Optical absorption and fluorescence properties of Tm3+-doped K–Mg–Al phosphate glasses for laser applications

Journal of Alloys and Compounds 496 (2010) 335–340

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Optical absorption and fluorescence properties of Tm3+ -doped K–Mg–Al phosphate glasses for laser applications R. Praveena a , Kyoung Hyuk Jang a , C.K. Jayasankar b , Hyo Jin Seo a,∗ a b

Deapartment of Physics, Pukyong National University, 599-1 Daeyeon 3-Dong, Namgu, Busan 608-737, South Korea Department of Physics, Sri Venkateswara University, Tirupati 517-502, India

a r t i c l e

i n f o

Article history: Received 3 November 2009 Received in revised form 30 January 2010 Accepted 5 February 2010 Available online 11 February 2010 Keywords: Fluorescence properties Tm3+ Phosphate glasses Judd–Ofelt analysis Laser applications

a b s t r a c t Metaphosphate glasses doped with different concentrations of Tm3+ ions have been prepared and characterized through absorption, emission and decay measurements at room temperature. The peak positions in the absorption spectra have been identified and analyzed using free-ion Hamiltonian model. Judd–Ofelt analysis has been carried out on the absorption spectrum of 1.0 mol% Tm3+ -doped phosphate glass and the intensity parameters have been determined. These intensity parameters have been used to predict the radiative properties for the florescence levels of the Tm3+ ion. The emission spectra in the UV–VIS region have been recorded with an excitation wavelength of 355 nm. The branching ratios and stimulated emission cross-sections show that the 1 D2 → 3 F4 transition of Tm3+ ions can be used as the potential laser emission at ∼451 nm. Decay profiles for the 1 D2 level of Tm3+ ions have been measured by monitoring the 1 D2 → 3 F4 transition. The decay curves exhibit non-exponential nature for all concentrations and are analyzed under the framework of Inokuti–Hirayama model. The results obtained in this paper have been compared with other reported Tm3+ -doped glasses. © 2010 Elsevier B.V. All rights reserved.

1. Introduction In recent years, advancements in Tm3+ -doped materials for telecommunication devices have been used to expand the transmission bandwidth of optical fibers beyond the range available from Er3+ -doped fiber amplifiers (C-band), by taking advantage of the 1.4 ␮m emission wavelength of Tm3+ ion [1,2]. In addition to the 1.4 ␮m emission band, a number of studies have also been carried out to develop the 1.8 and 2.0 ␮m emission band of Tm3+ ion, which are of potential interest for eye-safe LIDAR, remote sensing and medical laser applications [3,4]. On the other hand, Tm3+ ion is one of the most efficient lanthanide (Ln) ions for obtaining blue upconversion emission to be used in visible lasers, optical amplifiers, color display, under-sea optical communications, biomedical diagnostics, sensors, etc. [5–10]. The important 1 D2 → 3 F4 transition of Tm3+ ion gives blue emission at ∼450 nm in both down- and upconversions [11,12] which is useful for high capacity data storage optical devices. The spectroscopic investigations and intensity analysis play a prominent role in characterizing the Ln-doped glasses as well as to understand some fundamental phenomenon and interactions involving radiation and matter. Among the phosphate glasses, alkali and alkaline earth metaphosphate (O/P ≈ 3.0) glasses are

very attractive hosts due to their high transparency to ultraviolet light. Moreover, the P2 O5 –K2 O–BaO–Al2 O3 glass composition when doped with significant amount of Nd2 O3 (well known commercial laser glass composition as LG-750, Schott Glass Technologies, USA) has its own significance in the market for high average energy and high peak power solid-state laser applications [13–15]. In our earlier paper, the detailed UV–VIS spectroscopic properties of Tm3+ -doped P2 O5 –K2 O–BaO–Al2 O3 (PKBAT) glasses at room temperature (RT) and 15 K have been reported [16]. In the present investigation, the optical properties and energy transfer studies of Tm3+ -doped P2 O5 –K2 O–MgO–Al2 O3 (PKMAT) glasses in the UV–VIS–NIR range have been studied through their optical absorption and emission spectra and decay measurements at RT. The optical absorption spectrum of 1.0 mol% Tm3+ -doped glass has been analyzed in the light of Judd–Ofelt (JO) theory and radiative properties such as radiative transition probabilities, radiative lifetimes, branching ratios and stimulated emission cross-sections have been determined. The Inokuti–Hirayama (IH) model has been applied to the decay curves of the 1 D2 level. A careful analysis of the laser transitions has been carried out to unravel the nature of nonradiative processes which limit the radiative properties in these glasses. 2. Experimental

∗ Corresponding author. Tel.: +82 51 629 5568; fax: +82 51 629 5549. E-mail address: [email protected] (H.J. Seo). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.02.007

The metaphosphate glasses with the composition (in mol%) of (where x = 0.01, (60 − x/2)P2 O5 + 14K2 O + (16 − x/2)MgO + 10Al2 O3 + xTm2 O3 0.1, 0.5, 1.0 and 2.0 mol%) were prepared and characterized through the procedure

336

R. Praveena et al. / Journal of Alloys and Compounds 496 (2010) 335–340 Table 1 Experimental (Eexp , cm−1 ) and calculated energies (Ecal , cm−1 ) and experimental (fexp , ×10−6 ) and calculated (fcal , ×10−6 ) oscillator strengths of 1.0 mol% Tm3+ -doped PKMAT glass. Level

Energies Eexp

3

H6 F4 H5 3 H4 3 F3 3 F2 1 G4 1 D2 1 I6 3 P0 3 P1 3 P2

0 5,811 8,258 12,610 14,556 15,152 21,325 27,933 – – – –

3 3

Oscillator strengths Ecal 17 5,806 8,241 12,649 14,544 15,119 21,319 27,950 35,032 36,042 36,974 38,317

fexp

fcal

– 4.74 3.88 5.78 4.73 0.78 2.02 3.36

– 3.03 2.00 2.28 1.56 0.20 0.96 2.13

Fig. 1. FT-IR transmittance spectrum of 1.0 mol% Tm3+ -doped PKMAT glass.

described in our earlier paper [16]. The absorption spectrum was measured on spectrophotometer U-3400 with a resolution of 1.0 nm. The FT-IR spectrum was recorded using Perkin-Elmer FT-IR spectrophotometer in the spectral range of 400–4000 cm−1 . Infrared emission spectrum was measured with a Spex Fluorolog-3 spectrophotometer using an excitation wavelength of 700 nm. All the measurements were made at RT.

3. Results and discussion Fig. 1 shows the FT-IR transmittance spectrum of 1.0 mol% of Tm3+ -doped PKMAT glass. The spectrum consists of 10 bands centered at 753, 886, 1088, 1255, 1730, 1977, 2028, 2159, 2926 and 3749 cm−1 . The bands at 753 and 886 cm−1 are associated to symmetric and asymmetric stretching vibrations of P–O–P groups, respectively. The peak at 1088 cm−1 is due to the PO3 symmetric stretching mode while the peak at 1255 cm−1 is assigned as PO2 asymmetric stretching vibration mode [17]. The bands from 1730 to 3749 cm−1 belong to OH groups [18–23]. The RT optical absorption spectrum originating from the 3 H6 ground state to various excited 3 F4 , 3 H5,4 , 3 F3,2 , 1 G4 and 1 D2 levels of 1.0 mol% Tm3+ -doped PKMAT glass, is shown in Fig. 2. The band positions in this spectrum are similar to those reported in earlier papers [1,2,4,5,16] except minor variations in their relative positions, broadening and intensities due to changes of chemical environment around the Tm3+ ion. The observed energy levels obtained from the absorption spectra have been analyzed using the free-ion Hamiltonian model as explained in our earlier

Fig. 2. Absorption spectrum of 1.0 mol% Tm3+ -doped PKMAT glass at room temperature.

paper [16]. In the present systematic energy level analysis, only Coulomb (Fk , k = 2, 4 and 6) and spin–orbit () interaction parameters have been varied using the values of PKBAT [16] glass as initial parameters. The experimental and calculated energy levels are presented in Table 1. The best fitted free-ion parameters (cm−1 ) obtained for PKMAT (PKBAT [16]) glass are EAVG = 17,976 (18,002), 4 6 F2 = 102,906 (103,291), (50,680),  kF = 75,036 (75,297), F =250,428 F = 228, 370 (229, 268), F /F4 = 1.37 (1.37),  = −2645 (−2642), F2 /F6 = 2.04 (2.04) with r.m.s. deviation of ±35 (±16) for 8 (8) levels. The other parameters, ˛ = 18.19, ˇ = −745,  = 1583, M0 = 7.97, M2 = 0.56M0 , M4 = 0.38M0 , P2 = 982, P4 = 0.75P2 and P6 = 0.50P2 , were fixed to the values of Tm3+ :LaCl3 [24]. From these best fitted values, it is clear that there is no much variation in the magnik tudes  of ‘F ’ and ‘’ interaction parameters, net electrostatic effect ( F k ) and hydrogenic ratios (F2 /F4 , F2 /F6 ) of PKMAT and PKBAT [16] glasses. This indicates that the radial integral part of the forbital of the Tm3+ ions remains unchanged even though the glass compositions are changed, which is mainly due to shielding of 4f electrons by the 5s2 and 5p6 orbitals. The bonding parameter (ı, cm−1 ) [25,26] is defined as



ı=

¯ 1−ˇ ¯ ˇ



× 100



(1)

¯ =( where ˇ ˇ)/N and ˇ (the nephelauxetic ratio) = c /a , ‘c ’ N and ‘a ’ are the energies of the corresponding transitions in the complex and aquo-ion, respectively, and ‘N’ refers to the number ¯ values. ‘ı’ may be positive or of levels that are used to compute ‘ˇ’ negative depending on environmental field, which indicates the covalent or ionic bonding between the Ln3+ ion and the ligand. The ‘ı’ values for PKMAT, tellurite [12] and PKBAT [16] glasses are found to be −0.9214, −1.3112 and −1.0041, respectively. The negative quantities of ‘ı’ indicate that the bonding of Tm3+ ions with the surrounding ligands is ionic in these glasses. Based on the relative values of ‘ı’ among the various glasses, it is found that the ionic character decreases in the order of tellurite < PKBAT < PKMAT glasses. The tellurite glass exhibits higher ionic character whereas PKMAT glass exhibits lower ionic character. On the other hand, the covalency is large for PKMAT glass than PKBAT and tellurite glasses. Oscillator strengths have been estimated from the optical absorption spectrum of 1.0 mol% Tm3+ -doped PKMAT glass and are shown in Table 1. The oscillator strengths of different absorption transitions are found to be relatively larger than those of phosphate [1], tellurite [12], PKBAT [16] and fluorophosphate [27] glasses which indicate that relatively higher non-symmetric component of electric field is acting on the Tm3+ ions in PKMAT glass. Using these oscillator strengths and refractive index (1.512), JO analysis has been carried out following the same procedure given in

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Table 2 Judd–Ofelt parameters (˝ , ×10−20 cm2 ) for 1.0 mol% Tm3+ -doped PKMAT glass and some of the reported Tm3+ :glasses. System

˝2

˝4

˝6

PKMAT (present work) Phosphate [1] Germanate [2] Oxyfluoroborate [11] Tellurite [12] PKBAT [16] Fluorophosphate [27] Borate [28] Tellurite [29] ZBLAN [30] ZBLALi [31] Indium-fluoride [31] CaBaMg aluminate [32] Fluoro-indate [33]

9.32 ± 0.18 8.08 5.55 8.37 3.37 5.23 4.12 9.18 5.04 2.21 2.09 2.05 4.38 2.36

1.82 ± 0.05 3.05 2.03 3.20 1.03 1.95 1.47 2.91 1.36 1.71 1.74 1.58 1.12 1.59

3.21 ± 0.08 1.16 1.26 4.34 8.51 0.72 0.72 1.87 1.22 0.92 0.91 1.12 0.82 1.21

our earlier paper [16] and the intensity parameters (with an r.m.s. deviation ± 0.54) for 1.0 mol% of PKMAT glass has been determined and are presented in Table 2 along with some reported values. The reduced matrix elements are taken from the values of Tm3+ doped phosphate and tellurite [29] glasses. All the bands of the absorption spectrum are dominated by electric-dipole transitions except the transition 3 H6 → 3 H5 , which contains large electricdipole and small magnetic-dipole contributions. It is worth noting that the magnetic-dipole oscillator strengths are host independent. As the ratio between the magnetic-dipole and electric-dipole is quite negligible, all the absorption transitions are assumed as induced electric-dipole allowed and magnetic-dipole contributions were not taken into consideration for calculating JO parameters. The estimated errors for JO parameters in Table 2 do not include the systematic error due to uncertainties in the concentration of Tm3+ ions and the thickness of the sample but taken into account only the uncertainties in the integration of the absorption spectra due to noise in the measurements and determination of the peak values and the resolution/accuracy of the spectra. The estimated uncertainties of the JO parameters shown in Table 2 were found to be larger than those estimated for Er3+ :oxyfluoride glass [34] but smaller than those found for Er3+ :LiNbO3 crystal [35]. As can be seen from Table 2, it is observed that the trend of JO parameters (˝2 > ˝4 > ˝6 ) is found to be same for phosphate [1], germanate [2], PKBAT [16], fluorophosphate [27], borate [28], tellurite [29], ZBLAN [30], ZBLALi [31], indium-fluoride [31], CaBaMg aluminate [32] and fluoro-indate [33] glasses. The trend of ˝2 > ˝6 > ˝4 is noted for PKMAT similar that of oxyfluoroborate [11] glasses and the trend of ˝6 > ˝2 > ˝4 was observed for tellurite [12] glass. It is well known that ‘˝2 ’ is the most sensitive to local structure and glass composition and its value is indicative of the amount of covalent bonding between Ln3+ ions and ligand anions [36]. From Table 2, it is also clear that the magnitude of the ‘˝2 ’ for PKMAT glass is found to be higher than those of the other reported glass systems. The larger value of ‘˝2 ’ suggests more asymmetry at the Tm3+ ion site and the larger Tm–O covalency in the PKMAT glass than the other reported glasses. The larger Tm–O covalency in the PKMAT glass is also evident from the nephelauxetic effect. Both nephelauxetic effect and JO theory demonstrates that Tm3+ ions in the PKMAT glass experience relatively higher covalency and therefore both approaches describe similar conclusion and are on a physically sound basis. The JO parameters can be used to predict the radiative lifetime ( R ) for the 1 D2 and 1 G4 levels of the Tm3+ ion. In the present PKMAT glass, the ‘ R ’ for 1 D2 and 1 G4 levels are found to be 24 and 273 ␮s, respectively, which are found to be less than those of other Tm3+ -doped phosphate [1], PKBAT [16], fluorophosphate [27] and fluoro-indiate [33] glasses. Fig. 3 shows the emission spectra of PKMAT glasses for different concen-

Fig. 3. Normalized emission spectrum (UV–VIS) of Tm3+ -doped PKMAT glasses for different concentrations of Tm3+ ions at RT. The excitation wavelength (exc ) = 355 nm. The inset shows the variation of intensity for the 1 D2 → 3 F4 transition of Tm3+ ions in PKMAT glasses with respect to the concentration.

trations measured exactly under the same experimental conditions. The spectra contains two strong bands at ∼365 and ∼451 nm and a weak band at ∼478 nm, which correspond to 1 D2 → 3 H6 , 1 D2 → 3 F4 and 1 G4 → 3 H6 transitions, respectively. It is well know that this weak band is caused by the non-radiative decay due to the multiphonon relaxation from the 1 D2 to 1 G4 level. Among these three transitions the blue (∼451 nm) band is more intense. The intensity of 1 D2 → 3 F4 transition is plotted against Tm3+ ion concentration and is shown in the inset of Fig. 3. From Fig. 3, it is observed that the intensity increases with increasing concentration of Tm3+ ions up to 1.0 mol% and then decreases with further increase in concentration. This implies that the 1 D2 → 3 F4 transition has optimum intensity for the 1.0 mol% of Tm3+ -doped PKMAT glass. The effective linewidth (eff ), emission cross-section ((p )) and branching ratios (ˇR ) for the 1 D2 → 3 H6 and 1 D2 → 3 F4 transitions of Tm3+ ion in PKMAT glass are calculated from the emission spectra and are collected in Table 3. The predicted ‘ˇR ’ values using the JO theory are also presented in Table 3. It is clear from Table 3 that the experimental and calculated ‘ˇR ’ values are in good agreement. The ‘ˇR ’ values are found to be higher for 1 D2 → 3 F4 transition whereas lower for 1 D2 → 3 H6 transition in PKMAT glass than that of PKBAT [16] glass. It is well known that an emission level with ‘ˇR ’ value of more or equal to 50% can be a potential laser emission transition [37]. Hence, the present glass with an experimental ‘ˇR ’ of 0.78 may be considered for blue laser (451 nm) emission under UV (355 nm) pumping. Furthermore, there is large difference observed in ‘eff ’ values for 1 D2 → 3 F4 and 1 D2 → 3 H6 transitions of PKMAT and PKBAT [16] glasses but very slight difference has been noticed in their ‘(p )’ values. This may be due to the higher radiative transition probabilities for these transitions in the PKMAT glass than that of PKBAT [16] glass. Fig. 4 shows the infrared emission Table 3 Emission peak positions (p , nm), radiative transition probabilities (A, s−1 ), effective bandwidths (eff , nm), experimental and calculated branching ratios (ˇR ) and stimulated emission cross-sections ((p ), ×10−20 cm2 ) of the 1 D2 → 3 F4 and 3 H6 transitions of 1.0 mol% Tm3+ -doped PKMAT glass. Transition

p

A

eff

D2 → 3 F4 1 D2 → 3 H6

452 365

25,927 8,353

13.7 9.00

1

ˇR

(p )

exp

cal

0.78 0.22

0.76 0.24

4.59 0.96

338

R. Praveena et al. / Journal of Alloys and Compounds 496 (2010) 335–340 Table 4 Physical parameters (concentration, density (d, gm/cc), path length (l, cm)), experimental lifetime ( exp , ␮s), quantum efficiency ( , %), energy transfer parameter (Q), critical distance (R0 , A0 ) and donor–acceptor interaction parameter (CDA , ×10−50 m6 /s) of Tm3+ -doped PKMAT glasses. Concentration

Fig. 4. Normalized IR emission spectrum of 2.0 mol% of Tm3+ -doped PKMAT glass at RT. The excitation wavelength (exc ) = 700 nm.

spectrum of 2.0 mol% Tm3+ -doped PKMAT glass in the range of 1300–1600 nm. The maximum peak position is at 1453 nm which corresponds to the 3 H4 → 3 F4 transition. The value of ‘eff ’ for this transition is found to be 115 nm. It is broader by nearly 40 nm compared to fluoride glasses [2] which makes them attractive for broadband amplifiers especially in the wavelength range that overlaps the conventional band of erbium-doped fiber amplifiers. Fig. 5 shows the decay profiles for the 1 D2 level of Tm3+ ions in the PKMAT glasses. The decay curves are found to be non-exponential for all concentrations of Tm3+ ion. Hence, the experimental lifetimes ( exp ) for the 1 D2 level in all PKMAT glasses have been calculated from the expression,

 tI(t) dt exp =  I(t) dt

(2)

The values of  exp calculated from the above expression are shown in Table 4. The  exp values decreases from 24 to 7 ␮s when the concentration of Tm3+ ions increases from 0.01 to 2.0 mol%. The non-exponential decay curves for the PKMAT glass indicate the presence of energy transfer processes between Tm3+ ions

mol%

(×1020 ) ions/cc

0.01 0.1 0.5 1.0 2.0

0.05 0.25 1.26 2.52 5.05

d

l

 exp



Q

R0

CDA

4.759 2.519 2.543 2.576 2.638

0.310 0.228 0.298 0.320 0.297

24 20 16 11 7

100 83 67 46 29

0.44 0.89 1.36 2.03 3.06

23 17 11 10 9

493.53 80.46 7.05 3.40 1.77

even at its lower concentration (0.01 mol%). The non-exponential behavior of the decay curves is found to increase with increase in Tm3+ ion concentration associated with reduced lifetimes. This concentration dependent quenching of lifetime is attributed to the increase of the energy transfer processes between Tm3+ ions through one of the cross-relaxation channels: 1 D2 + 3 H6 → 1 G4 + 3 F4 and 1 G4 + 3 H6 → 3 F2,3 + 3 F4 [16]. At very low concentrations of dopant ions, the interaction between the optically active Ln3+ ions is negligible, the fluorescence decay curves can be fitted to a single exponential. However, when the concentration is large enough, the interaction between these ions becomes so prominent that energy transfer takes place from an excited Ln3+ ion (donor) to a non-excited Ln3+ ion (acceptor), leading to a non-exponential shape of the decay curves. The non-exponential nature of the fluorescence decay curves is fitted in the framework of IH model [38] to reveal the dominant mechanism of interaction. According to this model, the fluorescence decay intensity, I(t), is given by



I(t) = I0 exp

t − −Q 0

 t 3/S 0

where ‘t’ is the time after excitation, ‘ 0 ’ is the intrinsic decay time of the donors in the absence of acceptors. The value of ‘S’ (=6, 8 or 10) depends on whether the dominant mechanism of the interaction is dipole–dipole, dipole–quadrupole or quadrupole–quadrupole, respectively. The energy transfer parameter (Q) is defined as Q =

4

 3



1−

3 S



N0 R03

(4)

and depends at first on ‘S’ and the gamma function  (x), which is equal to 1.77 for dipole–dipole (S = 6), 1.43 for dipole–quadrupole (S = 8) and 1.3 for quadrupole–quadrupole (S = 10) interactions. ‘N0 ’ is the concentration of acceptors, which is almost equal to the total concentration of Ln3+ ions, and ‘R0 ’ is the critical distance defined as donor–acceptor separation for which the rate of energy transfer to the acceptors is equal to the rate of intrinsic decay of the donor. The donor–acceptor energy transfer parameter (CDA ) is related to the ‘R0 ’ by the relation, CDA = R06 0−1 If the distance between optically active Ln3+

Fig. 5. Decay profiles for the 1 D2 level of Tm3+ ions in PKMAT glasses at RT (exc = 355 nm). IH fit for S = 6 is also shown in the figure for all concentrations. The inset shows the lifetime of the 1 D2 level with respect to Tm3+ ion concentration.

(3)

(5)

ions decreases (due to increase in concentration) then one may expect an increase in the ‘Q’ parameter leading to faster fluorescence decays. Eq. (3) is valid for the special case of pulsed excitation and a random distribution of optically active ions (donors and acceptors) in the sample. This is because of the fact that during the pulsed excitation (in the order of nanosecond) there is no enough time for the donors to transfer the excitation energy to other ions (acceptors). The non-exponential decay curves of PKMAT glass have been well-fitted to the IH model for S = 6 indicating that the nature of interaction for energy transfer process between Tm3+ ions is of dipole–dipole type. From the IH model fitting, ‘Q, R0 and CDA ’ values have been evaluated. Here ‘ 0 ’ is obtained by freely varying the

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339

4. Conclusions

Fig. 6. Variation of quantum efficiency and energy transfer parameter with Tm3+ ion concentration in PKMAT glasses.

‘ 0 ’ value also for best fit during IH fitting (with S = 6) of the decay curve for 0.01 mol% Tm3+ -doped PKMAT glass as the decay curve for 0.01 mol% is also non-exponential. This value of ‘ 0 ’ (30 ␮s) has been fixed for the rest of the fittings of decay curves following the similar procedure as reported in our earlier paper [16]. The energy transfer between donor–acceptor can be of two types, one is static transfer and another one is fast diffusion. In the former case, the energy is transferred from an excited donor to an unexcited acceptor whereas in the later, the energy is transferred from an excited acceptor after migration to donors [39]. In the present investigation, static transfer process is expected to give the main contribution because pure exponential decay has not been observed for any concentration. Fast diffusion may be important for larger Tm3+ ion concentrations. The values of ‘ exp , Q, R0 and CDA ’ are presented in Table 4 along with the concentration, density and path length. As expected the values of ‘ exp , R0 and CDA ’ are decreasing with increasing concentration whereas ‘Q’ values are increasing with increasing concentration (Fig. 6). The values of ‘R0 ’ decreases drastically up to 0.5 mol% and then the variation is marginal for 1.0 and 2.0 mol% of Tm3+ ion concentrations. This may be due to the commencement of fast diffusion (energy migration) which involves resonant energy transfer among donors and finally to an impurity such as a transition metal ion or some other quenching center in the glass. The same trend in the values of ‘R0 ’ has also been observed in the PKBAT [16] glasses. Though the values of ‘ exp ’ and ‘R0 ’ in the PKMAT glasses are found almost similar to those of PKBAT [16] glasses, the ‘Q’ values are very high in the PKMAT glasses than that of PKBAT [16] glasses which clearly indicates the enhancement of energy transfer processes through cross-relaxation between Tm3+ ions in the PKMAT glasses. The quantum efficiency ( ) is defined as the ratio of the number of photons emitted to the number of photons absorbed and it can be calculated from the expression [6,40], =

exp × 100 R

(6)

The values of are presented in Table 4 which is found to decrease from 100 to 29 when the concentration of Tm3+ ions increased from 0.01 to 2.0 mol%. Although the JO analysis has an inherent inaccuracy of 20–30%, which has not been taken into account in our calculations, this can only be partially responsible for the difference in measured and calculated lifetimes. The efficiency is found to be more in the present PKMAT glasses than that of PKBAT [16] glasses.

The optical properties of Tm3+ ions in PKMAT glasses have been determined and are compared with those reported for PKBAT and other Tm3+ -doped glasses. The electronic structure of the Tm3+ ions in theses glasses has been deduced from the optical absorption spectra through free-ion Hamiltonian model. From the intensity analysis of absorption spectrum the JO intensity parameters have been evaluated. The value of ‘˝2 ’ is found to be larger compared to other reported values which indicates the higher asymmetry at the Tm3+ ion site in the PKMAT glasses. Both the nephelauxetic effect and JO analysis reveal that Tm3+ ions in PKMAT glass experience higher covalency indicating that both approaches are on a physically sound basis. The radiative transition probabilities for the 1 D2 → 3 F4 and 1 D2 → 3 H6 transitions are higher in PKMAT glass than that of PKBAT glass. The 1 D2 → 3 F4 transition is found to be most intense and its effective linewidth and branching ratios are very high in the PKMAT glass than that of PKBAT glass. The 3 H4 → 3 F4 emission at 1453 nm is important to achieve a band extension in the spectral range corresponding to the Sband amplifier, on the shorter wavelength side of the conventional erbium-doped amplifier (1530–1570 nm). The non-exponential decay curves are well-fitted to the IH model for S = 6, indicating that the energy transfer between Tm3+ ions is of dipole–dipole type. The ‘ exp , R0 and CDA ’ values are decreasing with increasing concentration whereas ‘Q’ value is increasing with increasing Tm3+ ions concentration. The values of ‘Q’ and ‘ ’ are found to be more in the PKMAT glasses compared to PKBAT glasses. Acknowledgements This study was financially supported by Pukyong National University in the 2008 Post-Doc Program and by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Government (Ministry of Education, Science and Technology, MEST) (No. 2009-0078682). CKJ is grateful to DAE-BRNS, Government of India (No. 2007/34/25-BRNS/2415, dt. 18-01-2008) for financial assistance. References [1] A. Kermaoui, F. Pelle, J. Alloys Compd. 469 (2009) 601. [2] R. Balda, L.M. Lacha, J. Fernández, M.A. Arriandiaga, J.M. Fernández- Navarro, D. Munoz-Martin, Opt. Exp. 16 (2008) 11836. [3] H. Lin, X.Y. Wang, C.M. Li, H.X. Yang, E.Y.B. Pun, S. Tanabe, J. Lumin. 128 (2008) 74. [4] H. Xia, Q. Lin, J. Zhang, Q. Zhang, J. Rare Earths 27 (2009) 781. [5] J. Li, X. Wang, Z. Jiang, Opt. Commun. 282 (2009) 4249. [6] K.S.V. Sudhakar, T. Satyanarayana, L. Srinivasa Rao, M. Srinivasa Reddy, N. Veeraiah, Solid State Commun. 146 (2008) 441. [7] C.P. Ortiz, D.M. Boye, J. Lumin. 128 (2008) 894. [8] R.P. Rao, J. Lumin. 113 (2005) 271. [9] M. Higuchi, T. Shimizu, J. Takahashi, T. Ogawa, Y. Urata, T. Miura, S. Wada, H. Machida, J. Cryst. Growth 283 (2005) 100. [10] T. Soukka, K. Kuningas, T. Rantanen, V. Haaslahti, T. Lovgren, J. Fluoresc. 15 (2005) 513. [11] K.S. Lim, P. Babu, C.K. Jayasankar, S.K. Lee, V.T. Pham, H.J. Seo, J. Alloys Compd. 385 (2004) 12. [12] G. Poirier, F.C. Cassanjes, C.B. de Araujo, V.A. Jerez, S.J.L. Ribeiro, Y. Messaddeq, M. Poulain, J. Appl. Phys. 93 (2003) 3259. [13] P.R. Ehrmann, J.H. Campbell, J. Am. Ceram. Soc. 85 (2002) 1061. [14] J.H. Campbell, T.I. Suratwala, J. Non-Cryst. Solids 263–264 (2000) 318. [15] J.H. Campbell, T.I. Suratwala, C.B. Thorsness, J.S. Hayden, A.J. Thorne, J.M. Cimino, A.J. Marker III, K. Takeuchi, M. Smolley, G.F. Ficini-Dorn, J. Non-Cryst. Solids 263–264 (2000) 342. [16] P. Babu, H.J. Seo, K.H. Jang, R. Balakrishnaiah, C.K. Jayasankar, A.S. Joshi, J. Phys.: Condens. Matter 17 (2005) 4859. [17] A.A. El-Kheshen, F.A. Khaliafa, E.A. Saad, R.L. Elwan, Ceram. Int. 34 (2008) 1667. [18] G.A.C.M. Spierings, Glastechn. Ber. 56K (1983) 1130. [19] P.F. McMillan, R.L. Remmele, Am. Mineral. 71 (1986) 772. [20] R. Ravindra, K.R. Krovvidi, A.A. Khan, A. Kameswara Rao, Macromolecules 30 (1997) 3288. [21] H. Kandori, Y. Shichida, J. Am. Chem. Soc. 122 (2000) 11745.

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