Spectroscopic studies of Nd3+ doped lead tungsten tellurite glasses for the NIR emission at 1062 nm

Optical Materials xxx (2014) xxx–xxx

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Spectroscopic studies of Nd3+ doped lead tungsten tellurite glasses for the NIR emission at 1062 nm M. Venkateswarlu a, Sk. Mahamuda a, K. Swapna a, M.V.V.K.S. Prasad a, A. Srinivasa Rao a,b,⇑, A. Mohan Babu c, Suman Shakya d, G. Vijaya Prakash d a

Department of Physics, KL University, Green Fields, Vaddeswaram 522 502, Guntur (Dt.), AP, India Department of Applied Physics, Delhi Technological University, Bawana Road, New Delhi 110 042, India Department of Physics, Chadalawada Ramanamma Engineering College, Renigunta Road, Tirupati 517 506, AP, India d Nanophotonics Laboratory, Department of Physics, Indian Institute of Technology-Delhi, Hauz Khas, New Delhi 110 016, India b c

a r t i c l e

i n f o

Article history: Received 18 June 2014 Received in revised form 1 October 2014 Accepted 16 October 2014 Available online xxxx Keywords: Amorphous materials Glasses JO parameters Optical materials Luminescence Optical properties

a b s t r a c t Lead Tungsten Tellurite (LTT) glasses doped with different concentrations of Nd3+ ions were prepared by using the melt quenching technique to study the absorption, emission and decay spectral profiles with an aim to understand the lasing potentialities of these glasses. From the absorption spectra, the Judd–Ofelt (J–O) parameters are evaluated and in turn used to calculate the transition probability (AR), total transition probability (AT), radiative lifetime (sR) and branching ratios (bR) for prominent emission levels of Nd3+. The emission spectra recorded for LTT glasses gives three emission transitions 4F3/2 ? 4I9/2, 4F3/2 ? 4I11/2 and 4 F3/2 ? 4I13/2 for which effective band widths (DkP) and stimulated emission cross-sections (rse) are evaluated. Branching ratios (bR) measured for all the LTT glasses show that 4F3/2 ? 4I11/2 transition is quite suitable for lasing applications. The intensity of emission spectra increases with increase in the concentrations of Nd3+ up to 1.0 mol% and beyond concentration quenching is observed. Relatively higher emission cross-sections and branching ratios observed for the present LTT glasses over the reported glasses suggests the feasibility of using LTT glasses for infrared laser applications. From the absorption, emission and decay spectral measurements, it was found that 1.0 mol% of Nd3+ ion concentration is aptly suitable for LTT glasses to give a strong NIR laser emission at 1062 nm. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction In recent years, rare earth doped materials are playing a vital role in the modern optical technology as an active constituents to produce low price integrated laser sources, integrated optical amplifiers, 3D display devices, sensors, up-conversion fibres and low loss components [1]. The above mentioned applications possessed by the rare earth doped materials have stimulated the research on rare earth doped glasses [2–6]. Among different rare earth ions, neodymium ion is one of the utmost efficient ions used to prepare solid-state lasers because of its intense emission at 1060 nm [7]. For this reason, trivalent neodymium (Nd3+) ion doped variety of crystals and glasses were studied extensively under 808 and 885 nm laser diode excitation with an aim to develop high power NIR (at 1060 nm) solid state lasers [8,9]. ⇑ Corresponding author at: Department of Applied Physics, Delhi Technological University, Bawana Road, New Delhi 110 042, India. Tel.: +91 85860 39007; fax: +91 01127871023. E-mail address: [email protected] (A. Srinivasa Rao).

Different glass hosts like borates, phosphates, germanates, vanadates and tellurite families have been studied extensively for this purpose [10–14]. Among all oxide glasses, Lead Tungsten Tellurite (PbF2–WO3–TeO2) glass system (LTT) has unique optical properties. Tellurite based glasses besides having high linear and third order nonlinear optical constants (nonlinear refractive indices), can possess high mechanical stability, good corrosion resistance and low phonon energies equal to 750 cm1. All these novel properties make them to have their transitions range extended up to mid IR (5–6 lm) region. Tellurite based glasses are also having good capacity to accept lanthanide dopants at different concentrations [15]. The low phonon energy, extended transmission range and high refractive index of tellurite glasses allow the observation of laser emission from rare earth ions in a wide spread spectral range [5,16–20]. Such materials are also useful in optical waveguides. Tungsten oxide (WO3) is a precise noble semi conducting and transition-metal oxide which has fascinated considerable attention for several years. It is also one of the most examined and used material for electro-chromic and photo-chromic devices in which colouration and bleaching can be reversibly obtained by

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an electro-chemical process [20] and has extensive applications in smart windows, display devices and sensors. Tungsten ions are also capable to influence the luminescence characteristics of rare earth ions in tellurite glasses, for the simple reason that these ions can exist in different valence states i.e., W6+, W5+ and W4+ irrespective of their starting oxidation state in glasses [21–25]. Hence, the environment around the doped rare earth ions in tellurite glass network can be changed effectively by introducing WO3 at different concentrations. Consequently, interesting changes in the luminescent characteristics of lasing ions is expected. Now a days lead based (PbO and PbF2) glasses are believed to be of special interest because of their effective use in IR fibre optics, laser windows and multi-functional optical components. These glasses are highly transparent in the mid infrared region up to 8 lm and more stable against atmospheric moisture. They are also believed to be more favourable material for electrochemical applications in the field of solid state batteries as power sources [26]. While both lead oxide (PbO) and lead fluoride (PbF2) are efficient glass modifiers, the latter is having glass forming nature with strong network arrangement of PbO2F4 form [27,28]. Lead fluoride (PbF2) especially with its less phonon energies (346 cm1) added to tungsten tellurite glass can reduce the phonon energies of the host glass drastically and eventually increases their radiative transition rates. It is well known that a glass host with less phonon energies encourages radiative transition rates very much and is much useful for the design and development of photonic devices. Over and above, fluoride compounds present in oxide glasses can react and remove OH group from the glass and reduces the redundant phonon energies of the base glass to enhance radiative emissions. In the backdrop of the scientific patronages offered by lead fluoride, tungsten oxide and tellurium oxide, in the present work we prepared a germane system namely Lead Tungsten Tellurite (LTT) glass by taking the aforementioned chemicals as constituent elements and doped it with Nd3+ ions at different concentrations to study their optical absorption, NIR emission and decay spectral profiles in order to explore the possibility of using these new materials for future photonic devices. Particularly the absorption spectra have been analysed on the basis of Judd–Ofelt (J–O) theory as it is a standard tool in evaluating the radiative properties of the doped rare earth ions in glass hosts. Such measured radiative properties combined with emission spectral data are useful in understand the lasing potentialities of the prepared LTT glasses.

2. Experimental 2.1. Glass preparation The LTT glasses of composition (in mol%) 15 PbF2 + 25 WO3 + (60  x) TeO2 + x Nd2O3 (here x = 0.1, 0.5, 1.0 and 1.5 mol%) were prepared by melt quenching technique from reagents of analar grade (purity more than 99.9%) in 10 g batches. These glasses are labelled as LTTNd01, LTTNd05, LTTNd10 and LTTNd15 depending on Nd3+ ion concentration 0.1, 0.5, 1.0 and 1.5 mol% respectively. Batches containing different concentrations of Nd3+ ions in base glass compositions were thoroughly mixed in an agate mortar to get a smooth powder. Such powders were then collected in a silica crucible and heated in a furnace at 730 °C temperature for 30 min. The melt was quenched by pouring onto a circular shaped copper plate and pressed quickly by another plate. The LTT glasses thus prepared were annealed at 400 °C for two hours to remove thermal strains and to improve the mechanical strength of the prepared glasses. Finally, the glasses were polished with emery paper before going for experimental measurements.

2.2. Physical, absorption, emission and decay spectral measurements Archimedes principle is used to determine the densities of the Nd3+ doped LTT glass samples with water as an immersion liquid. Brewster’s angle method with He–Ne laser operating at 632 nm is used to measure the refractive index of the prepared glasses. From the measured density and refractive indices, various other physical properties of the LTT glasses were evaluated using the relevant expressions given in our previous paper [29] and are given in Table 1. Absorption spectra for Nd3+ doped LTT glasses were recorded at room temperature in the wavelength range 500–900 nm with a spectral resolution of 0.1 nm using a JASCO model V-670 UV–vis– NIR spectrophotometer. The photoluminescence (PL) emission and time-resolved PL measurements were carried out using home-built set ups with 808 nm CW laser as an excitation source. The emission from sample was coupled into a monochromator (Acton SP2300) coupled to CCD (charge coupled detector) through appropriate lenses and filters. For time resolved PL measurements, a frequency generator (5 Hz), lock-in amplifier, digital storage oscilloscope and a monochromator (Acton SP2300) coupled to InGaAs detector through the appropriate lenses and filters are used. 3. Results and discussion 3.1. Physical properties The variation of density (g/cm3), mean atomic volume (g/cm3/ atom), refractive index, average molecular weight (g), inter ionic distance (Å) and field strength (1015 cm2) as a function of Nd2O3 concentration in LTT glasses have been shown in Figs. 1a–1c. From Fig. 1a it is observed that with increase in Nd3+ ion concentration in LTT glasses, the density and mean atomic volume values are also increasing indicating more rigid nature of the prepared LTT glasses. This is further confirmed by increase in average molecular weight and decrease in inter ionic distance values with Nd3+ ion concentration as shown in Figs. 1b and 1c respectively. The tendency of decrease in inter ionic distance in the present glasses indicates that the atoms are more tightly packed as the Nd3+ ion concentration increases in these glasses. From Fig. 1b, it is observed that refractive index of LTT glasses increases with increase in dopant ion concentration. As the density of the LTT glasses are increasing with dopant ion concentration, the denser nature of the glasses relatively increases and will increase the refractive index of the medium with dopant ion concentration. From Fig. 1c, it can also be seen that the field strength values are increasing with increase in Nd3+ ion concentration. This is obvious because, when the rare earth ion concentration in glass increases, the number of ions available per unit volume will also increase and this will increase the field strength. From Table 1, it is also observed that these glasses have molecular electronic polarizability of the order 1023 which is a very low value and hence the present LTT glasses are said to be more stable. In general, the optical basicity of the oxide medium is nothing but the average electron donating power of all oxide atoms present in the medium. For the present system of glasses, the values of optical basicity are increasing with the increase in Nd3+ ion concentration. This results in increasing negative charge on the oxygen atom and thus increases covalency in the cation–oxygen bonding. 3.2. Optical absorption spectra Optical absorption spectra for all the LTT glasses doped with Nd3+ ions were measured in visible and near infra-red (vis–NIR) regions at room temperature in the wavelength range 500–900 nm. Each spectrum has same resemblance in band positions with slight difference in intensities of various absorption bands. Fig. 2 shows the

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M. Venkateswarlu et al. / Optical Materials xxx (2014) xxx–xxx Table 1 Various physical properties of Nd3+ ions doped LTT glasses.

Density q (g/cm ) Refractive index (nd)  ðgÞ Average molecular weight M Nd3+ ion concentration N (1021 ions/cm3) Mean atomic volume (g/cm3/atom) Dielectric constant (e) Optical dielectric constant (e  1) Reflections losses (R%) Molar refraction (Rm) (cm3) Polaron radius (Å) Inter ionic distance (Å) Molecular electronic Polarizability, a (1023 cm3) Field strength (1015 cm2) Optical basicity ðKth Þ

LTTNd05

LTTNd10

LTTN15

6.607 2.190 190.6 2.086 8.870 4.796 3.796 0.139 18.89 3.150 7.838 7.500 3.016 0.464

6.612 2.201 191.3 10.40 8.874 4.844 3.844 0.140 19.02 1.845 4.588 1.510 8.804 0.468

6.619 2.213 192.2 20.73 8.880 4.897 3.897 0.142 19.18 1.466 3.646 0.761 13.94 0.473

6.626 2.221 193.1 30.98 8.884 4.932 3.932 0.143 19.31 1.283 3.189 0.511 18.23 0.477

6.630

20

8

Mean atomic volume

6.625

16

o

-2

Inter Ionic distance (A )

7

Density

8.883

6.620

8.880 8.877

6.615

8.874 6.610 8.871

14

Field Strength 12

6

10 8

5

6 4

4

2 8.868

6.605 1.0

1.5

0

3

2.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Nd3+ Ion Concentration (mol %)

Nd3+ ion concentration (mol %) Fig. 1a. The variation of density (g/cm3) and mean atomic volume (g/cm3/atom) parameters as a function of Nd3+ ion concentration (mol%) in LTT glasses.

Fig. 1c. The variation of inter ionic distance (Å) and field strength (1015 cm2) parameters as a function of Nd3+ ion concentration (mol%) in LTT glasses.

193.5

2.225

2.5

Refractive index

192.5

2.210 192.0 2.205 191.5 2.200 191.0

2.195

Avg.Molecular weight

1.5 4

F5/2

4

G7/2 2

H11/2

4

G9/2

4

F9/2

4

F7/2

1.0

4

F3/2

0.5 0.1

190.5

0.5

0.50

0.75

1.00

1.25

1.50

1.75

1.5

3+

Nd Ion Concentration (mol %) Fig. 1b. The variation of refractive index and average molecular weight (g) parameters as a function of Nd3+ ion concentration (mol%) in LTT glasses.

optical absorption spectra of Nd3+ ions doped LTT glasses in vis–NIR region. As shown in Fig. 2, the Nd3+ ions in all the LTT glasses exhibits eight absorption bands in vis–NIR region at 513, 526, 584, 627, 681, 748, 803 and 877 nm [30,31] corresponding to the transitions 4 I9/2 ? 4G9/2, 4G7/2, 4G5/2, 2H11/2, 4F9/2, 4F7/2, 4F5/2, and 4F3/2 respectively [32,33]. Because of the strong absorption of the host glass in UV region, few absorption bands have disappeared in that region.  From the identified band positions (cm1), nephelauxetic ratio ðbÞ and bonding parameters (d) are evaluated and are given in Table 2. The essential mathematical formulae needed for the determination of nephelauxetic ratio and bonding parameter are collected from

500

600

700

800

900

Co nc en t

0.25

ra t

1 0.00

io

n

2.190

2.0

Absorbance (au)

2.215

G5/2

(m ol

193.0

Avg. Moleculer Weight (g)

2.220

4

)

0.5

%

0.0

Refractive Index

18

Interionic distance

8.886

3 Density (g/cm )

Mean atomic volume ((g/cm3 )/atom))

8.889

LTTNd01

15

3

Field Strength (X10 cm )

Physical properties

Wavelength (nm) Fig. 2. Absorption spectra of Nd3+ ions in LTT glasses.

literature [34]. Depending on the field environment, the bonding parameter (d) may be positive or negative indicating covalent or ionic bonding. From Table 2, it is observed that the bonding parameter (d) for all the LTT glasses is found to be positive i.e., the prepared glasses are covalent in nature and the covalent nature decreases gradually with increase in Nd3+ ion concentration. The intensity of absorption bands, which are usually called as oscillator strength (fexp) is directly proportional to the area under the absorption band and can be determined from the absorption spectral features using the relevant equation given in literature

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M. Venkateswarlu et al. / Optical Materials xxx (2014) xxx–xxx

Table 2  and bonding Assignment of absorption bands, experimental and calculated oscillator strengths (fexp and fcal) (106), rms deviation (drms) (106), nephelauxetic ratio ðbÞ parameters (d) for Nd3+ ions doped LTT glasses. Transition 4I9/2?

4

G9/2 4 G7/2 4 G5/2 2 H11/2 4 F9/2 4 F7/2 4 F5/2 4 F3/2 drms  ðbÞ

Wavelength (nm)

513 527 584 627 682 747 803 878

d

LTTNd01

LTTNd05

LTTNd10

LTTNd15

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

0.975 4.650 16.83 0.113 0.321 1.426 7.023 3.867 ±0.45 0.9946 0.537

2.068 5.064 16.80 0.156 0.524 1.791 6.696 3.857

2.330 6.340 27.9 0.510 1.220 4.130 10.63 4.52 ±0.22 0.9948 0.515

2.776 6.660 27.881 0.294 1.006 4.206 10.572 4.405

3.284 7.610 29.520 0.547 1.959 5.206 11.290 5.08 ±0.42 0.9950 0.498

3.0521 7.263 29.540 0.328 1.119 4.685 11.738 4.842

2.130 5.16 18.100 0.270 0.72 3.530 7.27 3.430 ±0.29 0.9951 0.488

2.015 4.732 18.123 0.207 0.721 2.984 7.644 3.246

[34]. From the absorption spectra, it is noticed that one particular transition namely 4I9/2 ? 4G5/2 is more intense than other transitions and having large oscillator strengths in all the glasses under investigation. This transition is known as the hypersensitive transition, for which the selection rules DJ 6 2; DL 6 2 and DS = 0 holds good. The calculated oscillator strengths (fcal) for the f–f transitions of Nd3+ ions from its ground state ðwJ Þ to the excited state ðw0J0 Þ were determined using J–O theory [35,36]. The experimental and calculated oscillator strengths reported in Table 2 are in good agreements with each other with less rms deviation. In the process of evaluating J–O parameters (Xk) through a least squares fitting procedure, the necessary reduced matrix elements kU k k2 were collected from literature [32]. The J–O intensity parameters (Xk) and spectroscopic quality factors of Nd3+ doped LTT glasses are given in Table 3 along with the reported values [37–51]. J–O intensity parameters are highly useful in explaining the environment around the doped rare earth ions in the host glass. The local structure and bonding nature of rare earth ions can be studied on the basis of magnitudes of the J–O intensity parameters. The bulk properties of the host medium such as rigidity and viscosity of the medium in which the rare earth ions are situated can be explained on the basis of the magnitudes of X4 and X6. From Table 3, it is observed that X4 is higher than X6 and X2 and follows the same trend (X4 > X6 > X2) for all the Nd3+ doped LTT glasses. Supplementarily it is perceived that the J–O parameters are increasing from 0.5 to

1 mol% of dopant ion concentration and then decreasing beyond in these glasses. Ideally, the J–O intensity parameters need not show any variation with concentration in the same host owing to the occupation of similar dopant sites. But, in the present work, the observed variation in J–O intensity parameters may be attributed to the tendency of clustering of dopant ions. Also, increasing concentration may produce site-to-site variation and subsequently the local crystal field as well as the asymmetry around the dopant ions may change and may cause variations in J–O parameters. The J–O intensity parameter X2 increases from glass LTTNd01 to LTTNd10 and then decreases for LTTNd15. The intensity of hypersensitive transition and X2 are high for LTTNd10 glass indicating that it has the highest covalence and asymmetry around Nd3+ ions. 3.3. Photoluminescence spectra and radiative properties The NIR photoluminescence spectra of LTT glasses doped with Nd3+ ions when excited under 808 nm CW laser source is shown in Fig. 3. The Nd3+ ions excited to 4F5/2 level in LTT glasses are immediately falling to 4F3/2 metastable state through fast non-radiative relaxation as indicated in Fig. 4. The resultant PL spectra shown in Fig. 3 consist of three emission bands at 878, 1062 and 1339 nm corresponding to the transitions 4F3/2 ? 4I9/2, 4F3/2 ? 4I11/2 and 4 F3/2 ? 4I13/2 respectively. From the PL spectra, it is observed that, the intensity of all the emission transitions increases with increase

Table 3 Judd–Ofelt intensity parameters (Xk where k = 2, 4 and 6) (1020 cm2) for Nd3+ ions doped LTT glasses and other reported glasses. Name of the glass

X2

X4

X6

Trend

v = X4/X6

References

LTTNd01 LTTNd05 LTTNd10 LTTNd15 Tellurite Germanate LBNNd10 with HST NaTFP Nbl LaF3 Tellurite Germanate CaNb glass LBTAFNd05 ZBLAN SPB1 LBT 30 B2O3–70 PbO Fluoride Silicate Phosphate TeO2–WO3–PbO–Nd2O3

1.81 4.46 4.54 2.60 3.80 3.09 4.81 3.01 3.12 0.35 3.62 3.09 4.40 5.82 5.09 4.81 1.05 3.52 1.95 4.71 6.30 4.88

5.27 5.31 5.79 3.91 4.94 5.54 5.55 5.26 4.84 2.57 4.21 5.54 5.20 1.88 3.12 1.97 1.50 2.98 3.65 4.54 4.00 4.05

2.17 5.14 5.69 3.59 4.54 4.48 3.73 4.21 3.28 2.50 2.95 4.80 2.70 4.76 7.16 3.94 4.20 5.48 4.17 5.05 4.30 3.82

X4 > X6 > X2 X4 > X6 > X2 X4 > X6 > X2 X4 > X6 > X2 X4 > X2 > X6 X4 > X6 > X2 X4 > X2 > X6 X4 > X6 > X2 X4 > X6 > X2 X4 > X6 > X2 X4 > X6 > X2 X4 > X6 > X2 X6 > X4 > X2 X2 > X6 > X4 X2 > X6 > X4 X2 > X6 > X4 X6 > X4 > X2 X6 > X2 > X4 X6 > X4 > X2 X6 > X2 > X4 X2 > X6 > X4 X2 > X4 > X6

2.42 1.03 1.01 1.08 1.08 1.23 1.48 1.25 1.03 1.03 1.43 1.15 1.92 0.39 0.43 0.50 0.35 0.54 0.87 0.89 0.93 1.06

Present Present Present Present [37] [37] [38] [39] [39] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51]

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work work work work

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M. Venkateswarlu et al. / Optical Materials xxx (2014) xxx–xxx

transitions originating from an excited state are given in Table 4 along with experimental branching ratios. The experimental branching ratios (bexp) can be determined from the relative areas of the emission transitions. From Table 4, it can be seen that the experimental branching ratios are higher than radiative branching ratios for all the LTT glasses. The highest value of branching ratio (bR) is an attractive feature for low threshold and high gain applications of lasers [34]. From Table 4, it can be observed that, the LTTNd10 glass has highest branching ratio for the transition 4 F3/2 ? 4I11/2 and can be recommended as an efficient lasing transition. The stimulated emission cross-section for all the luminescence transitions have been calculated using the formula given in the literature [34] and are tabulated in Table 5. The stimulated emission cross-section is an important parameter that influences the potential laser performance and its value signifies the energy extraction from the lasing material. From Table 5, it is observed that LTTNd10 glass possess highest stimulated emission crosssection than the other glasses for the transition 4F3/2 ? 4I11/2 at 1062 nm. In order to understand optical amplification performance of the LTT glasses, we have evaluated gain band width (rse  DkP) and optical gain parameters (rse  sR) and are given in Table 5. From Table 5, it is observed that, among all the LTT glasses studied here, the LTTNd10 glass possesses highest values of optical gain parameters and gain band width for 4F3/2 ? 4I11/2 transition and hence can be suggested for optical amplification also. Hence in the present system of LTT glasses, the LTTNd10 glass can be recommended as a suitable host for lasing emission at 1062 nm and also for optical amplification.

F3/2

0.9 0.8

4

I11/2

0.7 0.6 0.5

4

4

I13/2

I9/2

0.4 0.3

Normalized Intensity (a u)

1.0

λexc=808 nm

4

LTTNd01 LTTNd05 LTTNd10 LTTNd15 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350

Wavelength (nm) Fig. 3. Emission spectra of Nd3+ ions in LTT glasses.

in Nd3+ ion concentration up to 1.0 mol% and then decreasing beyond showing concentration quenching, which can be attributed to the interaction between the excited Nd3+ ions and energy transfer observed between Nd3+ ions. Among all the emission transitions, a transition 4F3/2 ? 4I11/2 observed at 1062 nm has highest intensity for LTTNd10 glass. The absorption, excitation and emission mechanisms are shown in Fig. 4 along with the possible cross-relaxation channel. The J–O intensity parameters are coupled with the emission spectral data, to evaluate different radiative properties such as radiative transition probability (AR), total transition probability (AT), branching ratios (bR) and radiative lifetimes (sR) using the relevant expressions given in literature [34]. In general, the stimulated emission cross-section (rse) is independent on the J–O parameters and half band widths (Dkeff) of the emission bands, which are effected by host glass composition. In addition, the intensity of 4F3/2 ? 4I11/2 laser transitions at 1062 nm depends only on the X4 and X6 parameters because of triangle rule jJ 0  Jj  k  jJ 0 þ Jj [39]. The AR, AT and sR values observed for all the emission transitions of Nd3+ ions in LTT glasses are given in Table 4. These values are useful to determine how fast an excited level gets depopulated. The radiative branching ratios (bR) used to calculate the relative intensities of all the luminescence

3.4. Decay curve analysis The decay curves recorded for the luminescence from 4F3/2 level of Nd3+ ion in LTT glasses are shown in Fig. 5. From Fig. 5, it can be observed that the decay profiles for all the LTT glasses are single exponential in nature irrespective of Nd3+ ion concentration. This may be due to fast decay of Nd3+ ions (or) negligible effect of ligands on Nd3+ ions. From the decay curves, the experimental lifetimes (sexp) are evaluated by using the relevant expression given in literature [52] and are given in Table 6 along with the radiative lifetimes (sR). From Table 6, it can be observed that the experimental lifetimes were found to be decreasing with increasing in Nd3+ ion concentration in LTT glasses. The quantum efficiency, an another

20000

4

G9/2 G7/2 4 G5/2 4 H11/2 4 F 9/2 4 F 4 7/2 F 5/2 4 F 3/2 4

16000

4000

0

Cross-Relaxation channels 4

F3/2

-1

5344 cm

878 nm

1062 nm

808 nm

8000

1339 nm

12000

876 nm 805 nm 748 nm 682 nm 826 nm 584 nm 526 nm 514 nm

Energy (cm-1)

Non-Radiative

4

I 15/2

4

I 13/2

4

I 11/2

4

I 9/2

Absorption Channels Excitaion Emission Channels

4

4

I15/2

I15/2

-1

5912 cm

4

I9/2

Fig. 4. Schematic energy level scheme of Nd3+ ions in LTTNd10 glass along with the absorption, excitation, emission and cross-relaxation channels.

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M. Venkateswarlu et al. / Optical Materials xxx (2014) xxx–xxx

Table 4 Radiative transition probabilities (AR) (s1), total transition probabilities (AT) (s1), experimental and radiative branching ratios (bexp and bR) and radiative lifetimes (sR) (ls) for the emission transitions of Nd3+ ions doped LTT glasses. Transition 4F3/2

Spectral parameters

LTTNd01

LTTNd05

LTTNd10

LTTNd15

4

AR AT bexp bR

4020.12 7342.46 0.226 0.547 136

4663.76 10866.82 0.327 0.429 92

5148.49 12040.22 0.200 0.427 83

3467.25 7926.80 0.220 0.437 126

2896.56 7342.46 0.701 0.394 136

5169.21 10866.82 0.606 0.475 92

5742.55 12040.22 0.709 0.476 83

3727.74 7926.80 0.687 0.470 126

405.68 7342.46 0.072 0.055 136

984.97 10866.82 0.066 0.090 92

1095.14 12040.22 0.095 0.091 83

697.46 7926.80 0.091 0.088 126

I9/2

sR 4

I11/2

AR AT bexp bR

sR 4

I13/2

AR AT bexp bR

sR

Table 5 Emission peak wavelength (kP) (nm), effective band widths (DkP) (nm), stimulated emission cross-sections (rse) (1019) (cm2), gain band width (rse  Dkp) (1026) (cm3) and optical gain parameters (rse  sR) (1023) (cm2 s) for the emission transitions of Nd3+ ions doped LTT glasses. Transition 4F3/2

Spectral parameters

LTTNd01

LTTNd05

LTTNd10

LTTNd15

4

kp Dk p

878 5.56 1.19 6.61 1.62

878 5.28 1.44 7.59 1.32

878 4.72 1.76 8.29 1.46

878 5 1.17 5.83 1.47

1062 6.94 1.47 10.23 2.00

1062 5.55 3.24 18.01 2.98

1062 4.16 4.75 19.80 3.94

1062 5.55 2.30 12.80 2.89

1339 3.75 0.96 3.61 1.31

1339 3.33 2.60 8.67 2.39

1339 3.19 2.99 9.54 2.48

1339 3.611 1.67 6.03 2.10

I9/2

rse rse  Dkp rse  sR 4

I11/2

kp Dk p

rse rse  Dkp rse  sR 4

I13/2

kp Dk p

rse rse  Dkp rse  sR

Table 6 Radiative and experimental lifetimes (sR and sexp) (ls), quantum efficiency (g) and multi-phonon relaxation rate (WNR) (ls1) for 4F3/2 ? 4I11/2 emission transition of Nd3+ ions doped LTT glasses.

Logarithmic Intensity (a.u)

1 LTTNd01 LTTNd05 LTTNd10 LTTNd15

9

50

100

150

200

250

300

-6

350x10

Time (s) Fig. 5. Emission decay profiles for 4F3/2 ? 4I11/2 transition of Nd3+ ions in LTT glasses. Solid lines are from exponential curve fits.

important parameter used to understand the lasing efficiency of a host material is defined as the ratio between emitted light intensity and absorbed pump intensity and is evaluated using the expression given in our previous paper [52]. Such quantum efficiency values along with radiative and measured lifetimes are given in Table 6. From Table 6, it is observed that, among all the LTT glasses, LTTNd10 glass possess highest quantum efficiency for the transition 4F3/2 ? 4I11/2 (1062 nm). The experimental lifetimes for LTT are found to be less than the radiative lifetimes. The relaxation from excited state is represented by both radiative

Name of the sample

sR

sexp

g (%)

WNR

LTTNd01 LTTNd05 LTTNd10 LTTNd15

136 92 83 126

91 73 68 61

67 79 82 48

3636 2829 2657 8456

and non-radiative decay modes. The total transition probability, i.e., the reciprocal of the fluorescence decay rate measured, has the relations with the radiative and non-radiative lifetimes as follows:

The radiative decay rate

  1 WR ¼

ðA:1Þ

sR

The non-radiative decay rate W NR ¼



1

sexp



1

sR

 ðA:2Þ

The radiative decay rate is affected by the local crystal-field symmetry around the Nd3+ ion and to some extent due to local vibration density of the states of the host. The non-radiative decay rate is due to multi-phonon relaxation process. The prominent processes contributing to the reduction of radiative decay should be

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M. Venkateswarlu et al. / Optical Materials xxx (2014) xxx–xxx Table 7 Comparison of branching ratio (bR), stimulated emission cross-section (rse  1022 cm2) and quantum efficiency (g) (%) of LTTNd10 glass for 4F3/2 ? 4I11/2 emission transition with commercial glass systems and other Nd3+ doped glass hosts. Name of the glass system

bexp

rse

g (%)

References

Commercial glass systems LTTNd10 LHG-80 LG-770 Q-88 LHG-8 LG-750

0.709 – – – -

4.75 4.2 3.9 4.0 3.7 3.6

82 – – – -

Present work [53,54] [53,54] [53,54] [53,54] [38,55]

Other glass systems BNa4 ZnAlBiB (Glass C) NaTFP LiTFP KTFP TZNLN LBTAFNd05 TeO2WO3PbONd2O3 PKMAFN MgTP CaTP SrTP NdF3 based glass CLiBT SFB

0.52 0.72 0.735 0.764 0.734 0.77 0.62 0.45 0.51 0.73 0.73 0.73 – 0.52 0.50

4.94 3.67 3.89 3.75 4.50 4.28 2.60 – 4.40 3.82 3.95 4.03 4.68 3.90 3.46

– – 71 62 68 – 23 – – 72 75 84 – – –

[30] [34] [39] [39] [39] [40] [43] [51] [53] [56] [56] [56] [57] [58] [59]

considered in order to explain the mechanism of non-radiative process. This non-radiative process (WNR) is an undesirable for amplifying devices as they need population inversion. In the present work, the quantum efficiency of the 4F3/2 ? 4I11/2 transition increases up to 1 mol% of Nd3+ ion concentration and then decreases. The non-radiative relaxation rates measured for the LTT glasses are given in Table 6. From Table 6, it is observed that, among all the LTT glasses, the LTTNd10 glass possess lowest nonradiative relaxation rate and highest quantum efficiency. Hence the LTTNd10 glass is aptly suitable for NIR laser emission at 1062 nm. Table 7 gives the comparison of experimental branching ratios, stimulated emission cross-section and quantum efficiency for LTTNd10 glass with commercial systems and other hosts [30,34,38–40,43,51,53–59]. Hence the present system LTTNd10 possess better values of branching ratios, stimulated emission cross-section and quantum efficiency than the other reported glass systems. Hence LTTNd10 glass can be recommended as suitable host to produce efficient lasing action in NIR region at 1062 nm. 4. Conclusion Lead Tungsten Tellurite (LTT) glasses doped with different concentration of Nd3+ ions were prepared by employing melt quenching method and characterized by using the spectroscopic techniques such as optical absorption, emission and decay spectral measurements to optimise the concentration of the doped rare earth ions for better luminescence efficiency. From the measured branching ratios, emission cross-sections and quantum efficiency, it is concluded that the LTT glasses are better suited for NIR laser emission applications. The gain band width (rse  Dkp) (1026) (cm3) and optical gain parameters (rse  sR) (1023) (cm2 s) measured for LTT glasses suggests the suitability of these devices for optical amplification applications. Among all the LTT glasses studied here, the LTTNd10 glass possessing better lasing potentialities and optical amplification with more branching ratio, emission cross-section, quantum efficiency, optical gain and gain band width values. Based on the luminescence characteristic parameters measured for all the LTT glasses, it is suggested that LTTNd10 glass is aptly suitable for optical amplification as well as for efficient NIR emission at 1062 nm.

7

Acknowledgments Two of the authors, Mahamuda Shaik (File No. SR/WOS-A/PS53/2011) and Swapna Koneru (File No. SR/WOS-A/PS-35/2011) are very much grateful to Department of Science and Technology (DST), Government of India, New Delhi, for endowing them with a Project under Women Scientist’s scheme (DST/WOS-A program). This work is also partly supported by High impact Research initiative of IIT – Delhi, UK India Education Research Initiative (UKIERI) and DST, Govt. of India, New Delhi. One of the authors, Dr. Mohan Babu is expressing his sincere thanks to ADE-BRNS (Project No. 2012/34/72) and DST-SERB (Project No. SR/FTP/PS-109/2012) for their financial support to carry out this work. References [1] Gang Fua, Shiling Li, Xifeng Qin, Xiuquan Zhang, Nucl. Instrum. Methods Phys. Res., Sect. B 315 (2013) 325–327. [2] K. Swapna, Sk. Mahamuda, A. Srinivasa Rao, M. Jayasimhadri, T. Sasikala, L. Rama Moorthy, Visible fluorescence characteristics of Dy3+ doped zinc alumino bismuth borate glasses for optoelectronic devices, Ceram. Int. 39 (2013) 8459– 8465. [3] Sk. Mahamuda, K. Swapna, P. Packiyaraj, A. Srinivasa Rao, G. Vijaya Prakash, Lasing potentialities and white light generation capabilities of Dy3+ doped oxyfluoroborate glasses, J. Lumin. 153 (2014) 382–392. [4] Sk. Mahamuda, K. Swapna, M. Venkateswarlu, A. Srinivasa Rao, Suman shakya, G. Vijaya Prakash, Spectral characterisation of Sm3+ ions doped oxyfluoroborate glasses for visible orange luminescent applications, J. Lumin. 154 (2014) 410–424. [5] K. Swapna, Sk. Mahamuda, A. Srinivasa Rao, T. Sasikala, P. Packiyaraj, L. Rama Moorthy, G. Vijaya Prakash, Luminescence characterization of Eu3+ doped zinc alumino bismuth borate glasses for visible red emission applications, J. Lumin. 156 (2014) 80–86. [6] K. Swapna, Sk. Mahamuda, A. Srinivasa Rao, M. Jayasimhadri, Suman Shakya, G. Vijaya Prakash, Tb3+ doped zinc alumino bismuth borate glasses for green emitting luminescent devices, J. Lumin. 156 (2014) 180–187. [7] C. Madhukar Reddy, N. Vijaya, B. Deva Prasad Raju, NIR fluorescence studies of neodymium ions doped sodium fluoroborate glasses for 1.06 lm laser applications, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 115 (2013) 297–304. [8] Antonio Agnesi, Paolo Dallocchio, Federico Pirzio, Giancarlo Reali, Compact sub-100-fs Nd:silicate laser, Opt. Commun. 282 (2009) 2070–2073. [9] I. Iparraguirre, J. Azkargorta, J.M. Fernandez-Navarro, M. Al-Saleh, J. Fernandez, R. Balda, Laser action and up-conversion of Nd3+ in tellurite bulk glass, J. NonCryst. Solids 353 (2007) 990–992. [10] M. Yamane, Y. Ashara, Glass for Photonics, Cambridge University Press, 2000. [11] Hrvoje Gebavi, Stefano Taccheo, Daniel Milanse, The enhanced two micron emission in thulium doped tellurite glasses, Opt. Mater. 35 (2013) 1792–1796. [12] Hrvoje Gebavi, Stefano Taccheo, Rolindes Balda, Joaquin M. Fernandez, Daniel Milanse, Francois Auzel, The effect of ZnF2 on the near-infrared luminescence from thulium doped tellurite glasses, J. Non-Cryst. Solids 358 (2012) 11497– 11500. [13] Guihau Liao, Qiuping Chen, Jianjun Xing, Hrvoje Gebavi, Daniel Milanse, Michael Fokine, Monica Ferraris, Preparation and characterization of new fluorotellurite glasses for photonic applications, J. Non-Cryst. Solids 355 (2009) 447–452. [14] Hrvoje Gebavi, Daniel Milanse, Guihaua Liao, Qiuping Chen, Monica Ferraris, Mile Ivanda, Ozren Gamulin, Stefano Taccheo, Spectroscopic investigation and optical characterization of novel highly thulium doped tellurite glasses, J. NonCryst. Solids 355 (2009) 548–555. [15] R. El-Mallawany, Tellurite Glasses Hand Book. Physical Properties and Data, CRC Press, USA, 2002, p. 376. [16] Nadia G. Boetti, Joris Lousteau, Alessandro Chiasera, Maurizio Ferrari, Emanuele Mura, Gerardo C. Scarpignato, Silvio Abrate, Daniel Milanese, Thermal Stability and spectroscopic properties of erbium-doped niobictungsten–tellurite glasses for laser and amplifier devices, J. Lumin. 132 (2012) 1265–1269. [17] Animesh Jha, Billy Richards, Gin Jose, Toney Teddy-Fernandez, Purushottam Joshi, Xin Jiang, Joris Lousteau, Rare-earth ion doped TeO2 and GeO2 glasses as laser materials, Prog. Mater Sci. 57 (2012) 1426–1491. [18] Shaoxiong Shen, Mira Naftaly, Animesh Jha, Tungsten–tellurite-a host glass for broadband EDFA, Opt. Commun. 205 (2002) 101–105. [19] X.Y. Wang, H. Lin, C.M. Li, S. Tanabe, Derivation of quantum yields for visible emission transitions of Sm3+ in heavy metal tellurite glass, Opt. Commun. 276 (2007) 122–126. [20] D. Barreca, S. Bozza, G. Carta, G. Rossetto, E. Tondello, P. Zanella, Structural and morphological analyses of tungsten oxide nanophasic thin films obtained by MOCVD, Surf. Sci. 532 (2003) 439–443. [21] G. Poirier, F.C. Cassanjes, Y. Messaddeq, S.J.L. Ribeiro, Crystallization of mono clinic WO3 in tungstate fluorophosphate glass, J. Non-Cryst. Solids 355 (2009) 441–446.

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