Absorption and emission spectral studies of Sm3+ and Dy3+ ions in PbOPbF2 glasses

IOffRNI~L OF

IiIQIMlmg ELSEVIER

Journal of Non-Crystalline Solids 217 (1997) 215-223

Absorption and emission spectral studies of in PbO-PbF 2 glasses

S m 3+

and Dy 3+ ions

P. Nachimuthu a, R. Jagannathan a,*, V. Nirmal Kumar b, D. Narayana Rao b a School of Chemistry, University of Hyderabad, Hyderabad 500 046, India b School of Physics, University ofHyderabad, Hyderabad 500 046, India Received 7 October 1996; revised 11 March 1997

Abstract The optical properties of lanthanide ions, Sm 3+ and Dy 3+ doped in the PbO-PbF 2 oxyfluoride glass matrix are similar to those of fluoride glasses as shown by the Judd-Ofelt parameters and emission spectral properties. The stimulated emission cross-sections (%) indicative of potential for lasers have been obtained. They are an order of magnitude less than the values reported for transitions in other rare earth ions. For Dy 3+ O-p is found to be the highest ( % × 1024 cm 2 = 1191 ___60) for the hypersensitive transition 4F9/2 --'-~6HI3/2. This transition also has the largest transition probability (A = 390 + 12 s - l ) and branching ratio (/3 R = 0.608 + 0.024) signifying most favored emission. The ratio (yellow/blue) of its intensity relative to the non-hypersensitive electric dipole transition is affected by the nature of the glass matrix and has the value of 1.58 + 0.02 for our glass sample. The quantum efficiencies for of Sm 3+ (0.2 wt%) and for 4F9/2 of Dy 3+ (0.2 wt%) are 52 ___2 and 49 + 2%, respectively. The fluorescent decay processes for both the ions follow single exponential at lower concentrations (0.2 wt%). At larger dopant concentrations (1.35 wt%) while Dy 3+ exhibits a single exponential decay, Sm 3+ does not, since energy transfer involving cross-relaxation becomes significant in the radiative decay process. Our studies show that for some applications oxyfluoride glasses can be a better substitute for fluoride glasses. © 1997 Elsevier Science B.V.

4G5/2

I. Introduction Optical properties of lanthanide ions doped in glasses in general are being extensively studied on account of their potential in the field of lasers and fiber optics [1-11]. However, samarium and dysprosium containing glasses have received relatively less attention than other lanthanide ions despite many features of interest. Samarium containing phosphate

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glass is known to have an unusual elastic behavior due to valence instability [12]. Samarium has promising characteristics for spectral hole burning studies [13,14]. Codoping of Sm 3+ as a sensitizer with Eu 3+ affects the emission of the lasing transition 5D 0 ~ 7F2 in the latter [15]. The decay of excited states in Sm 3+ involves different mechanisms depending on the matrix [16]. The 3.02 p~m 6H13/2 -->6H15/2 transition in Dy 3+ has been identified as a laser transition [17]. The hypersensitive t r a n s i t i o n 6Fll/2---') 6H15/2 of Dy 3+ at 1.34 p~m has potential for fiber amplifiers [6]. The yellow to blue intensity ratio of the emission lines in Dy 3+ is affected by metal-

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P. Nachimuthu et al. / Journal of Non-Crystalline Solids 217 (1997) 215-223

216

ligand bonding and site symmetry [18]. Sm 3+ and Dy 3+ in general have smaller emission efficiencies compared to other rare earth ions [19]. It is therefore of interest to understand the quenching mechanism and the influences of the matrices and codoped Ln 3÷ ions present as donors or acceptors on optical properties. We have chosen for the present study, the heavy metal oxyfluoride, PbO-PbF 2 glass, as the parent matrix for the following reasons. The oxyfluoride glasses can be prepared with relative ease and have good thermal stability and insensitivity to moisture [20]. Our own studies on the optical properties of Pr 3+, Eu 3+, Tb 3+ and Tm 3÷ ions doped in PbO-PbF 2 glasses show that these systems have features tending towards those of fluoride glasses [21-23]. Previous studies suggest that the rare earth ions doped in these glass matrices permit observation of fluorescence over a wide spectral region [2-5,23,24]. In this work the Judd-Ofelt parameters for Sm 3+ and Dy 3+ ions doped in PbOPbF 2 glass have been evaluated from their absorption spectra and the radiative properties derived. The variations in the relative intensities of the emission lines of Dy 3+ is compared with Eu 3÷ in different glass matrices. The stimulated emission cross-sections for a number of transitions for trivalent samarium and dysprosium are reported. The calculated radiative properties are compared with the results of emission spectra and experimentally measured life-

times. The influence of dopant concentration on the lifetimes are presented as well.

2. Experimental procedures The samples with compositions 30PbO-70PbF 2xSm 3+ and 30PbO-70PbF2-xDy 3+ ( x = 0 . 2 and 1.35 wt%) were prepared as described earlier [2023]. The starting materials used were reagent grade Pb304 (Sisco), PbF 2 (BDH) and respective rare earth oxides (Aldrich). Typical weight loss on melting under experimental conditions indicated that the actual compositions should be within + 0.5% of the values quoted for the components. The glass samples were annealed at a temperature below T~ and subsequently polished with commercial media and water free lubricant. The samples were obtained with a good transparency and uniform thickness of ~ 0.2 cm and ~ 2 cm diameter. The absorption spectra were measured in ultraviolet, visible and near infrared regions with spectrophotometers (Jasco 7800 and Cary 17D) using undoped glass as reference. The excitation and emission spectra were measured with a spectrofluorometer (Jasco FP-777). For measurement of decay times, the samples were excited with first (Asl) or second (As2) anti stokes line from a home built H e Raman

Table 1 Observed and calculated oscillator strengths of Sm 3+ in 30PbO-70PbF 2 glass Peak positions (nm + 1)

Transition from 6Hs/2

6H15/2 6Fi/2 a 6F3/2

6F5/2 6F7/2 6F9/2 6FI1/2 4G5/2

4F3/2 4 4 4 4 4 b 4G7/2419/2 MI5/2 4111/2 113/2 6F5/2 4 M17/2 4 G9/2 4 Ii5/2 ~, 4 4 4 ( P P)5/2 LI3/2 F7Z2 P3/2 KII/2 LI5/2 GI1/2 4 6 4 a4 4 ( D I / 2 P7/2 F9/2) L17/2 KI3/2 4 4 6 4 D3/z ( D

rms a Hypersensitive transition. b See text.

P)5/2 Hv/z

1505 1390 1225 1082 946 560 527 469 439 407 374 362

P ( l O -8) obs.

calc.

148 183 247 123 21 3

152 173 250 156 24 1

1

0

147 11 410 103 148

74 11 425 94 96 ~28

217

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Table 2 Observed and calculated oscillator strengths of Dy 3+ in 30PbO-70PbF2 glass Transition from 6HI5/2 Peak positions (nm ± 1) 6HII/2 6 a6 FII/2 H9/2 6 6 F9/2 H7/2 6 6 F7/2 H5/2 bF 413/2 a 15/2 4Gll/2 a 4 4 4 F7/2 K17/2 It3/..2 4MI9/2 6p5/2 (4p ,ID)3/2

P (10 -s)

1665 1285 1090 902 803 753 452 425 381 364

obs.

calc.

39 382 207 154 91 11 31 12 105 91

64 381 213 130 51 10 21 8 106 117 ± 19

rills a Hypersensitivetransition.

shifter operating at 3500 KPa. The pump source for the Raman shifter is the second harmonic (532 nm) of Nd3+:YAG laser (Continuum, USA, Model No. 660B-10, 6 ns, 10 Hz). Asl and As2 for the H 2 gas are 368.9 and 435.7 nm, respectively. The spectra recorded with a monochromator (Jobin Yvon Hrs2) were found to be similar to those recorded with spectrofluorometer (Jasco FP-777). The decay curves were recorded with a camera (DCS01GPH) and oscilloscope (Tektronix 2465B) controlled by a personal computer. The density measurements were made by Archimedes' method using xylene as the immersion liquid. The refractive index (r/) was measured by the Brewster angle method using H e - N e laser as a light source. The r/ value is found to be 1.779 __+0.001 for the undoped 3 0 P b O - 7 0 P b F 2 glass and is not influenced significantly by doping with Ln 3+ ions.

3. Results The room temperature absorption spectra of Sm 3+ and Dy 3+ doped in 3 0 P b O - 7 0 P b F 2 glasses had features identical to those reported for fluoride glass matrices [16,25,26]. The oscillator strengths for the observed transitions were deduced as described earlier [21,22,27]. The oscillator strengths were a least square fit to a linear combination of the Judd-Ofelt parameters and the matrix elements composed of U ~ (A = 2, 4, 6), the unit tensor operator connecting the

initial and final states of fN configuration. The values of U x (A = 2, 4, 6) parameters given by Camall were used for this purpose [28]. The values of oscillator strengths obtained from experimental spectra and those calculated using the Judd-Ofelt parameters are given in Tables 1 and 2 for Sm 3+ and Dy 3+, respectively, along with the respective root mean square (rms) deviations. The Judd-Ofelt parameters (g2x, A = 2, 4, 6) obtained are given in Table 3. These Judd-Ofelt parameters were used for deriving the radiative properties viz., radiative transition probabilities (A), lifetimes (r R) and branching ratios (/3R). The relevant expressions for obtaining the radiative parameters are given in our earlier works [21,22,27]. The values of these parameters are given in Table 4. The emission spectra of Sm 3+ and Dy 3+ in P b O - P b F 2 glass are reproduced in Figs. 1 and 2, respectively. The experimental branching ratios were obtained by the area method from the emission spectra [25,26]. These are reproduced along with

Table 3 The Judd-Ofelt parameters for lanthanide ions doped in 30PbO70PbF2 glass. The values in parenthesis are for fluoride glasses [4,18,19]. Lanthanide 02 (10 -20 cm 2) J']4 (10-2° cm2) 06 ( 10-2° crn2) ion + 0.02 + 0.2 + 0.3 Sm3+ Dy 3÷

1.16 (0.68) 2.13 (2.70)

2.6 (3.7) 2.1 (1.8)

1.4 (2.2) 1.0 (2.0)

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Table 4 The radiative properties of Sm 3+ and Dy 3+ in 30PbO-70PbF 2 glass, Transition

Ap ( n m + 1)

Samarium(III) 4G5/2 ~ 6H5/2 6H7/2 6H9/2 6Htl/2 6H13/2 6F3/2 6F5/ 2 6F7/2 6F9/ 2

563 599 646 706 781 917 947 1054 1208

Dysprosium(liD 4F9/2 ~6H15/2 6HI3/2 6H1~/2 6 6 Hg/2 Fll/2 6H7/2 6F9/2 66H 5/2 6F 7/2 Fs/2

481 575 662 758 877 1031 1200

A (s -I ) _+3%

/3R (calc.) _+4%

/3R (obs.) _+4%

9 92 71 26 2 1 8 2 1

0.044 0.432 0.332 0.123 0.012 0.004 0.038 0.010 0.005

0.118 0.636 0.188 0.032 0.025

30 304 295 139 41

4.46 + 0.15

136 390 35 25 22 7 3

0.220 0.630 0,057 0.040 0,035 0.012 0,005

0.384 0.608 0.004 0.004

162 1191

1.60 5:0.05

calculated branching ratios in Table 4. The stimulated emission cross-section (o-o) for a transition from ~0'J' to q~J is given by [29] (Fp

8OTC,~2 A Ap

a( ~J,q~'J'),

(1)

Ion

Conc. (wt%) +0.5%

r (ms) +0.01

xz

Remarks

Sm 3÷

0.20 1.35 1.35

2.33 1.87 2.28

3.0× 10-5 0.230 0.089

single exponential single exponential fitted with Eq. (2)

6H7/2

G 61"19/2

1 550

Dy 3+

0.20 1.35

0.79 0.52

5.0× 10 -5 9.7× 10 -5

single exponential single exponential

The error is given as X 2 = ~ ( O - E ) 2 / E where O is the observed data point and E the expected data point.

7 R (ms)

The excited state lifetimes (~-) of t h e 4G5/2 fluorescent level of Sm 3+ a n d 4F9/2 of Dy 3+ have been measured at different dopant concentrations. The fluorescent decay curves are reproduced in Figs. 3 and 4 for Sm 3+ and Dy 3+, respectively. For Dy 3+

where Ap is the average emission wavelength, A Ap is the effective emission band width, r/is the refractive index and A(q~J, q~'J') is the radiative transition probability. The values of stimulated emission cross-sections (O'p) are also given in Table 4.

Table 5 The measured lifetimes (r) of the 4G5/2 fluorescent level of Sm 3+ and 4F9/2 level of Dy 3+ ions in PbO-PbF 2 glasses at different dopant concentrations.

o-p (10 -24 cm 2) ± 5 %

=

L 600

I

650

6Y.113/2,

I

700

I

750

790

Wavelength (nm) Fig. 1. Emission spectrum of 0.2 wt% of Sm 3+ at room temperature in 30PbO-7OPbF 2 glass. The transitions are from the 4G5/2 to the indicated levels.

P. Nachimuthu et al./ Journal of Non-Crystalline Solids 217 (1997) 215-223

219

6H

c 6Hl112

I 450

500

L

550

1 600

J

1

I

650

700

Wavelength

i

I

750

,

I

#00

(nm)

Fig. 2. Emission spectrum of 0.2 wt% of Dy 3+ at room temperature in 30PbO-70PbF 2 glass. The transitions are from the 4F9/2 to the indicated levels.

the decay curves were analyzed in terms of single and double exponentials as previous studies show the decay to follow one of these behaviors [26,30]. In the case of Sm 3+, the data were analyzed in terms of single exponential and the kinetic expression that includes cross-relaxation as well [16]. If cross-relaxation is responsible for non-exponential decay, the expression derived by Inokuti and Hirayama [31] and Eisenthal and Siegel [32] for donor-acceptor transfer can be used. In the present case the non-excited

Sm 3+ ions act as acceptors. Assuming that the number of excited ions are small compared to the total number of active ions, the emission intensity versus time is given by [16]

-- [/:exp(- (t'/r)

-( 47r/3) NA(t' C~ )3/~F( 1 - 3/s) ) dt'] /[foexPi-(t'/"c) _ (4~r/3)~r 1*AI," (,,r(s))3/51-(1_ "-'DA

3/s))dt']

1

(2) where s = 6, 8 or 10 depending on whether the dominant mechanism of the interaction is dipole-dipole, dipole-quadrupole or quadrupole-quadrupole, respectively, NA is the concentration of acceptor ions, F ( x ) is the gamma function and

o

C(S) DA

50.1

'

0.000

I

'

0.002

I

'

0.004 Decoy

time

I

0.006

is the parameter of the dominant contribution to the donor acceptor transfer related to the transition probability by

i

0.008

(seconds)

Fig. 3. Luminescence decays at room temperature in 30PbO70PbF 2 glasses with 0.2 and 1.35 wt% of Sm 3+. The lines represent the best fits of the experimental results to single exponential and Eq. (2) for 0.2 and 1.35 wt% of Sm 3+, respectively.

WDA = ('-,(s) /1;?s ,

(3)

where R is the distance between ions involved in the transfer. The best fits were judged by means of error analyses. The lifetimes (~-) for 4G5/2 level of Sm 3+

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P. Nachimuthu et al. / Journal of Non-Crystalline Solids 217 (1997) 215-223

"2". vO @

_z

0.1 '

0.0000

I

0.0004

'

I

Decoy

I

t

0.0008 time

0.0012

b

I

0.0016

'

0.0020

(seconds)

Fig. 4. Luminescence decays at room temperature in 30PbO70PbF2 glasses with 0.2 and 1.35 wt% of Dy 3+. The lines represent the best fits of the experimental results to single exponential for both 0.2 and 1.35 wt% of Dy 3+, respectively.

and 4F9/ 2 level of Dy 3+ obtained from different fits to experimental decay curves along with the errors are given in Table 5. The best theoretical fits are superimposed on experimental data in Figs. 3 and 4.

4. Discussion The small rms values given in Tables 1 and 2 along with oscillator strengths for Sm 3+ and Dy 3+, respectively, show good agreement between observed and calculated values. The overlapping transitions marked b in Table 1 principally contribute to the rms deviation. This contribution is found to be the case in the earlier works as well [16,25]. The values of Judd-Ofelt parameters are of the same order as those reported for Sm 3+ and Dy 3+ in fluoride glasses and significantly less than the values reported for oxide glasses (Table 3) [16,25,26,33]. These results show that the dopant ions in the fluoride rich 30PbO-70PbF 2 glass are situated in sites with crystal field and metal ligand bonding similar to those in the fluoride glass matrices [34]. The emission spectra for Sm 3+ and Dy 3+ show the following features of interest. Unlike the case of phosphate and borate glasses the emission lines cor• 4 .._.) 6 6 3+ respondmgto G 5;./2 H61 i / 2, H t3/ 2 f o r S m and 4 F 9 / 2 "--')6H11/2 , etH9/2,

FII/2

f o r Dy 3+ c o u l d

be

observed even at room temperature [35]. In this

respect these systems behave more like the fluoride glasses [16,25,26]. Such a similarity between the present samples and the fluoride glasses is also exhibited in the emission studies of Eu 3÷ and Tb 3+ doped PbO-PbF 2 glasses [22,23]• For these ions the high energy transitions were observed with significant intensities in the oxyfluoride and fluoride glasses whereas they were absent in borate and phosphate glass matrices [22,23,35,36]• The close similarity between Ln 3+ doped oxyfluoride and fluoride glasses arises due to absence of any interactions between the rare earth ions at small dopant concentrations ( < 0.1 wt%). For the sites proposed for the rare earth ions in the oxyfluoride and fluoride glasses, even when the Ln 3+ ions occur as near neighbors, are far apart and preclude energy transfer [21,23,24]. The ionic environment and the consequent low phonon energy maxima of ~ 650 and ~ 930 cm-~ in the vibrational spectra of fluoride and oxyfluoride glasses, respectively, prevent multiphonon non-radiative decay [16,21-26]. The two dominant emissions for Dy 3+ occur due to 4F9/2 ---~6H13/2 at 575 nm in the yellow region and 4F9/2 --)6H15/2 at 481 nm due to blue emission. S u e t al. have shown in crystalline systems that the yellow to blue ( Y / B ) ratio of Dy 3+ emissions follows a trend parallel to the red to orange ( R / O ) intensity ratio (SD 0 ---~7F2/SD 0 ---7F1) of Eu 3+ as these ratios are influenced by site asymmetries and electronegativities of the ligand atoms in a similar manner [ 18]. It would be of interest to compare the trends in the above ratios in glass systems. The Y / B value for Dy 3÷ for the present system is found to be 1.58 + 0.02 compared to 1.23 ___0.02 for fluoride glasses [26]. Comparison of R / O of Eu 3+ and Y / B in Dy 3÷ in glasses show different trends, viz., silicate > fluoride > phosphate > borate > oxyfluoride for Eu 3+ and oxyfluoride > silicate ~ fluoride > phosphate > borate for Dy 3÷ [22,26,30,37,38]. The small values of ~2 for the two ions indicate that they occupy ionic sites with small distortions from cubic symmetry. It is therefore unlikely that these ions occupy very different sites in the oxyfluoride glass leading to the above discrepancy with respect to different glass matrices. The differences in the relative trends of the R / O ratio of Eu 3+ and Y / B ratio of Dy 3+ may arise due to the differences in the nature of the transitions involved. In the case of

P. Nachimuthu et aL / Journal of Non-Crystalline Solids 217 (1997) 215-223

5D1~0) transition is purely magnetic dipole and its intensity is not affected by the metal ligand interactions. Only the 7F0~2)~SD1¢2) transition being electric dipole in nature and hypersensitive, is affected. As a result Eu 3+ shows R / O ratio characteristic of covalency of the ligand atoms and distortions from cubic symmetry at Eu 3+ site. In Dy 3+, on the other hand both the transitions are electric dipole one of which is hypersensitive [34]. Richardson has shown that intensity calculations of f - f transitions in such cases is affected by the differences in charges and polarizabilities of the ligand atoms in the axial and equatorial positions even when the coordination geometries are similar [39]. In the present glass the Ln 3÷ ions are bound to two different types of ions, viz., monovalent F- and divalent 0 2- ions. In such cases it is unlikely that the R / O ratio of Eu 3+ and Y / B ratio of Dy 3+ in different glass matrices will follow a parallel trend• In Table 4, the radiative transition probabilities for Sm 3÷ and Dy 3+ ions in the present system are comparable with those reported for fluoride glasses and less than the values reported for borate and phosphate glasses [16,25,26,33-35]. The observed branching ratios were measured by area method excluding the transitions which are occurring in near infrared region [16,25,26]. The exclusion of the transitions in the near infrared region may result in a systematic error in the observed branching ratios. The errors should however be negligible as the emission lines in the near infrared region have negligible intensities compared to the transitions taken into account [16,25,26]. The branching ratios calculated from absorption spectral data show good agreement with those obtained from emission spectral data. To our knowledge the trpS for different transitions for Sm 3+ and Dy 3+ ions have not been reported so far. The trpS are an order of magnitude less than the O-pS reported for various transitions in this region of wavelengths for other rare earth ions [21,22]. The low emitting efficiencies reported for Sm 3÷ and Dy 3÷ (1%) compared to other rare earth ions such as Eu 3÷ (10%) and Tb 3+ (70%) is in accordance with the low o-ps obtained [19]. The lifetimes calculated from Judd-Ofelt parameters are larger than the measured values. The corresponding values for quantum efficiencies are 52 + 2 and 49 + 2% for Sm 3+ (0.2 wt%) and Dy 3÷ (0.2 E u 3+ t h e 7F0(I) ~

221

wt%) ions, respectively. The corresponding values for these ions in fluoride glasses are 92 and 64%, respectively [16,25,26]. Thus the agreement between measured and calculated lifetimes in the case of fluoride glasses is better [16,25,26]. The PbO-PbF 2 glasses have a phonon energy maximum of 930 cm-1 compared to the low phonon energy maximum of 650 cm -1 in the spectra of fluoride glasses [16,21-26,40]. In such a case the multiphonon nonradiative decay will have a larger probability in the present glass than in fluoride systems and the agreement between calculated and measured lifetimes will be less. For samples with 0.2 wt% of Ln 3+ (Sm 3+, Dy3+), good fits of the decay curves to single exponential are obtained as shown in Figs. 3 and 4 for Sm 3+ and Dy 3+ respectively. The decay curves for the emission l e v e l 4F9/2 of Dy 3+ follows single exponential for both the concentrations studied although the larger concentration of the dopant ion resulted in a reduction of the lifetime to 0.52 _+ 0.01 ms (Fig. 4) due to concentration quenching. The decay of Sm 3÷ when present in a larger concentration (1.35 wt%) is complex. The attempted fits gave rise to a larger X 2. The best fit is obtained using Eq. (2) with s = 6, CDA = 5.63 X 10 -41 c m 6 s - ! and the energy transfer probability WDA = 5.9 + 0.3 S-~ (Fig. 3). The fact that the data fits better than single exponential as shown by the marked reduction in g 2 shows that there is strong cross-relaxation. Cross-relaxations in Sm 3÷ • • 4 6 _..+6 occur Involving the levels Gs/2, H 9 / ~ F9/~ 6F7/2,

6r 6r 1"9/2 r g / 2 ,

6~ r7/2

6~ - . r9/2 ano

6 ~ -~

rs/2

6~ "~ r11/2.

The results also show that the dominant interaction in the relaxation mechanism is dipole-dipole in the present case as in the fluoride glass [16]. In the earlier studies of Sm 3÷ doped glasses other mechanisms such as dipole-quadrupole and even quadmpole-quadrupole interactions have also been proposed [ 16,41 ].

5. Conclusions The results of our studies o n Sm 3+ and Dy3+ in 30PbO-70PbF 2 glass matrix show that the JuddOfelt parameters and radiative transition probabilities (A) derived from their absorption spectral data are similar to fluoride glass hosts and less than the

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P. Nachimuthu et al. / Journal of Non-Crystalline Solids 217 (1997) 215-223

values for the less ionic borate and phosphate glasses. These observations are substantiated by the similar trends exhibited by their emission properties as well. The Judd-Ofelt parameters give rise to reliable branching ratios (/3 R) for the emission transitions as evidenced by the good agreement between values calculated from these parameters and those derived from the emission spectra (Table 4). The stimulated emission cross-sections (o-p) for Sm 3÷ and Dy 3+ ions are an order of magnitude less than the values for other rare earth ions. These cross-sections account for the low emission efficiencies observed for these ions compared to other rare earth ions such as Eu 3+ and Tb 3+. For Dy 3+, O-p is found to be the highest (trv × 10 24 c m 2 = 1191 __+60) for the hypersensitive t r a n s i t i o n 4F9/2 ---)6H13/2. This transition also has the largest transition probability (A = 390 + 12 s -1 ) and branching ratio (/3 R = 0.608 ___0.024) signifying most favored emission. The ratio (yellow/blue) of its intensity relative to the non-hypersensitive electric dipole transition is affected by the nature of the glass matrix and has the value of 1.58 + 0.02 for our glass sample. The fluorescent decay processes for both the ions follow single exponential at lower concentrations (0.2 wt%). At larger dopant concentrations (1.35 wt%) while Dy 3÷ exhibits a single exponential decay, Sm 3÷ does not, since energy transfer involving cross-relaxation becomes significant in the radiative decay process. Our earlier reports on other rare earth ions and the present studies show that the similarity between the emission properties of Ln 3÷ in fluoride and oxyfluoride glasses is of general validity for the whole rare earth series [22,23]. Thus the oxyfluoride glasses can be good substitute for fluoride glasses as host matrices especially as they offer certain advantages over the latter due to relative ease with which they can be prepared and moisture insensitivity and thermal stability.

Acknowledgements P.N. and V.N.K. gratefully acknowledge the CSIR and UGC, New Delhi for the award of Senior Research Fellowships. Financial support from the DST, New Delhi and National Laser Programme (India) is gratefully acknowledged.

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