Yb3+ codoped tellurite glasses for photonic applications

Journal Pre-proof Spectroscopic investigations on Yb for photonic applications

3+

3+ 3+ doped and Pr /Yb codoped tellurite glasses

M. Seshadri, M.J.V. Bell, V. Anjos, Y. Messaddeq PII:

S1002-0721(19)30766-5

DOI:

https://doi.org/10.1016/j.jre.2019.12.006

Reference:

JRE 667

To appear in:

Journal of Rare Earths

Received Date: 16 September 2019 Revised Date:

26 November 2019

Accepted Date: 12 December 2019

Please cite this article as: Seshadri M, Bell MJV, Anjos V, Messaddeq Y, Spectroscopic investigations 3+ 3+ 3+ on Yb doped and Pr /Yb codoped tellurite glasses for photonic applications, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.12.006. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © [Copyright year] Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.

Spectroscopic investigations on Yb3+ doped and Pr3+/Yb3+ codoped tellurite glasses for photonic applications M. Seshadri1,*, M. J. V. Bell1,2 and V. Anjos1,2** 1

Grupo de Engenharia e Espectroscopia de Materiais, Departamento de Fisica – ICE, Universidade Federal de Juiz de Fora, Juiz de Fora – MG 36036-900, Brazil. Y. Messaddeq2

2

Centre d’Optique, Photonique et Laser, 2375 Rue de la Terrasse, Université Laval, Québec, QC, G1V 0A6, Canada

Abstract In this work, we report on structural and spectroscopic properties of Yb3+ doped and Pr3+/Yb3+ co-doped TeO2-Bi2O3-ZnO-Li2O-Nb2O5 (TBZLN) tellurite glasses. Bending and stretching modes of TeO2 and Te-OH bond (strong and weak) were analysed from the deconvolution of observed Raman and FT-IR spectra. Based on the absorption measurements, the energy bands of Yb3+ and Pr3+ ions are assigned. The spectroscopic properties for the radiative transitions of Yb3+ and Pr3+ ions were reported using McCumber and Judd-Ofelt theories. Visible emission bands originated from 3P1 and 3P0 to lower lying levels of Pr3+ were registered under 447 nm excitation. The emission band around 1334 nm is assigned to the Pr3: 1G4→3H5 was observed when excited at 980 nm. The stimulated emission cross-section (σemi(λ)) and effective linewidth (∆λeff) for the 3P1→3H6, 3P1→3H5, 3P0→3H6, 3

P0→3F2, 3P1→3F3, 3P1→3F4, 3P0→3F4 and 1G4→3H5 transitions of Pr3+ are reported.

Upconversion luminescence in Pr3+/Yb3+ co-doped glass upon 980 nm excitation was measured. Possible resonant transfer processes between Yb3+ and Pr3+ ions are presented and discussed. The chromaticity co-ordinates were also evaluated from the visible emission spectra resulting that Pr3+/Yb3+ co-doped glass may be suitable for the development of Yellow-Orange (λexc = 447 nm) and near white light (λexc = 980 nm) emitting devices in photonics.

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Keywords: Rare earth ions; Tellurite Glass; Spectroscopic properties; Upconversion luminescence Foundation item: Project supported by Programa Nacional de Pos-Doutorado (PNPD/CAPES – 086/2013) – Brazil. Corresponding author. E-mail: [email protected] (M. Seshadri); [email protected] (V. Anjos); Tel.: +55-32-2102-3307 Ext: 237 (office) or 234 (lab)

1. Introduction Over the past few decades, great attention has been given in the development of rare earth (RE3+) doped glasses to be used as functional material in visible and infrared lasers, color displays, amplifiers for telecommunications, switching and wavelength converting devices, etc.

[1-4]

. Rare earth ions produce extremely stable spectral characteristics due to the

position of electron in 4f and 5d orbitals. Important sharp emission lines in ultraviolet, visible and infrared regions arise due to 4f-4f and 4f-5d transitions. Among rare earth ions, praseodymium (Pr3+) doped materials can easily achieve simultaneous blue and red emission lines because of the rich energy level diagram that offers visible light from the 3P0,1,2, 1D2 and 1

G4 metastable states[5], as well as infrared light at 1.3 µm, that is useful for

communication[6]. Pr3+ ions possess three level laser action associated with the transitions from the 3P0 state in certain activated host systems which are potentially usable for visible laser generation[7]. Upconversion luminescence in Pr3+ ions doped materials has attracted attention for blue laser achievement[8]. Considering the blue emission from 3P0 (Pr3+) states, low concentration is needed to achieve laser threshold because of three level laser action. Additionally, high pump absorption is also required for compact devices. However, codoping with Yb3+ ions can increase the upconversion efficiency due to the energy transfer processes among donor and acceptor ions. Yb3+ ion is commonly used as sensitizer because of the large absorption cross-section around 980 nm (2F7/2 → 2F5/2) which lead to an efficient absorption of pump photons resulting in a large enhancement of upconversion luminescence

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in RE3+ doped materials[9]. Superposition of the absorption of silicon and emission of Yb3+, RE3+-Yb3+ co-doped systems (Pr3+ - Yb3+; Tm3+ - Yb3+; Nd3+ - Yb3+; Tb3+ - Yb3+; Ce3+ - Yb3+ and Er3+ - Yb3+) have also been used for solar cells to enhance conversion efficiency[10-14]. Therefore, spectroscopic investigations are needed to understand up and down conversion processes. Low phonon energy glasses are required to enhance the RE3+ ions emission due to the reduction of the nonradiative relaxation mechanisms. Among oxide glasses, tellurite glasses have low phonon energy than borate, phosphate and silicate ones, relatively high linear (n ~ 1.8 - 2.3) and nonlinear (20 - 50 × 10-20 m2/W) refractive index, low glass transition and melting temperature, good chemical and mechanical stability and high rare earth ions solubility. In recent years, these glasses have paid much attention for optoelectronic and photonic applications[15-19]. Zhang et al.[20] studied effect of GeO2 on spectral (absorption and emission) behaviour of Yb3+ doped tellurite glasses. Souza et al.[21] investigated optical characteristics of Te4/Yb3+ doped TeO2+Li2O glass for luminescent solar concentrators. Venkataiah et al.[22] reported laser properties of Yb3+ doped TeO2+ZnO+ZrO2 glasses. Lin et al.[23] studied spectroscopic properties of Yb3+ doped TeO2+Bi2O3+Nb2O5 glasses. Zhou et al.[24] achieved super broadband near-IR photoluminescence in the wavelength range 1.30 – 1.67 µm from Pr3+-doped fluorotellurite systems. Belançon et al.[25] investigated spectroscopic properties in visible and near-infrared, and achieved 155 nm of full-width at half maxima (FWHM) for the Pr3+:1G4→3H5 transition in Pr3+/Yb3+ co-doped tellurite glasses. Rajesh et al.[26] reported Pr3+/Yb3+ doped TeO2-ZnO-YF3-NaF glasses for manipulation of the solar spectrum, using up and down conversion processes to increase the absorption efficiency of currently used c-Si photovoltaic solar cells.

In this paper, we

prepared TeO2-Bi2O3-ZnO-Li2O-Nb2O3 (TBZLN) tellurite glass doped with Yb3+ and Pr3+/Yb3+ ions, and analysed their structural and spectroscopic properties based on Raman, FT-IR, absorption and luminescence techniques. Focus has been placed on calculation of

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laser properties among optical transitions of Pr3+ and Yb3+ ions improving efficiency of solid state lasers that are not explored[26]. Visible and NIR emission properties have been successfully obtained demonstrating via lifetime of the Yb3+ transference of energy to Pr3+ ions, and the estimated energy transfer efficiency.

2. Experimental The glasses were prepared by melt-quenching method with the chemical composition: 77TeO2 + 4.5 Bi2O3 + 5.5 ZnO + 10.5 Li2O + 1.5 Nb2O5 + 1.0 Yb3+; 76.5TeO2 + 4.5 Bi2O3 + 5.5 ZnO + 10.5 Li2O + 1.5 Nb2O5 + 0.5Pr3+ + 1.0 Yb3+. The reagents, TeO2 (99.995%), Bi2O3 (99.995%), ZnO (99.995%), Li2CO3 (99.995), Nb2O5 (99.995%), Yb2O3 (99.99%) and Pr6O11 (99.995%), mixture of 20 g appropriate proportion of each glass composition were melted in a platinum crucible covered with silica plate at temperature between 850 0C for 2 h using electrical furnace. The melts were cast into pre-heated brass plate, and subsequently, annealed at glass transition temperature (Tg = 280 oC) for 2 h before cooling down to room temperature at a rate of 10 oC/min. The obtained samples were polished for optical characterization. IR transmittance spectra were obtained with FT-IR spectrometer (BRUKER, VERTEX 70). Optical absorption spectra were measured in the range 350 – 900 nm with UV – 2550 UV-Vis spectrophotometer (SHIMADZU). The visible emission and lifetime measurements were carried out using a Jobin Yvon Fluorolog-3 spectrofluorimeter (Horiba FL3-22iHR320) with 450 W Xenon arc and 150 W Xenon flash lamps as pump sources. The near infrared to visible up-conversion emission spectra were measured using a 980 nm laser line. All the measurements were carried out at room temperature. According to the Archimedes principle, the densities were measured using distilled water as an immersion liquid. Refractive index was estimated by prism coupling method

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(Metricon Model 2010) at 633 nm. Table SM1 in the Supplementary Material summarizes some physical parameters of the studied glasses.

3. Results and Discussion 3.1. Raman and FT-IR spectral analysis Generally, tellurite network consists of TeO4 trigonal bipyramid (Tbp) units and TeO3 trigonal pyramid (Tp) units with a long pairing of electrons. The Tbp units were converted into Tp units while adding alkali, alkaline earth and metal oxides[27]. Fig. 1 shows Raman spectra of TBZLN:Yb3+ tellurite glass. The Raman spectra were deconvoluted by symmetrical Gaussian functions. Typical Raman spectra of such glass are composed basically by three modes at 50–300, 300–550 and 550–850 cm-1 due to torsion or rotation, bending and stretching modes of TeO2, respectively[28]. The Raman bands due to stretching modes were deconvoluted into four peaks at 597, 658, 737 and 792 cm–1, denoted as S1, S2, S3 and S4, respectively. The band S1 is due to the antisymmetric stretching of TeO4 Tbp units. The band S2 is assigned to the antisymmetric vibrations of Te–O–Te linkages with two unequal Te–O bonds. The band S3 is related to the stretching modes of Te–O– and Te=O bonds with non-bridging oxygens (NBOs) in TeO4 trigonal bipyramid (Tbp) units and TeO3 trigonal pyramid (Tp) units. The band S4 is due to the presence of some deformed TeO4 units in the γTeO2 polymorph and to the stretching vibrations of tellurium and NBO atoms in TeO3 Tp units. The band at 869 cm–1 is related to the presence of NbO6 octahedra. The band at 442 cm–1 is assigned to the symmetrical stretching or bending vibrations of Te–O–Te linkages at corner sharing sites and is larger in glasses containing higher concentration of TeO2. The bands between 100–300 cm–1 are related to torsional or rotation modes. The water content can be characterized by Fourier transform infrared (FT-IR) analysis. The water band is due to the OH– ion which has not only risen at its fundamental

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absorption close to 2900 cm–1 but also exhibited several other absorption bands. The newly exhibited band frequencies can be written as[29] =

+

,

(1)

where, m and n are positive integers, ν0 is the fundamental absorption frequency and ν1 is the fundamental TeO4 stretching vibrational frequency. The overtones will appear at frequencies closer to mν0 while OH– ions are linked to TeO4 fundamental stretching vibration. Fig. 2 shows FT-IR transmittance spectra of Yb3+ and Pr3+/Yb3+ doped TBZLN glasses. It is observed that the bands around 3000 and 2240 cm–1 are due to the presence of OH– in the glass; and the band at 1750 cm–1 is related to the multiphonon vibration edge characteristic of the glass network. The absorption coefficient of OH– (αOH–) (∼3000 cm–1) are calculated using equation given in literature[28] and are 2.39 and 3.11 cm–1 for Yb3+ and Pr3+/Yb3+ glasses, respectively. Inset of Fig. 2 shows the Gaussian deconvoluted absorption coefficient spectrum of TBZLN: Yb3+ glass. The five fitted Gaussian bands observed at 2287, 2656, 2936, 3200 and 3384 cm–1 are denoted as W1, W2, W3, W4 and W5, respectively. The band W1 is related to the stretching mode of the strongly bonded Te-OH groups. The band W2 is attributed to the stretching mode of Te-OH group that forms the weaker hydrogen bonding with non-bridging oxygens (NBOs). The highest band W3 is due to the free OH– linked to TeO3 Tp units. The band W4 is related to the stretching mode of free Te–OH groups and that of molecular water. The W5 is due to OH– linked to TeO4 Tbp units. Beyond the several structural units, the bonding of O–H with TeO3 is the stronger one[29].

3.2. Optical absorption spectra and energy level analysis Fig. 3 shows optical absorption spectra of Yb3+ doped and Pr3+/Yb3+ co-doped TBZLN glasses. In the case of Pr3+/Yb3+ sample, the bands assigned at 2343, 1959, 1548, 1454, 594, 485, 474 and 448 nm are related to the transitions of Pr3+:3H4→3H6, 3H4→3F2, 6

3

H4→3F3, 3H4→3F4, 3H4→1D2, 3H4→3P0, 3H4→3P1+3I6 and 3H4→3P2. The band at 978 nm is

attributed to Yb3+:2F7/2→2F5/2 transition and is present in two glass matrices. The optical absorption edge is connected with theory of electronic structure of amorphous materials and can be used to determine the optical band gaps for direct and indirect transitions. According to Tauc’s relation[30], the indirect allowed transitions are valid for amorphous materials due to their high absorption coefficient in the range of 104 -105 cm-1. In turn, the absorption coefficient (α(ν)) is related to the incident photon energy (hν) and can be expressed for indirect transitions as follows[30]:

(ν ) =

( ν

)

ν

,

(2)

where, A is a constant and Eopt is the optical energy bandgap. The (αhν)1/2 as a function of photon energy (hν), is illustrated in Fig. SM1 of the Supplementary Material. The optical gaps are estimated by extrapolation from the linear region of the plots of (αhν)1/2 versus hν and are 3.15 and 2.89 eV for Yb3+ doped and Pr3+/Yb3+ co-doped TBZLN glasses. The decrease trend of optical band gap in our glasses indicates increase of non-bridging oxygens with addition of RE3+ ions. The energies of the observed absorption bands were calculated using Taylor series expansion and least square fit method as described in Ref. [31] (See Section 3.2.2 of the Supplementary Material). Our energy levels for Pr3+ ions (3F2, 3F3+3F4, 1D2, 3P0, 3P1+1I6, 3P2) and parameters such as Racah (E1, E2 and E3), spin-orbit interaction and hydrogenic ratios are comparable to those calculated for other host glasses (see Table SM2 in the Supplementary Material). 3.3 Spectroscopic properties of Yb3+ doped TBZLN glass Normally, the 2F7/2 and 2F5/2 Stark levels of Yb3+ ions split into several sublevels due to crystal field effect. The larger/smaller stark splitting of Yb3+ is useful to operate a quasi-

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four/three level laser scheme. In the present work, the normalized absorption spectrum is fitted by Lorentz fitting and is shown in Fig. 4(a). It can be seen that the spectrum fitted to four absorption bands around 931, 957, 977 and 995 nm which are attributed to transitions between the ground states of 2F7/2 and the stark splitting energy levels of 2F5/2. Due to the unique f-f transition of Yb3+ ions, the radiation trapping can occur due to spectral overlap of their absorption and emission bands which may influence the performance of an Yb3+ glass laser. In order to know the lasing performance, the absorption and emission cross-sections should be estimated. The absorption cross-section of Yb3+: 2F7/2→2F5/2 transition can be obtained from[22], ( )=

.

× OD( ) ,

(3)

where, OD(λ) is the optical density, l is the thickness of the sample and N is the Yb3+ ion concentration in the sample. The stimulated emission cross-section was evaluated from the reciprocity method (McCumbers theory) which is more accurate than Fuchtbauer-Ladenburg (for emission spectra) and can be expressed as[22], ( )

!(

)

#

= "# $ & exp "

+,$

%

- ./

01

&,

(4)

where Zl and Zu are the partition functions of the lower and upper levels involved in the relevant optical transition, εZl is the zero line energy, h and K are the Planck’s and Boltzmann’s constants and εZl = hc/λp, the energy corresponding to the peak wavelength (λp) of the absorption. Table 3 shows the estimated absorption and emission cross-sections of Yb3+ host glasses. The obtained cross-sections, σabs(λp = 977 nm) is 3.35 × 10–20 cm2 and

σemi(λ0 = 1002 nm) is 0.78 × 10–20 cm2, are higher than tellurite[22, 32-34], borophosphate[35, 36], phosphate[37-40] and heavy metal oxide[41,

42]

glasses. Fig. 4(b) depicts the wavelength

dependent absorption and emission cross-sections (using eq. (6)) for the Yb3+:2F7/2 ↔ 2F5/2 transition.

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According to Fuchtbauer-Ladenburg expression[15], the stimulated emission crosssection (σemi) is proportional to spontaneous radiative transition probability (Arad) which is also an important parameter for laser action operation. The estimated Arad for the TBZLN glass (2002 s–1) is lower than heavy metal oxide glasses[42]. Table 1 presents the laser parameter βmin, which is defined as the minimum fraction of Yb3+ ions that must be excited to balance the gain exactly with the ground-state absorption at λ0 and given by[42], βmin =

σabs(λ0)/(σemi(λ0)+σabs(λ0)). In Yb3+ glass lasers, high doping concentration, high fluorescence lifetime and small emission cross-section factors can determine the depletion of storage energy due to the spontaneous emission and the pump power, which is essential to generate population inversion in gain material and is given as, Usat = hν0/(σabs(λ0)+ σemi(λ0)). The Usat is around 16.84 J/cm2 for the TBZLN glass and is higher than lead fluoroborate (16.20 J/cm2)[42], niobium tellurite (14.02 J/cm2)[42] and silicate (SACF0.2 (8.45 J/cm2); SACF0.4 (11.2 J/cm2))[43] glasses, respectively. The wavelength dependent gain cross-section for Yb3+ sample was obtained based on[15], σg(λ) = γσe(λ) – (1–γ)σa(λ), where γ is the ratio of the inverted ions to the total Yb3+ ions concentration, as shown in fig. SM2 of Supplementary material. The γ = 0 and γ = 1 correspond to the absorption and emission cross-sections (see Fig. SM2). The positive gain appears at γ = 0.5 with wide tunable wavelength possibility from 977 to 1035 nm which would generate IR lasers. 3.4 Spectroscopic properties of Pr3+/Yb3+doped TBZLN glass 3.4.1 Judd-Ofelt analysis and radiative properties The radiative transitions within the 4f configuration of Pr3+ ions can be analysed from the Judd-Ofelt theory[44, 45] using absorption spectra of the Pr3+/Yb3+ glass. The oscillator strengths (f) for the observed transitions of Pr3+ ions are given in Table 2 along with r.m.s deviation. The low r.m.s deviation between experimental and calculated oscillator strengths

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indicates the validity of Judd-Ofelt theory. Using experimental oscillator strengths, the observed absorption bands and Judd-Ofelt theory, the Ωλ (λ = 2, 4 and 6) intensity parameters are estimated and reported (Table 2).

As noted in the literature[46], the

conventional Judd-Ofelt theory applied to Pr3+ ion can cause discrepancy between experimental and calculated oscillator strengths, and negative magnitude of Ω2 values by considering the 3H4→3P2 hypersensitive transition in least square fit method. The effect can be due to a small energy difference between 4f and 5d orbitals, which lead to strong 4f-5d mixing between these orbitals for Pr3+ ion. In the present work, we took into account the intensity parameters with and without considering the 3H4→3P2 hypersensitive transition (HST) through least square fit method. Significant improvement is found in the root mean square deviation (δrms) when HST is not considered. Positive Ω2 intensity values (with and without HST) indicate that J-O theory is adequate to Pr3+ tellurite glasses. The Ω2 parameter is an indicative of covalent bond between RE-O. Large values represent polarized and asymmetric sites in the glasses. From Table 2, the magnitudes of Ω2 and ΣΩλ (5.07 × 10–20 cm2 and 13.94 × 10–20 cm2) of our glass is higher than other host glasses: Tellurite (TWPLa) (4.77 × 10–20 cm2 and 11.14 × 10–20 cm2)[47], ZnBPr (1.52 × 10–20 cm2 and 12.34 × 10–20 cm2)[48], Zinc cadmium fluoride (0.72 × 10–20 cm2 and 13.45 × 10–20 cm2)[49], LBTAF (2.42 × 10–20 cm2 and 9.19 × 10–20 cm2)[50], indicating strong asymmetry site and high covalence between Pr–O bond. The Ω4 and Ω6 intensity parameters are related to the rigidity of the host glass. The order of magnitude of J-O parameters is Ω6>Ω2>Ω4. Using determined Ωλ* (λ = 2, 4 and 6) intensity parameters (see Table 2), the radiative transition probabilities (AT), radiative lifetimes (τ) and branching ratios (β) for the excited states of Pr3+ ion were estimated through formulas given in Supplementary Material (Eqs. (3), (5) and (6)). Table 3 reports total radiative transition probabilities (AT), radiative lifetimes (τ) of excited states 3P1, 3P0, 1D2, 1G4 and 3F3 of Pr3+ ion and branching ratios (β) of

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our glass. It is observed that the trend of lifetimes decreases as 1G4 > 3F3 > 1D2 > 3P0 > 3P1. The fluorescence branching ratio, βR, predicts the relative intensity of lines from given excited states and characterizes the lasing potential of that particular transition. There is a decreasing tendency of branching ratios for the emission transitions, 3F3→3H4 > 1G4→3H5 >1D2→3F4 > 3P0→3F2 > 3P0→3H4 > 3P1→3F3. Thus, the stronger fluorescence intensity may be found for 3F3→3H4 and 1G4→3H5 transitions in near infrared region and 3P0→3F2 transition in visible region.

3.4.2 Visible and NIR luminescence spectra The excitation wavelength plays an important role to analyse the emission spectra of rare earth doped glasses. In this way the excitation spectrum was measured by monitoring emission at 645 nm for the Pr3+ ions in Pr3+/Yb3+ co-doped TBZLN glass as shown in Fig. 5(a). Three characteristic excitation bands corresponding to 3H4→3P0 (484 nm), 3H4→3P1+1I6 (470 nm) and 3H4→3P2 (447 nm) transition of Pr3+ ion are observed. In order to obtain intense emissions, the band, 3H4→3P2 (447 nm), is used to pump the samples. Luminescence spectra and lifetimes in the wavelength range between 510–750 nm for the TBZLN:Pr3+/Yb3+ glass are shown in Fig. 5(b). The observed emission bands at 529, 544, 615, 646, 683, 706 and 730 nm are assigned to the 3P1+1I6→3H6, 3P1→3H5, 3P0→3H6, 3P0→3F2, 3P1→3F3, 3P1→3F4 and 3

P0→3F4 transitions, respectively. For Pr3+ ion, the energy gap between 3P multiplet excited

states is very small. Therefore, excited 3P2 state can decay very fast to lower excited states, 3

P1 and 3P0, through non-radiative relaxation. In the sequence these states are depopulated via

radiative recombination to lower states of Pr3+ (see Fig.6). Moreover, few ions are depopulated from 3P0 to 1D2 state by non-radiative relaxation followed by subsequent orange emission at 590 nm due to 1D2→3H4 transition. This orange emission is not well resolved as found in other host systems such as borate, phosphate and silicate glasses[51-53].

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The non-radiative transitions can take place due to energy transfer and multiphonon processes. The multiphonon relaxation rate has been reported for a number of crystals and glasses and is given by[54]: WMRP = Cexp(–α∆E)(n+1)p (5) where C and α are positive, host dependent constants which can be taken from Ref. 66. The n = 1/(exp(ħω/KT)–1) and p = ∆E/ħω, where p is the effective phonon number, ∆E is the energy gap between two successive electronic levels and ħω is the phonon energy of host glass. The above equation concluded that the host glass matrix play an important role on multiphonon relaxation rates. It is observed that the multiphonon relaxation rate for 3P1 and 1

D2 states are 3270 s–1 and 4.83 × 10–20 s–1 in TBZLN glass and are lower compared with

other glasses[52]. This indicates that negligible phonon assisted depopulation from 3P0 to 1D2 within the delay time occurs. Thus, a strong 613 nm emission was observed concomitantly with negligible 590 nm emission (see Fig. 5(b)). The inset of Fig. 5(b) shows the measured lifetime for the Pr3+:3P0 and 3P1 levels. As both levels are very close, they are possibly thermally coupled and present the same lifetime, 2.54 µs. Considering the 1D2 →3H4 emission, the high Pr3+ concentration and the high probability of occupation of level 1D2 leads to different relaxation mechanisms (migration and cross-relaxation)[14] as can be depicted in Fig. 6. Near-infrared (NIR) emission spectrum in the wavelength range 1200–1500 nm by pumping sample at 980 nm, is shown Fig. 5(c). The observed band position at 1334 nm is related to the 1G4 → 3H5 transition of Pr3+ ions. The Yb3+:2F5/2 level is slightly bigger than Pr3+:1G4 level and the absorption cross-section for the former is higher than that of the latter. Therefore, the 1G4 level is populated by transfer of energy from the Yb3+, as shown in the inset (left) of Fig. 5(c). The full-width at half maximum (FWHM) of the 1G4 → 3H5 emission transition is 96 nm for the TBZLN glass and is comparable with the oxyhalied tellurite glass

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(100 nm)[55]. The inset (right) of Fig. 5(c) shows the measured lifetime of the Pr3+:1G4 level, given by 3.60 µs. Based on the known radiative transition probabilities (from J-O theory) for the observed emission (visible and NIR) transitions, the stimulated emission cross-section (σemi) is calculated using the formula given in Ref.15 which is presented in Table 5 along with some emission properties. The higher cross-section of 3P0→3F2 (among visible transitions) and 1G4→3H5 transitions are most suitable for laser action. Fig. 7 exhibits the lifetime of the Yb3+: 2F5/2 level in the Pr3+ /Yb3+ and Yb3+ doped glasses. Clearly, there is a significant reduction of the lifetime from 491 µs (Yb3+) to 60 µs (Pr3+/Yb3+), in the presence of Pr3+. It is important to note that the lifetime of the Yb3+ doped sample corresponds practically to the radiative lifetime (as shown in Table 1), indicating that phonons are not a significant path for nonradiative losses for the Yb3+: 2F5/2 → 2F7/2 transition. It can be concluded that Yb3+ is transfering energy to Pr3+ ions, and the estimated energy transfer efficiency ηET = 1 – τYb/τYb–Pr corresponds to about 85%.

3.4.3. Upconversion luminescence spectra Fig. 8(a) shows room temperature upconversion luminescence of Pr3+/Yb3+ doped TBZLN glass at 980 nm excitation. The observed emission bands around 486, 531, 618, and 648 nm are due to 3P0→3H4, 3P0→3H5, 3P0→3H6 and 3P0→3F2 transitions. The pumping mechanism is accomplished by means of combination of ground state absorption (GSA) of Yb3+ sensitizer, energy transfer (ET) from Yb3+→Pr3+ and excited state absorption (ESA) of Pr3+ acceptor, as shown in Fig. 8(b). In the case of Pr3+, the direct excited state absorption (ESA) of Pr3+:1G4 level is difficult because of small-excited state population and short lifetime of 1G4 level. Thus, as a first step, we excite the Yb3+ ions to 2F5/2 level upon 980 nm excitation. This is followed by non-resonance energy transfer to a neighbour Pr3+:1G4 level (ET: 3H4+2F5/2→1G4+2F7/2). Subsequently, Pr3+ ions are promoted to the 3P0 excited level by the absorption of a second photon, via excited state absorption, ESA: 1G4+2F5/2→3P0+2F7/2.

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Finally, the excited Pr3+ ions at 3P0 level decay radiatively to generate visible light corresponding to 3P0→3H4, 3P0→3H5, 3P0→3H6 and 3P0→3F2 transitions, respectively [26, 56-58]. In order to further understand the energy transfer mechanism, the pump power dependence of up-conversion luminescence was analysed considering that the up-converted 7 emission intensity Iup is proportional to mth power of IR excitation intensity IIR, i.e. 234 256 ,

where m is the number of IR photons involved in the process. Fig. 9 illustrates a plot of logarithmic emission intensities in function of the logarithmic pump power. It can be seen that the fitted slopes found for the 486, 531, 618 and 647 nm bands are 1.07, 1.41, 1.20 and 1.09, respectively. From Fig. 8(b), the observed four visible up-conversion transitions occurs due to the aid of two photons. One comes from the Yb3+ excitation and the second comes through the absorption process of Pr3+ excited state. According to Pollnau et al.[59], in the case of three level system, when linear decay of the intermediate state is the dominant depletion mechanism Iup is proportional to Iexc2 and when the upconversion mechanisms (ESA and ETU) are dominant Iup is proportional to Iexc. Therefore the competition between linear decay and upconversion processes, concerning the depletion of the intermediate excited states, results in a reduction of Fig. 8 slope at higher pump power. This is our case as the observed slope, m < 2.

3.4.4. CIE chromaticity coordinates Considering the above visible luminescence spectra, the Pr3+/Yb3+ codoped TBZLN glass exhibited red-green-blue (RGB) light emissions (except the blue emission absence under 447 nm). Based on the RGB emissions, it is worth to know better the color evolution. In the Commission Internationale de L’Eclairage (CIE), chromaticity coordinates are the standard reference for color definition considering the sensitivity of the human eye. The CIE coordinates have been calculated from visible emission spectra and are found to be (x = 0.541, y = 0.450) and (x = 0.245, y = 0.376) for the Pr3+/Yb3+ co-doped TBZLN glass. The

14

CIE coordinates fall in yellow-orange (λexc = 447 nm) and near white light (λexc = 980 nm) color regions, as shown in Fig. 10. These results suggest that the Pr3+/Yb3+ co-doped TBZLN glass is favourable candidate of yellow-orange and white light emitting glass for photonic devices.

4. Conclusions In summary, Raman spectra have been used to understand the bending and stretching modes of TeO2 in TBZLN glasses. The OH– - concentration was estimated for the Yb3+ doped and Pr3+/Yb3+ co-doped systems using FT-IR spectra. The decrease trend of optical energy bandgaps, 3.15 eV (Yb3+) → 2.89 eV (Pr3+/Yb3+) increase the non-bridging ion concentration with addition of RE3+ ions. Racah (E1, E2 and E3), spin-orbit interaction (ξ4f) and hydrogenic ratios (E1/E3 and E2/E3) for Pr3+ ion in Pr3+/Yb3+ co-doped system are estimated by the interpretation of free-ion model and are compared with other host glasses. For Yb3+ doped TBZLN glass, spectroscopic and laser parameters such as absorption and emission ((σabs(λp) and (σemi(λ0)) cross-sections, minimum fraction of Yb3+ ions (βmin) and depletion of storage energy (Usat) were reported. The observed σabs(λp) = 3.35 × 10–20 cm2 and σemi(λ0) = 0.78 × 10–20 cm2 for the studied glass is higher than other host glasses. For Pr3+/Yb3+ co-doped TBZLN glass, using absorption spectra of Pr3+ in Pr3+/Yb3+, positive J-O (Ωλ, λ = 2, 4 and 6) intensity parameters were estimated with and without consideration of the 3H4→3P2 hypersensitive transition. Ωλ have been used to calculate radiative transition probabilities (AT), radiative lifetimes (τ) and branching ratios (β) for the excited states of Pr3+ ion. Several emission transitions of Pr3+ were observed upon 447 nm excitation and NIR emission at 1334 nm is observed when exciting at 980 nm in Pr3+/Yb3+ co-doped glass. The 3P0→3F2 (among visible transitions) and 1G4→3H5 emission transitions possess high stimulated emission cross-sections and are most suitable for laser action. Under 980 nm excitation, the upconversion visible luminescence was observed in Pr3+/Yb3+ co-

15

doped TBZLN glass. The possible emission transitions and mechanisms have been discussed from schematic energy level diagram. Using visible emission intensity, the CIE coordinates were determined (x = 0.541, y = 0.450) or (x = 0.245, y = 0.376) when sample is excited with 447 nm or 980 nm, respectively. These coordinates fall in yellow-orange and near white light color regions. Finally, some optical properties of Yb3+ doped and Pr3+/Yb3+ co-doped TBZLN glasses were obtained in order to evaluate the potentiality as a laser material.

Acknowledgments The authors like to thankfully acknowledge Brazilian Funding Agencies: CNPq, CAPES and FAPEMIG. M. Seshadri thanks the support of Physics Department - UFJF by the PNPD-CAPES Postdoctoral fellowship. Prof. V. Anjos with this article honors the memory of Renata Ferreira who left us so early.

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Figure captions

19

Fig. 1. Raman spectrum of Yb3+ doped TBZLN glass with deconvolution by symmetrical Gaussian functions.

Fig. 2. FT-IR spectra of doped TBZLN glasses. Inset of figure shows deconvoluted absorption coefficient of Yb3+ doped TBZLN glass.

Fig. 3. Optical absorption spectra of Yb3+ doped and Pr3+/Yb3+ co-doped TBZLN glasses.

20

Fig. 4. (a) Deconvolution absorption spectrum of Yb3+ ions; (b) Absorption cross-section and emission cross-section spectra for Yb3+ doped TBZLN glass. λp and λ0 are the primary and secondary peak positions.

Fig. 5. Photoluminescence spectra of Pr3+ in Pr3+/Yb3+ co-doped TBZLN glass. (a) Excitation spectrum (λemi = 645 nm); (b) Visible emission spectrum. Inset shows decay curve for for the Pr3+:3P0 and 3P1 levels (λexc = 447 nm); (c) NIR emission spectrum. Inset of figure (left) show possible transfer of energy from Yb3+→ Pr3+, and inset of figure (right) show decay curve for the 1G4 level (λexc = 447 nm).

21

Fig. 6. Schematic energy level diagram of Pr3+, visible emissions with and possible crossrelaxation channels.

Fig. 7. Decay curves of the Yb3+:2F5/2 level for the Yb3+ doped and Pr3+/Yb3+ codoped TBZLN glasses (λexc = 365 nm).

Fig. 8. Upconversion spectra of Pr3+/Yb3+ co-doped TBZLN glass (a) and energy level diagram for NIR to Visible luminescence with possible energy transfer mechanisms (b).

22

Fig. 9. A plot of lg (intensity) vs lg(power) for blue, green and red emission peaks in Pr3+/Yb3+ co-doped TBZLN glass upon λexc = 980 nm.

Fig. 10. CIE diagram of overall emission of Pr3+/Yb3+ co-doped TBZLN glass under 447 nm and 980 nm excitations.

23

Table 1 Spectroscopic properties of Yb3+ doped glasses. Glass

σa(λP) –20

TBZLN* TWZYb10 [22] TWZ4 [32] TPNY1 [33] TBaBaFLa [34] BPYb10 [35] Borophosphate (x = 0) [36] Borophosphate (x = 11) [36] Borophosphate (x = 22) [36] PKBAYb10 [37] PKFBAYb10 [38] PKSAYb10 [39] QX-Kigre [40] GP [41] Lead fluoroborate [42] Heavy metal oxide [42] * Present work.

2

(×10 cm ) 3.35 1.50 -1.20 1.93 2.88 1.89 2.00 1.99 1.47 1.28 1.85 0.50 1.20 2.50 2.20

σe(λo) –20

2

(×10 cm ) 0.78 -1.32 --0.52 0.35 0.34 0.39 -0.61 -0.70 0.60 0.22 0.13

24

Arad (s–1) 2002 1822 3004 -2701 715 ---521 511 812 --3515 3000

βmin 0.332 0.190 ---0.281 0.150 0.146 0.146 -0.55 0.157 0.171 -0.171 0.148

Table 2 Oscillator strengths (fexp and fcal) and Ωλ (λ = 2, 4 and 6) parameters for Pr3+ ions in various host glasses. Transitions

3

→ 3F2 3 F3+3F4 1 D2 3 P0 3 1 P1+ I6 3 P2

Oscillator strengths (×10–6) TBZLN* TWPLa [47] fexp fexp fcal fcal+

2.48 2.49 5.90 5.81 6.83 6.73 1.09 1.09 1.87 1.87 2.16 -1.22 0.04 δrms J-O intensity parameters (×10–20 cm2) 4.94 5.00* Ω2 2.69 2.69* Ω4 5.51 5.42* Ω6 13.14 13.11 Σ Ωλ H4

2.33 5.42 1.42 1.38 1.44 4.33

ZnBPr [48] fexp

L5BP [49] fexp

LBTAF [50] fexp

6.01 12.37 3.34 2.29 1.99 --

2.02 5.36 1.53 1.55 2.89 4.55

1.47 4.09 0.92 1.87 2.52 5.28

4.01 7.88 1.29 2.56 2.60 --

4.77 1.21 5.16 11.14

1.52 3.85 6.97 12.34

0.13 4.61 3.92 8.66

2.42 2.54 4.23 9.19

*

Present work. Oscillator strengths (fcal) without considering the 3H4→3P2 (HST).

+

25

Table 3 Matrix elements (89 ( ) 82), Energies (ν) (cm–1), electric dipole line strengths (Sed×10–22/e2) (cm2), radiative transition probabilities (Ared) (s–1) and branching ratios (β) for excited states of Pr3+ ion in Pr3+/Yb3+ co-doped TBZLN glass. ‖9 ; ‖

‖9 < ‖

P1→ 3P0 1 D2 1 G4 3 F4 3 F3 3 F2 3 H6 3 H5 3 H4

0 0.0825 0 0 0.5714 0.2683 0 0 0

0 0 0.0843 0.2621 0.1964 0 0 0.2857 0.1714

0 0 0 0 0 0 0.1246 0.0893 0

P0→ 1D2 1 G4 3 F4 3 F3 3 F2 3 H6 3 H5 3 H4

0.0148 0 0 0 0.2954 0 0 0

0 0.0559 0.1075 0 0 0 0 0.1719

0 0 0 0 0 0.0726 0 0

D2→1G4 3 F4 3 F3 3 F2 3 H6 3 H5 3 H4

0.2937 0.6077 0.0332 0.014 0 0 0.0022

0.0519 0 0.0187 0.0891 0.0711 0.0022 0.0181

0.0779 0.0201 0 0 0.0066 0.0004 0.0534

G4→3F4 3 F3 3 F2 3 H6 3 H5 3 H4

0.0785 0.0041 0.0001 0.2583 0.0375 0.0012

0.1448 0.0059 0.0154 0.2592 0.0997 0.007

0.3513 0.0509 0.0063 0.2429 0.4169 0.0258

F3→3F2 3 H6 3 H5 3 H4

0.0209 0 0.6285 0.0653

0.0509 0.3182 0.3467 0.3465

0 0.8459 0 0.6982

3

3

Sed (×10–20)

‖9 ‖

Transition

1

1

3

ν

(cm–1)

Arad (s–1)

0 4314.68 0 41.25 11315.9 1382.6 22.68 14202 1502.8 70.5 14660.4 5138.6 338.53 16025.8 32231.1 134.15 16867.7 14892.7 68.65 19049.9 10978.3 126.06 21132.7 27520.6 46.11 21132.7 10066.4 –1 ΣArad = 103713 s ; τR = 9.64 µs 7.4 3758.1 27.3 15.04 10759.3 1300 28.92 13645.4 5099.2 0 14103.8 0 147.7 15469.3 37943.1 40 16311.1 12046.4 0 18493.3 0 46.24 20576.1 27954.6 –1 ΣArad = 84371 s ; τR = 11.85 µs 203.73 7001.21 970.3 314.93 9887.27 4224.5 21.63 10345.7 332.4 30.97 11711.2 690.4 22.76 12553 624.8 0.81 14735.2 36 35.39 16818 2336.4 –1 ΣArad = 9215 s ; τR = 108.52 µs 271.77 2885.24 50.3 31.68 3343.7 9.1 7.66 4709.12 6.2 332.71 5551 438.8 275.28 7733.2 981.6 16.7 9816 121.8 ΣArad = 1608 s–1; τR = 621.97 µs 24.14 1365.42 0.6 551.69 2207.3 58.8 407.51 4389.5 341.8 510.57 6472.3 1372.7 ΣArad = 1744 s–1; τR = 563.73 µs

26

β 0 0.013 0.014 0.05 0.311 0.144 0.106 0.265

0 0.015 0.06 0 0.45 0.143 0 0.331 0.105 0.458 0.036 0.075 0.068 0.004 0.254 0.031 0.006 0.004 0.273 0.61 0.076 0 0.033 0.193 0.774

Table 4 Emission peak wavelength (λP), radiative transition probability (Arad), effective linewidth (∆λeff), and stimulated emission crosssection (σemi) for emission transitions of Pr3+ in Pr3+/Yb3+ codoped TBZLN glass. Transitions

λP Arad (nm) (s )

∆λeff (nm)

σemi×10–20 (cm2)

529 544 615 646 683 706 730 1334

9.08 23.97 13.11 7.65 20.46 19.99 10.67 106.47

7.15 0 3.96 26.03 10.33 5.58 4.09 0.88

–1

P1→3H5 3 P0→3H5 3 P0→3H6 3 P0→3F2 3 P1→3F3 3 P1→3F4 3 P0→3F4 1 G4→3H5 3

27521 0 12046 37943 32231 14893 5099 982

Graphical Abstract Power dependent upconversion luminescence in Pr3+/Yb3+ co-doped TeO2-Bi2O3-ZnO-Li2ONb2O5 (TBZLN) tellurite glass sample under 980 nm laser excitation.

27

Conflicts of interest The authors declare that there is no potential conflicts of interest regarding the publication of this paper.