Studies of radiative and mechanical properties of Nd3+-doped lead fluorosilicate glasses for broadband amplification in a chirped pulse amplification based high power laser system

Journal of Luminescence 188 (2017) 558–566

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Studies of radiative and mechanical properties of Nd3+-doped lead fluorosilicate glasses for broadband amplification in a chirped pulse amplification based high power laser system

MARK

P. Manasaa, D. Ramacharia, J. Kaewkhaob, P. Meejitpaisanb, E. Kaewnuamc, A.S. Joshid, ⁎ C.K. Jayasankara, a

Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Center of Excellence in Glass Technology and Materials Science (CEGM), Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand Physics Program, Faculty of Science and Technology, Muban Chombueng Rajabhat University, 70150, Thailand d Laser Plasma Division, Raja Ramanna Center for Advanced Technology, Indore 452 013, India b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Nd3+:glasses Lead flourosilicate glass Optical properties X-ray fluorescence CPA Scheme

Lead fluorosilicate (SPbKNLFNd: SiO2 + PbF2 + K2O + Na2O + LiF + Nd2O3) glasses doped with different concentrations of Nd3+ ions have been prepared by the melt quenching technique and characterized the compositional, mechanical and spectroscopic properties. The elements present in the chemical composition of lead fluorosilicate glass were analyzed by X-ray fluorescence spectra (XRF). The structural analysis and OHcontent has been evaluated from FTIR spectra. Judd–Ofelt (JO) theory has been applied to the absorption spectrum of 1.0 mol% Nd3+-doped lead fluorosilicate glass to estimate the JO intensity parameters (Ωλ, λ=2, 4 and 6) which are in turn used to calculate the radiative properties of luminescent level of Nd3+ ions. The near infrared emission spectra recorded with 808 nm laser diode excitation for different concentrations of Nd3+ ions show that the emission intensity is highest for the 4F3/2→4I11/2 transition at 1056 nm. The measured decay times for the 4F3/2 level decreased with increasing Nd3+ ions concentration due to the concentration quenching. The laser parameters such as stimulated emission cross-section (σe =4.11×10–2° cm2) and Vickers's hardness (3.59 GPa) were found to be higher for the 1.0 mol% of Nd3+:SPbKNLF glass along with wider fluorescence bandwidth (Δλeff ~ 36 nm) at Δλp ~ 1056 nm. The obtained values are strong evidence that the SPbKNLFNd glasses could be useful for broadband amplification in a chirped pulse amplification (CPA) based high power laser systems. The results obtained in the present study have been compared with similar results obtained for Nd3+:glasses.

1. Introduction Nd3+-doped laser materials are very attractive and are extensively studied for a wide varity of applications due to their easier 4-level laser operation mode in various crystalline and amorphous hosts [1–5]. Demand for ultrafast high energy and high power (HEHP) chirped pulse amplification (CPA) based laser chains with Ti: sapphire oscillator and Nd: glass amplifiers in MOPA configuration is increasing day-by-day because of applications in the fields of extreme field science [4] leading to charged particle acceleration and studies relevant to fast ignition [6]. Such lasers suffer from problems of gain narrowing of the spectrum causing a larger pulse duration of the compressed pulse [7]. An efficient amplifier gain medium for laser operation in such HEHP lasers should exhibit suitably large emission cross-section to provide high gain, high

⁎

Corresponding author. E-mail address: [email protected] (C.K. Jayasankar).

http://dx.doi.org/10.1016/j.jlumin.2017.04.065 Received 3 February 2017; Received in revised form 27 April 2017; Accepted 28 April 2017 Available online 04 May 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.

hardness for good mechanical strength, large bandwidth for preservation of pulse duration of laser pulses on compression. In the case of Nddoped glass regenerative amplifiers used in CPA scheme to amplify the large spectral bandwidth pulses from a Kerr lens mode locked (KLM) pulses from a Ti: sapphire oscillator, it is necessary to have a Nd-doped glass rod gain media having different glass hosts to obtain a large gain bandwidth of the amplified pulse. The gain media, where one of the gain medium is Nd-doped phosphate glass (matching the large diameter amplifiers of the laser chain that use phosphate laser glass amplifiers equivalent to commercial phosphate laser glass amplifiers of LHG-8 and LG-770 gain media used in HEHP laser chains), the other suitable glass having a λp within ~5 nm of λp of the phosphate laser glass is desirable. In addition to this, the gain medium should have a large fluorescence spectrum width.

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ray fluorescence analysis of the present SPbKNLFNd10 glass. From Fig. 1, it is clear that the elements present in the XRF is in good agreement with the chemical composition of the glass under study, i.e., SiO2+PbF2+K2O+Na2O+LiF+Nd2O3. The X-ray fluorescence spectra recorded for different concentrations of Nd3+ ions doped in SPbKNLF glass is more or less similar and the chemical composition of each element analyzed by XRF for the SPbKNLFNd glass is collected in Table 1.

Therefore, to achieve good lasing action, verity of Nd3+-doped materials have been investigated [8–15]. In this direction, for the present work, oxyfluorosilicate glass host has been choosen. Because, among all rare earth doped glass matrices, the oxyfluorosilicate glass host can potentially offer the best features of both fluorides and silicates. The incorporation of heavy metal fluorides such as PbF2 into oxyfluorosilicate glasses, enhances the chemical durability and mechanical stability with flexible optical properties and minimizes the phonon energy of the host glasses [16–18]. Also, K2O is often used to modify the field strength of cations to improve the mechanical strength which is prerequisite for a good laser glass [19]. The luminescent properties of the rare-earth ions are being investigated either by varying the concentration of rare earth ions or by changing chemical composition [8–18]. In the present case, the authors investigated the concentration dependent luminescent properties to optimize the concentration in this particular host. However, relatively the large gain bandwidth of Nd3+-doped glasses has not yet been found. In this paper, the spectroscopic studies carried out on lead fluorosilicate glasses doped with different concentrations of Nd3+ ions (SPbKNLFNd) have been reported. The obtained values are strong evidence that the SPbKNLFNd glasses are promising for applications like gain medium in a regenerative amplifier for bandwidth preservation to overcome problem of gain narrowing in CPA scheme.

3.2. FTIR spectra – evaluation of hydroxyl content The FTIR spectra in the range of 500–4000 cm−1 for the undoped and SPbKNLFNd10 glasses along with band assignments is shown in Fig. 2. As seen from Fig. 2, the strong band at 992 cm−1 is attributed to Si-OH symmetric starching vibrations [22], the band at 1626 cm−1 corresponds to OH- molecular bending vibrations [23], the peaks around 2351 cm−1 and 2918 cm−1 are due to the presence of hydrogen bonding [24]. The broad and strong band around 3436 cm−1 is attributed to the presence of O–H stretching vibration [22–25]. In order to estimate the quenching effect due to OH ions, the free OHcontent, NOH (ions/cm3) in the glasses has been evaluated by using the following equation [25].

NOH = 2. Experimental procedure

N 1 ln ε. L T

(1)

Where N is the Avagadro's number, L is the glass thickness (cm), T is the transmittance, ε is the molar absorptivity of the free OH groups in the glass. In general, for silicate glasses, the ε value is 49.1×103 (cm2 mol−1) [24,25]. The calculated OH content for all the concentrations of the present glass host is found to be more or less similar, which is around 2.29×1020 ions/cm3. The OH content for the present glass is very much less than 3.70×1019 ions/cm3 found in germanotellurite [24], 5.82×1020 ions/cm3 in zinc tellurite [25], and more or less similar to the value of 2.01×1020 ions/cm3 observed in fluorophosphate [26] glasses. Hence, the present glass exhibits lower OH content resulting in less quenching effect on Nd3+ ions. This is one of the reason, that the SPbKNLFNd glasses exhibit relatively longer lifetimes of the 4F3/2 level.

2.1. Glass preparation Nd3+-doped lead fluorosilicate glasses with the molar composition of 45SiO2 +(20-x) PbF2 + 20K2O +5Na2O +10LiF + xNd2O3 (where x=0.1, 0.5, 1.0, 2.0 and 3.0 mol% and are referred as SPbKNLFNd01, SPbKNLFNd05, SPbKNLFNd10, SPbKNLFNd20 and SPbKNLFNd30, respectively) were prepared by melt quenching technique. About 20 g of batch composition was thoroughly crushed in an agate mortar and this homogeneous mixture was taken into a platinum crucible and heated in an electric furnace at 1250 °C for 2 h at an ambient conditions. The melt was poured onto a preheated brass mold and annealed at 4200C for 10 h to remove thermal strains. The glass samples are slowly cooled down to room temperature (RT) and polished for optical measurements.

3.3. Optical properties In order to investigate the laser quality parameters of lanthanide doped materials, it is essential to measure optical properties. The optical properties of Nd3+:SPbKNLF glass have been determined as follows:

2.2. Measurements Refractive index was measured using digital Abbe refractometer at sodium wavelength (589.3 nm) with 1-bromonaphthalin (C10H7Br) as contact liquid. The density of the glass was determined by Archimedes's method, using distilled water as an immersion liquid. The compositional analysis is made by PANanalytical miniPal 4 energy dispersive Xray fluorescence spectrometer by using Rhodium (Rh) as an excitation source. The structural analysis is carried out by using Perkin–Elmer Paragon 500 FTIR spectrometer at a resolution of 4 cm−1 in the range of 400–4000 cm−1. Absorption spectrum was recorded on a Perkins Elmer Lambda-950 UV–Visible-NIR spectrophotometer in the wavelength region of 380–2400 nm with a spectral resolution of 1 nm. The excitation, emission and decay measurements were carried out by using Edinburgh FLS-980 fluorescence spectrometer by using 808 nm laser diode as an excitation source. The micro hardness was measured by using digital Vickers hardness micro indentation tester HVS-50P.

3.3.1. Optical band gap The optical band gap is one of the important parameter to describe a solid state laser material. The optical absorption edges are not sharply defined which characterize the glassy nature of samples. In amorphous materials, the relation between the absorption coefficient (α(hυ)) and photon energy (hυ) for direct and indirect optical transitions can be expressed as [27,28].

⎡ (hυ − Eg )n ⎤ α (hυ) = A ⎢ ⎥ ⎣ ⎦ hυ α (hυ) = 0

for hυ < Eg

for hυ > Eg

(2) (3)

where the exponent, n =1/2, is for allowed indirect transition, while n =2 is for allowed direct transition, A is constant, h υ is the photon energy and Eg is the optical energy gap. Plotting (αhυ )1/2 and (αhυ )2 against photon energy (hυ ) gives a straight line with intercept equal to the optical energy band gap for indirect and direct transitions, respectively. Figs. 3(a) and 3(b) shows the Tacu's plot of the (αhυ )1/2 and (αhυ )2 as a function of photon energy (hυ ) used to determine the optical band gap (Eg) and is found to be 3.57 eV and 3.74 eV for indirect

3. Results and discussion 3.1. Compositional analysis - X-ray fluorescence spectrometer The X-ray fluorescence (XRF) is one of the non-destructive analytical technique that has been used for the elemental analysis of major and trace elements of many types of objects [20,21]. Fig. 1 shows the X559

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Fig. 1. X-Ray fluorescence analysis of SPbKNLFNd10 glass.

and 4F3/2 transitions, respectively [10–14]. The hypersensitive transition 4I9/2→2G7/2 centered at 17,064 cm−1 (586 nm) is the most intense one among the observed absorption bands. The band at 806 nm is most commonly used for the optical pumping of neodymium based lasers, either by flash lamps or by semiconductor GaAs laser diodes [19]. The Nephelauxetic ratio ‘β’ and the bonding parameter δ are defined as [29].

Table 1 The chemical composition of lead fluorosilicate glass (SPbKNLF: Nd3+) for different concentrations of Nd3+ ions analyzed by X-ray fluorescence spectroscopy (XRF). S. No.

1. 2. 3. 4. 5.

Glass label

SPbKNLFNd01 SPbKNLFNd05 SPbKNLFNd10 SPbKNLFNd20 SPbKNLFNd30

Chemical composition (mol%) SiO2

PbF2

Na2O

K2O

Nd2O3

13.081 12.251 11.331 10.512 9.753

60.976 51.740 45.633 36.475 29.730

15.055 23.717 29.571 39.177 47.081

8.937 8.629 7.670 6.987 5.346

1.951 3.663 5.795 6.849 8.090

⎛1 − β ⎞ δ=⎜ ⎟ × 100 ⎝ β ⎠

(4) υc where β = ∑N β / N and β = , where υc and υa are the energies of the υa corresponding transitions in the complex and aquo-ion [30], respectively, ‘N′ refers to number of levels that are used to compute ‘β’ values. Depending on the surrounding environment, ‘δ’ may be positive or negative indicating the covalent or ionic nature. The values of ‘β’ and ‘δ’ for the SPbKNLFNd10 glass are presented in Table 2. The negative quantity of bonding parameters ‘δ’ for the SPbKNLFNd10 glass exhibits relatively higher ionic character (δ=−19.0218) [31]. The experimental oscillator strengths (fexp) for each absorption band of SPbKNLFNd10 glass has been evaluated by using the following relation [31].

fexp = 4.318

∫α

(υ) dυ

(5)

where α (υ) is the molar absorptivity of a band at wavenumber (υ) in cm−1. According to Judd- Ofelt (JO) theory [32,33], the calculated oscillatory strengths (fcal) for the absorption bands corresponding to the electronic transitions from an initial state ΨJ to a final state Ψ΄J΄ can be estimated by the following equation

fcal (ψJ → ψ ′J ′) =

Fig. 2. FTIR spectra for undoped and SPbKNLFNd10 glasses.

and direct transitions, respectively, for SPbKNLFNd10 glass.

8π 2mcυ (n2 + 2)2 ∑ Ωλ (ΨJ Uλ Ψ ′J ′)2 3h (2J + 1) 9n λ =2,4,6

(6)

where ‘m′ is the mass of the electron, c is the velocity of light in vacuum, ‘h′ is the Planck's constant, ‘n′ is the refractive index, (n2+2)2/9 n is the Lorentz local field correction for the absorption band, Ωλ (λ=2,4 and 6) are the host dependent JO intensity parameters and U λ are the doubly reduced matrix elements of the unit tensor operator. The experimental oscillator strengths ( fexp) for various transitions are evaluated by using Eq. (5) and are used in Eq. (6) for JO analysis. A least-square fitting approximation is then used to determine the Ωλ parameters which give the best fit between experimental and

3.3.2. Optical absorption spectrum The optical absorption spectrum of the SPbKNLFNd10 glass is shown in the Fig. 4, and the spectrum consists of 11 bands due to 4 f3 −4 f3 intra electronic transitions of Nd3+ ion which are centered at 351, 431, 472, 514, 530, 586, 626, 685, 739, 806 and 878 nm and can be attributed to 4I9/2 → (4D3/2, 4D1/2), (2D5/2, 2P1/2), 2D3/2, 2G9/2, 2 K15/2, 4G9/2, 4G7/2, 4G5/2, 2G7/2, 2H11/2, 4F9/2, 4S3/2, 4F7/2, 4F5/2, 2H9/2 560

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Fig. 3. Tauc's plots for the SPbKNLFNd10 glass (a) Indirect and (b) Direct optical bad gap.

glass systems. The value of Ω2 generally indicates the covalence between Nd3+ ions and ligands, also reflects the asymmetry of the local environment at the Nd3+ ion site. Whereas Ω4 and Ω6 parameters indicate the rigidity of the glass matrix. Specifically, Ω6 intensity parameter gives information on the electron-phonon coupling intensity between rare-earth ion and anion ligands [34]. Higher value of Ω6 parameter indicates a stronger interaction between 4 f and 5d orbitals [35,36] which favors the admixture of opposite parity states achieving to intensify the 4f–4 f absorption transitions. This strong electron-phonon coupling also enlarges the emission band widths. As can be seen from Table 3, the JO intensity parameters for the SPbKNLFNd10 glass were found to be Ω2 =6.89 ± 0.02×10−20 cm2, Ω4 =10.09 ± 0.02×10−20 cm2 and Ω6 =9.09 ± 0.02×10−20 cm2 which follows the trend as Ω4 > Ω6 > Ω2, indicating that Nd3+ ions experience stronger rigidity besides ionic nature in SPbKNLFNd10 glass. Table 3 shows the characteristic spectroscopic parameters of Nd3+: glasses [37–57]. From Table 3, it is clear that the glass systems such as 20ZnO+79 Bi2O3 +0.5SiO2 +0.5Nd2O3 (ZBSN1) [40], 1.0Nd2O3+6 La2O3 +3Na2O+25ZnO +65TeO2 (TZLNd) [45], 41P2O5 + 17K2O +9.0CaO +8Al2O3 +24CaF2 +1.0Nd2O3 (PKCFAN) [47], 60TeO2 +39ZnO +1.0Nd2O3 (TZN10) [50], 60Bi2O3 +39.5 (4ZnO+3B2O3) +0.5Nd2O3 (BiZNd) [51], 59TeO2 +20ZnO +7.5Na2O +7.5 Li2O +5 Nb2O5 +1.0Nd2O3 (TZNLN) [52], 15PbF2+25WO3+59TeO2+1.0Nd2O3 (LTTNd) [55], 19ZnO +80TeO2 +1Nd2O3 (TZnNd) [57] exhibit strong rigidity and low covalence and other reported glasses such as 20 Nb2O5 + 20K2O+19 ZnF2 +10LiF +30SiO2 +1.0Nd2O3 (NKZLSNd) [38], 97.0SiO2 +2.1B2O3 +0.8Al2O3 +0.1 (Na2O+CaO) (SBANC) [39], 30Li2O +9 Nb2O5 +5ZrO2 +55Si2O+1 Nd2O3 (LNZSN) [41], 30Li2O +9Ta2O5 +5ZrO2 +55Si2O+1Nd2O3 (LTZSN) [41], 30Li2O +9La2O3 +5ZrO2 +55Si2O+1 Nd2O3 (LLZSN) [41], SiO2 + NdF3 + Al(NO3)3.9H2O (SAlNd) [42], 39 PbO +6 Al2O3 +54SiO2 +1.0Nd2O3 (AN10) [44], 49.5B2O3+24.75Na2O+24.75NaF+1Nd2O3(BNaNf) [54], 35Ga2O3+45SrO+10BaF2+10(LaF3+YF3+AlF3)+1Nd2O3 (GSBNd) [56] exhibit higher covalence and lower rigidity. On the other hand, as can be seen from Table 3, the Ω4 intensity parameter obtained for the SPbKNLFNd10 glass is relatively higher (10.09×10−20 cm2) compared to all the Nd3+-doped glasses. According to Jacobs and Weber [58], the emission intensity of the 4 F3/2→4I11/2 transition of Nd3+ is characterized by the ratio of Ω4 and Ω6 parameters, the so called spectroscopic quality factor (χ). The smaller the ratio, more the intensity of the 4F3/2→4I11/2 laser transition and vice versa. The χ value for the SPbKNLFNd10 glass is found to be 1.1. From the Table 3, it is clear that the obtained χ value is more or less similar to those reported glass hosts of NKZLSNd [38], ZBSN1 [40],

Fig. 4. Optical absorption spectra of SPbKNLFNd10 glass. Table 2 The absorption band positions (λ, nm), complex (υc) and aquo-ion (υa) energy levels (cm−1), average nephelauxetic ratio ( β ) and the bonding parameter (δ) of SPbKNLFNd10 glass. Transition from 4I9/2→ 4

4

D3/2 + D1/2 D5/2+ 2P1/2 2 D3/2 2 G9/2 2 K15/2 4 G9/2 4 G7/2 4 G5/2 2 G7/2 2 H11/2 4 F9/2 4 S3/2 4 F7/2 4 F5/2 2 H9/2 4 F3/2 β δ 2

λ

υc

υa

351 431 461 472 478 514 530 573 586 630 685 739 748 799 806 878

28490 23202 21692 21186 20920 19455 18868 17452 17065 15873 14598 13532 13369 12516 12407 11389

23465 18693 17095 16764 17642 15705 15443 14187 13888 12495 11762 10950 10859 10138 10033 9371 1.2349 −19.0218

calculated oscillator strengths. The relative magnitudes of the Ωλ parameters are useful for explaining the bonding, symmetry and stiffness of the host matrices. The evaluated JO intensity parameters for the present SPbKNLFNd10 glass have been collected in Table 3 along with reported

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Table 3 Judd-Ofelt parameters (Ωλ, x 10–20 cm2), spectroscopic quality factor (χ=Ω4/Ω6), experimental (τ exp, µs) and radiative (τ rad, µs) lifetime, stimulated emission cross-section (σemi, x 10–20 cm2), radiative transition probabilities (AT, s−1), experimental (βexp, %) and calculated (βcal, %) branching ratios, emission peak position (λemi, nm), effective bandwidth (Δλeff, nm), quantum efficiency (η, %), gain bandwidth (σemi x Δλeff, x 10–27 cm3), optical gain (σemi x τ rad, 10–25 cm2s), figure of merit (σemi x τ exp, 10–25 cm2s), non-radiative relaxation rate (WNR, s−1), density (ρ, g/cm3), refractive index (n),) and saturation intensity (IS, x108 W/m2) of 4F3/2→ 4I11/2 transition of SPbKNLFNd10 glass along with reported Nd3+: glasses. Glass Label

Ω2

Ω4

Ω6

χ

τ

SPbKNLFNd10 SBiLNd [37] NKZLSNd [38] SBANC [39] ZBSN1 [40] LNZSN [41] LTZSN [41] LLZSN [41] SAlNd [42] SFBNd [43] AN10 [44] TZLNd [45] LCB [46] NCB [46] KCB [46] PKCFAN [47] BBONd [48] ZnAlBiB [49] TZN10 [50] BiZNd [51] TZNLN [52] TZONd [53] BNaNf [54] LTTNd [55] GSBNd [56] TZnNd [57]

6.89 3.21 10.26 5.26 1.4 11.7 10.42 8.5 6.27 8.06 9.36 4.49 3.53 3.74 4.04 5.4 9.95 5.31 3.8 2.79 2.13 3.86 5.33 4.54 6.42 4.27

10.09 2.85 0.38 3.4 4.29 6.5 6.44 6.01 3.32 4.83 0.56 5.03 4.21 4.48 4.62 7.03 8.91 3.84 4.94 4.53 3.29 4.01 2.84 5.79 4.16 4.76

9.09 3.23 6.06 3.68 3.61 8.8 7.34 7.18 3.16 11.37 8.42 4.31 5.04 5.27 5.83 6.51 12 4.61 4.54 4.49 3.83 3.79 4.9 5.69 3.71 4.59

1.1 0.88 1.05 0.92 1.18 0.73 0.87 0.83 1.05 0.42 0.06 1.17 0.835 0.85 0.792 1.08 0.75 0.83 1.08 1 0.86 1.05 0.57 1.01 1.12 1.03

253 290 173 619.65 532 139 161 177 636 181 191 164 399 379 355 254 318 232 153 169 154 198 347 83 290 130

rad

τ

exp

240 263 135 315 – 62.97 95.5 106.4 – 122 169 162 – – – 237 250 – 104 83.3 136 158 151 68 191 –

σemi

AT

βcal

βexp

λemi

Δλeff

η

σemi x Δλeff

σemi x τ

4.11 2.33 4.3 2.38 2.42 – – – 2.36 3.46 – 4 2.965 2.465 3.195 3.42 2.49 3.67 4.27 3.36 4.27 4.2 4.18 4.75 2.1 4.27

3910 3401 5715 810.56 1881 3216 3218 3347 742.73 2666 3731 6112 2501 2635 2814

63 33 63 50 45 45 52 59 47 55 63 45 – – – 47 51 72 60 48 – 45 38 71 46 47

62 – 48 – – – – – – 50 – – – – – 65 – 50 60 62 – 47 41 48 – –

1056 1058 1065 1059 1060 1060 1060 1060 1058 1075 1060 1063 1058 1060 1061 1056 1062 1060 1061 1065 1061 1062 1062 1062 1070 1062

36.4 34 38 41.82 – – – – 24 27 – – 30 38 32

94 91 78 51 – 45 59 60 – 67 78 98 – – – 93 79 – 60 49 88 79 43 82 66 –

1.49 0.79 1.63 0.99 – – – – 0.56 0.93 – – 0.88 0.93 1.02 – 0.98 1.1 1.32 1.24 1.21 0.1 0.73 1.94 1.02 1.28

103.98 67.57 74.39 147.5 128.7 – – – 150.09 62.62 – 65.6 118.3 93.42 113.4 86.86 79.18 85.14 65.33 56.78 65.75 83.16 145.04 39.42 60.9 55.5

3145 2136 5920 6494 2301 – 1204 1576 3311.9

39.4 30 31 37 28.35 24 18 41 49 30.2

rad

σemi x τ exp

WNR

ρ

n

IS

98.64 61.27 58.05 74.97 – – – – – 42.21 – 64.8 – – – 81 62.25 – 44.4 27.9 58.07 66.36 63.11 32.3 40.11 –

215 354 1627 1562 – – – – – 2050 – 75 – – – 282 885 – 3079 6088 860 1279 3740 2657 1785 –

4.01 – – – 7.25 3.07 3.64 3.23 – – 4.301 – 4.53 4.37 4.21 2.896 4.22 3.29 – 6.45 – – 2.58 – – –

1.651 – – 1.462 2.23 1.77 1.79 1.78 – – 1.645 – 1.531 1.534 1.532 1.536 1.62 1.805 – – – – 1.629 – – –

1.91 3.06 3.21 2.5 2.23 – – – – 4.37 – – – – 2.32 3 – 4.21 6.66 3.22 2.82 2.96 5.79 4.63 –

SAlNd [42], TZLNd [45], PKCFAN [47], TZN10 [50], BiZNd [51], 85TeO2 +15ZnO + Nd2O3 (TZONd) [53], LTTNd [55], GSBNd [56] and TZnNd [57] glasses and less than those reported for 54 SiO2 +25Bi2O3 +20Li2O +1.0Nd2O3 (SBiLNd) [37], SBANC [39], LNZSN [41], 25Na2O +5LaF3 +10CaF2 +10AlF3+49 B2O3+1.0 NdF3 (SFBNd) [43], AN10 [44], 20CdO+20Li2O+59.5H3BO3+0.5Nd2O3 (LCB) [46], 20CdO+20 Na2O+59.5H3BO3+0.5Nd2O3 (NCB) [46], 20CdO+ 20K2O+59.5H3BO3+0.5Nd2O3 (KCB) [46], 45B2O3+50BaO+5Nd2O3 (BBONd) [48], 20ZnO+10Al2O3+9 Bi2O3+60B2O3+1.0 Nd2O3 (ZnAlBiB) [49] and BNaNf [54] glasses indicates that the 4F3/2→4I11/2 transition possesses more probability for laser action in the present SPbKNLFNd10 glass.

3.3.3. NIR emission and radiative properties The NIR emission spectra of SPbKNLF glasses doped with different concentrations of Nd3+ ions were measured with 808 nm laser diode excitation are shown in Fig. 5. The spectra consists of three emission bands centered at 881, 1056 and 1325 nm attributed to 4F3/2→4I9/2, 4 F3/2→4I11/2 and 4F3/2→4I13/2 transitions, respectively [19,36,44,50]. Among the three transitions, the 4F3/2→4I11/2 transition is most intense one than the other two transitions. The radiative properties such as effective band width (Δλeff), radiative transition probability (AR), experimental (βexp) and calculated (βcal) branching ratios and stimulated emission cross-section (σemi) determined from the emission spectrum of SPbKNLFNd10 glass for the 4F3/2→4I9/2,11/2,13/2 transitions are collected in Table 4. The radiative transition probability (AR) can be calculated using the following equation [58,59].

⎛ n (n2 + 2)2 ⎞ 64π 4 A (ΨJ , Ψ ′J ′) = ×⎜ Sed + n3Smd ⎟ 3 ⎠ 3hλ (2J + 1) ⎝ 9

Fig. 5. Emission spectra (λexe =808 nm) of SPbKNLF glasses for different Nd3+ ion concentrations.

Table 4 Emission peak positions (λp, nm), effective bandwidths (Δλeff, nm), radiative transition probabilities (AR, s−1), stimulated emission cross-section (σemi, x 10–20 cm2), experimental (βexp) and calculated branching ratios (βR) for the SPbKNLFNd10 glass. Transition 4F3/2→

λP

(Δλeff)

AR

σemi

βexp

βR

4

881 1056 1325

43 36 47

820 2473 617 3910 253

0.47 4.11 1.94

0.13 0.62 0.23

0.20 0.62 0.15

I9/2 I11/2 4 I13/2 AT (s−1) τrad (µs) 4

(7)

where Sed and Smd are electric-dipole and magnetic-dipole line strengths, respectively, which are expressed using the following equations. 562

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

Smd =

e 2h 2 Ωλ (ΨJ L + 2S Ψ ′J ′)2 16π 2m2c 2

(8)

(9)

The sum of the radiative transition probability (AT) for all the terminal level gives the total radiative transition probability (AT) and is given by

AT (ΨJ ) =

∑ A (ΨJ , Ψ ′J ′)

(10)

The radiative lifetime (τR) of an emitting state ΨJ is calculated using the following equation

τR (ΨJ ) = [AT (ΨJ )]−1

(11)

The fluorescent branching ratio (βR) can be obtained from the following equation

βR (ΨJ , Ψ ′J ) =

A (ΨJ , Ψ ′J ′) AT ΨJ

(12)

The experimental branching ratios are obtained from the relative areas under the emission peaks. The peak stimulated emission crosssection (σemi) can be determined from the following expression

σemi =

λp 4 8πcn2Δλeff

AR (ΨJ , Ψ ′J ′)

Fig. 6. Branching ratio and gain bandwidth for different Nd3+-doped glasses.

(13)

Where n is the refractive index, λp is the emission transition peak wavelength and Δλeff is the effective linewidth of the 4F3/2→4I9/2, 11/2, 13/2 transitions and is given by

Δλeff =

1 Imax

∫ I (λ) dλ

(14)

Where I is the fluorescence intensity and Imax is the intensity at band maximum. From the Table 4, it is noticed that the 4F3/2→4I11/2 transition exhibits higher stimulated emission cross-section (4.11×10–20 cm2). The stimulated emission cross-section is an important parameter that influences the potential laser performance and its value signifies the energy extraction from the lasing material. From Table 3, it is noticed that the stimulated emission cross-section is higher compared to all other reported glasses except for TZN10 [50], TZNLN [52], LTTNd [55] and TZnNd [57] which clearly indicates that the present SPbKNLFNd10 glass could be useful for the laser emission at 1.056 µm. Moreover, the experimental and calculated branching ratios are more or less similar for the SPbKNLFNd10 glass and are comparable to those of other reported glasses listed in Table 3. The highest value of branching ratio (βR) is an attractive feature for lower threshold and higher gain of lasers. The fluorescence bandwidth (Δλeff) obtained for 4F3/2→4I11/2 laser transition of SPbKNLFNd10 glass at 1056 nm is 36 nm. The gain bandwidth (σemi x Δλeff) defines the range of frequencies into which the optical amplification can occur while the optical gain (σemi x τR) evaluates the laser threshold. The gain bandwidth and optical gain for the 4F3/2→4I11/2 laser transition in the SPbKNLFNd10 glass is found to be relatively higher than those presented in Table 3. Fig. 6 shows branching ratio (βR) and gain bandwidth (σemi x Δλeff) for different Nd3+-doped glasses. In Fig. 6, initially gain bandwidth has been arranged from higher to lower values, with respect to glass composition and then branching ratios are arranged accordingly. Hence, the SPbKNLFNd10 glass is suitable for a broadband optical amplification since it exhibits relatively larger value of gain bandwidth. For good lasers, the figure of merit (σemi x τexp) should be as large as possible to attain high gain. In the present study, the figure of merit was found to be 98.64×10−5 cm2s for the SPbKNLFNd10 glass which is highest compared to all other reported glasses (see Table 3). Also, Fig. 7 shows the figure of merit for different Nd3+:glasses which indicates that

Fig. 7. Figure of merit versus Nd3+-doped glasses.

the SPbKNLFNd10 glass has possibility to attain high gain for good laser applications, where the glass compositions are arranged with decreasing order of figure of merit. Moreover, as seen from Table 3, most of the laser parameters for the 4F3/2→4I11/2 transition of the SPbKNLFNd10 glass are found to be better to those reported for other Nd3+-doped glasses. Therefore, a suitable broadband optical amplification at 1056 nm (4F3/2→4I11/2) might be achieved from the SPbKNLFNd10 glass.

3.3.4. Decay time analysis The fluorescence decay curves of 4F3/2 level of SPbKNLF glass-doped with different concentrations of Nd3+ ions, measured by monitoring emission at 1056 nm are shown in Fig. 8. The decay curve exhibits single exponential nature at lower concentrations (0.1 and 0.5 mol%) and turned into bi-exponential at higher concentrations (1.0, 2.0 and 3.0 mol%). Lifetimes of the 4F3/2 level have been determined by finding the first e-folding and average lifetime (τavg) for the single and biexponential decay curves, respectively. The average lifetime has been determined by the formula

τavg = A1 τ12 +A2 τ2 2 / A1 τ1 +A2 τ2 563

(15)

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3.3.5. Non-radiative processes The non-radiative transition rate (WNR) is an important parameter to obtain information on the non-radiative processes that could affect the optical amplification. The non-radiative transition rate (WNR) can be evaluated from the fluorescence and radiative lifetimes of the 4F3/2 emitting level from the expression

WNR =

1 1 − τF τR

(17)

The above equation contains the main non-radiative effects as multiphonon relaxation, concentration quenching and energy transfer among optically active and surrounding ions. The hydroxyl content (OH-) for the present glass is found to be low value, consequently, the non-radiative transition rate is also low (2.29×1020 ions/cm3). Therefore, the effect of hydroxyl ions on non-radiative process in this particular host is less. Non-radiative transition rates of the 4F3/2 emitting level for the SPbKNLFNd10 glass along with some other Nd3+-doped glasses are listed in Table 3. The less non-radiative relaxation rate of the 4F3/2 level in the SPbKNLFNd10 glass (215 s−1) indicates that it can be used as a laser material for amplification at 1056 nm.

Fig. 8. Decay curves for the 4F3/2 level of SPbKNLF glasses for different Nd3+ ion

3.3.6. Saturation intensity The pump power to reach threshold for cw laser operation is directly proportional to the saturation intensity (IS) which depends on the material properties such as σ(λp) and τexp. The IS is given by [60].

IS =

hc λp σ (λp ) τexp

(18)

From the above expression, it is clear that the material with a larger product of σ(λp) and τexp results in lower value of the IS which in turn yield lower laser threshold. The IS value for the SPbKNLFNd10 glass is calculated to be 1.91×108 W/m2 which is very low compared to other reported Nd3+ doped glasses listed in Table 3. All these results strongly confirm that the SPbKNLFNd10 glass could be used as 1.056 µm laser gain media with relatively lower threshold power than the other reported glasses. 3.4. Mechanical properties-Vickers's hardness measurement Fig. 9. Experimental lifetime for 4F3/2 level of Nd3+-doped glasses.

The hardness of a material is its ability to withstand an applied load without failure or plastic deformation. The Vickers's hardness measures the material ability to resist permanent deformation induced by a harder material. The Vickers's hardness number (HV) is calculated by using well known expression as follows [61].

where A1 and A2 are constants, τ1 and τ2 are the short and long decay times, respectively. The lifetimes (τ) of 4F3/2 level are found to be 391, 344, 240, 83 and 41 µs for the 0.1, 0.5, 1.0, 2.0 and 3.0 mol%, respectively, for Nd3+:SPbKNLF glass. The lifetime is decreasing with increase of Nd2O3 concentration due to concentration quenching through the energy transfer among the Nd3+ ions [59]. The radiative lifetime calculated for the SPbKNLFNd10 glass is 253 µs. The experimental and radiative lifetimes for 4F3/2 level of Nd3+ ion in the SPbKNLF glass were compared with other Nd3+-doped glasses in Table 3. Fig. 9 shows the variation of experimental lifetimes of different Nd3+-doped glasses, where the glass compositions are arranged with increasing order of lifetime. The quantum efficiency (η) is defined as the ratio of the number of photons emitted to the number of photons absorbed. For trivalent rare earth (RE3+) ion systems, it is equal to the ratio of the τexp and τrad for respective levels and is given by

η = τexp / τrad

HV =

1.8544F d2

(19)

Where ‘F′ is the force in kg and ‘d′ is the average diagonal length of the indentation mark in mm. The hardness value is measured from the observed size of the impression remaining after a loaded indenter has penetrated and removed from the surface. All the measurements were taken at the load of 50 g and dwell time of 5 s at room temperature. The values are specified as the average values of 10 independent indentations made on the sample under identical loading conditions. The Vickers's hardness value for the present SPbKNLFNd10 glass is found to be 3.59 GPa, which is in good agreement with the reported glass systems [61–66] and concluded that the present glass have high mechanical strength and could be useful for many practical applications.

(16)

As can be seen from Table 3, the obtained ‘η’ value for the SPbKNLFNd10 glass is found to be 94% which indicates that the present glass could be used as laser material. The quantum efficiency of the present glass is high compared to other reported glasses except TZLNd [45], suggesting the less influence of non-radiative processes in the present glass.

4. Conclusions Nd3+-doped leadfluorosilicate glasses (SPbKNLFNd) were prepared by melt quenching technique and studied their compositional dependent, structural, near infrared emission, and mechanical properties. The 564

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