Effect of P2O5 addition on structural and luminescence properties of Nd3+-doped tellurite glasses

Accepted Manuscript 3+ Effect of P2O5 addition on structural and luminescence properties of Nd -doped tellurite glasses K. Linganna, R. Narro-García, H. Desirena, E. De la Rosae, Ch. Basavapoornima, V. Venkatramu, C.K. Jayasankar PII:

S0925-8388(16)31404-9

DOI:

10.1016/j.jallcom.2016.05.082

Reference:

JALCOM 37601

To appear in:

Journal of Alloys and Compounds

Received Date: 13 January 2016 Revised Date:

22 April 2016

Accepted Date: 8 May 2016

Please cite this article as: K. Linganna, R. Narro-García, H. Desirena, E. De la Rosae, C. Basavapoornima, V. Venkatramu, C.K. Jayasankar, Effect of P2O5 addition on structural and 3+ luminescence properties of Nd -doped tellurite glasses, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.05.082. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Effect of P2O5 addition on structural and luminescence properties of Nd3+doped tellurite glasses K. Linganna1, 2, R. Narro-García3, H. Desirena4, E. De la Rosa4, Ch. Basavapoornima1, V. Venkatramu5, C.K. Jayasankar1,* 1

Department of Physics, Sri Venkateswara University, Tirupati – 517 502, India School of Information and Communications, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 500-712, South Korea 3 Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, Querétaro 76230, Querétaro México 4 Centro de Investigaciones en Óptica, A. P. 1-948, León Gto. 37150, Mexico 5 Department of Physics, Yogi Vemana University, Kadapa -516 003, India Corresponding author: [email protected]

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Abstract

Effect of P2O5 addition on spectroscopic properties of trivalent neodymium (Nd3+)-

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doped tellurite glasses was studied through Raman, absorption, emission and decay measurements. Raman spectra (350-1250 cm-1) of the Nd3+-doped tellurite glasses with various P2O5 concentrations, were obtained upon 514 nm Ar+ laser excitation and found that bandwidth of the intense band increased with the addition of P2O5 from 0 to 20 mol %,

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indicating the transformation of TeO4 into TeO3+1 and TeO3 units. Judd-Ofelt intensity parameters and radiative properties of luminescent 4F3/2 level were evaluated using absorption spectra for all the glasses. Luminescence spectra were measured in the region 840-1500 nm

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under 806 nm diode laser excitation. The decay curves for the 4F3/2 level of Nd3+ ion were recorded under 806 nm diode laser excitation by registering the emission at 1058 nm and the

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lifetime has been found to increase from 188 µs to 249 µs when P2O5 concentration increased from 0 to 20 mol%. The figure of merit and quantum efficiency were evaluated and compared to the other reported Nd3+-doped glasses. The results indicate that the investigated phosphotellurite glasses could be useful as laser gain media around 1060 nm. Keywords: Phospho-tellurite glasses; Nd3+ ion; Raman spectra; Judd-Ofelt parameters; Quantum efficiency.

ACCEPTED MANUSCRIPT 1. Introduction Rare-earth (RE)-doped glasses have been found to be potential candidates for the development of optical devices such as solid state lasers, fiber lasers, fiber amplifiers, up- and

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down-converting glass layers for solar cells, optical fiber switching, etc. [1-6]. For the past three decades, trivalent neodymium (Nd3+)-doped glasses have been largely investigated for solid state lasers since Nd3+ ion exhibits prominent emission at around 1060 nm

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corresponding to 4F3/2 → 4I11/2 transition [7,8]. The development of Nd3+-doped glasses for laser applications requires hosts with a low maximum phonon frequency to minimize the

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multiphonon relaxation rates which in turn enhance the quantum efficiency of emitting levels. Among oxide glasses, TeO2-based glasses have been attracted as gain media because of their promising properties such as high transmission from UV to IR range, low maximum phonon energy, high linear and nonlinear refractive indices, and good thermal stability and

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chemical durability [9-11]. Despite a tremendous progress has been made, unfortunately, there were some difficulties such as relatively low thermal stability (158 °C) [12], low tensile strength (2.5-3.2 GPa) [13] and strong up-conversion luminescence of tellurite glasses that

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have so far been hindering the tellurite glass devices for practical applications. Some of these drawbacks could be overcome by adding a second glass former to the glass matrix such as the

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P2O5 compound which have shown better thermal stability (171-212 °C) [14,15] and tensile strength (4.2-7.2 GPa) [16] values than tellurite glasses. It is expected that this phosphotellurite glass exhibits a combination of properties from both glass formers. However, adding P2O5 to the tellurite matrix could also lead to a change in some interesting optical properties of the tellurite matrix such as refractive index, lifetime and quantum efficiency. In addition, the combination of the two glass formers TeO2 and P2O5 is an intrinsically interesting subject of study and are distinct from the properties of either pure tellurite or phosphate networks

ACCEPTED MANUSCRIPT [17-19]. Hence, the motivation of the present study tries to elucidate the structural and optical properties of Nd3+-doped tellurite glasses with varying P2O5 concentration. Raman analysis

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was carried out to see the effect of P2O5 addition on vibrational modes of the tellurite glass network. Judd-Ofelt parameters were determined from the optical absorption spectra and in turn used to evaluate the radiative properties for the 4F3/2 level of Nd3+ ions in the present

tellurite glasses with varying P2O5 concentration.

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2. Experimental procedure

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glasses. The lifetime for the 4F3/2 level was obtained from the measured decay curves of the

The glass samples studied here with the composition: (75-x) TeO2-xP2O5-6Al2O318K2O-0.5La2O3-0.5Nd2O3, where x = 0, 10, and 20 mol%, referred to as TAKLNP0, TAKLNP10, and TAKLNP20, respectively, were prepared by a conventional melt-quenching

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technique. The batch materials of 20 g were weighed and ground thoroughly in an agate mortar with a pestle for 1 hour to obtain homogeneous mixture. The well-mixed powder of all the samples was taken in a platinum crucible and melted in an electric furnace in the range of

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900-1000 °C for 3 hours. Then, the melt was rapidly quenched by pouring it onto a preheated

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brass plate maintaining in the range of 270-290 °C to avoid excess thermal shocks and cracking. After the quenching, the prepared glass samples were immediately transferred to another furnace and subsequently annealed in the range of 270-290 °C for 12 hours and then slowly cooled down to ambient temperature. The annealing process was made with an objective of minimizing the internal mechanical stress and strain. For optical measurements, the glass samples were cut and polished carefully to get good transparency. The thickness of the glass samples was around 2.38 mm. Raman spectra of the samples were recorded by Raman microscope (Renishaw

ACCEPTED MANUSCRIPT inVia) using Ar+ laser as an excitation source (514 nm). The density of the glasses was measured according to the Archimedes’ principle, where the weight of a volume of distilled water equivalent to that of the glass sample was obtained to a precision of 0.0001 g. Linear

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refractive index measurements (±0.001) of the glass samples were carried out using a prism coupler system at λ=632.8 nm. The UV-VIS-NIR absorption spectra (350 nm to 950 nm) of the glass samples were measured using spectrophotometer (Agilent Technologies) with

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wavelength resolution of 1.0 nm. The emission spectra were recorded by exciting the samples at 806 nm from a continuous wave laser diode (CWLD). The signal emitted was focused onto

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a SP-2357 monochromator (Acton Research) and detected by an InGaAs detector (Thorlabs DET10C). The system was controlled with a personal computer where emission spectra were obtained. Special care was taken to maintain the alignment of the setup in order to compare the intensity of the emission signal between different glass samples. The decay time

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measurements were conducted using a standard setup and coupler impedance with a rise time of 20 µs and comprising a pulsed laser diode at 250 Hz and an InGaAs detector to register the 1058 nm emission.

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3. Results and discussion

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3.1. Physical and structural properties Table 1 reports the physical properties of the Nd3+-doped tellurite glasses with

varying P2O5 concentration. By the addition of P2O5 with respect to TeO2 into the glass network, the density decreased with an increase in P2O5 content. The density values decreased because molecular weight of P2O5 (141.94 g/mol) is 0.8 times lower than the molecular weight of TeO2 (159.6 g/mol). Fig. 1 shows the normalized Raman spectra of the tellurite glasses with varying P2O5 concentration. As shown in Fig. 1, 75TeO2-6Al2O3-18K2O-0.5La2O3-0.5Nd2O3 (TAKLNP0)

ACCEPTED MANUSCRIPT glass showed a typical Raman spectrum of tellurite glass with major vibrational bands at 465, 667 and 749 cm-1. The structure of tellurite glass is similar to that of α-TeO2 [20]. The αTeO2 consists of a three dimensional network of infinite chains of TeO4 trigonal bipyramid

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(tbp) linked together by shared vertices. Pure TeO2 hardly forms the glass under normal quenching conditions. The addition of metal oxides to TeO2 seems to break the axial Te-O bonds because of the strong polarizability of tellurium lone pair electrons and leads to the

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formation of glass structure. As a result, more non-bridging Te-O bonds are formed and the TeO4 units are transformed gradually into TeO3+1 and TeO3 type polyhedra when the metal

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oxides are added more.

The basic structural unit of P2O5 glass is the PO4 tetrahedron, which contains one double-bonded oxygen and three bridging oxygen single bond atoms [21]. The tetrahedral units of phosphate glasses are expressed with the terminology Qi, where ‘i’ represents the number of bridging oxygens per PO4 tetrahedron. In addition to the characteristic peaks at

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465, 667 and 749 cm-1, the additional bands at 967, 1019 and 1129 cm-1 were observed due to the P2O5 addition in the tellurite glasses. The assignment of the Raman peaks was performed

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based on the literature available on the tellurite, phosphate, and phospho-tellurite glasses in combination with the detailed analysis of various compositions [18-23]. The band at 465 cm-1

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corresponds to the symmetrical bending and stretching modes of Te-O-P or Te-O-Te linkages. The band at 667 cm-1 corresponds to the asymmetric vibrations of the continuous network structure by the connection of TeO4 trigonal bipyramids and P-O-P bonds. The most intense band at 749 cm-1 results from the asymmetric stretching vibrations of P-O-P and stretching vibrations of TeO in TeO3 trigonal pyramidal units or TeO3+1 units. The presence of Raman bands in the regions of 935-975, 1040-1080 and 1130-1200 cm−1 are important in identifying the type Qi phosphate units available in the glass network. As seen from Fig. 1, the Raman bands at 967, 1019 and 1129 cm−1 were observed with the

ACCEPTED MANUSCRIPT addition of P2O5 in tellurite glasses. The Raman bands at 967, 1019 and 1129 cm-1 are due to the symmetric stretching mode of non-bridging oxygen atoms (PO4) in Q0 phosphate tetrahedra, P-O stretching between Q1 and Q0 phosphate tetrahedra, P-O stretching between

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Q2 and Q1 tetrahedra, respectively [21,22]. Intensity of these bands increased with increasing P2O5 concentration, which clearly indicating that the addition of P2O5 results in the increase of more Q2 units in the present glass system. The intensity variation of the Raman peaks at

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749 cm-1 to 667 cm-1 was studied in the present glasses with increase of P2O5 concentration. It was noticed that with the addition of P2O5 from 0 to 20 mol %, the value of intensity ratio

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increased from 0.12 to 1.28, indicating the transformation of TeO4 into TeO3+1 and TeO3. The change can be attributed to two reasons: on the one hand, with substituting TeO2 by P2O5, will make more Te-O-Te linkages cleaved; on the other hand, P2O5 breaks the Te-O-Te linkages, decreases the number of bridging oxygen contents and transforms TeO4 into TeO3+1 and TeO3 [24,25]. The bandwidth of the major peak at 749 cm-1 increased from 186 to 218

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cm-1 with the addition of P2O5. This bandwidth tuning indicates that the tellurite glass with P2O5 content would be promising candidate as a new gain media for broadband Raman fiber

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amplifier.

3.3. Absorption spectra and Judd-Ofelt analysis

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Fig. 2 shows the absorption spectra of the 0.5 mol % Nd2O3-doped tellurite glasses with varying P2O5 concentration. As shown in Fig.2, the absorption spectra exhibited 10 inhomogeneous absorption bands at 431, 470, 515, 527, 586, 629, 685, 749, 805 and 878 nm attributed to 4I9/2 → (2P1/2, 2D5/2), (2G9/2, 2D3/2, 2K15/2), 4G9/2, 4G7/2, (4G5/2, 2G7/2), 2H11/2, 2F9/2, (4F7/2, 4S3/2), (4F5/2, 2H9/2) and 4F3/2 transitions, respectively, obtained due to the 4f-4f transitions of Nd3+ ion. The prominent broadband absorption peak locating at 805 nm corresponding to 2H9/2 + 4F5/2 transition was observed and the absorption cross-section (σabs) for this transition was calculated to be 3.13, 3.48, and 3.72 (× 10−20 cm2) for 0 %, 10 % and

ACCEPTED MANUSCRIPT 20 % P2O5 concentrations, respectively, which could lead to an efficient energy capture. Therefore, the commercial ~ 800 nm diode lasers can be adopted as suitable pumping sources for the intense 4F3/2 → 4I9/2, 4F3/2 → 4I11/2 and 4F3/2→4I13/2 emissions, as observed and depicted

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in Fig. 3. Judd-Ofelt (JO) theory [26,27] is used to calculate the oscillator strength for the transitions from the ground state to the excited states for the RE ions implanted in different

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hosts. The experimental oscillator strength ( f exp ) of each absorption band can be determined by the following expression: mc 2 α (λ )dλ πe 2 N ∫

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f exp =

(1)

where ‘m’ and ‘e’ are the mass and charge of an electron, ‘c’ is the velocity of the light in vacuum and ‘N’ is the Nd3+ ion concentration per unit volume, α (λ ) = 2.303D0 (λ ) / d is the

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absorption coefficient at a given wavelength ‘λ’ and D0 (λ ) is the optical density [log I / I 0 ] . According to JO theory [26,27], the calculated oscillator strengths ( f cal ) of an electric-dipole allowed transition ΨJ → Ψ′J′′ is given as

Ω λ (ψ J || U λ || ψ ' J ' ) ∑ λ

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where e 2

 8π 2 mcυ (n 2 + 2) 2  2  e ∑ Ωλ (ψ J || U λ || ψ ' J ' ) 2  2 3he (2 J + 1) 9n  λ = 2 , 4 , 6 

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f cal =

2

(2)

is the line strength of electric-dipole transitions, ‘h’ is the

=2 , 4, 6

Planck’s constant, ‘υ’ is the wavenumber of the transition in cm-1, ‘n’ is the refractive index, J and J′ are the total angular momenta for the ground and upper levels, respectively, Ωλ (λ = 2, 4 and 6) are the JO intensity parameters that are characteristic of a given RE3+ ion, and

|| U λ ||2 are the doubly squared reduced matrix elements of the unit tensor operator of the rank λ = 2, 4 and 6 which are calculated from the intermediate coupling approximation for a

ACCEPTED MANUSCRIPT transition ψJ → ψ ′J ′ . JO intensity parameters, Ωλ and f cal were derived from the f exp by a least-square fitting approach. The matrix elements were taken from the Ref. [28]. The f exp and f cal of

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TAKLNP glasses were presented in Table 2 along with the Ωλ parameters. As seen from Table 2, the root-mean-square deviation (δrms) clearly indicates that the fitting process is reliable. As well known, the intensity parameter Ω2 has been identified to be associated with the

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asymmetry and the covalency of the RE sites, and Ω4 and Ω6 are related to the rigidity of the samples [29]. The decrease in the Ω2 parameter after adding P2O5 is an indication of local

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structure modification of the glass host, which will be discussed as follows. As TeO2 and P2O5 both are glass formers, there is a distortion of the tellurite network with the addition of the phosphate. The formation of P-O-Te bonds as well as less distorted TeO6 polyhedra is reported in P2O5-TeO2 glasses with the increase in concentration of P2O5 [30]. The obtained

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Ωλ parameters were compared to the reported Nd3+-doped glasses that include zinc-borobismuthate [31] (Ω2= 2.42, Ω4= 3.76, and Ω6=3.45 ×10-20 cm2), bismuth borate [32] (Ω2= 3.28, Ω4= 3.88, and Ω6=4.24 ×10-20 cm2), and lead fluorosilicate [33] (Ω2= 3.66, Ω4= 1.81,

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and Ω6=2.10 ×10-20 cm2) and found that the present investigated glasses exhibited higher

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covalency between Nd-O bond and asymmetry in the vicinity of Nd3+ ions. Using Ωλ parameters and refractive index, several important optical properties, i.e.,

the radiative transition probability, the branching ratio, and the radiative lifetime were evaluated. The spontaneous emission probability (A) of a transition can be obtained by the following equation:

A (ψ ' J ' ,ψJ ) =

 n(n 2 + 2) 2  64π 4 Sed + n3 S md  3  3h(2 J + 1)λ  9 

(3)

where the factor n(n 2 + 2) 2 / 9 is the local field correction for Nd3+ ions in the initial ‘J’

ACCEPTED MANUSCRIPT manifold, ‘Sed’ and ‘Smd’ represent the line strength for the induced elctric and magneticdipole transitions, respectively, which are expressed by

Ωλ (ψJ U λ ψ ' J ' ) ∑ λ

2

(4)

=2 , 4 , 6

 e2h2  2 (ψJ L + 2S ψ ′J ′) S md =  2 2 2   16π m c 

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Sed = e 2

(5)

The total radiative transition probability ( AT ) for an excited level is given by the sum

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of the A(ψ ' J ' ,ψJ ) terms calculated over all terminal levels,

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AT = ∑ A(ψ ' J ' ,ψJ )

(6)

The radiative lifetime ( τ rad ) of an excited level ψ ' J ' is given by the reciprocal of the AT (ψ ' J ' ,ψJ ) , 1 AT (ψ ' J ' ,ψJ )

(7)

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τ rad =

The branching ratio ( β R ) corresponding to the emission from an excited level ψ ' J '

follows:

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to a lower level ψ J can be calculated from the transition probabilities by using the relation as

A(ψ ' J ' ,ψ J ) AT (ψ ' J ' ,ψJ )

(8)

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β R (ψ ' J ' ,ψJ ) =

The obtained A, τrad and β R for the different transitions of the Nd3+ ions were presented in Table 3. It can be seen that the value of the A for all the three emission transitions decreased with increasing P2O5 concentration. The β R was not shown any consistent variation for all the transitions with the increase in P2O5 content but the τrad for the 4F3/2 level increased with the increase in P2O5 concentration. It was also noticed that the A, τrad and β R values for the 4

F3/2 → 4I11/2 transition were higher than those of the other transitions, indicating that this 4F3/2

ACCEPTED MANUSCRIPT → 4I11/2 transition could be useful for laser application around 1.06 µm. The τrad for the 4F3/2 level of present glasses also enlisted in Table 4 along with the reported zinc boro-bismuthate 60Bi2O3-39.5(4ZnO-3B2O3)-0.5Nd2O3 [31], bismuth borate 99.5(40Bi2O3-60B2O3)-0.5Nd2O3

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[32], lead fluoro-silicate 67SiO2-10PbO-2PbF2-12K2O-8Na2O-0.24As2O3-0.76Nd2O3 [33] for comparison.

3.4. Luminescence spectra

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Luminescence spectra of the Nd3+-doped tellurite glass samples with varying P2O5 concentration are shown in Fig.3. The emission spectra consisted of three bands centering at

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879, 1058, and 1331 nm, which were assigned to the 4F3/2 → 4I9/2, 4I11/2, and 4I13/2 transitions of Nd3+, respectively. It was noted that the luminescence intensity of the observed transitions slightly increased with increasing P2O5 concentration. The effective bandwidth (∆λeff) slightly increased for the 4F3/2 → 4I11/2 transition but no considerable variation was noticed for the rest

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of the transitions (see Table 3). It was reported that inhomogeneous broadening can result from structural disorder of the glass, which causes differences in ligand electric field at various sites of Nd3+ ions depending on the composition of the glass host [34]. The addition

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of P2O5 into TAKLNP glasses changed the local environment of Nd3+ ions. The PO4 units, which participate in the network formation, rupture the Te-O-Te chain in the P2O5-TeO2 glass

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network [11]. The local ligand fields of the Nd3+ ions are influenced by a combination of TeO3+1 units, Te-O-P-groups, PO4 tetrahedra and the terminal groups of PO3 and PO2. The increased bandwidth of 4F3/2 → 4I11/2 transition in phospho-tellurite glasses is a culmination of two factors, the formation of TeO3 and TeO3+1 units, the formation of PO4 tetrahedra, and the PO3 and PO2 terminal groups, which also lead to the breaking of tellurite network. The peak stimulated emission cross-section (σe(λp)) can be determined using the following equation:

ACCEPTED MANUSCRIPT  λp4  σ e (λ p ) =  8πcn 2 ∆λeff 

  A (ψ ' J ' ,ψJ )  

(9)

where λ p is the peak wavelength of the emission band and ∆λeff is its effective bandwidth I ( λ ) dλ ) . The σe(λp) is a very crucial parameter for a laser gain medium. It affects several I max

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(∫

laser parameters of active medium such as the maximum achievable gain, saturation power, laser threshold, etc. For solid-state four level laser systems, the threshold pump power is

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inversely proportional to the effective σe(λp) of the lasing material [35]. In the present glass

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samples, the σe(λp) decreased for all the emission transitions with increasing P2O5 concentration (see Table 3). The σe(λp) values for the 4F3/2 → 4I11/2 transition of the investigated glasses are tabulated in Table 4 along with the reported glasses for comparison. The present glasses showed higher σe(λp) values than the reported glasses [31-33] (see Table

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4). The product of the emission cross-section and the lifetime of a laser transition is recognized as a figure of merit (σe(λp)×τexp) of the laser transition, because σe(λp)×τexp is proportional to the slope efficiency and inversely proportional to the threshold pump power

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of a laser. The σe(λp)×τexp for the 4F3/2 → 4I11/2 transition was calculated and presented in Table 4 along with the reported glasses [31-33]. It can be seen that the Nd3+-doped phospho-

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tellurite glasses possessed larger figure of merit for 1058 nm emission than the reported glasses [31-33]. Other important properties for the development of fiber laser gain media are non-linear refractive index and high solubility of rare earth. High non-linear refractive index reduces the energy extraction efficiency of the laser and promotes the risk of laser induced damage [36-38]. Also high solubility allow the introduction of large rare earth concentration in a small volume which open the possibility to design smaller laser devices. Since phosphate glasses show low non-linear refractive index and very high solubility of rare earth, it is

ACCEPTED MANUSCRIPT expected that phospho-tellurite glasses show better properties than tellurite glasses for fiber laser. Hence, the phospho-tellurite glasses are the preferable as laser gain media at 1058 nm.

3.4. Decay curves

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Fig.4 presents the decay curves for the 4F3/2 level of Nd3+ ion in the phospho-tellurite glasses. Decay curves were found to be quite single exponential nature indicating the negligible influence of P2O5 concentration even though the concentration of Nd3+ ion is lower. Experimental lifetime (τexp) was obtained from the first e-folding times of the decay intensity.

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The τexp of the 4F3/2 level of Nd3+ ion was found to increase from 188 µs to 249 µs with the

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increase of P2O5 concentration from 0 to 20 mol %. The fluorescence lifetime of Nd3+ ion can be described as the decrease in the number of excited ions with time following optical excitation with an infinitesimally short light pulse. As the number of the excited ions is proportional to the fluorescence intensity, integration between t=0 and t=t yields a single exponential function. In the case of heterogeneous samples, the fluorescence decay could be

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better described by applying a multiexponential function due to the different mechanism of energy transfer between the ions. However, the decay curves from the samples were found to

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be quite single exponential nature due to low power excitation of laser. Such fact indicates the negligible influence of different mechanism of energy transfer between the Nd3+ ions or

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additional routes of depopulation from the 4F3/2 state by non-radiative process. The addition of P2O5 to the tellurite glasses may lead to an increase in phonon energy of the glasses since the maximum phonon energy of P2O5 is higher than that of TeO2, which will result in a decrease in lifetime of the 4F3/2 level. However, in the present study, the τexp increased with the increase of P2O5 concentration and this may be due to the decrease in the refractive index “n”. According to JO theory, the τrad is inversely proportional to “n” and thus a decrease of refractive index brings about an increase of the lifetime and the higher symmetry around the Nd3+ ion (see inset of Fig. 4). On the other hand, the addition of P2O5 to the present tellurite

ACCEPTED MANUSCRIPT glasses leads to an increase in the phonon energy of the glasses from 749 cm-1 to 1129 cm-1 which result from the asymmetric stretching vibrations of P-O-P and stretching vibrations of TeO in TeO3 trigonal pyramidal units; and due to symmetric stretching vibrations in O-P-O

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groups, respectively. However, the intensity of the Raman band around 749 cm-1 is higher than the intensity of the band at 1129 cm-1. The intensity of the Raman bands is proportional to the number of scattering units and their scattering efficiency (Raman cross-section). It may

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be concluded that the number of O-P-O groups increase as P2O5 concentration increases up to 20 mol% but the number of TeO3 trigonal pyramidal units which are associated with low

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phonon energy are greater than the O-P-O groups. Thus, the lower the phonon energy of glass host, the larger the phonons needed to bridge the energy gap between the 4F3/2 level and the next-lower 4I11/2 level, and consequently the lower the probability for non-radiative decay. The experimental results that the τexp increased with the increase of the maximum phonon

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energy of the phospho-tellurite glasses clearly indicates that emission of the 4F3/2 → 4I11/2 transition is more sensitive to effect the symmetry than to the non-radiative processes due to the increase of the maximum phonon energy.

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The τexp for the 4F3/2 level of Nd3+ ion in the present glasses are tabulated in Table 4 along with the reported glasses for comparison. As seen from Table 4, the τexp for the 4F3/2

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level of the present glasses was higher than that of zinc-boro-bismuthate [31] and bismuthborate [32] glasses but lower than that of lead fluorosilicate [33] glass. Based on the experimental lifetimes, the quantum efficiency (η) for 4F3/2 level can be obtained from the ratio between τexp and τrad (η = τexp/τrad), where τexp is the experimentally measured lifetime and τrad is the calculated radiative lifetime. The quantum efficiencies for the 4F3/2 level of Nd3+ were obtained to be 93 %, 86 %, and 91 %, for TAKLNP0, TAKLNP10, and TAKLNP20 glasses, respectively, and were compared to the other reported glasses [31-33].

ACCEPTED MANUSCRIPT 4. Conclusion Nd3+-doped tellurite glasses with varying P2O5 concentration were investigated and characterized for their spectroscopic properties. Raman analysis confirmed the presence of

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phosphate units in the studied tellurite glasses. Judd-Ofelt analysis reveals that the intensity parameters of the Nd3+-doped tellurite glasses decreased with increasing P2O5 concentration, showing decrease in the covalent nature of Nd-O bond and increase of symmetry in the

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vicinity of Nd3+ ion. Decay curves for the 4F3/2 level of Nd3+ ion in the prepared glasses exhibited single exponential and the lifetime was found to increase with increasing P2O5

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concentration. Figure of merit for the 4F3/2 → 4I11/2 transition increased with increasing P2O5 concentration and it was found to be higher than the reported glasses. Quantum efficiency of the 4F3/2 level was found to be between 93 % and 86 % favoring lasing action at 1058 nm. Hence, it can be concluded that these studies are useful and may be considered to enhance the

Acknowledgments

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laser properties of Nd3+-doped tellurite glasses.

The authors would like to thank the Mexican agency CONACYT (No. 164373) and DST,

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Govt. of India (No. DST/INT/MEXICO/P-102012), for the sanction of Indo-Mexican Joint Research Project. Dr. Ch. Basavapoornima is also grateful to University Grants Commission,

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New Delhi, for the award of Post Doctoral Fellowship for Women for the year of 2011-12 (F.15-1/2011-12/PDFWM-2011-12-OB-AND-9964 (SA-II), dt. 1-11-2013).

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[37]

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Table 1. Physical properties of the Nd3+-doped tellurite glasses with varying P2O5 concentration.

TAKLNP0

TAKLNP10

TAKLNP20

Refractive index

1.8901

1.7872

1.7144

Thickness (cm)

0.238

0.238

4.5362

4.0231

Concentration (× 10 ions/cm )

1.87

1.68

Dielectric constant

3.57

3.19

Reflection loss (%)

9.48

7.98

Interionic distance (Å)

17.14

18.02

18.65

7.05

7.30

7.52

5.98

5.63

5.31

20

3

Polaran radius (Å) Field strength (cm , × 10 ) 14

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0.243

3.6467

1.54

2.94

6.93

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Density (g/cm )

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Property

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Table 2. Experimental (fexp) and calculated (fcal) oscillator strengths and Judd-Ofelt intensity parameters (Ωλ, × 10-20 cm2) of absorption bands in the Nd3+-doped tellurite glasses with varying P2O5 concentration. 4

I9/2→

4

F3/2

4

TAKLNP0

TAKLNP10

fexp

fcal

fexp

fcal

2.26

2.44

2.03

2.10

2

8.07

8.20

7.22

7.32

4

F5/2 + H9/2 F7/2 + S3/2

8.74

8.70

7.90

7.88

2

F9/2

0.78

0.66

0.89

0.60

2

H11/2

0.37

0.18

0.35

2

G5/2 + G7/2

28.82

28.84

25.03

4

G7/2

4.63

4.25

3.71

4

G9/2

2.81

2

G9/2 + 2D3/2 + 2K15/2

2

2

P1/2 + D5/2

1.27 0.49

fcal

2.04

2.25

7.38

7.34

7.63

7.71

0.58

0.59

0.16

0.19

0.16

25.04

23.30

23.31

3.67

3.71

3.69

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fexp

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4

TAKLNP20

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Transition

1.68

2.49

1.48

2.79

1.52

1.16

1.08

1.20

1.18

1.05

0.63

0.30

0.53

0.27

0.59

6.26

Ω4

3.53

Ω6

4.74

δrms (N)a

± 0.394 (10)

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Ω2

5.93

5.47

3.23

3.79

4.69

4.83

± 0.350 (10)

± 0.425 (10)

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Table 3. Radiative properties and stimulated emission cross-section of the Nd3+-doped tellurite glasses with varying P2O5 concentration.

A -1

βR

∆λeff

4

F3/2

(cm )

(s )

(%)

(nm)

4

I9/2

11429

1894

38

45

4

I11/2

9570

2511

51

33

4

I13/2

7588

517

10

36

4

I9/2

11429

1455

37

45

4

I11/2

9570

2009

51

4

I13/2

7588

420

11

4

I9/2

11429

1432

39

4

I11/2

9570

1844

4

I13/2

7588

375

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TAKLNP20

-1

σ(λp)

τrad (µs)

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TAKLNP10

Energy

(× 10-20 cm2) 0.94

202

3.58

1.70

0.81

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TAKLNP0

Transition

35

3.00

36

1.52

45

0.88

50

35

2.97

10

37

1.44

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256

272

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σ(λp)

τexp

τrad

η

σe(λp)×τexp

TAKLNP0 [This work]

3.58

188

202

93

6.73

TAKLNP10 [This work]

3.00

221

256

86

6.63

TAKLNP20 [This work]

2.96

249

272

91

7.37

Zinc boro-bismuthate [31]

3.95

96

131

73

3.79

Bismuth borate [32]

3.90

95

176

54

3.70

Lead fluoro-silicate [33]

0.87

586

845

69

5.10

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Glass host

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Table 4. Stimulated emission cross-section (σe(λp), × 10-20 cm2), experimental (τexp, µs) and radiative lifetime (τrad, µs), quantum efficiency (η, %), and figure of merit (σe(λp) × τexp, ×10-24 cm2s) of 4F3/2 → 4I11/2 transition in the Nd3+-doped tellurite glasses with varying P2O5 concentration along with the reported glasses.

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Figure captions Figure 1. Raman spectra of the Nd3+-doped tellurite glasses with varying of P2O5 concentration.

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Figure 2. Optical absorption spectra of Nd3+-doped tellurite glasses with varying P2O5 concentration.

Figure 3. Emission spectra of the Nd3+-doped tellurite glasses with varying P2O5 concentration.

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Figure 4. Decay curves for the 4F3/2 level of Nd3+ ion in tellurite glasses with varying P2O5

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concentration.

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1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

P2O5=10%

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P2O5=0%

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0.0 0.0 400 600 800 1000 1200 400 600 800 1000 1200 1.0 P2O5=20% 0.8 0.6 0.4 0.2 0.0

400 600 800 1000 1200 -1 Raman Shift (cm )

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Normalized intensity (arb. units)

1.0

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Fig.2

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500

Wavelength (nm)

600

700

F3/2

G G9/2 7/2 4

4

P2O5= 10%

P2O5= 20%

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4 F7/2 + S3/2

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2 G5/2 + G7/2

4I → 9/2

4

4

F9/2

4

1.0

2

H11/2

1.5

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0.5 2 P1/2 + D5/2 2 2 2 G9/2 + D3/2+ K15/2

2

Absorbance (arb. units)

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P2O5= 0%

800 900

1.2 4F 3/2

P2O5 = 0 %

4

I11/2

P2O5 = 10 %

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1.0

P2O5 = 20 %

0.8 0.6 0.4

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4

I13/2

4

I9/2

0.2 0.0

900

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Intensity (arb. units)

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1050

1200

1350

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Fig.3

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Wavelength (nm)

1500

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1 250

F3/2

230 220 210 200 190

0.1

180

P2O5 = 0 %

200

400

600

800

Time (µs)

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Fig.4

10

15

20

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P2O5 = 20 %

0

5

P2O5 concentration (%)

P2O5 = 10 %

0.01

0

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Lifetime (µ s)

240

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Normalized intensity

4

1000

1200

ACCEPTED MANUSCRIPT

Highlights Nd3+-doped phospho-tellurite glasses were investigated and characterized




Raman analysis confirmed the incorporation of P2O5 addition in tellurite glass network




Lifetime (τexp) for 4F3/2 level increased with the increase of P2O5 concentration




Figure of merit (σe(λp)×τexp) increased with the increase of P2O5 concentration




Higher quantum efficiency (η) has been noticed for the present glasses

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