Influence of sesquioxides on fluorescence emission of Yb3 + ions in PbO–PbF2–B2O3 glass system

Journal of Non-Crystalline Solids 378 (2013) 265–272

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Influence of sesquioxides on fluorescence emission of Yb3 + ions in PbO–PbF2–B2O3 glass system D. Rajeswara Rao a,b, G. Sahaya Baskaran a,⁎, V. Ravi Kumar c, N. Veeraiah b a b c

Department of Physics, Andhra Loyola College (Autonomous), Vijayawada 520 008, A.P., India Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar 522 510, A.P., India Department of Physics, Krishna University, Nuzvid Campus, Nuzvid 521 201, A.P., India

a r t i c l e

i n f o

Article history: Received 5 May 2013 Received in revised form 27 June 2013 Available online 30 August 2013 Keywords: PbO–PbF2–B2O3: Yb3 + glasses; Sesquioxides; Emission spectra

a b s t r a c t Yb3+ ions doped lead oxyfluoro borate glasses mixed with varying concentrations of three interesting sesquioxides (viz., M2O3 = Al2O3, Sc2O3, Y2O3) were synthesized. The emission spectra of each series of glasses were recorded and characterized as a function of concentration of sesquioxides. The luminescence spectra of the three series of glasses excited at 970 nm have exhibited intense band in the NIR region corresponding to 2F5/2 → 2F7/2 transition. The emission spectra of all the three series of glasses found to be partially overlapped with the absorption band. The comparison of Stokes shift indicated the highest value for Y2O3 mixed glasses. From these spectra, the absorption and emission cross-sections and also life time of 2F5/2 level of Yb3+ ions have been evaluated. The luminescence cross section is found to be the highest for Y2O3 mixed glasses among the three series of the glasses studied. The reasons for such variations have been discussed in the light of varying degree of structural modifications at the vicinity of Yb3+ ions in the glass network with the support of IR spectral results of these glasses. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Lead oxyfluoride based glass systems act as attractive hosts for rare earth ions for lasing emission because of their high density, high refractive index and low phonon energies. In view of such characteristics, these glasses are being extensively used in IR fiber optics, laser windows and multifunctional non-linear optical devices [1–3]. Further, the addition of sesquioxides like scandium, yttrium and aluminum oxides that have got high melting point makes these glasses more efficient for lasing emission as these oxides increase the thermal conductivity [4], lower the phonon energies [5] of the host glass and pave the way for high luminescence efficiency [6]. Among different solid-state laser materials, Y2O3 containing glasses are proved to have an efficient operation both in continuous wave operation and in pulsed systems. The mixing of Y2O3 to PbO–PbF2–B2O3 glasses is expected to increase the mechanical, thermal and chemical stability and constitutes as a good candidate for biomedical and photonic applications [7,8]. In fact the addition of Y2O3 to borate glass systems widens the region of transparency, increases refractive index and optical band gap. In view of these reasons the presence of Y2O3 in lead oxyfluorborate glasses makes them as excellent host material for rare earth ions doping and makes the glass as useful for integrated optics [9,10].

⁎ Corresponding author. Tel.: +91 9490658088. E-mail address: [email protected] (G. Sahaya Baskaran). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.07.001

Similarly, rare earth doped scandia (Sc2O3) mixed glasses are found to have exciting technological applications in the field of optical devices such as luminescent displays, optical amplifiers and solid state lasers [11]. Recently, scandium oxide containing glasses in particular, have attracted the attention of many researchers owing to their interesting physical and chemical properties. Their high chemical stability together with a high bulk refractive index value and high ultraviolet cut-off [12] makes them as interesting for numerous applications in the field of photonics and optoelectronics [13,14]. Likewise, considerable amount of literature is available on the influence of Al2O3 on the emission features of different rare earth ion laser hosts [15–19] as the aluminum ions are found to de-cluster the rare earth ions in the glass matrices and minimize non-radiative losses due to quenching. Among several rare earth ions, Yb3+ doped laser materials are of interest in laser technology for next generation nuclear fusion [20,21]. The materials containing high concentrations of Yb3+ are being used as gain media in the microchip laser [22]. Additionally, Yb3+ ion is also being used extensively as an effective sensitizer of energy transfer for infrared-to-visible up-conversion [23]. Yb3+ ion energy level consists of a ground state manifold, 2F7/2, that may Stark-split into four sublevels and an excited-state manifold, 2F5/2, split into three sublevels. Thus, excited state absorption at both the pump and signal wavelength, is absent. Further, these ions are free from cross-relaxation process or any other internal mechanism that can reduce the effective laser cross section. The upper manifold of this ion lies approximately at about 10,500 cm−1 above the ground level [24]. This large energy gap

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precludes significant multiphonon-nonradiative decay [25]. The broad absorption spectrum of this ion due to Stark-splitting permits a wide choice of pump wavelengths. The broad emission spectrum of Yb3+ ion, and its large saturation fluence permit lasing to be achieved over a wide range of NIR wavelengths (1–1.2 μm). These characteristics make these glasses as attractive media for the generation and amplification of ultra-short pulses [26,27]. Three most important fundamental optical parameters of Yb3+ ions in the glass matrices are the absorption cross-section, the emission cross-section and the fluorescence lifetime of the 2F5/2 level. A considerable number of recent studies on emission properties Yb3+ ions in different glass systems are available in the literature [28,29]. Zhang et al. have done a commendable work on Yb3+ ion spectroscopy in tellurite glasses mixed with PbF2 or ZnF2. From this study these authors have concluded that the co-existence of TeO2–PbF2 or TeO2–ZnF2 is an effective way to enhance the spectroscopic, lasing and physical properties of Yb3+ doped fluorophosphate glasses. In this work the authors have also concluded that PbF2 has advantages in improving the crystallization properties of the glass while ZnF2 is preferable in enhancing spectroscopic and lasing properties [30]. The same group of authors have also proved that Yb3+ ions are effective sensitizers to enhance luminescence efficiency of Er3+ ions in oxyfluoride germanate glasses, oxyfluoride bismuth–germanium glasses and in tellurite glasses [31–33]. When Yb3+ containing PbO–PbF2–B2O3 glasses are mixed with different sesquioxides like scandium, yttrium, and aluminum oxides, different degrees of structural modification and local field variations around Yb3+ ion are expected in the glass network. As a result there may be significant changes in the luminescence efficiency of ytterbium ions. The objective of this investigation is to characterize the optical absorption and the fluorescence spectra of Yb3+ ions in lead fluoroborate glasses mixed with different concentrations of three interesting sesquioxides, viz., Y2O3, Sc2O3 and Al2O3, to compare the luminescence efficiency of Yb3 + ions in the three series of glasses and to throw some light on the suitability of these glasses for NIR laser emission. 2. Experimental The following series of compositions are chosen for the present study: 40PbO–10PbF2–(49 − x)B2O3 − x M2O3: 1Yb2O3 (where M = Sc, Y, Al with x = 2 to 10 in steps of 2 (all in mol%) and the samples are labeled as M2 (x = 2), M4 (x = 4), M6 (x = 6), M8 (x = 8) and M10 (x = 10). Appropriate amounts of Analytical grade reagents of PbO, PbF2, Yb2O3, Sc2O3, H3BO3, Al2O3, and Y2O3 powders (Metall, China) all in mol% were thoroughly mixed in an agate mortar and melted in a platinum crucible in the temperature range of 900 to 1000 °C in an automatic temperature controlled furnace for about 1/2 h. The resultant bubble free melt was then poured in a brass mold and subsequently annealed at 200 °C. While preparing the samples, it was ensured that the samples were free from visible inhomogeneities such as inclusions, cracks or bubbles. The samples prepared were ground and optically polished to the dimensions of 1.0 cm × 1.0 cm × 0.2 cm. The amorphous nature of samples was verified by recording XRD using

a Rigaku D/Max ULTIMA III X-ray diffractometer with CuKα radiation. The density d of the bulk samples was determined (to an accuracy of ± 0.0001) by the standard principle of Archimedes' using o-xylene (99.99% pure) as the buoyant liquid. The mass of the samples was measured to an accuracy of 0.1 mg using a Shumatchu digital balance, Model AR2140 to evaluate the densities. The refractive index, nd, of the samples was measured (at λ = 589.3 nm) using an Abbe refractometer with monobromo naphthalene as the contact layer between the glass and the refractometer prism. The optical absorption spectra of the glasses were recorded to a resolution of 0.1 nm at room temperature using a JASCO Model V-670 UV–vis–NIR spectrophotometer. The photoluminescence spectra of the samples were recorded at room temperature on a Photon Technology International (PTI) Spectrofluorometer. This instrument contains auto calibrated quadrascopic monochromator for wavelength selection and quadracentric sample compartment. The light source is the high intensity continuous xenon lamp with a high sensitivity TE-cooled In GaAs detector possessing lock-in amplifier and chopper for noise suppression and an additional emission mono with a 600 groove grating blazed at 1.2 µm. The system provides unmatched NIR luminescence recording capability from 0.2 to 2.2 nm. The spectral resolution is 0.1 nm. Infrared transmission spectra of the samples were recorded on a JASCO-FT/IR-5300 spectrophotometer with spectral resolution of 0.1 cm− 1 in the spectral range 400–2000 cm− 1 using potassium bromide pellets (300 mg) containing pulverized sample (1.5 mg). These pellets were pressed in a vacuum die at ~ 680 MPa. The other details of the measurements used for recording optical absorption, and luminescence can be found in our earlier papers [34–36]. 3. Results From the measured values of density d and calculated average molecular weight M , various physical parameters such as ytterbium ion concentration Ni, mean ytterbium ion separation ri and polaron radius rp of these glasses are evaluated using the conventional formulae [37] and are presented in Table 1A for Sc2O3 mixed glasses, whereas in Table 1B the comparison of the physical parameters of the three glasses mixed with 2.0 mol% of sesquioxides is presented. The optical absorption spectra recorded at ambient temperature in the spectral wavelength range 300–1200 nm of PbO–PbF2–B2O3: Yb2O3 glasses mixed with different concentrations of Sc2O3 are presented in Fig. 1. The spectra exhibited absorption band corresponding to 2 F7/2 → 2F5/2 transition with two distinct peaks between 900 and 1000 nm [38]. As the concentration of Sc2O3 is increased up to 8.0 mol% the band is observed to grow gradually and beyond this concentration, a decrease in the intensity of the band is observed. The spectra of Y2O3 and Al2O3 mixed glasses exhibited similar behavior. Inset of Fig. 1 represents, the comparison plot of optical absorption spectra containing 2.0 mol% of three sesquioxides. The comparison of the spectra indicated a slight shift in the spectral positions of the absorption bands by the replacement of sesquioxides one with other in the glass matrix. The intensity of the band, however, is observed to

Table 1A Physical parameters of PbO–PbF2–Sc2O3–B2O3: Yb2O3 glasses. Glass

Refractive index (±0.001)

Density (g/cm3) (±0.001 g/cm3)

Ni (1021 ions/cm3) (±0.01 ions/cm3)

ri (Å) (±0.001 Å)

rp (Å) (±0.001 Å)

Mol. vol (cm3/mol) (±0.001 cm3/mol)

Sc0 Sc2 Sc4 Sc6 Sc8 Sc10

1.627 1.629 1.632 1.634 1.637 1.638

5.418 5.598 5.690 5.731 5.777 5.823

23.30 23.26 23.22 23.18 23.15 23.11

7.54 7.54 7.55 7.55 7.55 7.56

3.039 3.041 3.042 3.044 3.046 3.047

27.042 27.087 27.132 27.175 27.219 27.262

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267

Table 1B Comparison of physical parameters of Yb3+ doped lead oxyfluoro borate glasses mixed with 2.0 mol% of sesquioxides. Glass

Density (g/cm3) (±0.001 g/cm3)

Ni (1021 ions/cm3) (±0.01 ions/cm3)

ri (Å) (±0.001 Å)

rp (Å) (±0.001 Å)

Mol.vol (cm3/mol) (±0.001 cm3/mol)

Y2 Sc2 Al2

5.792 5.598 5.641

23.09 23.26 23.37

7.56 7.54 7.53

3.048 3.041 3.035

27.287 27.087 26.949

2

F7/2

12.0

2

F5/2

Absorbtion coefficient (cm-1)

Absorbtion coefficient (cm-1)

.

12.0

8.0

8.0

Al2

4.0

0.0 800

Sc2

Y2

1200

1000

Wavelength (nm)

4.0 Sc 8 Sc 10 Sc 6 Sc Sc 2 4 Sc 0

0.0 850

900

950

1000

1050

Wavelength (nm) Fig. 1. Optical absorption spectra of Sc2O3 mixed PbO–PbF2–B2O3–Yb2O3 glasses recorded at room temperature. Inset represents the comparison plot of optical absorption spectra mixed with 2.0 mol% of three sesquioxides.

(a) of Fig. 2). Similar plots were also drawn for the other three series of glasses and the values of Eo obtained are presented in Table 2 along with cut-off wavelength. The inset (b) of Fig. 2 represents the comparison of Tauc plots for the three glasses mixed with 2.0 mol% of sesquioxides. The comparison indicates the highest value of Eo for Al2O3 mixed glasses.

be the highest for the glasses mixed with Y2O3. From the observed absorption edges we have evaluated the optical band gaps (Eo) of these glasses by drawing Tauc plots between (αℏω)1/2 and ℏω as per the equation, α(ω)ℏω = C(ℏω − Eo)2. Fig. 2 represents such plots for Sc2O3 mixed PbO–PbF2–B2O3: Yb2O3 glasses. The value of Eo is found to be the lowest for the glasses mixed with 8.0 mol% of Sc2O3 (inset

9.0

6.0

3.2

2.4 410 1.6 390 0.8

(a) 370

Bandgap (E o)

Cut-off wavelength (nm)

0.0 0.0

2.0

4.0

6.0

8.0

10.0

(b)

Sc 0

Sc 2

Sc 6

Sc 8

Sc

Al2

4.0

-1

Sc 4

Y2

Sc 10

2.0

1/2

3.0

(cm eV

Conc. Sc2O 3 (mol %)

(αhν)

(αhν)1/2 (cm-1 eV)1/2

430

0.0 1.5

2.5

3.5

hν

0.0 1.5

2.0

2.5

3.0

3.5

4.0

hν (eV) Fig. 2. Tauc plots of Sc2O3 mixed PbO–PbF2–B2O3–Yb2O3 glasses. Inset (a) presents the variation of optical band gap with the concentration of Sc2O3. Inset (b) presents the comparison of Tauc plots of the studied glasses mixed with 2.0 mol% of sesquioxides.

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Sc2O3 and Al2O3. The inset (b) of Fig. 3 represents the emission and absorption spectra of PbO–PbF2–B2O3: Yb2O3 glasses mixed with 2.0 mol% Y2O3. The figure indicates that the emission band spectrally overlaps partially with the higher wavelength absorption line but a considerable Stokes shift is visualized. The Stokes shift is found to be the highest for Y2O3 mixed glasses (Table 3) when compared with that of other two series of glasses. The infrared transmission spectra (Fig. 4) of PbO–PbF2–B2O3: Yb2O3 glasses have exhibited two main groups of bands: (i) in the region 1300–1400 cm−1, (ii) in the region 1100–1200 cm−1 and another band at about 710 cm−1; these bands are identified as being due to the stretching relaxation of B\O bond of the trigonal BO3 units, vibrations of BO4 structural units and due to the bending vibrations of B\O\B linkages, respectively [39–41]. In the region of vibrations of BO4 structural units a band due to octahedral distorted PbO2F4 vibrational groups is also expected [42]. Another band due to PbO4 structural units is also observed in the spectra of all the glasses at about 467 cm−1 [43]. With the introduction of Sc2O3 up to 8.0 mol% into the glass network, the intensity of the second

Table 2 Optical parameters PbO–PbF2–B2O3: Yb2O3glasses mixed with different concentrations of sesquioxides (M2O3). M2O3 conc. (mol%)

0 2.0 4.0 6.0 8.0 10.0

Cut-off wavelength (nm) (±0.1 nm)

Optical band gap Eo (eV) (±0.01 eV)

Y2O3

Sc2O3

Al2O3

Y2O3

Sc2O3

Al2O3

398 408 419 432 444 440

373 388 397 407 423 418

360 375 384 394 410 405

2.81 2.64 2.45 2.33 2.05 2.17

2.9 2.88 2.85 2.81 2.75 2.79

3.06 2.89 2.87 2.84 2.78 2.82

The emission spectra (Fig. 3) of Sc2O3 mixed PbO–PbF2–B2O3: Yb2O3 glasses recorded at room temperature (excited at 970 nm) have exhibited two strong emission bands at about 978 and 1003 nm due to 2F5/2 → 2F7/2 transition of Yb3+ ions. The spectral intensity of these bands is found to increase with the content of Sc2O3 up to 8.0 mol%. However, when the concentration of Sc2O3 is

2

F7/2

Intensity(arb. (arb. Intensity

F5/2

Intensity (arb. units)

2

2

(a)

F 5/2

2

F 7/2

Y2

900 900

Sc2

Al2

Intensity (arb. units)

268

Y2 Sc 2 Al2

1000 1100 1000 1100 Wavelength (nm)

(b)

emission spectra

Emission Optical absorption

OA

900 1000 1100 900 1000 1100 Wavelength (nm)

Sc 8 Sc 0 Sc 2

900

950

1000

1050

Sc 4

1100

Sc 6

Sc 10

1150

1200

Wavelength (nm) Fig. 3. The emission spectra of Sc2O3 mixed PbO–PbF2–B2O3–Yb2O3 glasses recorded at room temperature (excited at 970 nm). Inset (a) presents the comparison spectra of glasses mixed with 2.0 mol% of sesquioxides. Inset (b) represents the emission and absorption spectra of PbO–PbF2–B2O3–Yb2O3 glasses mixed with 2.0 mol% Y2O3.

raised beyond 8.0 mol%, a decrement in the intensity of the emission band could be visualized. The emission spectra of other two series of glasses exhibited a similar trend. For the sake of comparison, the spectra of glasses mixed with 2.0 mol% of sesquioxides are presented as inset (a) of Fig. 3. The comparison indicates the highest intensity for the glasses mixed with Y2O3. Additionally, a slight blue shift in the emission peak is observed with the replacement of Y2O3 successively by

group of bands (band due to BO4 units) is observed to decrease with the shifting of meta-center towards slightly lower wavenumber, whereas the intensity of the first group of bands (bands due to the BO3 structural units) is observed to increase. The IR spectra of the other two series of glasses exhibited similar behavior. For the sake of comparison, the IR spectra of the glasses mixed with 2.0 mol% of sesquioxides are presented in Fig. 5. In the inset of Fig. 5 the

Table 3 Comparison of spectroscopic data of Yb3+ doped lead oxyfluoro borate glasses mixed with 2.0 mol% of sesquioxides. Sample

Refractive index (nd) (±0.001)

λp (nm) (±0.1 nm)

λe (nm) (±0.1 nm)

Stokes shift (nm) (±1.0 nm)

σabs (λp) (×10−18 cm2) (±0.01 cm2)

σem (λe) (×10−18 cm2) (±0.01 cm2)

τr (ms) (±0.01 ms)

βmin (±0.001)

Y2 Sc2 Al2

1.639 1.629 1.603

985 982 980

992 987 984

7.0 5.0 4.0

1.16 1.06 0.91

1.96 1.63 1.32

0.15 0.20 0.23

0.371 0.394 0.406

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PbO4 structural units

B-O-B linkages BO3 structural units

269

BO4 structural units

Sc10

Transmitance %

Sc8 Sc6 Sc4 Sc2 Sc0

1400

1200

1000

800

Wavenumber

600

400

(cm-1)

Fig. 4. IR spectra of PbO–PbF2–Sc2O3–B2O3: Yb2O3 glasses.

comparison of variation of intensities of BO3 and BO4 structural units of the glasses mixed with 2.0 mol% of sesquioxides is presented. The comparison indicates the highest intensity of the band due to BO3 structural units in the case of Y2O3 mixed glasses. Additionally, the spectra of Al2O3 mixed glasses are expected to exhibit bands at about 760 cm−1 and 460 cm−1 due to AlO4 and AlO6 structural units, respectively [44] These bands, however, seemed to be overlapped with the band due to B\O\B linkages and PbO4 structural units, respectively. It may be noted here that similar to Sc2O3, when the concentrations of Al2O3 and Y2O3 are increased up to 8.0 mol% a considerable decrease in the intensity of the band due to BO4 and AlO4 structural units is observed. The pertinent data related to the IR spectra are presented in Table 4. 4. Discussion Out of different constituents of the studied glass system, B2O3 is a well known glass former and participates in the glass network with

BO3 and BO4 structural units as evidenced from the IR spectra. Each BO4 unit is linked to two such other units and one oxygen from each unit with a metal ion giving rise to a structure that leads to the formation of long chain tetrahedrons. Earlier NMR investigations by different researchers [45,46] on fluoroborate glasses indicate that in addition to BO4 units there exits B(O, F)4 or (BO3F) and BO2F2 units in this type of systems. PbO in general is a glass modifier and enters the glass network by breaking up the B\O\B linkages However, PbO may also participate in the glass network with [PbO4/2] pyramidal units connected in puckered layers when lead ion is linked to four oxygens in a covalent bond configuration. The fraction of the four fold coordinated boron in PbO–B2O3 is ~ 0.53 [47]. Accordingly, the structural–chemical composition of this glass can be represented in the form of a combination 2 − of the following structural–chemical units: 0.5 Pb+ + 0.5 1/2 [BO4/2] − +2 Pb1/2 [O BO2/2]. When PbF2 is added, additional non-bridging Pb\F bonds are expected to be formed at the expense of the bridging B\O\B and

AlO6 structural units/ PbO4 structural units

B-O-B linkages / AlO4 structural units

BO4 structural units

Sc2

Al2

BO3

BO4

Arb. units

Transmitance %

Y2 BO3 structural units

Y2

1400

1200

1000

Wavenumber

800

600

Sc2

Al2

400

(cm-1)

Fig. 5. Comparison plots of IR spectra Yb3+ doped lead oxyfluoro borate glasses mixed with 2.0 mol% of sesquioxides. Inset indicates the comparison of variation of intensities of BO3 and BO4 structural units of the glasses mixed with 2.0 mol% of sesquioxides.

270

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Table 4 Summary of band positions in the IR spectra of Yb3+ doped lead oxyfluoro borate glasses mixed with different concentrations of Sc2O3. (Band positions were measured to an accuracy of ±1.0 cm−1). Glass

Sc0 Sc2 Sc4 Sc6 Sc8 Sc10

Borate groups (cm−1) BO3

BO4

B\O\B units

1330 1331 1332 1333 1334 1335

1090 1089 1088 1087 1085 1083

694 695 698 700 702 704

Band due to PbO4 units (cm−1) 467 467 467 467 467 467

B\O\Pb linkages and may also introduce more non-bridging oxygen ions. Considering the participation of sesquioxides in the glass network, yttrium ions enter in to the silicate glass network with YO6 octahedral structural units linking either by sharing corners or edges with BO4 structural units [48,49]. Earlier structural investigations on aluminum silicate glasses have indicated that these ions occupy mainly tetrahedral (AlO4) and octahedral (AlO6) sites [50]: 2Al2O3 → [Al3+]o + 3[AlO4/2]t. Some previous studies on other glass systems containing Al2O3 have pointed out that Al(6) dominates the glass structure when Al2O3 is present in low concentrations and Al(4) structural units prevail when Al2O3 is present in higher concentrations [51]. The decreasing intensity of the band due to AlO4 structural units in the IR spectra with an increase in the concentration of Al2O3 up to 8.0 mol% suggests the decreasing presence of the concentration of Al\O\B linkages in the glass network. Thus the IR spectra

suggest that there is an increase in the degree of disorder in the glass network with an increase in the concentration of Al2O3 up to 8.0 mol%. Similarly structural studies on several Sc2O3 mixed oxide glasses indicated that these ions occupy octahedral positions in the glass network without direct Sc\O\Sc linkages [52,53]. However, small percentage of eight-coordinate Sc atoms is also detected in some of the glasses containing Sc2O3 [54]. Overall, the three ions viz, Al3+, Sc3+ and Y3+ are expected to be coordinated by three BO4 tetrahedral ligands. Two of the oxygen ions associated with each tetrahedron are assumed to be non-bridging ions forming ionic bonds with these trivalent ions (Fig. 6). However, in the case of Y3+ ions the borate tetrahedra must be moved more outward since the ionic radius of this ion is more when compared with that of Al3+ and Sc3+ ions. In view of this, although three ions are linked in triangle with BO4 structural units, the octahedra of yttrium ion is more distorted when compared with that of the other two ions. The highest disorder in the network of Y2O3 mixed glasses is also evident from the IR spectra. The lowest intensity of band due to BO4 structural units for this series of glasses among the three series studied obviously suggests this inference. A similar conclusion can be drawn from the observed trend of optical band gap value Eo. The decreasing value of Eo with an increase in the concentration sesquioxides (up to 8.0 mol%) suggests a growing degree of disorder in the glass network. The lowest value of Eo for Y2O3 mixed glasses suggests the highest degree of disorder in these glasses among the three series of glasses investigated. Under these conditions, if we consider the sesquioxides (Al2O3, Sc2O3 and Y2O3) to be incorporated between the long chain molecules in the vicinity of Yb3+ ion, then the symmetry and or the covalency of the glass at the Yb3+ ions should be different for different sesquioxides mixed glasses. Such variations might be responsible for the observed

Entry of PbO modifier

Fig. 6. A structural fragment of Al3+ ions and Yb3+ ions in octahedral coordination surrounded by BO4 structural units.

D. Rajeswara Rao et al. / Journal of Non-Crystalline Solids 378 (2013) 265–272

difference of the luminescence intensity of Yb3+ transition in these samples. Further it may be noted here that for all the three series of glasses, we have observed anomalies both in optical absorption and luminescence spectra at about 8.0 mol% of sesquioxides. From the IR spectra, it is clear that, up to 8.0 mol% of sesquioxides, there is an increasing degree of disorder in the glass network and beyond that there is an increasing rigidity of glass network due to the cross linking of various structural units. Yet, it seems there is also a possibility for the gradual transformation of sesquioxides from octahedral to tetrahedral coordination from this concentration. Tetrahedral ions once again participate in the network forming and make the glass network more rigid. Such higher rigidity obviously leads to higher phonon energies causing more absorption as well as emission losses. These might be the reasons for the observed anomalous behavior both in absorption and emission spectra. The Yb3+ absorption band in the spectra of all the glasses seemed to be consisting of two distinct peaks at about 950 nm and 980 nm corresponding to the lowest sub-levels of 2F7/2 → 2F5/2 transition. According to the crystal-field theory, the maximum allowed splitting number of J = 5/2 level is three hence we expect Stark splitting of 2 F5/2 level into three sublevels. In view of this, absorption spectrum should be consisting of three broad spectral bands. Hence the weak broader band observed at shorter wavelength may be considered as a combination of two unresolved bands. It is further noticed that when Al3+ is successively replaced by 3+ Sc and Y3+ in the glass matrix, the position of absorption band due to 2F7/2 → 2F5/2 transition is shifted to slightly lower energy and the absorption oscillator strength under given peak is found to be increased. This is possible when the associated electrons are trapped at slightly shallow sites within the main band gap with smaller wave-function radii. Enhancement of absorption under the absorption peak, points out the fact that the interaction between the ytterbium ions and the neighboring ligand ions is weaker in Y3+ mixed glasses and causes to lower the crystal-field splitting for 2F7/2 and 2F5/2 energy levels in these samples. Using the absorption spectra we have evaluated emission cross-section σem(λ) with the relation [55]: σ em ðλÞ ¼ σ abs ðλÞ

Z1 ε−ε′ exp Z2 kT

! ð1Þ

where σ abs ðλÞ ¼ 2:303

logðI0 =IÞ ; NL

ð2Þ

and presented in Table 3 for one of the samples in each series. The emission cross sections are found to be the highest for Y2O3 mixed glasses. The values obtained are found to be comparable with those obtained for various other glass systems, reported earlier [56–59]. In Eqs. (1) and (2), ε is the absorption energy determined from the most intense absorption peak of Yb3+ ions and ε′ is the energy corresponding to emission peak. The remaining terms have the usual meaning as reported in our earlier papers [60]. Zl/Z2 represents the ratio of partition functions of the lower and upper levels of Yb3+ ions which is approximately equal to 1.2 [61]. The highest emission cross section (Table 4) observed in the case of Y2O3 mixed glasses is well in accordance with the results of the IR spectra, which indicated the highest degree of disorder in the glass network of these series of glasses. Such disorder leads to low phonon losses and enhances luminescence emission as observed. Normally, the life time of Yb3+ ions evaluated from fluorescence spectra is highly erroneous because of the larger contribution of radiation trapping [62]. To avoid such difficulty, calculated lifetime in general, is being considered as a reference in deciding the actual value of measured lifetime [62]. The inaccuracy originated from the radiation trapping can be

271

minimized by computing radiative lifetime τr with the following equation: τr ¼

Z 8πcn2

Λ4 σ em ðλÞdλ

;

ð3Þ

where, 4

Z

Λ ¼

4

λ gðλÞdλ:

The comparison (Table 3) of the values of τr of the three series of glasses indicated the highest values for Y2O3 mixed glasses. The highest Stokes shift observed for Y2O3 mixed glasses (Table 3) indicates that radiative trapping is comparatively high and this leads to low values of radiative lifetime. The partial overlapping of the absorption and emission bands indicates the re-absorption and as a result there may be resonant absorption loss of Yb3+ ions. Such loss is denoted by βmin which is defined as the minimum fraction of Yb3+ ions that must be excited to balance the gain exactly with the ground-state absorption at the emission peak wavelength. The value βmin is calculated using the absorption and emission cross sections at the emission wavelength λ0 by the equation: βmin ¼

σ abs ðλ0 Þ : σ abs ðλ0 Þ þ σ em ðλ0 Þ

ð4Þ

The value of βmin is found to be the highest for Y2O3 mixed glasses. This observation indicates low absorption loss for these series of glasses when compared with that of other two series of glasses investigated. 5. Conclusions The optical absorption and photoluminescence spectra of Yb3+ ions in PbO–PbF2–B2O3 glasses mixed with varying concentrations of three interesting sesquioxides (viz., Al2O3, Sc2O3, Y2O3) have been studied. From these spectra, the absorption and emission crosssections and also the fluorescence lifetime of Yb3+ ions have been evaluated. The emission spectra of all the three series of glasses have exhibited a broad emission band at about 1000 nm with degeneracy due to 2F5/2 → 2F7/2 transition. The analysis of results has indicated the highest PL light output for the glasses mixed with yttrium oxide. The reasons for the highest PL emission of Yb3+ ions in Y2O3 mixed glasses have been explored and discussed in the light of variations in the degree of disorder around Yb3+ ions in the glass network. Acknowledgment One of the authors, N. Veeraiah wishes to thank DAE-BRNS, Govt. India for financial support to carry out this work. References [1] R.G. Fernandes, J. Ren, A.S.S. de Camargo, A.C. Hernandes, H. Eckert, J. Phys. Chem. C 116 (2012) 6434. [2] O.B. Petrova, A.V. Popov, V.E. Shukshin, Yu.K. Voron'ko, J. Opt. Technol. 78 (2011) 659. [3] B.C. Jamalaiah, T. Suhasini, L. Rama Moorthy, Il-Gon Kim, Dong-Sun Yoo, Kiwan Jang, J. Lumin. 132 (2012) 1144. [4] S. Chénais, F. Druon, F. Balembois, P. Georges, R. Gaumé, P.H. Haumesser, B. Viana, G.P. Aka, D. Vivien, J. Opt. Soc. Am. B 19 (2002) 1083. [5] A. Fukaboria, V. Chania, K. Kamadaa, T. Yanagidaa, Y. Yokotaa, F. Morettic, N. Kawaguchid, A. Yoshikawaa, J. Cryst. Growth 318 (2011) 823. [6] L. Fornasiero, E. Mix, V. Peters, K. Peterman, G. Huber, Cryst. Res. Technol. 34 (1999) 225. [7] M. Tokurakawa, K. Takaichi, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, A.A. Kaminskii, Appl. Phys. Lett. 90 (2007) 071101.

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