Enhanced frequency upconversion in Er3+-Yb3+ ...

Methods and Applications in Fluorescence

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Enhanced frequency upconversion in Er3+-Yb3+ codoped heavy metal oxides based tellurite glasses To cite this article before publication: Vineet Kumar Rai et al 2018 Methods Appl. Fluoresc. in press https://doi.org/10.1088/2050-6120/aaa5e8

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Enhanced frequency upconversion in Er3+-Yb3+ codoped heavy metal oxides based

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tellurite glasses

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Mohd Azam, Vineet Kumar Rai*

Laser and Spectroscopy Laboratory

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Department of Applied Physics

Indian Institute of Technology (Indian School of Mines) Dhanbad, 826004

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Jharkhand, India

*Author’s to whom correspondence is made: [email protected]; [email protected]

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AUTHOR SUBMITTED MANUSCRIPT - MAF-100378.R1

Phone. No.: +91-326-223-5404


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Abstract The spectroscopic investigations on the Er3+/Yb3+ ions doped/codoped TeO2-ZnO (TZ), TeO2-

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ZnO-WO3 (TZW) and TeO2-ZnO-WO3-TiO2 (TZWTi) heavy metal oxide glasses have been made. The absorption, photoluminescence, decay curve and Judd-Ofelt analysis have been

performed to optimize the optical properties of the Er3+/Yb3+ ions. The effect of incorporation

of heavy metal oxides like WO3 and TiO2 in the Er3+/Yb3+ doped/codoped TZ glass on its

optical properties have been investigated. The enhancement in upconversion (UC) emission intensity has been explained on the basis of efficient energy transfer and inhomogeneous local field generation around the rare earth ions. The spectroscopic quality factor, absorption and

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stimulated emission cross-sections, optical gain, quantum efficiency (~17.53%), energy transfer efficiency (~61.64%), colour purity (~94.7 %) and ionic nature of the bonding have been determined. The Er3+-Yb3+-TZWTi glass can be used in visible lasers, yellowish green

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optical devices and home appliances.

Keywords: Glass, Quantum efficiency, Upconversion, Judd-Ofelt analysis, Nephelauxetic

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ratio, Colour purity.

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1. INTRODUCTION Frequency upconversion is a nonlinear, anti-stokes emission process of combining two or more

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low energy photons to observe the emitted radiation of higher energy photons. Frequency upconversion (UC) in the rare earth ions (REs) doped glassy and crystalline materials have

been extensively studied by the researchers due to their wide photonic applications [1, 2]. The spectroscopic properties of REs doped solid materials provide the position of energy levels, absorption and stimulated emission cross-sections, non-radiative and radiative relaxation rates,

transition probabilities, and branching ratios of different excited states for the development of lasers, upconverters, optical fibre amplifiers, etc. Among the lanthanides, the erbium (Er3+) ion

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provides the infrared (IR) luminescence at ~ 1.53 μm for optical amplification and visible light for solid state visible laser applications. The Er3+/Yb3+ ions doped/codoped glassy, crystalline materials and nanocrystalline (NC) materials have many potential applications as optical data

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storage, colour display devices, optical temperature sensors, biomedical diagnostics, efficient nanotransducer for NIR light to enhance the energy transfer efficiency and undersea optical transmission, etc. [3–13]. The sequential growth of CaF2:Yb, Er@CaF2:Gd nanoparticles for efficient magnetic resonance angiography and tumor diagnosis has been studied by Kun Liu et

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al. [14]. Rumin Li et al. reported that the core-shell structured Gd2O3:Ln@mSiO2 hollow nanospheres shows strong and multicolour emissions upon excitation with ultraviolet and NIR radiations and they concluded that the developed materials have wide applications in biomedical field [15]. During the investigation of good UC materials, researchers are looking for the host materials having low phonon energy, high refractive index and good glass stability [8, 9]. Due to the lower phonon energy (≤ 800 cm-1), the HMO based tellurite glasses reduce

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the non-radiative multiphonon relaxation of the excited RE ions which improves the radiative light emission. The Er3+ doped HMO based glasses are most interesting because they can easily be pumped by photons of wavelength around 800–1000 nm region. Apart from this, the concentrations of RE ions play an important role because at high concentration the distance between the RE ions become smaller and causes concentration quenching [16]. The HMO

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based glasses are non-hygroscopic in nature and highly stable in comparison to crystalline materials. Among the HMO hosts, the tellurite based glasses exhibit more interesting physical and optical properties. The tellurite based glasses possess high refractive index, better thermal stability as well as chemical durability as compared to fluoride and chalcogenide glasses. They

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have high RE ions solubility and lower melting temperature as compared to borate, phosphate


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and silicate glasses [17-25]. Table 1 shows the comparison of tellurite based glasses to other glasses in terms of phonon energy, refractive index and melting temperature.

refractive index and melting temperature. Phonon

Refractive

Melting

energy (cm-1)

index

temperature

Tellurite based glass

~800

1.97 – 2.14

Fluoride Glass

~ 510

1.49 - 1.65

450–600 0C

[17, 19]

Chalcogenide glass

~ 450

~ 2.4

> 900 0C

[18, 20]

Borate glass

~ 1400

1.53 – 1.60

1100 0C

[22, 23]

Silicate glass

~ 1050

1.59 – 1.61

1400 0C

[23, 24]

Phosphate glass

~ 1280

1.53 – 1.56

1350 0C

[23, 25]

Present work

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~ 800 0C

Reference

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Glasses

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Table 1. Comparison of the tellurite based material to other glasses in terms of phonon energy,

The Er3+ ions having lower absorption cross-section corresponding to the 4I11/2←4I15/2 transition show poor luminescence under NIR excitation. Therefore, to enhance the luminescence

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intensity further a suitable sensitizer with high absorption cross-section corresponding to the NIR excitation is needed [26–32]. In the Er3+/Yb3+ codoped solid materials, Yb3+ ion shows a large absorption cross-section and a broad absorption band (850 – 1080 nm) corresponding to the 2F7/2→2F5/2 absorption transition compared to the weak absorption band (4I15/2→4I11/2) of Er3+ ion [33].

In the present study, TeO2-ZnO (TZ), TeO2-ZnO-WO3 (TZW), TeO2-ZnO-WO3-TiO2

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(TZWTi) glasses doped/codoped with Er3+/Yb3+ have been prepared by simple melting and quenching method. Judd-Ofelt theory has been applied to investigate the various the various radiative parameters and type of bonding involved between rare earth ions to oxygen atoms discussed. The frequency UC emission behaviour and processes involved upon 980 nm diode laser excitation have been discussed in detail. On changing the glass compositions and hence

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the optical and physical properties of the glass changes [3, 34–36]. Therefore, the incorporation of good glass modifiers such as ZnO, WO3 and TiO2 in the glass former (i.e. TeO2) may change the spectroscopic properties of the glass. Because, this may improve the glass stability, refractive indices, radiative properties and also the RE ions solubility [37 –40].

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2. EXPERIMENTAL STUDY Er3+/Er3+-Yb3+ doped/codoped heavy metal oxide (HMO) based tellurite glasses have been

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synthesized through melting and quenching technique [26, 41]. The chemical compositions of the glasses and their respective codes are given in Table 2. The reagents TeO2, ZnO, WO3, TiO2, Er2O3, Yb2O3 of high purity (in mol %) are used as starting raw materials [42]. To synthesize the Er3+/Er3+-Yb3+ doped/codoped TZ, TZW and TZWTi glasses, the starting raw

materials in powder form were weighed to get 3 gram of each glass compositions and ground in an agate mortar for 1.5 hours to get fine and homogeneous mixture. The raw mixed material

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compositions of Er–TZ, Er–Yb–TZ, Er–Yb–TZW and Er–Yb–TZWTi were placed Table 2 Glass compositions and their respective codes. Glass code

Glass Composition

(80-x) TeO2 + 20ZnO + xEr2O3, x= 1.0 (in mol%)

Er–Yb–TZ

(80-x-y)TeO2+20ZnO + xEr2O3+ yYb2O3, x =1.0, y=1.0 (in mol%)

Er–Yb–TZW

(74-x-y)TeO2+20ZnO+6WO3+ xEr2O3+ yYb2O3, x =1.0, y =1.0 (in mol%)

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Er–TZ

(69-x-y)TeO2+20ZnO+6WO3+5TiO2+xEr2O3+yYb2O3, x=1.0, y=1.0 (in

TZWTi

mol%)

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Er–Yb–

in the alumina crucibles and placed into a high temperature electric furnace one by one for melting purposes at 8000C, 8500C, 9000C and 9250C respectively until the whole composition

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was converted into the transparent liquid. The resultant melt was quenched quickly by pouring in the preheated circular mould brass plate and covered with preheated flat brass plate. These synthesized circular glasses have been polished carefully to remove the surface roughness and to increase the optical quality [41].

To measure the refractive indices of all the prepared samples, the Brewster’s angle polarization

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technique has been used by using the He-Ne laser source of wavelength 632.8 nm and output power of 5 mW. The buoyancy method based on Archimedes’ principle was used to calculate the densities of all the prepared glasses by using Xylene as an immersion liquid. The absorption spectra of all the doped/codoped TZ, TZW and TZWTi glasses respectively have been recorded

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in the wavelength 400 – 2200 nm region by using a double beam UV-Vis-NIR spectrophotometer having spectral bandwidth of 0.5 nm. The frequency UC emission


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measurements of all the prepared samples have been recorded upon 980 nm laser diode excitation by using a monochromator equipped with a photomultiplier tube (PMT). The lifetime measurements have been performed through the same monochromator attached with

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PMT and a digital storage oscilloscope. All the measurements have been performed at room temperature. 3. RESULTS AND DISCUSSION

3.1. Study of absorption spectra and Judd-Ofelt (J-O) analysis

Optical absorption spectra of the optimized 1.0 mol% Er3+/1.0 mol% Er3++1.0 mol% Yb3+ ions doped/codoped TZ, TZW and TZWTi glasses have been recorded in the wavelength range 400

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– 2200 nm (Fig. 1). Each spectrum of the singly Er3+ doped glasses consists of eight number of absorption bands centered at ~449 nm, ~489 nm, ~522 nm, ~546 nm, ~654 nm, ~797 nm, ~980 nm and 1519 nm for Er3+ ions corresponding to the transitions from the ground state

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(4I15/2) to different excited states viz. 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2 respectively. But in the Er3+-Yb3+ codoped glasses, along with the above said bands the band centered at ~980 nm appears broader as compared to the singly Er3+ doped glasses [26, 42, 43].

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The observed broad absorption band in the codoped glasses around ~ 980 nm is due to the superposition of two absorption bands corresponding to the 2F7/2 →2F5/2 (Yb3+) and 4I15/2 →4I11/2 (Er3+) transitions. The observed absorption bands in all the prepared glasses are inhomogeneously broadened due to site to site variations in the crystal field strengths. Also, the absorption band positions in the glasses arising from the RE ions do not vary due to the shielding effect created by 5s2 and 5p6 completely filled orbitals [16, 26, 43]. The band corresponding to the 4I15/2 →2H11/2 (~522 nm) transition is highest among the other observed

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absorption bands. This is due to the hypersensitive nature of the transition because, the intensity of the hypersensitive transition changes dramatically by the slight change in the surrounding

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environment which obeys the selection rule as |∆L| ≤ 2, |∆J| ≤ 2 and sometimes |∆S| ≤ 0 [44].

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Fig.1 Optical absorption spectra of optimized 1.0 mol% Er3+/1.0 mol% Er3++1.0 mol% Yb3+

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ions doped/codoped (a) Er-TZ (b) Er-Yb-TZ (c) Er-Yb-TZW and (d) Er-Yb-TZWTi glasses with assignments of various absorption transitions.

The intensity of f-f transitions of REs ions in the glass matrices can be easily determined by

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using the of J-O theory [45, 46]. According to this theory, the J-O intensity parameters Ωλ (λ= 2, 4, 6) of Er3+ ions in all the TZ, TZW and TZWTi glasses have been calculated by using the respective absorption spectra shown in Fig. 1. These intensity parameters are very important to calculate the various spectroscopic parameters such as the transition probabilities, absorption and stimulated emission cross-sections, electric dipole line strengths, radiative lifetime, optical gain and branching ratios, etc. of the excited states. The experimental and calculated oscillator

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strengths play a key role to calculate these J-O intensity parameters. The experimental oscillator strengths for the several observed absorption bands have been calculated by using the following formula as [26, 41],

𝑚𝑐 2 𝜇 2 𝜋𝑒 2 𝜒𝑁𝑅𝐸

∫ 𝑄(𝜈)𝑑𝜈

(1)

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fexp =

where, ‘∫ 𝑄(𝜈)𝑑𝜈 ’ represents the integrated area covered by the corresponding absorption bands shown in Fig.1. The parameters ‘m’ and ‘e’ represent the values of the mass and charge of the electron; ‘c’ is the velocity of light in vacuum. ‘χ’ [=(μ2 + 2)2/9μ] represents the local

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field correction factor, where ‘μ’ is the refractive index of the corresponding glass matrix. The refractive indices of all the polished glass samples have been determined experimentally


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through Brewster’s angle polarization method by using the He-Ne laser source of 5.0 mW power and lasing at 632.8 nm [26]. In this experiment, the polished glass samples of definite thickness were placed on the rounded circular table one by one to observe the appropriate

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polarizing angles (θp) at which the reflected ray spot was found of minimum intensity. By applying the expression μ = tan(θp), the values of refractive indices of all the TZ, TZW and

TZWTi glasses were determined. The calculated values of local field correction factors,

polarizing angles and refractive indices of all the prepared samples are listed in Table 2. The term ‘NRE’ represents the RE ions concentration present in all prepared glasses, it was calculated by using the expression as [41],

NA × ρ × mol% of rare earth ions

(2)

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NRE (ions/cm3) =

M

where, ‘NA’ is Avogadro’s number, ‘M’ and ‘ρ’ denotes the average molecular weight (g/mol)

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and the density (g/cm3) of the prepared glass samples. The glass density represents the geometrical configuration and the degree of structural compactness of the glass network. The density of all the glass samples were determined by using the buoyancy method based upon Archimedes’s principal having Xylene (density ~ 0.86 g/cm3) as an immersion liquid [41]. The

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calculated values of the RE ions concentration (NRE), molecular weight (M), densities (ρ) and corresponding physical parameters for different doped/codoped glasses are shown in Table 3. All the parameters vary gradually on introducing the WO3 and TiO2 in the TZ glass matrix. This may be due to change in the structure of the obtained glass [3]. Table 3 Measured values of density, polarizing angles, refractive indices, local field correction factors, rare earth ions concentrations, molecular weights, molar volume (Vm), ionic

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concentration of cations (Ni), molar polarizability (αm) and other related parameters for1.0 mol% Er3+/1.0 mol% Er3++1.0 mol% Yb3+doped/codoped glass samples.

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Physical Parameters

Er-TZ

Prepared glass samples Er-Yb-TZ

Er-Yb-TZW

Er-YbTZWTi

Density ’ρ’ (in g/cm3)

5.23

5.25

5.36

5.41

Polarizing angle (θp)

630 06`

630 12`

640 36`

650 54`

Refractive index (μ)

1.97

1.98

2.11

2.14

Field factor (χ)

1.95

1.97

2.18

2.43

NRE ×1020 (ions/cm3)

2.15

2.13

2.11

2.14

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146.21

148.56

152.89

152.10

Vm , (cm3/mol)

27.96

28.30

28.52

28.11

Ni (×1021 cm-3)

8.62

8.51

8.44

8.57

Ionic distance (R) (Ǻ)

4.88

4.90

4.91

4.89

Reflection loss (RL)

0.11

0.11

0.13

0.13

Molar refraction (Rm)

6.83

6.97

7.70

7.74

αm, (Ǻ3)

2.71

2.77

3.06

3.07

Metallization (Mc)

0.76

0.75

0.73

0.72

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Molecular weight (g/mol)

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Theoretically, the oscillator strengths (fcal) of an electric dipole absorption transition from the initial state < 𝛹𝐽 | to the excited state |𝛹′𝐽′ > can be calculated as [26, 41], 8𝜋2 𝑚𝑐𝜈 3ℎ (2𝐽+1)

2

(𝜇2 + 2)

[

9𝜇

] Sed (< 𝛹𝐽 ||𝛹 ′𝐽′ >)

(3)

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fcal =

where, ‘J’ is the total angular momentum of the ground state, (2J+1) represents the degeneracy of the ground state of Er3+ ion. The term ‘ν’ is the value of energy difference between the initial

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state (ground state) < 𝛹𝐽 | to the excited state |𝛹′𝐽′ > (in cm-1). ‘h’ represents the value of Planck’s constant and other parameters ‘m’ and ‘μ’ have their usual meanings. The symbol ‘Sed’ represents the electric dipole line strength of the absorption transition and can be represented by the relation as [26],

Sed = ∑𝜆=2,4,6 Ω𝜆 | (< 𝛹𝐽 ||𝑈 𝜆 ||𝛹′𝐽′ >)|2

(4)

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where, |Uλ|2 represents the reduced matrix element of Er3+ ions [39] of unit tensor operator of rank ‘λ’ and Ωλ (λ= 2, 4, 6) are the J-O intensity parameters. By using equations (1), (3) and (4), J-O intensity parameters have been calculated with the help of least square fitting approach for Er-TZ, Er-Yb-TZ, Er-Yb-TZW and Er-Yb-TZWTi glasses and compared with Er3+/Er3+-

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Yb3+ doped/codoped boro-tellurite glasses [48]. The values of J-O intensity parameters in different Er3+/Yb3+ doped/codoped glasses are shown in the Supporting Information (SI) Table 1. It has been observed that the intensity parameters of the Er3+/Er3+-Yb3+ doped/codoped TZ glasses are found similar due to the same host and crystal structure. But, on introducing the WO3 and TiO2 in the TZ glass the value of intensity parameters increases gradually. This may

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be due to change in the crystal structure and hence the crystal field strength as well as the physical parameters (Table 3). The value of ‘Ω2’ is very sensitive for the symmetry of the RE


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ions sites and strongly affected through covalency between REs and ligand ions. The value of ‘Ω6’ is closely related to the rigidity of the host in which the RE ions are situated [41, 49]. In the present work, the value of ‘Ω6’ increases from 1.020 × 10-20 cm2 to 2.104 × 10-20 cm2 as

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we move from TZ to TZWTi via TZW glass therefore, the rigidity of the TZ glass gradually

increases on introducing the WO3 and TiO2 modifiers. The ratio between Ω4 and Ω6 (i.e. Ω4/Ω6) is assumed as the spectroscopic quality factor of the host material to claim a good lasing

material among the synthesized glasses. The materials (crystalline or amorphous) having lower

value of the spectroscopic quality factor (Ω4/Ω6), show the higher laser emission intensity [50]. The Ω4/Ω6 value for the Er-TZ, Er-Yb-TZ, Er-Yb-TZW and Er-Yb-TZWTi glasses are found

TZWTi glass may be a good laser material [26, 48].

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to be 0.839, 0.857, 0.840 and 0.791 respectively (SI Table 1), therefore, the Er3+/Yb3+ codoped

By using the calculated J-O intensity parameters, Ωλ (λ= 2, 4, 6) in equations (4) and (5), the

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values of oscillator strengths (fcal) and electric dipole line strengths (Sed) for Er-TZ, Er-Yb-TZ, Er-Yb-TZW and Er-Yb-TZWTi glasses respectively have been calculated and compared with the experimental oscillator strengths (fexp) (SI Table 2). The root mean square deviations to find the accuracy in J-O intensity parameters for all the prepared glass samples have been

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determined by using the following formula [41], δrms = [

∑(𝑓𝑒𝑥𝑝 − 𝑓𝑐𝑎𝑙 )2 1/2 N−3

]

(5)

where, the summation is used over the absorption bands assumed to determine the J-O intensity parameters and ‘N’ denotes the number of absorption bands taken into consideration. The

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calculated values of ‘δrms’ for all of the prepared glasses are listed in SI Table 1 and also compared with Er3+/Yb3+ doped/codoped boro-tellurite glasses [48]. The J-O intensity parameters have been further utilized to calculate the various spectroscopic parameters such as spontaneous emission transition probabilities, total transition probabilities,

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radiative lifetime and branching ratios, etc. [26, 41, 50]. The numerical values of these parameters for the given transitions from different excited levels to lower lying levels of Er3+ ions in all the doped/codoped glasses are shown in SI Table 3. From SI Table 3, it has been found that the transition probability and the total transition probability gradually increases from Er-TZ glass to Er-Yb-TZWTi glass via TZW glass.

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3.2. Absorption and stimulated emission cross-section analysis The absorption cross-sections for the stimulated emission cause can be determined by using

σa (cm ) = 2

A (ΨJ ;Ψ′ J′ ) 8𝜋𝑐𝜇2 𝜈2

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the following Fuchtbouer- Landenberg equation [41, 51],

(6)

where, ‘ν’ and ‘A(ΨJ:Ψ’J’)’ represents the energy (in cm-1) between the corresponding

transition levels and spontaneous radiative transition probability of the observed absorption

bands shown in Fig.1 for all the doped/codoped glasses. The absorption cross-sections for the

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three absorption bands corresponding to the 4I15/2→2H11/2, 4I15/2→4S3/2 and 4I15/2→4F9/2 absorption transitions of the Er3+/Yb3+ doped/codoped TZ, TZW and TZWTi glasses have been estimated. The stimulated emission cross-sections for green and red emission bands

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corresponding to the 2H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions of the Er3+/Yb3+ doped/codoped TZ, TZW and TZWTi glasses have been also calculated by using the relation repotted elsewhere [41, 52]. The multiplication of stimulated emission cross-section (σE) to the radiative lifetime (τr) (i.e. σE×𝜏r) is called the optical gain of the material [53]. The numerical

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values of the absorption cross-sections, stimulated emission cross-sections and the optical gain of the Er3+/Yb3+ doped/codoped TZ, TZW and TZWTi glasses are shown in SI Table 4. 3.3. The nature of bonding between Er3+ ions and surrounding oxygen atoms The absorption spectrum of RE ions doped glassy/crystalline material is also an important tool to collect the information about the nature of bonding between the RE ions and its neighbouring

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oxygen atoms. The bonding between RE ions and its surrounding oxygen atoms can be identified by knowing the ‘nephelauxetic ratio’. The observed absorption bands of Er3+ ions in the Er3+/Yb3+ doped/codoped TZ, TZW and TZWTi glasses (Fig. 1) are slightly shifted by modifying the environment around the RE ions on complexion ‘called the nephelauxetic effect’. The change in nephelauxetic effect also affects the positions of the bands corresponding

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to the hypersensitive transitions [54]. The change arises through the expansion of the partially filled 4f-shell followed by the charge transfer from the ligand to the core of central RE ion in different hosts [55]. The nephelauxetic ratio ‘𝛽̅ ’ can be represented in the mathematical form as,

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𝜈𝑐𝑜𝑚𝑝 1 𝛽̅ = ∑ 𝑁

𝜈𝑓𝑟𝑒𝑒𝑖𝑜𝑛𝑠

(7)


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where, ‘νcomp’ and ‘νfreeions ’ represents the value of energies (in cm-1) of absorption bands in Er-TZ, Er-Yb-TZ, Er-Yb-TZW and Er-Yb-TZWTi glasses and free ion respectively [47], whereas, ‘N’ denotes the number of observed absorption bands in these glasses (Fig. 1). The

relation [26, 41], δ= [

̅ 1− 𝛽 ̅ ] 𝛽

and

b1/2 = [

̅ 1− 𝛽 2

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covalency (δ) and the bonding parameter (b1/2) can also be calculated by using the following

1/2

]

(8)

The nephelauxetic ratio (𝛽̅ ), covalency (δ) and bonding parameters (b1/2) [i.e. (𝛽̅ , δ, b1/2) ] of all the Er-TZ, Er-Yb-TZ, Er-Yb-TZW and Er-Yb-TZWTi glasses are found to be (1.0078, -

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0.0077, 0.0624i), (1.0077, -0.0076, 0.0620i), (1.0075, -0.0074, 0.0612i) and (1.0077, -0.0076,

0.0620i) respectively [48]. The bonding between the RE ion and its neighbouring oxygen atom in the host matrix may be either covalent or ionic in nature that depends upon the positive or

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negative value of covalency [41]. In the present case, the value of covalency in all the glasses is found to be negative, thereby justifying the ionic nature of the bonding between the Er3+ ions and oxygen atoms in all the glasses.

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3.4. The UC emission study upon 980 nm laser excitation

The UC emission bands are found to be of maximum intensity in the 1.0 mol% Er3+ /1.0 mol% Er3+ ions + 1.0 mol% Yb3+ combination in TZ, TZW and TZWTi glasses. But beyond this concentration of RE ions the intensity of all the UC emission bands decreases due to the concentration quenching process [26, 42]. The UC emission spectra of the Er3+/Yb3+ doped/codoped TZ, TZW and TZWTi glasses have been recorded by using the 980 nm diode

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laser excitation with low pump power density (i.e. 40.1 W/cm2) at room temperature within 400 – 800 nm wavelength range (Fig. 2). In the singly optimized Er3+ ions doped TZ glass three strong UC emission bands centered at ~532 nm, ~553 nm and ~669 nm corresponding to the 2

H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions respectively have been observed. The

bands corresponding to the 2H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions have the

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radiative transition probability as 7021.45 Hz, 2041.09 Hz and 1927.13 Hz respectively in Er3+ doped TZ glass as shown in SI Table 3. The radiative transition probability corresponding to the 2H11/2→4I15/2 transition is found to be maximum due to hypersensitive transition as compared to other 4S3/2→4I15/2 and 4F9/2→4I15/2 emission transitions (SI Table 3). Therefore, the

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Page 12 of 28

green band having a large value of transition probability is more intense than red band.

Page 13 of 28

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13

Fig. 2 The UC emission spectra of optimized (a). Er-TZ (b) Er-Yb-TZ (c) Er-Yb-TZW, and (d) Er-Yb-TZWTi glasses respectively upon 980 nm diode laser excitation keeping pump power

the samples.

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density ( 40.1 W/cm2). The inset of the figure shows the photograph of emitted colour from all

On codoping with Yb3+ ions in the singly Er3+ ions doped TZ glass, the UC emission bands centered at 532 nm, 553 nm and 669 nm have been enhanced significantly. In the Er3+-Yb3+ codoped TZ glass, enhancement in the green (2H11/2, 4S3/2→4I15/2) and red (4F9/2→4I15/2) UC

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emission bands are found to be ~ 7.6 times and ~ 14.0 times respectively as compared to the Er3+ ions doped TZ glass. The enhancement in the Er3+-Yb3+ codoped TZ glass is due to the efficient energy transfer from the Yb3+ to the Er3+ ions [26]. Since, in the Er3+-Yb3+ codoped TZ glass, Yb3+ ion shows a large absorption cross-section (12.67×10-18 cm2) around 980 nm corresponding to the 2F7/2→2F5/2 absorption transition compared to the absorption cross-section

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of weak absorption band of Er3+ ion (3.17×10-18 cm2) corresponding to the 4I15/2→4I11/2 absorption transition [26, 33]. Therefore, the rate of energy transfer from the excited (2F5/2) level of Yb3+ ion is maximum. On introducing the WO3 and TiO2 in the Er3+-Yb3+ codoped TZ glass, the position of the UC

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emission bands do not show any significant change but the UC emission intensities of the bands corresponding to the 2H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions increase by several


14

folds. The UC emission intensity enhancement for the green band (2H11/2, 4S3/2→4I15/2) in the Er-Yb-TZW and Er-Yb-TZWTi glasses are found to be ~ 12 times and ~ 18 times respectively and simultaneously ~ 29 times and ~ 36.6 times respectively for the red band (4F9/2→4I15/2) as

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compared to the Er3+ doped TZ glass. The enhancement observed for the green and red UC

emission bands in the Er3+-Yb3+ codoped TZW and TZWTi glasses occurs due to the inhomogeneous local field generated after incorporation of WO3 and TiO2 in the TZ glass

around the RE ions, and also by the efficient energy transfer from the Yb3+ ions to the Er3+ ions. In addition to the intensity enhancement the full width at half maxima (FWHM) of the

green and red UC emission bands also increases due the inhomogeneous local field generation around the RE ions caused by the introduction of the WO3 and TiO2 [33] (Table 4). Because

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on introducing the WO3 and TiO2 in the Er3+-Yb3+ codoped TZ glass, the refractive index

increases from 1.98 to 2.11 and 2.14. On account of which the local field correction factor [χ = (μ2 + 2)2/9μ] increases from 1.97 to 2.18 and 2.43 in the Er-Yb-TZW and Er-Yb-TZWTi

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glasses respectively (Table 3). Not only that, the absorption cross-sections (σa) and oscillator strengths for the 4I15/2→2H11/2 and 4I15/2→4S3/2 absorption transitions also increases as compared to that of Er3+/Yb3+ doped/codoped TZ glasses (SI Table 4). The numerical values

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of the absorption cross-section for the 4I15/2→2H11/2 absorption transition are noted to be ~ 6.54×10-18 cm2 in Er-TZ, ~ 6.70×10-18 cm2 in Er-Yb-TZ, ~ 11.22×10-18 cm2 in Er-Yb-TZW and ~ 14.14×10-18 cm2 in Er-Yb-TZWTi glasses respectively, where for the 4I15/2→4S3/2 absorption transition the values are found to be ~ 2.06×10-18 cm2 in Er-TZ, ~ 2.17×10-18 cm2 in Er-Yb-TZ, ~ 2.77×10-18 cm2 in Er-Yb-TZW and ~ 5.04×10-18 cm2 in Er-Yb-TZWTi glasses respectively (SI Table 4). The transition probabilities for the 2H11/2→4I15/2, 4S3/2→4I15/2 and F9/2→4I15/2 transitions are found to be larger in Er3+-Yb3+ : TZW / TZWTi glasses as compared

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4

to the Er3+-Yb3+: TZ glass. In the Er-TZ, Er-Yb-TZ, Er-Yb-TZW and Er-Yb-TZWTi glasses the S/H ratios (i.e. I553/I532) are found to be 8.02, 7.2, 9.73 and 7.86 respectively. The intensity ratios of green to red bands (i.e. Igreen/Ired) for the Er-TZ, Er-Yb-TZ, Er-Yb-TZW and Er-YbTZWTi glasses are found to be about 7.44, 4.09, 3.42 and 3.47 respectively (Table 4). The CIE

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diagram and the photographs of emitted light from doped/codoped glasses are shown in the inset of Fig.3 (d). The CIE colour coordinate for all the doped/codoped glasses are found similar lying in the yellowish green region in the CIE colour chromaticity diagram {Fig. 3 (d)}. From the above results, it is concluded that the Er3+-Yb3+ codoped TZWTi glass is suitable for

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Page 14 of 28

yellowish green display devices and good NIR to visible upconverter applications.

Page 15 of 28

15

Table 4 FWHM and intensity enhancement in green and red band, intensity ratios of the 4S3/2 →4I15/2 to 2H11/2 →4I15/2 (i.e. S/H) transitions and green to red bands upon 980 nm of all the

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doped and codoped glasses. FWHM

S/H

green/red

Green band

Red band

Samples

(Times)

Ratio

Ratio

(nm)

(nm)

Er-TZ

(Green, Red)

8.02

7.44

14.48

16.17

Er-Yb-TZ

(~7.6, ~14)

7.20

4.09

15.59

17.12

Er-Yb-TZW

(~12, ~29)

9.73

3.42

16.14

18.45

Er-Yb-TZWTi

(~18, ~36.6)

7.86

3.47

16.71

19.13

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Enhancement

To get the information about the number of NIR pump photons involved in the UC emission

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process the pump power versus UC emission intensity study has been performed. The intensity of green and red UC emission bands increase by varying the pump power density from 13.2 W/cm2 to 82.2 W/cm2. The number of NIR photons involved in the UC process can be observed

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by calculating the slope value of logarithmic plot of the pump power (Ppump) versus corresponding UC emission intensity (IUC) {inset Fig. 3 (a) and Fig. 3 (c)}. In the Er-TZ and Er-Yb-TZWTi glasses the slope values are noted to be 1.98 ± 0.04 and 1.99 ± 0.04 for the green band corresponding to the 2H11/2, 4S3/2→4I15/2 transition as well as 1.89 ± 0.08 and 1.96 ± 0.05 for the red band corresponding to the 4F9/2→4I15/2 transition respectively {Inset Fig. 3(a) and Fig. 3(c)}. From the observation of these slope values, it is concluded that the existence of

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green and red UC emission bands is basically two NIR pump photon excitation process. Also, on increasing the pump power density from 13.2 W/cm2 to 81.2 W/cm2 of the 980 nm laser excitation source, the ratio of the UC emission intensity of the thermally coupled 2

H11/2→4I15/2 (H) and 4S3/2→4I15/2 (S) transitions (i.e. IH/IS) in the Er3+-Yb3+ codoped TZWTi

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glass is not affected significantly [16, 26, 56].

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16

Fig.3 The upconversion emission intensity at different pump power densities of (a) 1.0 mol% Er3+ doped TZ and (b) (1.0 mol% Er3++1.0 mol% Yb3+ codoped TZWTi glasses upon 980 nm diode laser excitation. The inset figure (a) shows logarithmic pump power versus logarithmic UC emission intensity for green and red bands in Er-TZ glass. (c) Logarithmic pump power

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versus logarithmic UC intensity for green and red bands in Er-Yb-TZWTi glass. (d) The CIE colour coordinates diagram for all the samples at constant pump power. 3.4.1 Description of energy level diagram, efficiency of energy transfer and quantum efficiency

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UC emission bands observed in the visible region corresponding to the 2H11/2→4I15/2 (532 nm), 4

S3/2→4I15/2 (553 nm) and 4F9/2→4I15/2 (669 nm) transitions in all the Er3+/Yb3+ doped/codoped

glasses upon 980 nm laser excitation have been discussed through the energy level diagram shown in Fig. 4 (a). The energy corresponding to the ground state absorption (GSA) of the Yb3+

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Page 16 of 28

ion (2F7/2→2F5/2) and Er3+ ion (4I15/2→4I11/2) are synchronized with the energy of 980 nm laser photon. This is why, the 980 nm pump photons are directly absorbed by the Er3+ as well as

Page 17 of 28

17

Yb3+ ions. In the singly Er3+ doped glass, initially 980 nm pump photon helps to increase the population in the 4I11/2 level of Er3+ ion through the ground state absorption (GSA) process. Thereafter, some Er3+ ions are again excited by absorbing the second pump photon through

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excited state absorption (ESA) process and the higher 4F7/2 level is populated. After that these

Er3+ ions relax down to the 2H11/2 and 4S3/2 levels through the non-radiative relaxation (NRR)

process. The Er3+ ions in the 2H11/2 and 4S3/2 levels decay radiatively to the ground state by emitting highly intense green photons of wavelength around 532 nm and 553 nm corresponding

to the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions respectively [26]. A part of the population of the 4I11/2 level relaxes down non-radiatively to the 4I13/2 level and again by absorbing the 980 4

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nm pump photon get excited to the 4F9/2 higher level. The 4F9/2 level decays radiatively to the I15/2 ground level by emitting the red photons of wavelength around 669 nm corresponding to

the 4F9/2→4I15/2 transition. In case of the Er3+-Yb3+ codoped glasses (e.g. Er3+-Yb3+ codoped TZWTi glass here), the experimentally observe oscillator strength of Er3+ ion (~1.79 ×10-6)

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corresponding to the 4I15/2→4I11/2 transition is smaller than that of the Yb3+ ion (~ 5.98 ×10-6) corresponding to the 2F7/2 →2F5/2 absorption transition. Therefore, large number of 980 nm photons are absorbed by the Yb3+ ions as compared to that of the Er3+ ions in the Er3+-Yb3+ 4

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codoped TZWTi glass [26]. Hence, the low as well as high lying energy levels {as 4I13/2, 4I11/2, F9/2, 4F7/2, 2H11/2 and 4S3/2} are populated more and more due to the efficient energy transfer

from the Yb3+ to Er3+ ions. Thus, in the codoped glasses thermally coupled levels (2H11/2 and 4

S3/2) are populated via ESA, non-radiative relaxation from the 4F7/2 level and energy transfer

process from the Yb3+ to Er3+ ions. The 4F9/2 level is populated via the non-radiative relaxation from the 2H11/2 and 4S3/2 levels, ESA and energy transfer process from the Yb3+ to Er3+ ions. Due to the involvement of efficient energy transfer process from the Yb3+ to Er3+ ions and local

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field modification around the rare earth ions on incorporating the WO3 and TiO2 in the Er3+Yb3+-TZ glass the UC emission intensity corresponding to the 2H11/2,4S3/2→4I15/2 and 4

F9/2→4I15/2 transitions is significantly enhanced. As the enhancement in red UC emission band

is more than that of the green UC emission band. This is possibly due to the involvement of

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above mentioned processes along with the cross relaxation (CR) process 2F5/2 (Yb3+) + 2F7/2 (Yb3+)→4I13/2(Er3+) + 4F9/2(Er3+) upon 980 nm laser excitation.

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Fig.4 (a) Simplified energy level diagram of the Er3+-Yb3+ system upon 980 nm laser diode excitation. (b) and (c) shows the decay time analysis of green bands corresponding to the 4S3/2

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→4I15/2 transition of Er3+ doped TZ and Er3+- Yb3+ codoped TZWTi glasses respectively. The decay time analysis of the green emission band corresponding to the 4S3/2→4I15/2 transition has been performed at room temperature in both the Er3+ doped TZ and Er3+-Yb3+ codoped TZWTi glasses respectively upon 980 nm laser excitation attached with a mechanical chopper

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{Fig. 4(b & c)}. The decay time for the 4S3/2 level in Er3+ doped TZ glass is found to be 2.32 ± 0.13 ms, whereas, in the Er3+- Yb3+ codoped TZWTi glass it is 0.89 ± 0.08 ms. It is clear that the decay time of the 4S3/2 level in the Er3+-Yb3+ codoped TZWTi glass is reduced due to the incorporation of Yb3+ ions, WO3 and TiO2 in the TZ glass. It clearly supports the enhancement of the UC emission intensity by several folds in the Er3+- Yb3+ codoped TZWTi glass upon 980 nm laser excitation. The efficiency of energy transfer (ηET) from the Yb3+ to Er3+ ions in the

as [57],

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Er3+- Yb3+ codoped TZWTi glass in the UC emission process can be mathematically expressed

ηET (%) = (1 −

𝜏𝐸𝑟−𝑌𝑏 𝜏𝐸𝑟

) ×100

(9)

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where, ‘τEr-Yb’ and ‘τEr’ are represent the decay time of the 4S3/2→4I15/2 transition in the Er3+Yb3+ codoped TZWTi and Er3+ doped TZ glasses respectively. The efficiency of energy transfer from Yb3+ to Er3+ ions in the Er3+-Yb3+ codoped TZWTi glass is calculated ~ 61.64%. The decay curve analysis of the green emission band corresponding to the 4S3/2→4I15/2

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Page 18 of 28

transition in Er-TZ and Er-Yb-TZWTi glasses has been performed by using the microsecond

Page 19 of 28

19

pulsed flash lamp upon 450 nm excitation [Fig. 5]. The decay curves have been fitted by using the following single exponential function [16], I(t) = Bexp( - t/τ)

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(10)

where, ‘B’ represents a constant, ‘τ’ is the lifetime of the excited level. The lifetime of the

excited 4S3/2 level in Er-TZ and Er-Yb-TZWTi glasses is found to be ~ 23.91 ± 0.15 μs and ~

21.46 ± 0.15 μs respectively. The radiative lifetimes of the 4S3/2 level calculated by using the

J-O analysis in both the glasses are found to be ~ 353.5 μs and ~ 122.4 μs respectively. The quantum efficiency (η), defined as the ratio of number of photons emitted per second to the

η (%) =

𝜏𝑒𝑥𝑝 𝜏𝑟

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number of photons incident per second [16], can be calculated by using the following relation, ×100

(11)

where, ‘τexp’ and ‘τr’ represents the experimentally observed lifetime and radiative lifetime

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respectively. The quantum efficiency corresponding to the 4S3/2→4I15/2 transition in Er-TZ and Er-Yb-TZWTi glasses is found to be 6.78% and 17.53% respectively. Also, on the basis of quantum efficiency, it can be concluded that the Er3+-Yb3+ codoped TZWTi glass can be

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utilized for the practical purposes.

Fig.5 The decay time of green emission band (550 nm) corresponding to the 4S3/2 →4I15/2

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transition of (a) Er-TZ and (b) Er-Yb-TZWTi glasses upon 450 nm excitation.


20

The non-radiative relaxation rate (WNR) is also responsible for the change in UC emission band intensity in all the prepared glasses. The non-radiative relaxation rate can be calculated by using

WNR = 𝜏

1 𝑒𝑥𝑝

1

−𝜏

𝑟

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the formula as [58], (12)

where, all the parameters have their usual meanings. The non-radiative relaxation rates ‘WNR‘ for the 4S3/2 level in Er-TZ and Er-Yb-TZWTi glasses are found to be ~ 3.89×104 Hz and ~

3.84×104 Hz respectively. The reduction in the WNR values for the Er3+-Yb3+ codoped TZWTi

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glass confirms the appearance of emission band corresponding to the 4S3/2 →4I15/2 transition. 3.5. Study of Colour purity and Correlated Colour Temperature (CCT)

The CIE colour coordinates (XS, YS), CCT values and colour purity have been determined by

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using a GoCIE software with the help of frequency upconversion emission data recorded through a monochromator equipped with photomultiplier tube (PMT) upon 980 nm excitation for all the Er3+/Yb3+ codoped TZ, TZW and TZWTi glasses at room temperature and constant pump power density (~ 40.15 W/cm2). The CIE chromaticity diagram with positions of colour

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coordinates of UC emitted light in all the codoped glasses is shown in Fig. 3 (d). The above parameters have also been investigated on varying the pump power density (13.19 W/cm2 to 82.2 W/cm2) for highly sensitive Er3+-Yb3+ codoped TZWTi glass. The numerical values of these parameters are shown in Table 5. From the Table 5 it is found that the colour coordinate (0.32, 0.67) of the emitted colour from all the prepared codoped glasses lie in the yellowish green region and also do not vary significantly on varying the pump power density. Therefore,

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the Er3+-Yb3+ codoped TZWTi glass material showing good UC emission can be used to make the non-colour tunable yellowish green display devices [41, 59]. The correlated colour temperature (CCT) of all the codoped glasses have been measured by using the Mc-Camy empirical relation as,

(13)

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CCT = - 437p3 + 3601p2 – 6861p + 5514.31

where, p = (XS – Xe)/(YS – Ye), (XS, YS) is the sample colour coordinates observed upon 980 nm laser excitation, whereas, (Xe, Ye) indicates the coordinate of epicenter (i.e. 0.3320, 0.1858) [40]. The CCT values upon excitation at 980 nm laser radiation at different pump power

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Page 20 of 28

densities remain fixed (~ 5686 K) in the codoped glass on introducing the WO3 and TiO2. The commercially available light sources have the CCT value in the range from 2700 K to 6500 K

Page 21 of 28

21

[60]. According to the lighting industry, the lamps having low CCT value (2700 K to 4000 K) provide the light which appears “warm”, however, the lamps having CCT value (4000 K to 6500 K) appears “cool” [61]. Therefore, the Er-Yb-TZWTi glass having CCT value 5686 K

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may be applicable for cool yellowish green emission in home appliances [41]. The colour purity (CP) can be calculated by using the following expression as [41], (X −X )2 +(Y −Y )2

Color Purity = √(X s−Xi )2 +(Ys −Yi )2 ×100% d

i

d

i

(14)

where (Xd ,Yd) is the coordinate corresponding to the dominant wavelength (553 nm). The value of (Xd ,Yd) in the present case is found to be (0.30, 0.69) upon 980 nm, and the coordinate

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(Xi ,Yi) represents the illuminant point having value equivalent to (0.3101, 0.3162) [41, 62]. (XS, YS) is the coordinate of the sample illuminant point. The colour purity for the Er-YbTZWTi codoped glass showing large UC emission is found to be ~ 94.7% thereby indicating

devices and solid state lighting.

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the promising applications of the developed codoped glass in intense yellowish green display

Table 5 CIE color coordinates, correlated color temperature (in K) and color purity of

Glass Compositions Er-Yb-TZ Er-Yb-TZW Er-Yb-TZWTi

Pd (W/cm2)

XS

YS

CCT

CP (in %)

40.15

0.31

0.67

5833

94.6

40.15

0.32

0.67

5686

94.7

40.15

0.32

0.67

5686

94.7

13.19

0.32

0.67

5686

94.7

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Er-Yb-TZWTi

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different codoped glasses at different pump power density (Pd) upon 980 nm excitation.

23.65

0.32

0.67

5686

94.7

Er-Yb-TZWTi

34.88

0.32

0.67

5686

94.7

Er-Yb-TZWTi

45.08

0.32

0.67

5686

94.7

Er-Yb-TZWTi

55.87

0.32

0.67

5686

94.7

Er-Yb-TZWTi

66.91

0.32

0.67

5686

94.7

Er-Yb-TZWTi

81.20

0.32

0.67

5686

94.7

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Er-Yb-TZWTi

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3.6. Summary and Conclusion


22

In summary, the absorption and frequency upconversion emission study of the developed glasses have been performed and the various radiative parameters viz. transition probability, radiative lifetime, branching ratios, emission cross-sections, etc. determined using the Judd-

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Ofelt theory. The UC emission intensity arising from the Er3+ ion is significantly enhanced in the Er3+/Yb3+ TZWTi glass due to efficient energy transfer process and local field

modifications around the rare earth ions on introducing WO3 and TiO2. The ratio Ω4/Ω6, ‘known as spectroscopic quality factor’ and the most important predictor of stimulated

emission to claim a good laser material, is found (~ 0.791) better in Er3+/Yb3+ TZWTi glass as compared to other reported work. The bonding between rare earth ions and the surrounding

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oxygen atoms is found to be ionic in nature. Based on the experimental investigations, it has

been concluded that the Er-Yb-TZWTi glass may be used in making the good visible lasers, yellowish green optical display devices and in home appliances, etc.

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Acknowledgment

Funding: The funders [Science and Engineering Research Board (SERB) (Ref. no. EMR/001273/2014 and Department of Science and Technology, New Delhi, India

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(SR/FST/PSI-004/2013)] had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Authors acknowledge the financial support from Science and Engineering Research Board (SERB) and Department of Science and Technology, New Delhi, India in the form of a research project (Ref. no. EMR/001273/2014 and SR/FST/PSI-004/2013). One of the authors, Mr. Mohd Azam is thankful to Indian Institute of Technology (Indian School of Mines), Dhanbad- 826004, Jharkhand, India for providing the

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financial assistance in the form of fellowship. Conflicting interests

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The authors have declared that no conflicting interests exist.

References

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Page 22 of 28

Page 23 of 28

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F. E. Auzel, “Materials and devices using double -pumped phosphors with energy transfer”, Proc. IEEE, 61 (1973) 758–786.

2.

J. C. Wright, “ Radiationless processes in molecules and condensed phases”, Vol. 15,

3.

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Topic of Applied Physics, edited by F. K. Fong, Springer, New York, (1976) p. 239.

P. Rehana, O. Ravi, B. Ramesh, G. R. Dilip, C. M. Reddy, S.W. Joo, B. D. P. Raju,

“Photoluminescence studies of Eu3+ ions doped calcium zinc niobium borotellurite glasses”, Adv. Mater. Lett., 7 (2016) 170–174. 4.

P. P. Feofilov, V. V. Ovsyankin, “Cooperative luminescence of solids”, Appl. Opt., 6 (1967) 1828–833.

R. G. Smart, J. L. Zyskind, D. J. DiGiovanni, “Experimental comparison of 980 nm and

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