Tm3+ co-doped tellurite glass: Effects of Yb3+↔Tm3+ energy transfer and back transfer

Journal of Quantitative Spectroscopy & Radiative Transfer ] (]]]]) ]]]–]]]

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Contents lists available at ScienceDirect

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Journal of Quantitative Spectroscopy & Radiative Transfer

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journal homepage: www.elsevier.com/locate/jqsrt

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Enhanced 1.8 μm emission in Yb3 þ /Tm3 þ co-doped tellurite glass: Effects of Yb3 þ 2Tm3 þ energy transfer and back transfer

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Sathravada Balaji, Kaushik Biswas, Atul D. Sontakke, Gaurav Gupta, Kalyandurg Annapurna Glass Science and Technology Section, Glass Division, Central Glass and Ceramic Research Institute, CSIR, 196 Raja SC Mullick Road, Kolkata 700032, India

a r t i c l e i n f o Article history: Received 25 October 2013 Received in revised form 13 May 2014 Accepted 15 May 2014 Keywords: Tellurite glass Yb3 þ /Tm3 þ co-doping J–O analysis NIR emission Energy trasfer Upconversion

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abstract Q2 The  1.8 μm emission characteristics of Tm3 þ both by a direct excitation and through an energy transfer process upon sensitization with Yb3 þ ions in tellurite glass are reported. The spectroscopic properties of Tm3 þ ions have been evaluated by applying Judd–Ofelt theory on the measured absorption spectrum. The obtained intensity parameters, Ω2 ¼ 7.155  10  20 cm2, Ω4 ¼ 3.325  10  20 cm2, Ω6 ¼1.278  10  20 cm2 are used to estimate the radiative properties of Tm3 þ ions in the present glass host. A  10 fold enhancement in the Tm3 þ 1.8 μm emission observed with 16 fold reduced emission of Yb3 þ ions (1008 nm) in co-doped sample on Yb3 þ ions excitation illustrates the efficient energy transfer from Yb3 þ : 2F5/2-Tm3 þ : 3H5. The energy transfer process assisted by host phonon energy has been discussed by using relevant theoretical models and estimated the energy transfer micro-parameters. Effect of energy back transfer Tm3 þ Yb3 þ on NIR and upconversion emissions have been discussed. An efficient  1.8 μm with comparatively higher emission cross-section 1.115  10  20 cm2 on account of reduced upconversion emissions has been achieved in the present tellurite glass. & 2014 Elsevier Ltd. All rights reserved.

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1. Introduction Lasers operating close to  2 mm have numerous applications in medical, military and remote sensing [1,2]. Particularly rare earths such as Tm3 þ , Ho3 þ doped materials are being investigated immensely because of their unique energy level systems favorable for NIR emissions. Further, sensitized emission through co-dopant ions such as Yb3 þ , Er3 þ are gaining much attention in obtaining efficient and enhanced luminescence properties in infrared as well as visible spectral regions because of their high absorption cross-section at the excitation wavelength and

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E-mail addresses: [email protected] (S. Balaji), [email protected] (K. Annapurna).

availability of low cost high power diode lasers as suitable excitation sources. Tellurite glasses are being examined as host material because of their wide spectral transmission extended into MIR region and low phonon energy of the glass matrix useful for attaining efficient infrared emissions particularly from Yb3 þ /Tm3 þ , Yb3 þ /Ho3 þ co-doped systems [1,3–6]. However, the host glass composition and active ion concentration are crucial for obtaining better optical and spectroscopic properties. Since the NIR emissions are gaining much attention in co-doped systems, energy transfer effects among donor and acceptor ions are of particular interest in optimizing the emission properties. In the present investigation we have reported a detailed investigation on spectroscopic properties of Yb3 þ /Tm3 þ co-doped Ba–La-Tellurite glass. The energy transfer

http://dx.doi.org/10.1016/j.jqsrt.2014.05.025 0022-4073/& 2014 Elsevier Ltd. All rights reserved.

61 Please cite this article as: Balaji S, et al. Enhanced 1.8 μm emission in Yb3 þ /Tm3 þ co-doped tellurite glass: Effects of Yb3 þ 2Tm3 þ energy transfer and back transfer. J Quant Spectrosc Radiat Transfer (2014), http://dx.doi.org/10.1016/j. jqsrt.2014.05.025i

S. Balaji et al. / Journal of Quantitative Spectroscopy & Radiative Transfer ] (]]]]) ]]]–]]]

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Table 1 Measured density (ρ), rare earth ion concentration (NRE), linear refractive indices (nd, nF, nC), Abbe number (νd) of Yb3 þ /Tm3 þ co-doped, Tm3 þ and Yb3 þ singly doped tellurite glass. Property

ρ (g/cm3) NRE (1020 ions/cm3) N Tm3 þ N Yb3 þ nd (589.2 nm) nF (486.1 nm) nC (656.3 nm) νd

Yb3 þ /Tm3 þ co-doped

Tm3 þ doped

5.6594

5.6548

3.8954 3.9720 2.0338 2.0706 2.0195 20.23

3.8954 3.9720 2.0568 2.0958 2.0417 19.53

Yb3 þ doped 5.625 – 3.9720 2.0553 2.0919 2.0405 20.53

15 17 19 21 23 25

3þ

3þ

mechanism among donor, Yb and acceptor, Tm ions have been explained using relevant theoretical models by evaluating their energy transfer micro-parameters. The enhancement in Tm3 þ 1.8 mm emission under sensitized Yb3 þ ion excitation has been explained based on visible upconversion emission diminution which is originated from the effect of energy back transfer from Tm3 þ to Yb3 þ ions. The energy back transfer effect on NIR emission and vis-upconversion emissions have been discussed thoroughly by estimating the transfer rates and efficiency.

27 2. Sample preparation and characterization 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61

Tm3 þ , Yb3 þ singly doped and co-doped Ba–La-Tellurite glasses having chemical composition (mol%) 80 TeO2–15 (BaF2 þBaO) (5  x  y) La2O3  xTm2O3  yYb2O3, (where x ¼0, 1 and y¼0, 1) were prepared by melt quenching method. The size of batches was approximately 10 g, and all the chemicals used were of AR grade (Sigma-Aldrich) with 99.99% purity. The batches were melted using pure platinum crucible as a container in an electrical furnace at 700–750 1C for 1 h with intermittent stirring with thin platinum rod for attaining the homogeneity. The homogeneous melt was poured onto a pre heated graphite mold to form clear glass followed by annealing at glass transition temperature for about 2 h to avoid thermal stress in the glass. The density of the glasses was measured by Archimedes’ method using distilled water as buoyancy liquid at room temperature. Refractive indices of glasses were measured at five different wavelengths (473 nm, 532 nm, 632.8 nm, 1060 nm and 1552 nm) on Metricon M2010 Prism Coupler equipped with respective laser sources. The room temperature absorption spectra were recorded with polished plate shaped samples of average thickness 2 mm on an UV–vis–NIR absorption spectrophotometer (Model: UV-3101PC, Shimadzu, Japan). The emission, excitation spectra and fluorescence decay kinetics of the samples were recorded on spectrofluorometer (Model: QuantaMaster™-600-enhanced UV and NIR from PTI, USA) fitted with double monochromators on both excitation and emission channels. The instrument was equipped with LN2 cooled gated NIR photo-multiplier tube (Model: Hamamatsu IR-PMT-1.7) as well as InGaS (Model: J23TE2-66C-RO2M-2.4 from J23 series, Judson Technologies, USA) detectors for acquiring both study state

spectra and phosphorescence decay. Suitable low pass and high pass filters were used respectively at the source and detector side to avoid unwanted hormonic generations in the recorded emission spectrum. For decay measurements, a 60 W Xenon flash lamp was used as an excitation source. The upconversion emission spectra were recorded by exciting the sample with a Bragg grated, fiber pigtailed 980 nm laser diode (Model: PL980P200, THORLABS, USA) by varying the pump power from 20 to 200 mW. 3. Results and discussion 3.1. Physical and optical properties The measured densities of the glasses have been used to estimate the rare earth ion concentration (NRE) in the glass and the values are presented in Table 1. The measured refractive indices (n) of glasses at five different wavelengths were fitted to sellmeier dispersion equation to determine the linear refractive indices at nD, nF and nC to calculate the Abbe number (ν). The computed values are presented in Table 1. All these parameters are used in estimating the important J–O intensity parameters. 3.2. Absorption spectra and Judd–Ofelt analysis Fig. 1 shows the absorption spectrum of Yb3 þ , Tm3 þ doped and Yb3 þ /Tm3 þ co-doped tellurite glasses. The spectra exhibit a number of distinct absorption bands in the vis–NIR range. The absorption bands at 466, 661, 668, 793, 1212, 1706 nm and at 978 nm are identified as the transitions from ground state 3H6 to 1G4, 3F2, 3F3, 3H4, 3H5 and 3F4 excited states of Tm3 þ ions and 2F7/2-2F5/2 for Yb3 þ ions [7,8] respectively in the present tellurite glass. The Judd–Ofelt intensity parameters (Ω2, Ω4, Ω6) have been calculated from the base line corrected absorption spectrum of co-doped sample by deriving experimental electric dipole line strengths (Sed) using relevant expressions as reported in our earlier calculations [9]. The obtained J–O parameters are Ω2 ¼7.155  10  20 cm2, Ω4 ¼3.325  10  20 cm2, Ω6 ¼1.278  10  20 cm2. Among

Fig. 1. Absorption spectra of Yb3 þ , Tm3 þ doped and Yb3 þ /Tm3 þ co-doped tellurite glass.

Please cite this article as: Balaji S, et al. Enhanced 1.8 μm emission in Yb3 þ /Tm3 þ co-doped tellurite glass: Effects of Yb3 þ 2Tm3 þ energy transfer and back transfer. J Quant Spectrosc Radiat Transfer (2014), http://dx.doi.org/10.1016/j. jqsrt.2014.05.025i

S. Balaji et al. / Journal of Quantitative Spectroscopy & Radiative Transfer ] (]]]]) ]]]–]]]

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Table 2 mea cal Electric dipole line strengths (measured: Sed , calculated: Sed ), magnetic dipole strength (Smd), total oscillator strengths (measured: Pmea, calculated: Pcal) of different absorption transitions and root mean square values along with J–O intensity parameters (Ωt ¼ 2,4,6) of Tm3 þ ions in the co-doped tellurite glass. Transition 3H6-

Wavelength (nm)

Sedmea (10  20)

Sedcal (10  20)

Pmea (10  6)

Pcal (10  6)

Refractive index ‘n’

3

1706 1212 793 668 661 466

6.556 2.044 2.963 2.304 0.189 0.683 rms ΔSed ¼0.24

6.564 2.353 2.820 2.216 0.330 0.610

6.272 3.310 6.257 5.845 0.484 2.618 rms ΔP ¼0.47

6.280 3.728 5.946 5.394 0.846 2.340

1.9724 1.9800 2.0014 2.0174 2.0186 2.0813

F4 H5 3 H4 3 F3 3 F2 1 G4 3

H6-3H5: Smd ¼ 0.783  10  20 Ω2 ¼7.155  10  20 Ω4 ¼3.325  10  20 Ω6 ¼1.278  10  20 3

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Table 3 J–O intensity parameters of Tm3 þ ion in various hosts. Host

Ω2

Ω4

Ω6

GLS chalcogenide [1] ZBLAN fluoride [1] Silica [3] Germanate [12] Phosphate [1] Fluorophosphate [3] Oxyfluoride glass ceramics [11] TZB [3] TZN [1] T [1] TZC [1] TZPN [1] Present glass

5.7 2.57 6.23 7.32 5.63 2.75 4.60 3.30 5.11 5.04 2.51 2.51 7.15

1.7 1.90 1.91 1.88 1.75 2.28 3.69 2.38 1.17 1.36 1.71 1.33 3.32

1.4 0.84 1.36 2.86 1.11 1.18 2.46 1.28 1.08 1.22 1.46 0.95 1.28

were calculated and are listed in Table 4. The emission cross section for 1.8 mm emission corresponding to the transition 3F4-3H6 is estimated to be 1.115  10  20 cm2 which is higher than the values reported for other host glasses or glass ceramics [10–12] and even better than the other tellurite glass hosts [3] reported in the literature. Since, 3H6-3F4 is hypersensitive transition in 4f13 configuration of Tm3 þ and depends on the ligand field strength surrounding the active ion. As said earlier Ω2 value is sensitive to the hypersensitive transitions, the higher value of Ω2 in the present host also justifies the well defined crystal field surrounding the active ion which results in higher value of absorption and emission cross section of Tm3 þ : 3F4 transition in this particular tellurite glass host. 3.3. NIR emission characteristics

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the selected absorption transitions for the estimation of J–O parameters, 3H6-3H5 transition is magnetic dipole in nature according the spectral selection rules. So, its magnetic dipole line strength, Smd has been calculated and subtracted from the Sed value of that transition while estimating the J–O intensity parameters which should be derived only from electric dipole transitions. By using line strengths of transitions, the oscillator strength of respective transitions are determined and presented in Table 2 along with J–O intensity parameters. Further the minimum deviation of root mean square (rms) values for the measured and the calculated values of electric dipole line strengths (rms–ΔSed) and oscillator strengths (rms–ΔP) that are given in Table 2, suggest the good reliability of data in the J–O parameter calculations. Table 3 presents a comparative study of J–O intensity parameters of Tm3 þ ions obtained in the present glass system to that of Tm3 þ ions doped various other glass hosts reported in the literature. It is known from the literature that the J–O intensity parameter, Ω2 is closely related to the ligand symmetry and degree of covalency of the host material. The larger value of Ω2 in the present host glass represents a higher degree of covalency of the glass network bonding and lower symmetry in the vicinity of dopant ions [10]. Further, by using these Ω(t ¼ 2,4,6) values, various important radiative properties, such as spontaneous emission probability (Ar), branching ratios (βR) and radiative lifetime (τr) of the emissions transition

Fig. 2 presents the NIR emission spectra of Yb3 þ /Tm3 þ co-doped sample under 982 nm excitation through 2F5/2 energy state of sensitizer Yb3 þ ions and by direct Tm3 þ ion excitation with 804 nm through 3H4 energy level of Tm3 þ ions. Fig. 2 also presents the NIR emission spectrum of only Tm3 þ ions doped tellurite glass under 804 nm excitation for comparison. The inset (a) of Fig. 2 represents the excitation spectrum recorded by monitoring 1822 nm emission wavelength in the co-doped sample, while Inset (b) shows the emission spectrum revealing peak at 1476 nm under 982 nm excitation along with its corresponding excitation spectrum. The emission peaks detected at 1008, 1476 and 1822 nm are assigned to the transitions of Yb3 þ : 2F5/2-2F7/2, Tm3 þ : 3H4-3F4 and Tm3 þ : 3F4-3H6 respectively. The emission at 1476 nm corresponding to the transition Tm3 þ : 3H4-3F4 has been observed even under 982 nm excitation, but it is very less intense (  12 times) compared to the direct (Tm3 þ : 3H4) excitation by 804 nm in co-doped sample. The excitation spectrum (inset ‘b’) recorded by monitoring the 1476 nm emission clearly shows the presence of Yb3 þ ion excitation bands but are of less intense compared to Tm3 þ excitation bands. It is because, the 1476 nm emission is originating from Tm3 þ : 3 H4 level which is indirectly populated by Yb3 þ ions via two photon absorption and the branching ratio of this Tm3 þ : 3H4-3F4 (1476 nm) transition is lesser (7%; calculated from J–O analysis, Table 4) than that of the other

Please cite this article as: Balaji S, et al. Enhanced 1.8 μm emission in Yb3 þ /Tm3 þ co-doped tellurite glass: Effects of Yb3 þ 2Tm3 þ energy transfer and back transfer. J Quant Spectrosc Radiat Transfer (2014), http://dx.doi.org/10.1016/j. jqsrt.2014.05.025i

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Table 4 Electric dipole line strength (Sed), spontaneous emission probability (Ar), radiative rate (ΣAr), radiative lifetime (τr), branching ratio (βR) and emission crosssection (semi) for emission transitions of Tm3 þ ions in the co-doped tellurite glass. Emission transition

λ (nm)

Sed (10  20)

Refractive index ‘n’

Ar (s  1)

τr (ms)

βR

remi (10  20 cm2)

3

1822

6.449

1.9715

652.48

1.533

1.00

1.115

3

2135 1476 806

1.680 1.637 2.950

1.9689 1.9750 2.0021

82.46 313.63 3934.05 ΣAr:4330.14

0.231

0.02 0.07 0.91

5 F4-3H6

7 9

H4-3H5 -3F4 -3H6

11 13 15 17 19 21 23 25 27 29 31 33

3þ

3þ

Fig. 2. NIR Fluorescence spectra of Yb /Tm co-doped (black color under 982 nm excitation, red color under 804 nm excitation) and Tm3 þ singly (blue color under 804 nm excitation) doped tellurite glass; inset: (a) excitation spectra for λemi ¼ 1822 nm, inset: (b) 1476 nm excitation and emission spectrum. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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emission transition originating from Tm3 þ : 3H4 excited state (806 nm; Branching ration is 91%, calculated from J–O analysis, Table 4). Where as in case of Tm3 þ : 1822 nm emission, the energy transfer is taking place from Yb3 þ ions through host phonons assistance and is much efficient in the present case as can be seen from the intense Yb3 þ ion excitation bands (Inset ‘a’) compared to that of Tm3 þ excitation bands while monitoring the 1822 nm emission peak. The stimulated emission cross-section (se) of the intense NIR emission peak at 1822 nm has been calculated by using the following expression [13]:

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se ðλÞ ¼

λ4p 8πcn2 Δλef f

! Ar

ð1Þ

where λp is fluorescence peak wavelength, Δλeff is the effective band width (FWHM), n is refractive index, c is velocity of light and Ar is its spontaneous emission probability. The effective band width of 3F4-3H6 emission transition is estimated to be 220 nm in the present glass and is higher when compared to other host glasses [3,10– 12]. This can be attributed to the existence of varied structural units TeO4, TeO3 þ 1 and TeO3 of tellurite glass with different Te–O bond lengths results in multiplicity of ion-host field strengths and therefore, yield a range of electro-static fields surrounding rare earth ions in a

Fig. 3. Absorption and emission cross-sections (calculated from RM: McCumber Method using absorption transition; FL: Fuchtbauer–Ladenburg Method using emission transition) of Tm3 þ : 3F4 level in the present tellurite glass.

tellurite glass leading to an inhomogeneous broadening of the fluorescence band [9,14]. Also, it is observed that, the value of stimulated emission cross section (se) of 3 F4-3H6 transition in the present glass have the higher value (1.115  10  20 cm2) when compared to other host glasses. The higher emission cross-section is mainly due to the high spontaneous emission probability resulting from high refractive index in the present tellurite glass. It is highly beneficial for efficient laser output at 1.8 mm. It is observed that, the emission intensity of Tm3 þ : 3F4-3H6 has increased  10 fold under sensitized excitation with Yb3 þ : 982 nm when compared to the direct excitations of Tm3 þ under 804 nm. This can be ascribed to the high absorption cross section of Yb3 þ ions (sa ¼1.889  10  20 cm2 in the present glass) accompanied with efficient energy transfer from Yb3 þ to Tm3 þ ions. Fig. 3 shows the NIR absorption and emission crosssections of 3H623F4 transition for Tm3 þ ion in co-doped glass. Emission cross-sections were calculated by using McCumber theory (RM) from absorption transition and Fuchtbauer–Ladenburg equation (FL) from measured emission transition. A good agreement between these two methods has been observed as has shown in Fig. 3. For Tm3 þ ions, 3H623F4 transitions being hypersensitive (|ΔS|¼ 0,|ΔL|r2,|ΔJ|r2) in nature, the higher values of absorption and emission cross-sections in the present host can be attributed to the stronger ligand field strength surrounding the Tm3 þ ions as revealed from the J–O analysis as well as higher refractive index of the host material.

Please cite this article as: Balaji S, et al. Enhanced 1.8 μm emission in Yb3 þ /Tm3 þ co-doped tellurite glass: Effects of Yb3 þ 2Tm3 þ energy transfer and back transfer. J Quant Spectrosc Radiat Transfer (2014), http://dx.doi.org/10.1016/j. jqsrt.2014.05.025i

S. Balaji et al. / Journal of Quantitative Spectroscopy & Radiative Transfer ] (]]]]) ]]]–]]]

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3.4. Yb3 þ 2Tm3 þ energy transfer mechanism and decay analysis

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In order to understand the Yb3 þ -Tm3 þ energy transfer mechanism, the emission characteristics of Yb3 þ ions with and without Tm3 þ ions in the present glass system have been examined. Fig. 4 compares the room temperature emission spectra of Yb3 þ singly doped and co-doped samples. The emission spectra were recorded under second intense 924 nm excitation peak to obtain a full Gaussian shape of Yb3 þ : 2F5/2-2F7/2 transition at 1008 nm instead of intense 982 nm excitation peak. Insert of Fig. 4 presents the decay profiles of the same. It is observed from the emission spectra and the measured lifetimes that, a decrease of 16 fold in 1008 nm emission intensity and 8.6 fold in fluorescence lifetime of Yb3 þ ions for the co-doped sample in comparison with the Yb3 þ singly doped sample. This clearly indicates an efficient energy transfer taking place from Yb3 þ -Tm3 þ ions. The energy transfer rate (WET) and energy transfer efficiency (ηET) have been estimated by using the following expressions [9,15]:     1 1 W ET ¼  ð2Þ τ τ0 ηET ¼ 1 

  τ τ0

ð3Þ

where τ and τ0 are the lifetime of donor ions in the presence and absence of acceptor ions respectively. The estimated values of energy transfer rate (WET) and energy transfer efficiency (ηET) are found to be 12,111.3 s  1 and 88.4% respectively. To understand the donor–acceptor interactions in the present host, well established theoretical models like ‘Inokuti–Hirayama’ for direct energy transfer [16] and ‘Burshtein’ for donor–donor migration assisted energy transfer process [17] were applied on the decay profile of Yb3 þ ions in the presence of acceptor, Tm3 þ ions by assuming ion interactions as dipole–dipole in nature. At low donor concentration, the energy transfer

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Fig. 4. Emission spectra of Sensitizer, Yb3 þ ions in absence (black) and in the presence (red) of activator, Tm3 þ ions; inset: decay profiles for the same. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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follows direct donor to acceptor path and the Inokuti– Hirayama model can be used for decay analysis [16]     t 4π 3 IðtÞ ¼ I 0 exp   NA Γ 1  ðC DA tÞ3=s ð4Þ τ0 3 s where NA is the acceptor ion concentration, Г is Euler's gamma function, s is electrostatic interaction parameter (s ¼6 for dipole–dipole interactions) and CDA is the microscopic energy transfer parameter between donor and acceptor. CDX (X¼ A or D) is an intrinsic parameter of respective host material and is estimated using the spectral overlap relation [18] Z 3c X C DX ¼ 4 2 sD ð5Þ em ðλÞsabs ðλÞdλ 8π n where c is the velocity of light in vacuum, n is the X refractive index and sD em ðλÞ, sabs ðλÞ are the emission and absorption cross-sections of donor and acceptor/donor respectively. In present tellurite glass, the energy difference between Yb3 þ : 2F5/2 to Tm3 þ : 3H5 levels is around 1670 cm  1 which requires assistance of 2.8 phonons to bridge the energy gap for efficient energy transfer from Yb3 þ Tm3 þ . Since the energy transfer mechanism is phonon assisted, the same has been taken into consideration while calculating the energy transfer microparameters by constructing the Stokes phonon sidebands to the absorption and emission cross section spectra from an exponential law of Auzel as given below [18] sStokes ¼ select expð αS ΔEÞ

ð6Þ

where ΔE is the energy mismatch between electronic and vibronic transitions and αS is the host dependent parameter for Stokes transitions represented as αS ¼ ðhνÞ  1 ðln fðN=S0 Þ½1  expð  hνmax =kTÞg  1Þ

ð7Þ

where N is the number of phonons required for bridging the energy gap, S0 is the electron–phonon coupling constant ( 0.04), hνmax is the maximum phonon energy of the host and k is Boltzmann constant. An average of 2.5 phonons assistance has been assumed in energy transfer for constructing the phonon sidebands. Fig. 5 shows the absorption and emission cross-section spectra of Yb3 þ ions and absorption cross-section spectrum of Tm3 þ ions in the co-doped sample. Inset of Fig. 5 represents the emission and absorption cross-section spectra of Yb3 þ and Tm3 þ ions respectively by using Eq. (7) with corresponding phonon sidebands. By applying Eq. (6) to the spectral overlap function, the energy transfer microparameter (CDA) has been calculated and found to be 5.18  10  41 cm6 s  1 and the corresponding critical distance is found to be 5.63 Å. The CDD term defines the donor–donor energy migration microparameter. In case of Yb3 þ ions the absorption and emission falls in the same spectral region, while calculating CDA and CDD parameters the emission cross-section spectra calculated by Reciprocity method were utilized to avoid reabsorption losses. The donor–donor (Yb–Yb) energy migration parameter has been calculated by using Eq. (5) and the value is found to be 1.49  10  38 cm6 s  1 for a corresponding critical distance of 1.36 Å. It has been found

Please cite this article as: Balaji S, et al. Enhanced 1.8 μm emission in Yb3 þ /Tm3 þ co-doped tellurite glass: Effects of Yb3 þ 2Tm3 þ energy transfer and back transfer. J Quant Spectrosc Radiat Transfer (2014), http://dx.doi.org/10.1016/j. jqsrt.2014.05.025i

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in the present glass that, the CDD 4CDA signifying the donor–donor migration assisted energy transfer is the dominant mechanism at high donor concentration. Further to notice from the inset of Fig. 4 that the Yb3 þ ion decay profile fitted to Inokuti–Hirayama relation deviates much from the experimental data but Burshtein relation fits well, signifying that at this Yb3 þ ion concentration, the Yb–Tm energy transfer is not a direct donor–acceptor energy transfer instead it is assisted by donor–donor energy migration. The Burshtein hopping model has been applied on donor decay analysis by the relation [17]   pffiffi t IðtÞ ¼ I 0 exp  γ t Wt τ0

ð8Þ

where γ is the energy transfer parameter, W is the hopping migration rate. The donor–acceptor energy transfer parameters such as energy transfer rate (γ2), energy transfer micro-parameters (CDA) and critical distance (R0) have been derived from the fitting parameters using the relevant expressions [9,18,19]. The calculated values of the energy transfer parameters were tabulated in Table 5. The parameters obtained from Burshtein model are in concurrence with the calculated values through emission and absorption cross sections by using spectral overlap method (Eq. (5)) indicating the donor–acceptor energy transfer is mainly occurring through hopping assisted migration among donor ions. This efficient energy transfer from Yb3 þ to Tm3 þ is contributing to both normal fluorescence in NIR region and up-conversion visible emissions respectively.

The upconversion emission spectra as a function of laser pump power (upto 200 mW) are shown in Fig. 6. The inset of Fig. 6 represents the plot of log Iupc vs log Iexci to determine the number of photons involved in the upconversion process. The obtained upconversion emissions at 482 nm, 654 nm and 806 nm correspond to the transitions 1G4-3H6, 1G4-3F4 and 3H4-3H6 respectively [11,20–22]. It is important to mention that the upconversion emissions particularly at 482 nm and 654 nm are very low intense compared to 806 nm emission. Especially those visible upconversion emissions require a third photon absorption from Tm3 þ : 3H4 level to 1G4 level or an efficient co-operative sensitization from a pair of Yb3 þ ions simultaneously, which is efficient particularly at higher Yb3 þ ion concentrations [22]. Earlier we have demonstrated that, the Yb3 þ ion concentration effects the 1 μm emission in this particular glass host [23] and showed 1 mol% Yb2O3 doped glass is superior in optical and spectroscopic properties compared to higher Yb3 þ ions concentrations where concentration quenching is a predominant process. Hence, we have chosen the same Yb3 þ ion concentration in the present Tm3 þ /Yb3 þ co-doped system to avoid Yb–Yb quenching losses. Fig. 7 depicts the partial energy level diagram displaying energy level separation of donor and acceptor ions and various energy transfer mechanisms involved among them (Yb3 þ 2Tm3 þ ) particularly in this glass host. The upconversion emission corresponding to 3H4-3H6 transition at

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Fig. 5. Absorption and emission cross-section spectra; inset: Stokes phonon sidebands to the absorption and emission cross section spectra.

Table 5 Energy transfer parameters. Parameter

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Fig. 6. Upconversion emission spectra of Tm3 þ ions in the co-doped sample under 980 nm laser diode excitation for varied output powers; inset: log Iupc vs log Iexci.

20

3

Critical concentration, C0 (10 ions/cm ) Critical distance, R0 (Å) Energy transfer parameter, γ (s  1/2) Energy transfer rate, γ2 (s  1) Donor–acceptor energy transfer micro-parameter, CDA (10  41 cm6 s  1) Migration rate, W (s  1) Donor–donor energy transfer micro-parameter, CDD (10  40 cm6 s  1)

Inokuti–Hirayama

Burshtein

Spectral overlap

1.675 12.18 208.17 43,333 518.4 – –

16.746 5.57 22.03 400 4.78 8374.5 –

15.203 5.63 20 485.31 5.08 – 149.2

Please cite this article as: Balaji S, et al. Enhanced 1.8 μm emission in Yb3 þ /Tm3 þ co-doped tellurite glass: Effects of Yb3 þ 2Tm3 þ energy transfer and back transfer. J Quant Spectrosc Radiat Transfer (2014), http://dx.doi.org/10.1016/j. jqsrt.2014.05.025i

S. Balaji et al. / Journal of Quantitative Spectroscopy & Radiative Transfer ] (]]]]) ]]]–]]]

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Fig. 7. Partial energy level diagram of Yb3 þ /Tm3 þ co-doped and Tm3 þ doped tellurite glass showing the energy transfer mechanism under 982 nm (black) and direct 804 nm excitation (‘red’ in case of co-doped sample and ‘blue’ for Tm3 þ singly doped sample). Upconversion emission transitions under 982 nm excitation (green) in co-doped sample. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

806 nm represents the two photon absorption process i.e., it requires two lower energy photons (inset of Fig. 6) to visualize this transition. The upconversion energy transfer mechanism involves the excitation of first photon from Tm3 þ : 3H6-3H5 and a second photon from Tm3 þ : 3F4-3F2,3 level. On the other hand as said earlier, to obtain the 482 nm and 654 nm upconversion emissions from 1G4 excited level either a third photon absorption from Tm3þ : 3H4 level has to be absorbed or co-operative energy transfer from Yb3 þ to Tm3 þ : 1G4 state is mandatory. The slope of the emission intensity vs pump power curve for 482 nm and 654 nm emissions (inset of Fig. 6) are found to be 2.05 and 1.85 respectively and is too far from 3. Hence it supports our view that the these two upconversion blue and red emissions in visible region are not due to a three photon absorption process rather, by Yb3 þ sensitization through co-operative energy transfer to Tm3 þ ions. Further to elucidate the visible co-operative up-conversion emission process (VIS-CUC), the lifetime of Tm3þ : 1G4 excited state corresponding to the blue co-operative upconversion emission (482 nm) and Yb3 þ NIR emission (1008) have been examined under 982 nm excitation. It is well known [23,24] that, for CUC if IIR1Ne1exp(t/τIR), then ICUC1N2e 1exp(2t/τIR), where Ne is the population of excited Yb3þ ions. Fig. 8 presents the decay profile of Yb3þ : 1008 nm emission fitted to a single exponential function with a lifetime of 73.0370.37 ms and the inset of Fig. 8 represent the decay profile of Tm3þ : 482 nm CUC emission. The decay profile of 482 nm CUC emission is best fitted to double exponential function with faster lifetime (τ1) of 7.9770.13 ms and slower lifetime (τ2) of 38.6478.33 ms. the faster component can be ascribed to the energy migration among Tm3 þ –Tm3 þ or Tm3þ to impurity centers. From the measured lifetimes, it can be seen that the Tm3 þ VIS-CUC lifetime (τ2) is almost half of Yb3 þ IR lifetime and agrees well with the above theory [23,24] corroborating the mechanism of co-operative upconversion emission process. Though the co-operative energy transfer mechanism is occurring at this Yb3þ ion concentration, the observed very low intensities of these two transitions indicate that, this is not a dominant process which inturn is highly useful to obtain an efficient NIR 1.8 mm emission. Further investigation reveals that, there are two reasons for

Fig. 8. Decay profiles of Yb3 þ 1008 nm emission (red line: single exp. fit) under 982 nm excitation; inset: decay profile of Tm3 þ : 482 nm VIS-CUC emission under 982 nm excitation (red line: double exp. fit). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

obtaining very less intense blue and red upconversion emissions. Firstly, it may be due to the increased Tm3 þ (1 mol%) ion–ion interactions that restrict the third photon absorption from Tm3þ : 3H4 level. Secondly, energy back transfer (EBT) from Tm3þ to Yb3 þ , since the energy level difference between Tm3þ : 3H4 and Yb3þ : 2F5/2 (1708 cm  1) more or less similar to energy level difference between Yb3þ : 2F5/2 and Tm3þ : 3H5 (1650 cm  1), it may so happen that the energy back transfer can take place from Tm3 þ : 3H4 to Yb3þ : 2F5/2 with release of 2.5 to 3 phonons. To verify the manifestation of energy back transfer if any from Tm3þ -Yb3 þ , we have examined the emission spectra and lifetime of 3H4 excited state by monitoring 1476 nm emission for Tm3 þ singly doped Yb3 þ /Tm3þ co-doped samples under 804 nm excitation. Fig. 9a presents the excitation spectra monitoring 1476 nm emission and Fig. 9b shows the measured emission spectra under 804 nm excitation for Tm3 þ singly doped and Yb3 þ /Tm3 þ co-doped samples. Inset of Fig. 9b depicts the decay profiles of the 1476 nm emission corresponding to the transition Tm3 þ : 3H4-3F4. From the emission spectra, the observed emission transition at 1020 nm corresponding the Yb3 þ : 2F5/2-2F7/2 was observed only in co-doped sample which confirming the energy transfer is taking place from Tm3 þ -Yb3 þ under 804 nm excitation. But the intensity of this Yb3 þ emission is much lower (  7 times) than that of the emission under 982 nm excitation in the co-doped sample. A slight reduction in the Tm3 þ :1476 nm emission and excitation can be noticed along side with a considerable change in its lifetime from Fig. 9. Further to know the energy back transfer rate and its efficiency, the lifetime of the 3H4 excited state has been examined by using the Eqs. (2) and (3). The estimated energy transfer rate is 12,390 s  1 which is similar to the energy transfer rate of Yb3 þ -Tm3 þ (12,113 s  1) since the energy level difference is almost same. However, the energy transfer efficiency is just 37% which is much lower than the Yb3 þ -Tm3 þ energy transfer efficiency (88%). The energy back transfer (EBT)

Please cite this article as: Balaji S, et al. Enhanced 1.8 μm emission in Yb3 þ /Tm3 þ co-doped tellurite glass: Effects of Yb3 þ 2Tm3 þ energy transfer and back transfer. J Quant Spectrosc Radiat Transfer (2014), http://dx.doi.org/10.1016/j. jqsrt.2014.05.025i

S. Balaji et al. / Journal of Quantitative Spectroscopy & Radiative Transfer ] (]]]]) ]]]–]]]

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Fig. 9. Excitation and emission spectra of Tm3 þ singly doped (black) and co-doped (red) samples; inset: decay profiles for the 1476 nm emission fitted to single exp. function. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

25 3  10  21 cm2 can achieved over a wavelength range from 1800 nm to 1950 nm which is very promising for application of broadly tunable laser and ultrashort pulsed laser.

27 29 31

4. Conclusion

33 35 37 39 41

3þ Q6 Fig. 10. Gain measurement spectra of Tm : 1822 nm emission band in

the co-doped sample for various papulation inversions.

43 45 47

particularly at 804 nm excitation helps in restriction of third photon absorption leading to raise the upconversion emission from 1G4 level.

49

3.5. Gain measurements

51

Fig. 10 presents the gain cross sections for  1.8 mm emission of Tm3 þ ions in co-doped sample. For different fraction of ions in the excited state 3F4, the gain cross section sg can be expressed as [25]

53 55 57 59 61

αg ¼ βsemi  ð1 βÞsabs

ð9Þ

Detailed spectroscopic analyses have been performed on Tm3 þ /Yb3 þ co-doped tellurite glass by applying the standard Judd–Ofelt theory. The J–O intensity parameters have been used to estimate the oscillator strengths, radiative properties of emission transitions. The energy transfer mechanisms have been explained by adopting theoretical models. The energy transfer from Yb3 þ Tm3 þ and back transfer effects on NIR and upconversion emissions have been discussed elaborately. The observed visible upconversion emissions have been realised to be due to Yb3 þ sensitization through co-operative energy transfer process. Further, the stimulated emission crosssection of Tm3 þ : 3F4-3H6 transition in the present glass has been found to be comparatively high with he value of 1.115  10  20 cm2. The high energy transfer rate 12,111.3 s  1 with 88.4% efficiency and a 10 fold increase in the 1.8 mm emission intensity under 982 nm excitation clearly indicates the sensitizer excitation is better option than the direct excitation. An efficient NIR emission with reduced upconversion losses greatly demonstrates the present materials may be a suitable candidate for NIR laser at 1.8 mm.

3þ

where β is the ratio of the number of Tm ions in the excited state to the total Tm3 þ ions density. The semi and sabs are emission and absorption cross sections for β¼1 and β¼0 respectively. From Fig. 10 it can be observed that for a 60% of population inversion, a broad gain over

Acknowledgment The authors would like to thank the Director, CSIRCGCRI, and the Head, Glass Division, for their continued

Please cite this article as: Balaji S, et al. Enhanced 1.8 μm emission in Yb3 þ /Tm3 þ co-doped tellurite glass: Effects of Yb3 þ 2Tm3 þ energy transfer and back transfer. J Quant Spectrosc Radiat Transfer (2014), http://dx.doi.org/10.1016/j. jqsrt.2014.05.025i

S. Balaji et al. / Journal of Quantitative Spectroscopy & Radiative Transfer ] (]]]]) ]]]–]]]

Q4 1 Q3 support and encouragement. This work has been carried

out under CSIR Project ESC-0202 (WP-2.2). 3 Q5 References

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Please cite this article as: Balaji S, et al. Enhanced 1.8 μm emission in Yb3 þ /Tm3 þ co-doped tellurite glass: Effects of Yb3 þ 2Tm3 þ energy transfer and back transfer. J Quant Spectrosc Radiat Transfer (2014), http://dx.doi.org/10.1016/j. jqsrt.2014.05.025i