Influence of multiphonon and cross relaxations on 3P0 and 1D2 emission levels of Pr3+ doped borosilicate glasses for broad band signal amplification

Accepted Manuscript 3 1 Influence of multiphonon and cross relaxations on P0 and D2 emission levels of 3+ Pr doped borosilicate glasses for broad band signal amplification V. Naresh, Byoung S. Ham PII:

S0925-8388(15)32036-3

DOI:

10.1016/j.jallcom.2015.12.246

Reference:

JALCOM 36335

To appear in:

Journal of Alloys and Compounds

Received Date: 24 August 2015 Revised Date:

27 December 2015

Accepted Date: 30 December 2015

3 Please cite this article as: V. Naresh, B.S. Ham, Influence of multiphonon and cross relaxations on P0 1 3+ and D2 emission levels of Pr doped borosilicate glasses for broad band signal amplification, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2015.12.246. 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.

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Graphical Abstract

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Influence of multiphonon and cross relaxations on 3P0 and 1D2 emission levels of Pr3+ doped borosilicate glasses for broad band signal amplification V. Naresh1,2,* and Byoung S. Ham2, † 1

2

Dept. of Physics, Sri Venkateswara University, Tirupati-517502, A.P. India. Center for photon information and processing, School of information and communications, Gwangju Institute of Science and Technology, Gwangju, 500-712 Republic of Korea.

[email protected]; †[email protected] __________________________________________________________________________________

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*

Abstract

We discuss non-radiative relaxations of visible-near infrared (Vis-NIR) emissions

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originating from 3P0 and 1D2 levels of Pr3+ glasses. Thermal stability of host lithium aluminium borosilicate (LABS) glass is estimated from thermogravimetric and differential

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thermal analysis. The structural analysis of host LABS glass through Fourier transform infrared and Raman spectral profiles provide an insight to understand the effect of OH content and phonon energy on luminescence characteristics of Pr3+ ions. Visible emission spectrum of Pr3+ glass is composed of two prominent emission bands at 493 nm (3P0 →3H4) and 605 nm (1D2→3H4) when excited by 448 nm. In NIR region a narrow emission band at 1.06 µm (1D2

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→3F3) and ultra-broad emission band at 1.49 µm (1D2 →1G4) are observed for 488 nm excitation. For these emission transitions, emission decay curves are measured and analysed. The lifetime shortening due to non-radiative energy transfer is explained by multiphonon interactions and cross-relaxation routes, and later verified by Dexter’s model. Electric dipole-

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dipole mechanism is identified to be responsible for ion-ion interactions intervening in 3P0 and 1D2 states of Pr3+ causing quenching in emission intensities and lifetimes. The large

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absorption and stimulated emission cross-sections of Pr3+ ions around 1.49 µm suggests suitability of a host material operating for broadband signal amplifications at low-loss optical telecommunication windows. Besides, emission parameters like stimulated emission crosssection (σemi), effective band width (∆λeff) are calculated for 1.49 µm (1D2 →1G4) of Pr3+ doped silicate based glasses. Keywords: Energy transfer, Multi-phonon, Cross-relaxations and Pr3+ Vis-NIR emission.

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Introduction With expeditious development of wavelength division multiplexing, demand for nearand mid-infrared lasers operating in wavelength regions from O-band (1260-1360 nm) to U-

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band (1625-1675 nm) and beyond (1.7-3.5 µm) has been increased owing to various applications in optical communications, medical surgery, eye-safe laser radar, remote sensing, atmosphere pollution monitoring, and so on. It would be interesting to develop broadband

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optical amplifiers operating within low-loss wavelength region having extended/wide broad emission covering entire optical fiber communication windows ranging from 1.3 to 3.0 µm.

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Host glass matrix is instrumental in developing rare earth based optical devices for commercial and technological applications in the fields of optoelectronics, photonics and tele-communication. Among various oxide glasses, borosilicate glasses are considered to be prominent optical materials due to their wide glass forming range

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offering excellent opportunities for optimizing structural, optical, thermal and luminescence properties. In view of their chemical resistance nature, these glasses are reliable as sealing glass to store pharmaceutical and in immobilizing the radioactive nuclear

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wastes and also as an outer cover or piping for fibre optics components [1, 2]. The inclusion

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of alumina (Al2O3) to such stable glass decrease the number of non-bridging oxygens and increases

the

glass

forming

region

by

suppressing

the

phase

separation

and

devitrification/crystallization besides favouring homogenization of melts [3, 4]. Addition of Al2O3 also reduces rare earth ion clustering due to their association as Al3+ with rare earth (RE3+) ions forming Al-O-RE bonds instead of RE-O-RE, so that spacing between rare earth ions results in enhanced emission. The presence of lithium oxide in alumina borosilicate glass converts planar [BO3] units into [BO4] tetrahedral without non-bridging oxygen formation and also transforms octahedral Al3+ to [AlO4] in preference to BO4 2

ACCEPTED MANUSCRIPT tetrahedral. The excess negative charge on [BO4]– and [AlO4]– would be compensated by modifying cation lithium (Li+) ion and also the proportion of non-bridging oxygens are reduced. The SiO4, BO4 and AlO4 structural units forms a strong glass network exhibiting good chemical durability, high mechanical strength, thermal stability and

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transmittance due to strong mixed bonding linkages of Si+-O-B– and Si+-O-Al–. Keeping in view of this, structural (X-ray diffraction (XRD), Fourier transform infrared (FTIR), and Raman), and thermal (thermogravimetric and differential thermal analysis (TGA-DTA))

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properties, Li2O-Al2O3-B2O3-SiO2 (LABS) glasses have been studied alongside luminescent properties upon doping with rare earth ions. Near infrared (NIR) fluorescence of rare earth

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ions like Nd3+, Pr3+, Ho3+, Er3+ and Tm3+ are considered to be promising ions individually covering O-, E-, S-, C-, L- and U- amplification bands.

To achieve ultra-wide broadband and to increase the population inversion of certain energy levels, it has become a customary to co-dope different rare earth ions. However, this

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process has some serious features to concern like occurrence of severe concentration quenching and frequent cross-relaxations. So, it needs to explore and develop rare earthdoped systems for ultra-broadband signal amplification, while overcoming these problems in

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co-doped systems. Among the above said ions, trivalent praseodymium ion (Pr3+) is peculiar and interesting to study as it possesses multi-metastable states 3P0, 1D2 and 1G4 and also as it

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exhibits emissions from two different energy levels (3P0 & 1D2) extending from UV to NIR region due to inter-configurational (f-d) and intra-configurational (f-f) electronic transitions. The Pr3+ ion-contained glasses have gained significant weight as potential candidates for a laser medium in fibre optic amplifiers, medical applications, optoelectronic devices, and optical fibers operating in 1.3 – 1.6 µm region (low-loss region), and MIR region for optical communications [5-7]. In order to develop such credible optical devices it is important to understand the radiative/non-radiative energy transfer mechanism between Pr3+ ions 3

ACCEPTED MANUSCRIPT underlying behind fluorescence quenching of 3P0 and 1D2 emission levels. In the present work, we have chosen LABS (lithium alumina-borosilicate) glass matrix based on its structural compatibility, high thermal stability against crystallization and rare earth dissolution which could be suitable for studying Vis and ultra-broad NIR emission

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characteristics for achieving broad-band amplification upon doping with Pr3+ ion. The effect of dopant concentration on luminescence properties has been explained from fluorescence decay curves, energy level diagram and cross-relaxation channels.

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2. Experimental studies 2.1. Glass samples preparation

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Lithium alumina-borosilicate glasses with and without Pr3+ ion as a dopant have been prepared by a melt quenching method in the following chemical compositions: i.

30Li2CO3-20Al2O3-10B2O3-40SiO2 (reference glass).

ii.

30Li2CO3-20Al2O3-10B2O3-(40-x)SiO2-xPr2O3 (where x = 0.1, 0.5, 1.0 & 1.5 mol %).

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The starting chemicals such as Li2CO3, Al(OH)3, H3BO3, SiO2 and Pr2O3 are used in the preparation of optical glasses. All the chemicals are weighed in 10 g batch each

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separately, thoroughly mixed using an agate mortar and a pestle, and then each of those are collected into porcelain crucible and heated in an electric furnace for melting for an hour at

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1150 °C with intermittent stirring to remove bubbles and to ensure a total homogenization. Thus, obtained melts are then quenched in between two smooth surfaced brass plates in circular designs having 2-3 cm in diameter and a thickness of 0.3 cm. Theses circularly designed glasses were annealed below glass transition temperature at 450°C for 5 h inorder to remove internal stress. Further, to achieve smoothness it is desired to cut and polish the glass samples. Finally, these glass samples were characterized by employing different techniques to understand their potential applications.

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2.2. Measurements XRD profiles are recorded on a Seifert X-ray Diffractometer (model 3003TT) with Cu

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θ = 10º to 120º at Kα radiation (λ = 1.5406 Å) at 40 KV and 20 mA with a Si detector and 2θ the rate of two degrees per minute. Simultaneous measurements of TGA and DTA are carried out on NetZsch STA 409 N2 as purging gas at a heating rate of 10 oC/min for the precursor

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chemical mix. FTIR spectra for the glass samples are recorded on Nicolet -5700 FT-IR spectrometer using KBr pellet technique in the range of 4000-400 cm-1. Raman spectrum is

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measured using Jobin Yvon Horiba (LABRAM HR - 800) Micro Raman spectrometer attached with the He-Ne /Nd-YAG lasers as the excitation source having an output power of 15 mW with a laser beam spot size as 100 µm by employing an appropriate lens system. The optical absorption spectrum was recorded at room temperature in the spectral range of 250-2500 nm

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on a Varian-Cary-Win Spectrometer (JASCO V-570). The excitation and emission spectra of Pr3+ doped glasses in the Visible region were recorded at room temperature on a SPEX Flurolog-3 (Model-II) spectrophotometer attached with Xe-arc lamp of power 450

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W as an excitation source at λexci = 448 nm for λemi = 493 nm & 605 nm and NIR

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emission spectrum was measured on a Jobin-Yvon Fluorolog-3 spectrofluorimeter with 450 W Xenon arc and 35 W flash lamps as pump source (at λexci = 488 nm) attached with NIR photomultiplier tube as detector having range of 1000 nm-1700 nm. The emission lifetime curves were recorded on the spectrometer using pulsed Xe-lamp at room temperature. 3. Results and discussion 3.1 XRD spectrum

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ACCEPTED MANUSCRIPT In Fig. 1, XRD profile reveals a broad hollow-diffused peak at 2θ θ (10° - 120°) confirming that the LABS glass possess amorphous nature. XRD profiles of Pr2O3 glasses have also exhibited same amorphous natures which are not shown here.

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3.2 Thermal analysis (TGA-DTA)

Thermal properties play an important role in analysing nucleation and growth of crystal while using glasses for fiber fabrication. In Fig. 2, thermal analysis for the precursor

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chemical mix of Li2O-Al2O3-B2O3-SiO2 (LABS) has been carried out in terms of TGA-DTA, simultaneously measured in the temperature range of 30°C-1000°C. From TGA profile, it is

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observed that the thermal decomposition in the chemical composition takes place in multistep process. The initial weight loss of 3.6 % has been observed at 150°C due to decomposition of water and transformation of H3BO3 into HBO2, H2B4O7 and finally to B2O3 for higher temperatures [8]. The next weight loss starts at 150 °C and ends at 285°C, which is about 7.2

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% due to transformation of Al(OH)3 to α-, β-, γ- and δ- forms of Al2O3 upon reaching higher temperatures, corresponding to this weight loss an exothermic peak noticed in DTA profile [9]. A final weight loss of 21 % has been noticed at 674 °C due to decomposition of Li2CO3

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to Li2O by evolving of CO2 and beyond 700 °C a constant decrease in the profile is noticed

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confirming the formation of a stable compound of chemical composition. From DTA profile, two small endothermic peaks are noticed at 150°C and 279°C

which corresponds to the phase changes in borate groups from BO3 triangle to BO4 tetrahedral and SiO4 tetrahedrals. The glass transition temperature (Tg), onset crystallization temperature (Tx) and crystallization temperature (Tc) are noticed to be 457 °C, 661 °C and 679°C, respectively. The thermal stability ∆T = (Tx-Tg) and other stability parameters H = ∆T/ Tg and S = ∆T (Tc-Tg)/ Tg are calculated respectively to be 204 °C, 0.30 and 8.03,

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ACCEPTED MANUSCRIPT suggesting that lithium alumina borosilicate glasses are favourable for fiber drawing without devitrification or crystallization in the samples. 3.3 FT-IR & Raman spectral analysis

FT-IR spectrum has been carried out to understand the local structure and

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functional groups of the glass system. The FT-IR spectrum of the LABS glass has been shown in Fig. 3, exhibiting absorption bands at 3302 cm-1, 2888 cm-1, 2358 cm-1, 1646 cm-1, 1437 cm-1, 1314 cm-1, 1024 cm-1, 882 cm-1, and 538 cm-1 in the measured range of 400 cm-1

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to 4000 cm-1. The band centred at 3302 cm-1 is due to stretching and bending vibrational modes of O-H molecule and also due to the Si-OH stretching of silanol hydrogen group inside

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the glass matrix [10]. A couple of bands observed at 2888 cm-1 and 2358 cm-1 arise due to vibrations of O-H bonds formed at non-bridging oxygen sites [11]. The 1646 cm-1 band corresponds to vibrational modes of O-H group and also could be due to Si-O-Si antisymmetric stretching of bridging oxygen within the tetrahedral units. The band at 1437 cm-1

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corresponds to B-O asymmetric stretching vibrations in BO3 units, and the other band arising at 1024 cm-1 is due to vibrational modes of B-O bond in BO4 units in boroxol rings, tri-, tetra, penta- and pyro-borate and other borate structures. The bands in the range 1100 cm-1-500

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cm-1 are assigned to Si-O-Si, Si-O-Al, B-O-B, Si-O-B, Si-O-Li, SiO4 tetrahedral and Al-O

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bonds in Al3+ ions in the networks of tetrahedral structural units consisting of borate (BO4), silicate (SiO4) and (AlO4) groups [12, 13]. In Fig. 4, the Raman profile of LABS glass with the band positions identified at

363 cm-1, 501 cm-1, 570 cm-1, 834 cm-1 and 1013 cm-1 is shown. The band at 363 cm-1 is attributed to characteristic vibrations of cation-oxygen (Li-O & Al-O) bonds and bending vibrations of Si-O-H mode [14, 15]. A broad band centred at 501 cm-1 is due to the mixed rocking and bending modes of Si-O-Si bonds, and the other bands at 570 cm-1 and 667cm-1 arise as a result of breathing vibration of SiO4 and BO4 tetrahedra and Si-O-Si bonds [16]. 7

ACCEPTED MANUSCRIPT The band observed at 770 cm-1 is assigned to the replacement of BO3 triangles with BO4 tetrahedral units like di-, tri- and penta-borate groups [17]. A small shoulder peak at 835 cm-1 is assigned to symmetric stretching vibrations of silicate tetrahedra with four non-bridging oxygens [18]. A broad band originating at 1013 cm-1 is related to tetrahedral vibrations of Si-

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O-Si and Si-O-B bands. The band at 1389 cm-1 is assigned to B-O-B stretching vibrations as a result of BO3 triangles forming diborate group and presence of this band indicates the existence of bridging oxygens in glass matrix [17].

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3.4 Optical absorption spectrum of Pr3+: LABS glass

Optical absorption properties of rare earth ions in the glass matrix generally depend on

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neighbouring environment of the rare earth ion and interaction of rare earth ion with ligand. Fig. 5(a) shows the optical absorption spectrum of Pr3+: LABS glass exhibiting eight absorption bands at 446 nm, 470 nm, 484 nm, 593 nm, 1013 nm, 1438 nm, 1527 nm, and 1930 nm in Vis-NIR region assigned to electronic transitions originating from a ground state H4 to excited states 3P2, (3P1+1I6), 3P0, 1D2, 1G4, 3F4, 3F3, and 3F2. Of all these, 3H4 →3P2 and

3

H4 →3F2 transitions are hypersensitive in nature as they depend on neighbouring ligands and

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are governed by the selection rules ∆S = 0, │∆L│≤ 2, and │∆J│≤ 2 [19].

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3.5 Intra-configurational emission (f-f) transitions of Pr3+

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(a) Visible emission spectra of Pr3+: LABS glasses (at λexci = 448 nm) Figs. 6(a) and 6(b) present the excitation and emission spectra of the Pr3+ doped

LABS glass. The recorded excitation spectrum exhibits four excitation bands at 448 nm, 467 nm, 472 nm, and 482 nm assigned to electronic transitions 3H4 → 3P2, 3H4 → (3P1+1I6), and 3

H4 →3P0, respectively. Due to Stark splitting, the bands in the range of 465 nm-475 nm are

split into two at 467 nm and 472 nm. Of these bands, the most prominent one at 448 nm (3H4 → 3P2) is chosen to detect the emission spectrum in the measured range of 450 nm-750 nm. 8

ACCEPTED MANUSCRIPT Upon pumping at λexci = 448 nm, Pr3+ ions in the 3H4 ground state are excited to 3P2 state by means of ground state absorption. Latter, these excited ions cascade rapidly to populate 3P1, 3P0 and 1D2 metastable states by means of non-radiative relaxations. Therefore, Pr3+ ions from those metastable states decay radiatively to low lying multiplets 3FJ = (2, 3 & 4)

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and 3HJ = (5 & 4) with the emission of photons at 493 nm (3P0 →3H4), 530 nm (3P1 →3H5), 533 nm (3P0→3H5), 605 nm (1D2→3H4), 649 nm (3P0→3F2), 679 nm (3P1→3F3), 703 nm (3P0→3F3) and 731 nm (3P0→3F4) in the visible region.

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In Fig. 6(b), an anomalous situation is observed with emission bands arising from 3P0 and 1D2 states upon varying Pr3+ ion concentration. The emission intensity at 493 nm

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(3P0→3H4) band is found to exhibit enhanced emission for 1.0 mol % on increasing Pr2O3 concentration from 0.1 mol % to 1.5 mol % and beyond 1.0 mol % a decrement in its emission intensity has been noticed. Whereas with regard to emission from 605 nm (1D2→3H4), maximum emission intensity is noticed for 0.5 mol %, and thereafter emission

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intensity gradually decreases for varied concentration of Pr2O3 from 0.1 to 1.5 mol %. Such unusual trends in emission intensities could be due to: i) these emission bands arise from two distinct transitions 3P0 and 1D2 and ii) during non-radiative de-excitation process, unstable

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ions relax quickly from 3P0 state and populates 1D2 state through non-radiative relaxations.

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Therefore, with the availability of more number of ions, 1D2 state may decay to 3H4 state through fast multiphonon non-radiative relaxations, while 3P0 state decay rate is slow. These trends are further explained in emission decay curve sections. Besides this, luminescence quenching occur due to cross relaxations, non-radiative multiphonon relaxations (ion interaction with lattice vibrations), and energy transfer/migration mechanisms like ion-ion (Pr3+-Pr3+) interactions or Pr3+ ion interacting with OH group. The vibrational energies of OH group ranges from 2000 cm-1 to 3000 cm-1, which match with energy gap between energy levels of rare earth ions providing an efficient relaxation channels for decay of excited ions. 9

ACCEPTED MANUSCRIPT Luminescence quenching by OH– ions occurs due to their strong absorption bands and overtones in the NIR region causing energy transfer among OH bonded Pr3+ ions. But from the FTIR spectrum (see Fig.3), the intensity of absorption band (3302 cm-1) in higher wavenumber region is week suggesting that OH group has less influence on emission

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characteristics in NIR region. Thus, fluorescence quenching is needed to further be studied from multiphonon relaxations, lifetime decay profiles and energy transfer based crossrelaxations.

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Luminescence intensity of rare earth ions are sensitive to multiphonon relaxation rates, which depends on energy gap between the excited electronic states of rare earth ions

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and phonon energy of host matrix. Generally, the interaction between rare earth ions and the vibrating lattice makes these optically active ions occupied in the higher excited or metastable states of 4f

n

configuration to lose their energy and decay to lower levels as phonons due to

thermal energy of lattice vibrations of neighbouring atoms. At higher ion concentrations, non-

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radiative contribution is due to multi-phonon relaxation. The energy gap (∆E) between 3P0 and 1D2 states obtained from absorption spectra is found to be around 3785 cm-1, and the phonon energy of the silicate host considered from high vibrational energy (ħωmax) of [SiO4]

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structural unit is 1035 cm-1, suggesting that approximately three phonons are required to

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bridge the energy gap between (from previous literature) these levels. The multiphonon relaxation rate (WMPR) for an excited state due to stimulated emission of phonons can be obtained from the energy-gap law given by [20]:

W( MPR ) = β e−α ( ∆E −2 hωmax ) ,

(1)

where ∆E is the energy difference between the relaxing level and the very next level, ħωmax is the maximum optical phonon energy of host, α and β are the characteristic constants of the silica host material (given as α = 4.7 x10-3 cm-1 and β = 9.0x107 S-1) [21]. The rate of 10

ACCEPTED MANUSCRIPT multiphonon relaxation for 3P0 and 1D2 states is calculated to be 23548.5 S-1 and 3.79 x 10–13 S-1. The multiphonon relaxations taking place between 1D2 and 1G4 state can be ignored, as they need at least 7 to 8 phonons to bridge the large energy gap (9982 cm-1). Therefore, the involvement of multiphonon relaxations cannot be completely neglected in case of emission

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from 3P0. This suggests that, depopulation of 3P0 state takes place very fast through multi phonon relaxations by feeding the 1D2 state, and also WMPR has negligible influence on the depopulation of this state resulting in high emission efficiency at lower concentration (0.5

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mol %). In other words non-radiative multiphonon relaxations from the 3P0,1 states at lower concentrations (≤0.5 mol %) favours the emission from 1D2 state to be more dominant, while

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at higher concentrations (≥0.5 mol %) the energy transfer process becomes efficient and fast quenching in emission intensity takes place through cross relaxations due to enhanced nonradiative coupling between ions [22]. Based on this discussion, the multiphonon relaxation rates along with cross relaxations favours the depopulation of 3P0 state while multiphonon

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relaxations can be neglected in case of 1D2 state. Hence, cross relaxations could play a major role in the trends observed in emission spectra and decay profiles during emission from 1D2 state with respect to ion concentration. The above assumptions are further studied from the

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fluorescence decay curves.

Emission decay curves are useful in understanding the luminescence quenching due to

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energy transfer through mechanism in luminescence emitting rare earth ions. Figs. 7(a) and 7(b) show the emission decay profiles of the visible emission transition for 3P0→3H4 (λemi = 493 nm) and 1D2→3H4 (λemi = 605 nm) at λexci = 448 nm. Lifetime curves are fitted using a single exponential equation for all concentrations 0.1-1.5 mol % of Pr3+:

I = A exp  −t  ,  τ 

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(2)

ACCEPTED MANUSCRIPT where I is the photoluminescence intensity at time ‘t’, τ is the decay component, and A is a weighing parameter. From the decay profiles, the decay curves are initially exhibiting nonexponential nature. As the ion concentration increases, they tend to deviate to exponentially decreasing behaviour due to migration of excitation energy over donors to acceptors. The

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lifetime values are found to be decreasing in both cases with increase in Pr3+ ion concentration. It is also noticed that the measured life times of 3P0 and 1D2 states are quite different for same excitation wavelength (448 nm), which could be due to different selection

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rules and also could be due to different population mechanisms like multiphonon and crossrelaxation mechanisms involved. Both the fluorescence decay curves displayed exponential

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decreasing behaviour with their lifetimes decrease as a function of Pr2O3 concentration. This behaviour explains non-radiative energy transfer among Pr3+ ions suggesting enhanced crossrelaxation rates with increased concentration. Fig. 8 shows partial energy levels scheme of Pr3+ ion. The possible cross-relaxation channels responsible for quenching in emission

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intensities and reduction in lifetimes of Pr3+ in visible region are shown in Fig. 8 [22-25]: Donor emission (Pr3+ ion) 3

P0 → 1D2 (3797.7 cm-1)

:

3

H4 → 3H6 (4380 cm-1)

[B]

3

P0 → 1G4 (10795.33 cm-1)

:

3

H4 → 1G4 (9865.82 cm-1)

[C]

3

P0 → 3H6 (16271.15 cm-1)

:

3

H4 → 1D2 (16863.4 cm-1)

[A]

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(i) CR (3P0):

Acceptor absorption (Pr3+ ion)

(ii) CR (3P1): [D]

3

P1 → 1D2 (4368.02 cm-1)

:

3

H4 → 3F2 (4906 cm-1)

(iii) ET (1D2): [E]

1

D2 → 3H4

:

3

H4 → 1D2

The above cross-relaxation (CR) channels are explained as a Pr3+ (1st) ion in the

excited 3P0 state relaxes to the lower 3H6, 1G4 and 1D2 states, where a part of its energy is being transferred to the unexcited neighbouring Pr3+ (2nd) ion in the ground state of 3H4 to 1D2, 1

G4 and 3H6 states. Similar process occurs in case of 3P1 state and 1D2 state. On observing 12

ACCEPTED MANUSCRIPT these cross-relaxation channels attributed to 3P0 and 3P1, it has been understood that the energy levels are not in resonance, and one or more phonons are required to bridge the gap between these levels for energy transfer process to occur. Relaxation channel attributed to 1D2

ion) and 3H4 → 1D2 (1st Pr3+ ion) transitions of Pr3+ ions.

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presents the radiative energy transfer (ET) process taking place between 1D2 → 3H4 (2nd Pr3+

Fluorescence intensity ratio (R = O/B) has been calculated in order to understand the local symmetry and covalence nature between Pr3+ ion and surrounding ligand. The intensity

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ratio is obtained from orange and blue emission transitions i.e., (R=) 1D2 → 3H4/ 3P0 → 3H4. R (= O/B) values are calculated to be 0.1 Pr3+ (2.02); 0.5 Pr3+ (1.62); 1.0 Pr3+ (1.20); 1.5 Pr3+

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(2.46). It is understood that with increasing dopant ion concentration till 1.0 mol % of Pr3+, O/B values described that Pr3+ ion lies in asymmetric environment exhibiting low covalent nature while at higher concentration (1.5 mol %) Pr3+, lie in symmetric environment around the optically active ion with enhanced covalent bonding between Pr3+ and O2–.

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(b) NIR emission spectra of Pr3+: LABS glasses (at λexci = 488 nm) Fig. 9 shows the NIR emission spectra of (0.1- 1.5 mol %) Pr2O3 doped glasses pumped at 488 nm. NIR emission spectra exhibited a narrow and intense band at 1060 nm

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(1.06 µm: 1D2 →3F3, 3F4) and a low intense broad band at 1494 nm (1.49 µm: 1D2 →1G4)

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covering 1.30 µm to 1.67 µm emission region which includes E (1.36–1.46 µm) to U (1.62– 1.67 µm) bands. The inset in Fig. 9 shows the deconvolution (Gaussian multi-curve fitting) of NIR emission spectrum of Pr3+: LABS glass exhibiting the sum of four overlapping emission bands at 1343 nm, 1418 nm, 1497 nm and 1587 nm. Upon pumping at 488 nm, photons from 1D2 state are absorbed by the Pr3+ ions in the ground state in turn which are reemitted by reducing their number in the 3H4 ground state resulting in a broad emission at 1.49 µm emission. The existence of unique ligand of the Pr3+ environment in the borosilicate matrix enables in arising of broad emission band at 1D2 →1G4 transition [26, 27]. 13

ACCEPTED MANUSCRIPT Under same excitation, NIR emission band at 1.30 µm due to 1G4→3H5 transition could not be observed separately for the studied concentrations of Pr2O3 doped glasses. This is explained as, non-radiatvie multiphonon relaxation rates from 1G4 state to next lower lying 3

F4 state are comparatively much higher than the radiative transitions from 1G4 state; As a

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result Pr2O3 doped lithium aluminium borosilicate (LABS) glass cannot generate 1.30 µm (1G4→3H5) emission. From NIR emission spectra, it is evident that at higher concentrations of Pr2O3 emission intensity found to decline because of shortening in the inter-spacing between

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ions leading to luminescence quenching. This has been supported by the emission decay curves and spectral overlap of high energy side emission band and low energy side of

when

luminescence

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absorption band of Pr3+. It is noteworthy that, no appreciable change in profile is observed quenching

related

concentrations.

cross-relaxations

are

dominant

at

higher

Fig. 10 shows the emission decay profiles for NIR emission transition 1D2→1G4 (λemi

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= 1.49 µm) at λexci = 488 nm. Double exponential equation has been used to plot de-excitation curves of all studied glasses as a function of Pr3+ ion concentration: (3)

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I = A1 exp  −t  + A2 exp  −t  ,  τ1   τ2 

where I is the photoluminescence intensity at any time ‘t’ after switching off the excitation

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illumination, τ1 and τ2 are the slow and fast decay components (long and short lifetimes) respectively, and A1 and A2 are the fitting (weighing factors) parameters. Using the above two lifetime values (τ1 and τ2), the average lifetimes < τ > are calculated for Pr3+ emission states from:

A1τ 12 + A2τ 22 , (4) A1τ 1 + A2τ 2 The measured emission decay curves display similar behaviour of non-linearly < τ >=

decreasing nature confirming non-radiative decay of the higher populated state. The decay 14

ACCEPTED MANUSCRIPT profiles reveal two decay components: a non-exponential fast (short) decay with submicrosecond time constant and exponential slow (long) decay with several microseconds. The lifetime of short and long decay (radiative and non-radiative rates) components are found shorten on increasing Pr3+ concentration. The lifetimes as a function of Pr2O3 concentration

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plot is shown in inset of Fig. 10. The decrement in lifetimes depending on concentration suggests shortening of the average distance between Pr3+-Pr3+ ions resulting in a fast energy transfer. In Fig. 10, the average lifetimes calculated using Eq. (4) from fast and slow time

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components are given. This primary observation gives an evidence for non-radiative resonant energy transfer taking place from regular Pr3+ (donors) ions to perturbed Pr3+ (activator) ions.

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They exhibit non-radiative decay nature with decrement in their lifetime values depending on Pr3+ concentration due to interaction with OH content in the glass or could also be due to nonradiative energy transfer through cross-relaxation channels resulting in depopulation of 1D2 state for concentration quenching. Cross-relaxation channels responsible for self-quenching of

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emission due to 1D2 state in NIR region is given below:

Donor emission (Pr3+) (iv) CR (1D2):

Acceptor absorption (Pr3+)

D2 → 1G4 (6997.5 cm-1)

:

3

H4 → 3F4, 3F3 (6925.2 cm-1)

D2→ 3F4, 3F3 (9938.2 cm-1)

:

3

H4→ 1G4 (9865.8 cm-1)

1

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1

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Among 3P0, 3P1 and 1D2 cross-relaxation routes, channels arise from 1D2 state are in resonance (well matched) compared to other channels (3P0 & 3P1). This is because the energy gap between these levels is almost negligible with small energy difference, whereas the other levels are of high phonon assisted cross relaxation channels. Therefore the 1G4 state is not only populated by the 1D2 state but also with nearby ions in the 3H4 state. In our present study, channels origin from 1D2 state are more probable and responsible for dominant emission in visible and NIR regions. The involvement of these cross-relaxation routes in quenching of emission intensity and lifetime shortening has been evidenced from the 15

ACCEPTED MANUSCRIPT Dexter’s theory. Based on this theory, fluorescence and absorption spectra of Pr3+ doped glasses have been considered to overlap. In Fig. 11, spectral overlap of emission and absorption spectra of 1D2 transition of Pr3+ glasses in Visible and NIR region along with their cross-relaxation channels are also shown. Such spectral overlap explains the involvement of

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energy migration from excited to unexcited Pr3+ ions which takes place by means of resonant process. Unexcited Pr3+ ions in the ground state absorb radiations (photons) emitted by excited Pr3+ ions, reemit the absorbed photons, resulting in spectral broadening in the

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emission spectra. Energy migration from donors (Pr3+ ion) to acceptors (Pr3+ ion) through cross-relaxations in depopulating the 1D2 state and populating the 1G4 state. Thus, obtained D2 →1G4 emission spectrum of Pr3+ doped glass could cover most of the communication

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bands useful in telecommunication and three level laser systems depending on the excitation level.

Concentration quenching is generally caused by ion-ion interactions among dopant

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(Pr3+) ions, and the mechanism which governs non-radiative energy transfer process could be exchange interactions or multipole-multipole interactions [28, 29]. The fluorescence mechanism in Pr3+: LABS glass system is favoured by electric dipole transitions. Therefore

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energy transfer through exchange interaction can be neglected. According to Dexter’s theory

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multipole-multipole interactions are given as dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions. The emission intensity (I) can be obtained by considering the change in the emission intensity from the emitting level of the multipolar interactions. The emission intensity (I) per Pr3+ ion is given by the equation as follows [3032]: −1 S I = K 1 + β ( x) 3  ,   x

16

(5)

ACCEPTED MANUSCRIPT where I is emission intensity, S is electric multipole-multipole interaction constant whose values are 6, 8 and 10 for dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions respectively, x is concentration of Pr3+ ions, and K and β are constants for same

determined by plotting a linear fit between log (I/x) and log (x).

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excitation conditions for each interactions. The value of multipolar interaction constant (S) is

Fig. 12 shows the dependence of log (I/x) and log (x) according to Eq. (5). The profile exhibits a straight line fit with slope (S/3) = -1.98. The S value has been calculated to be 5.94,

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which is approximately equal to 6, where it demonstrates that dipole-dipole interaction plays a dominant role in concentration quenching compared with the other two interactions in Pr3+:

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LABS glasses. Stimulated emission cross-section (σemi) is an important parameter in evaluating signal amplification by [33-35]:

σ emi ( λ ) =

λ p4

8π cnd 2 ∆λeff τ m

,

(6)

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where λp is a peak emission wavelength, c is a velocity of light, nd is the refractive index of material, ∆λeff is the effective band width of emission, and τm is the life time of the emission band. Stimulated emission cross-section for the 1.49 µm emission spectrum has found to

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increase with increasing Pr3+ concentration. For NIR emission at 1.49 µm, luminescence parameters like stimulated emission cross-section (σemi), effective band width (∆ λeff), and

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amplification gain (σemi τPr) are calculated and tabulated in Table 1. Effective bandwidth (∆ λeff), and emission cross-section (σemi) for 1D2→1G4 emission transition of Pr2O3 doped glasses are compared in Table.2. The emission related parameters suggest the suitability of the host matrix as a gain media for broadband amplifiers.

Conclusion

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ACCEPTED MANUSCRIPT In conclusion, stable and transparent lithium aluminium borosilicate glasses with and without Pr2O3 in different concentrations (0.1-1.5 mol %) were prepared by employing melt quenching method in view of studying thermal, structural, optical and luminescence properties. Weight loss, glass transition temperature (Tg = 661 °C) and crystalline

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temperature (Tc = 670 °C) were identified from the simultaneous measurement of TG-DTA for the host precursor chemical mix. Amorphous nature of the host glass is confirmed from XRD profiles, while FTIR and Raman spectral profiles reveal various structural units

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attributed to intermixing of BO3, BO4, AlO4 and SiO4 groups. Pr3+ doped LABS glasses exhibited two visible emission bands at 493 nm (3P0 →3H4) and 605 nm (1D2→3H4) with

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pump at λexci = 448 nm, while two NIR emission bands at 1.06 µm (1D2 →3F3, 3F4) and 1.49 µm (1D2 →1G4) upon exciting at λexci = 488 nm. The hydroxyl group (OH–) and multiphonon rate have less influence on luminescence features of Pr3+ ions, when compared to cross relaxations. The involvement of cross relaxation routes for luminescence quenching and

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lifetime decrement in visible and NIR region due to non-radiative energy transfer among Pr3+ ions are explained on the basis of emission decay curves and also from spectral overlap of emission and absorption spectra (Dexter theory). The dipole-dipole mechanism is evaluated to

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be responsible for concentration quenching due to ion-ion interactions. The emission band at

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1.49 µm possesses broad stimulated emission cross-sectional area (σemi = 6.16 x 10-21 cm2) covering 1.35-1.65 µm (E to U bands) region with an effective linewidth of 207 nm, suggesting potential application of Pr3+ doped silicate based glasses operating as tunable lasers and optical gain medium for broadband signal amplification at low-loss optical telecommunication windows in NIR region.

Acknowledgments

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ACCEPTED MANUSCRIPT This work was supported by the “Basic research projects in high-tech industrial technology” project through a grand provided by GIST in 2015, and the ICT R&D program of MIST/IITP

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(R0190-15-2030).

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ACCEPTED MANUSCRIPT M.P. Belancon, J.D. Marconi, M.F. Ando, L.C. Barbosa, Near-IR emission in Pr3+ single doped and tunable near-IR emission in Pr3+/Yb3+ codoped tellurite tungstate glasses for broadband optical amplifiers, Opt. Mater. 36 (2014) 1020-1026.

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[34]

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Table Cation

Table.1: Emission parameters: Peak wavelength (λP); Effective bandwidth (∆ λeff); Luminescence lifetime (τEr); Stimulated emission cross-section (σEP); Figure of Merit (σEP x ∆ λeff) of various concentrations of Pr3+: LABS glasses for 1D2→1G4 emission transition.

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Figure Captions

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Table.2: Comparison of effective bandwidth (∆ λeff), and emission cross-section (σEP) for 1 D2→1G4 emission transition of various Pr2O3 doped glasses.

Fig.1: XRD spectrum of host Li2O-Al2O3-B2O3-SiO2 (LABS) glass

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Fig.2: TG-DTA profiles of host Li2O-Al2O3-B2O3-SiO2 (LABS) precursor chemical mix Fig.3: FTIR spectrum of host Li2O-Al2O3-B2O3-SiO2 (LABS) glass Fig.4: Raman spectrum of host Li2O-Al2O3-B2O3-SiO2 (LABS) glass Fig.5: Vis & NIR absorption spectrum of Pr3+: Li2O-Al2O3-B2O3-SiO2 (LABS) glass Fig.6 (a): Excitation spectrum of Pr3+: Li2O-Al2O3-B2O3-SiO2 (LABS) glass at λemi = 605 nm Fig.6 (b): Emission spectrum of (0.1- 1.5 mol %) Pr3+: Li2O-Al2O3-B2O3-SiO2 (LABS) glass at λexci = 448 nm Fig.7 (a & b): Emission decay profiles for the visible emission transition states 22

ACCEPTED MANUSCRIPT (a) 3P0→3H4 (λemi = 493 nm) & (b) 1D2→3H4 (λemi = 605 nm) at λexci = 448 nm. Fig.8: Energy level Scheme of Pr3+: LABS glass involving cross-relaxations in fluorescence quenching of multiplets 3P0 and 1D2 emission levels

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Fig.9: NIR emission spectra of (0.1- 1.5 mol %) Pr3+: Li2O-Al2O3-B2O3-SiO2 (LABS) glass under an excitation of 488 nm. Inset figure shows the de-convoluted emission spectrum at 1.49 µm of Pr3+ doped glass Fig.10: Emission decay profiles for NIR emission transition 1D2→1G4 (λemi = 1.49 µm) at λexci = 488 nm (Inset Figure shows concentration dependent lifetime values)

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Fig.11 (a, b &c): Spectral overlap of emission and absorption spectra of 1D2 transitions of Pr3+ doped glasses Fig.12: The dependence of log (I/x) of Pr3+ on log (x) of Pr3+ when θ = 6 for dipoledipole mechanism

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Figures

Fig. 1 23

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Fig. 2

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Fig. 3 25

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Fig. 4 26

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Fig. 5 27

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Fig. 6 (a) 28

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Fig. 6(b)

Fig. 7(a) 30

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Fig. 7(b)

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Fig. 8 32

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Fig. 9

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Fig. 10

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(a)

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(c)

Fig. 11 (a, b &c)

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Fig. 12

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ACCEPTED MANUSCRIPT Table.1 Emission parameters: Peak wavelength (λP); Effective bandwidth (∆ λeff); Luminescence lifetime (τPr); emission cross-section (σemi), amplification gain (σemi τPr) of various concentrations of Pr3+: LABS glasses for 1D2→1G4 emission transition.

λP

∆ λeff

τPr

(nm)

(nm)

(µs)

1494 1494 1494 1494

172 207 187 183

417.04 191.06 122.51 120.26

(10

–21

2

cm )

1.11 6.16 3.57 3.43

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0.1 Pr3+: LABS 0.5 Pr3+: LABS 1.0 Pr3+: LABS 1.5 Pr3+: LABS

σemi

σemi τPr

(10–25 cm2s)

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Sample

4.62 11.7 4.37 4.12

ACCEPTED MANUSCRIPT Table.2 Comparison of Emission parameters: Effective bandwidth (∆ λeff), emission cross-section (σemi) of Pr2O3 doped glasses for 1D2→1G4 emission transition.

∆ λeff (nm)

207 200 140 120

Refs.

0.61 0.89

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Pr2O3: Li2O-Al2O3-B2O3-SiO2 Pr2O3: SiO2-CaO-Na2O Pr2O3: TeO2-WO3-Na2O-Nb2O5 Pr2O3: BaF2-AlF3-BaO-La2O3-TeO2

σemi

(10–20 cm2)

[present work] [33] [34] [35]

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Research Highlights
Vis-NIR emission properties of Pr2O3: Li2O-Al2O3-B2O3-SiO2 glasses prepared by meltquenching technique are analyzed.

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NIR emission spectrum of Pr3+ (1.49 µm: 1D2 →1G4) covered 1.30 to 1.67 µm emission region which includes E to U bands.
The emission cross-section (σp) and effective bandwidth (∆λeff) of transition are 6.16x 10–21 cm2 and 207 nm.

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D2 →1G4

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The validity of ion-ion interactions & energy transfer through cross-relaxations is verified from Dexter’s model

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Energy transfer between Pr3+ ions is governed by dipole-dipole mechanism.