

F7/2 H11/2 4 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 rms f rms Δf rms error (%) N0 (×1020) Ω (2) Ω (4) Ω (6) Ω (4)/Ω (6) 2

fexp (×10−6)

TZEY (Yb3+)

TZY

fcal (×10−6)

Transition

λ (nm)

0.92 5.31 0.27 2.21 0.68 1.13 8.51

4

489 0.87 522 5.43 544 0.27 654 2.18 801 0.49 979 – 1533 8.12 3.82 × 10−6 0.7 × 10−6 18.47 1.12 8.02 × 10−20 3.76 × 10−20 1.71 × 10−20 2.19

F7/2 H11/2 4 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 rms f rms Δf rms error (%) N0 (x1020) Ω (2) Ω (4) Ω (6) Ω (4)/Ω (6) 2

fexp (×10−6)

where n is the refractive index, c is the speed of light, λ is the mean wavelength of the transition, Ωt(t = 2,4,6) are the J-O intensity parameters and U(t)(t=2,4,6) are the doubly reduced square matrix elements of the unit tensor operator calculated by Carnall [23] and Kaminskii [24] from the intermediate coupling approximation. The Ω(2) parameter is indicative of the amount of lanthanide – O covalent bonding and is very sensitive to the glass composition and local environment of the rare earth ion (REI) sites, while Ω(4) and Ω(6) are related to the rigidity of the host matrix. An increase in the asymmetric nature of REI site and an increase of the covalency of chemical bonds with the ligands cause an increase in the Ω(2) value. Generally, the position of the absorption edge depends on the strength

fcal (×10−6)

Transition

λ (nm)

fexp (×10−6)

Transition

λ (nm)

fexp (×10−6)

1.20 5.84 0.31 2.76 0.81 – 7.05

2

978

80.96

2

978

114.46

rms f rms Δf rms error (%) N0 (x1020) Ω (2) Ω (4) Ω (6) Ω (4)/Ω (6)

– – – 16.62 – – – –

F5/2

F5/2

rms f rms Δf rms error (%) N0 (x1020) Ω (2) Ω (4) Ω (6) Ω (4)/Ω (6)

– – – 15.91 – – – –

of the oxygen bond in the glass formation network. The higher value of Ω(2) representing less ionic nature of the chemical bond with the ligands is responsible for the increase in the covalent character. Since covalent character increased with increasing dopant concentration as seen in Table 4, the band gap values of the glasses is also expected to increase. Since the Judd-Ofelt parameters are larger in tellurite glasses, the electric dipole line strengths (Sed) are also expected to be larger which result in higher spontaneous emission probabilities (Aed). According to Jacobs and Weber [25], the emission intensity of the 4 F3/2 → 4 I11/2 laser transition of the Nd3 + is mainly characterized using the ratio of Ω(4) to Ω(6), the so-called spectroscopic quality factor (Ω(4)/Ω(6)). The host materials with larger values of spectroscopic

Table 4 Transition probabilities, branching ratios and the radiative lifetimes of the energy levels for all the samples. TZEY (Er3+)

TZE Transition

Aed (s−1)

β (%)

τR (ms)

Transition

4

351.6 449.8 56.4 581.2 139.0 3.2 5729.1 299.3 203.4 9.4 3540.7 1448.3 117.4 206.3 2.0 22773.7 455.9 276.4 317.6 75.0 0.2 9843.3 2291.8 936.7 433.8 23.0 0.1 2.5

1 88.85 11.15 80.34 19.21 0.45 91.8 4.79 3.26 0.15 66.62 27.25 2.21 3.88 0.04 95.29 1.9 1.16 1.32 0.31 0.0007 72.74 16.94 6.92 3.20 0.17 0.001 0.02

2.844 1.975

4

I13/2 → 4I15/2 I11/2 →4I15/2 4 I13/2 4 I9/2 → 4I15/2 4 I13/2 4 I11/2 4 F9/2 →4I15/2 4 I13/2 4 I11/2 4 I9/2 4 S3/2 →4I15/2 4 I13/2 4 I11/2 4 I9/2 4 F9/2 2 H11/2 → 4I15/2 4 I13/2 4 I11/2 4 I9/2 4 F9/2 4 S3/2 4 F7/2 →4I15/2 4 I13/2 4 I11/2 4 I9/2 4 F9/2 4 S3/2 2 H11/2 4

1.382

0.160

0.188

0.042

0.074

I13/2 → 4I15/2 I11/2 →4I15/2 4 I13/2 4 I9/2 → 4I15/2 4 I13/2 4 I11/2 4 F9/2 → 4I15/2 4 I13/2 4 I11/2 4 I9/2 4 S3/2 → 4I15/2 4 I13/2 4 I11/2 4 I9/2 4 F9/2 2 H11/2 → 4I15/2 4 I13/2 4 I11/2 4 I9/2 4 F9/2 4 S3/2 4 F7/2 → 4I15/2 4 I13/2 4 I11/2 4 I9/2 4 F9/2 4 S3/2 2 H11/2 4

TZEY (Yb3+)

TZY Aed (s−1) 387.8 515.2 74.4

6427.3 336.9 235.4 15.5 3772.8 1586.2 124.4 240.7 2.2 27886.3 543.6 332.2 381.7 92.6 0.2 11142.6 2743.5 1102.7 495.1 27.7 0.2 3.0

β (%)

τR (ms)

Transition

Aed (s−1)

β (%)

τR (ms)

Transition

Aed (s−1)

β (%)

τR (ms)

1 87.38 12.62

2.579 1.699

2

1279.2

1

0.782

2

1882

1

0.531

91.62 4.80 3.35 0.22 65.89 27.70 2.17 4.20 0.04 95.38 1.86 1.14 1.30 0.31 0.0006 71.82 17.68 7.11 3.19 0.18 0.001 0.02

0.143

0.175

0.034

0.064

F5/2 → 2F7/2

F5/2 → 2F7/2

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Fig. 4. Absorption and stimulated emission cross sections of 1.5 μm emission of Er3+ ions in TZEY glass system under 805.2 nm excitation.

lasing performance of the samples studied. The spontaneous emission probability and the stimulated emission cross section for Yb3+ doped samples have been determined using the equations below [26,27];

A

ed

¼

 0  8πcn2 2J þ 1

σ emis ¼

λ4p ð2J þ 1Þ

ð5Þ

  0 A J→J

λ4p 8πcn2 Δλ

∫α ðλÞdλ

ð6Þ

eff

Fig. 2. IR emission spectra of samples under 805.2 nm excitation.

Δλeff ¼ quality factors can have more potential stimulated emission. As it can be seen from table, TZEY has more potential for stimulated emission because of having highest quality factor of 2.19. A variety of spectroscopic parameters, such as spontaneous emission probabilities (Aed), luminescence branching ratios (β) and radiative lifetimes (τR) have been calculated using the J-O parameters to predict the

∫IðλÞdλ I max

ð7Þ

where λp is the peak position, Δλeff is the effective width of the emission band. The results are summarized in Table 4 Δλeff ; λp :

3.2. Emission properties under 805.2 nm excitation, absorption and emission cross sections The emission spectra of the glasses under 805.2 nm laser excitation with corresponding transitions are shown in Fig. 2. All measurements were conducted at room temperature in the 800–1700 nm wavelength range. It was observed that the intensity of the 1.5 μm emission of Er3+ is increased with the addition of the Yb3+ ions as a sensitizer to the host. We have determined the absorption and stimulated emission cross sections of the 1.5 μm transition under 805.2 nm excitation with and without the addition of the Yb3+ dopant. McCumber theory [28] was used to calculate the stimulated emission cross sections both from absorption and emission spectra. The results are shown in Figs. 3 and

Table 5 Effective bandwidth and stimulated emission cross sections values for the 1.55 μm transition of Er3+ ions under 805.2 nm and 980 nm excitations. Sample

Fig. 3. Absorption and stimulated emission cross sections of 1.5 μm emission of Er3+ ions in TZE glass under 805.2 nm excitation.

TZE TZEY

805.2 nm excitation

980 nm excitation

FWHM

σe

FWHM

σe

74.92 85.79

0.813 0.847

71.75 81.05

0.84 0.856

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3.3. Emission properties under 980 nm excitation, absorption and emission cross sections The emission spectra of the samples under 980 nm excitation with corresponding transitions are given in Fig. 5. The sharp intense spike seen in the spectra at 980 nm is due to the laser emission. It is clearly seen from the figure that the addition of the Yb3+ ions increased the intensity of 1.5 μm emission. The absorption and stimulated emission cross sections of the 1.5 μm transition under 980 nm excitation have also been determined before and after addition of Yb3+ dopant and given in Figs. 6 and 7, respectively. It was also determined that the FWHMs and the stimulated emission cross sections of the 1.5 μm transition increased with the addition of the Yb3+ ions. The FWHM and the stimulated emission cross section values are tabulated in Table 5. 3.4. Energy up-conversion processes under 980 nm excitation

Fig. 5. IR emission spectra of samples under 980 nm excitation.

4. It was determined that the FWHMs (full-width at half maxima) and the stimulated emission cross sections of the 1.5 μm transition increased with the addition of the Yb3 + ions. The FWHM and the stimulated emission cross section values are tabulated in Table 5.

Fig. 6. Absorption and stimulated emission cross sections of 1.5 μm emission of Er3+ ions in TZE glass under 805.2 nm excitation.

The up-conversion emission spectra of the samples under 980 nm excitation are given in Fig. 8 together with those obtained by using illuminance meter. There are four up-conversion luminescence peaks located at 480 nm (blue), 522 and 546 nm (green), 654 nm (red) and 780–850 nm (NIR), which are assigned to blue cooperative upconversion of Yb3+ and 2H11/2 + 4S3/2 to 4I15/2, 4F9/2 to 4I15/2 and 4I9/2 to 4I15/2 transitions, respectively. The measurements showed that the intensity of the up-conversion emission for the co-coped glass 7 (green) and 38 (red) times higher than that of Er3+ doped glass. This result is the indicative of the energy transfer processes between dopant ions. It is important to state here that, although the pumping power was 3 mW, all UC emissions from the samples can easily be seen by naked eye. Figs. 9 and 10 illustrates the energy level diagram of Yb and Er ions which describes the possible energy transfer processes, the up-conversion mechanism and the cooperative up-conversion processes. We have also determined the color quality parameters like CCT, CRI, CIE 1931 coordinates and illuminance values by using illuminance meter and obtained results are summarized in Fig. 11 and Table 6. It is clearly seen from Table that the obtained UC emissions from samples are cool in appearance (CCTs N4000 K) [13–16] and poor in CRI (b 80) [13–16]. The addition of another REI dopant into the glass system caused to decrease of the CCT and to increase of the CRI and illuminance values. The best values were obtained for the TZEY glass system. Results

Fig. 7. Absorption and stimulated emission cross sections of 1.5 μm emission of Er3+ ions in TZEY glass system under 805.2 nm excitation.

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Fig. 8. UC emission spectra of all samples under 980 nm excitation, obtained by both PL measurement system and illuminance meter. The insets are the image of the UC emissions.

Fig. 9. Energy level diagram of the energy transfer processes between Er3+ and Yb3+ ions.

G. Bilir et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 165 (2016) 183–190

Fig. 10. Energy level diagram for the blue cooperative UC processes between Yb3+ ions.

showed that we were achieved to obtain two main colors (green and red) from doubly doped glass system. 4. Conclusions Tellurite glasses doped singly and doubly with Yb3+ and Er3+ ions were synthesized using the conventional melt quenching method. The J-O parameters measured from the absorption and luminescence spectra of the samples were used to investigate the effect of the Yb3+ on the infrared and visible up-conversion luminescence properties of Er3+ ion in the doubly doped zinc tellurite glasses. Presence of Yb3+ ions in doubly doped glass improves the stimulated emission cross section (11–15%) and the bandwidth (1–4%) of the 1.5 μm emission of the Er3+. For example, the bandwidth of this emission was measured to be 74.92 nm and 85.79 nm in the sample doped only with Er3+ and co-doped with Yb3+ under 805.2 nm laser diode excitation, respectively. Two main colors (green and red) have been successfully obtained from doubly doped sample while a green emission at ~ 545 nm and a blue emission at ~475 nm were observed in singly doped glasses with Er3 + and Yb3 + ions, respectively. This indicates the improvement of the up-conversion processes due to the energy transfer processes with the addition of Yb3+ ions. Acknowledgement This study was funded with the project number 38162 by Support Branch of Research Projects of Istanbul Technical University (ITU —

Fig. 11. CIE coordinates for obtained UC emissions from the samples.

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Table 6 CIE coordinates, CCT, CRI and illuminance values for obtained UC emission from all samples. Sample

CIE coordinates

TZE

x = 0.2954 y = 0.5417 x = 0.2465 y = 0.2860 x = 0.3358 y = 0.6496

TZY TZEY

CCT (Kelvin)

CRI

Illuminance (Lux)

6249

63

5

15,425

65

3

5489

66

23

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