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Spectroscopy of Pr3+ ions in lithium borate and lithium fluoroborate glasses
Physica B 301 (2001) 326}340
Spectroscopy of Pr> ions in lithium borate and lithium #uoroborate glasses P. Babu, C.K. Jayasankar* Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Received 28 August 2000; received in revised form 8 November 2000
Abstract Optical absorption and emission spectra of 1 mol% Pr> doped lithium borate and lithium #uoroborate glasses have been recorded. The intensities of f}f transitions are parameterized in terms of Judd}Ofelt (JO) intensity parameters ("2, 4 and 6). The JO parameters obtained with modi"ed JO theory have been used to predict radiative properties H such as spontaneous emission probabilities (A), lifetimes ( ) and branching ratios ( ) for all the 12 excited states of these 0 0 Pr> doped glasses along with some of the reported Pr> : systems. The predicted are compared with the experimental 0 values for the emission from P and P levels. The stimulated emission cross-sections are also evaluated for all the observed emission transitions. The values for the potential laser transitions including the G PH (1.3 m) 0 transition useful for "bre ampli"er have been compared for 17 Pr> : systems. The e!ect of compositional changes of the glasses on the optical properties of Pr> ions have been discussed. 2001 Elsevier Science B.V. All rights reserved. PACS: 42.62.Fi; 76.30.Kg; 78.20; 78.40.Pg Keywords: Pr> ions; Optical properties; Borate glasses; Judd}Ofelt analysis; Laser spectroscopy
1. Introduction Praseodymium doped glasses "nd variety of practical applications such as UV-VIS-NIR lasers [1], up-converters [2], optical "bre lasers [3], "bre ampli"ers in 1.3 m region [4] etc. These applications can either be enhanced or optimized from systematic study of optical properties of Pr> ions in various environments. Generally the optical properties of rare earth ions in insulating matrices
* Corresponding author.Tel.:#91-8574-49666; fax:#918574-27499. E-mail address: [email protected] (C.K. Jayasankar).
are derived in the frame work of Judd}Ofelt theory [5,6]. However, it is known that the JO theory is less successful in the case of Pr> ion due to the small energy gap between the ground con"guration, 4f, and the "rst excited state con"guration, 4f5d [7]. The problems which arise due to the application of standard JO theory to the Pr> : systems are: (1) negative value for which has no physical signi"cance [8,9], (2) poor agreement between the calculated and measured oscillator strengths [10,11] and (3) inconsistency between predicted and measured radiative properties [8,12]. Kornienko et al. [13] tried to solve the problem of standard JO theory by modifying the equation for electric dipole line strength where the in#uence
0921-4526/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 2 3 9 - 3
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
of excited con"guration of Pr> ion has been taken into account. Later Quimby and Miniscalco [14] introduced the emission data along with the absorption data for the calculation of JO intensity parameters, . Alcala and Cases [15] applied the H modi"ed JO theory proposed by Kornienko et al. [13] to the absorption data and obtained positive value for with reasonable agreement between calculated and experimental oscillator strengths. They also considered the emission data along with absorption data for modi"ed JO theory and obtained positive value but the agreement between calculated and experimental oscillator strengths is little worse than the previous one. Arauzo et al. [16] also applied the modi"ed JO theory to the absorption data, absorption and emission data and found that the modi"ed JO theory yields positive values for and better agreement between the calculated and experimental values of oscillator strengths and branching ratios than the standard JO theory. In many cases, the experimental and calculated oscillator strengths for H PP "ts badly and therefore this transition is omitted in the standard JO analysis and found positive values for [11,17,18]. Florez et al. [9] used JO analysis for Pr> : #uoroindate glasses and obtained consistent set of JO parameters with the smallest root mean square (RMS) values of the "t when is elimi nated and odd rank parameters and have been introduced along with and besides suggestion for level assignment for Pr> ion. In the present paper, the JO intensity parameters have been derived by applying both the standard and modi"ed JO theories to the absorption data along with and without emission data. The relative merits of JO parameters obtained with these di!erent procedures have been discussed by taking in to account RMS deviations between experimental and calculated oscillator strengths along with trends of parameters. The JO parameters resultH ing from the best "t (smallest RMS deviation with consistent set of ), have been used to predict H radiative properties for all the 12 excited states of Pr> (4f) ion. Particular attention was given to lasing transitions and compared the values for 0 Pr> ion in various environments. This work will supplement the preliminary results of optical absorption studies in these glasses [19].
327
2. Experimental The molar compositions of Praseodymium doped lithium borate (LnBP, n"4}7) and lithium #uoroborate (LxFBP, x"2 and 5) glasses studied in the present paper are as follows: L4BP: 59.5Li CO #39.5H BO #1Pr O , L5BP: 49.5Li CO #49.5H BO #1Pr O , L6BP: 39.5Li CO #59.5H BO #1Pr O , L7BP: 29.5Li CO #69.5H BO #1Pr O , L2FBP: 24.75Li CO #24.75LiF #49.5H BO #1Pr O , L5FBP: 49.5LiF#49.5H BO #1Pr O . Above compositions (&5 g) are grinded homogeneously in an agate mortar and taken in a porcelain crucible and melted in an electric furnace in the temperature range of 900}9503C for 1 h. The melt is then air quenched to get a good optical quality glasses. The samples are annealed at 3503C for 5 h to remove thermal strains and then polished before measuring their optical properties. Absorption measurements were made on a Hitachi U-3400 spectrophotometer in the wavelength range of 400}2500 nm. Fluorescence spectra were recorded on a Hitachi 650}10 s #uorescence spectrometer at room and 25 K temperatures. Refractive indices, n, were measured on an Abbe refractometer at sodium wavelength (589.3 nm). Densities were measured by the Archimedes method using xylene as an immersion liquid.
3. Theoretical 3.1. Oscillator strengths*standard JO analysis 3.1.1. Absorption data The experimental oscillator strengths ( f ) of the f }f transitions in the absorption spectra are obtained by means of the usual expression [20],
f"4.318;10\ ()dv
(1)
328
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
where () is the molar absorptivity at a wave number . In the Judd}Ofelt f}f intensity model [5,6] of the static coupling between the radiation "eld and the electronic charge distribution of ligands, the oscillator strength, f, of an electric dipole f}f transition of trivalent rare earth ions from a level J to a level J is given by f(J, J)"[8mcv/3h(2J#1)][(n#2)/9n] ; (J ;H J) (2) H H where m is the mass of an electron, c is the velocity of light, is the wave number of the transition, h is the planck's constant, J and Jare the total angular momenta of the initial and "nal levels respectively, n is the refractive index, ("2, 4 and 6) are the H JO intensity parameters and ;H are the doubly reduced matrix elements evaluated in the intermediate coupling approximation for a transition JPJ. In general JO parameters can be obtained from the least square "t as carried out for Pr> : glasses [14}18]. 3.1.2. Both absorption and emission data The standard JO theory did not yield consistent JO parameters and shows discrepancy between experimental and calculated oscillator strengths for Pr> : systems [10,11]. Hence emission data have been included along with absorption data and obtained positive value as well as improved "t between experimental and calculated oscillator strengths [14}16]. In the present work, JO theory has been employed with and/or without emission data and obtained JO parameters. 3.2. Modixed Judd}Ofelt analysis The JO theory assumes that the energy gap between 4f and 5d orbitals is large and consequently the 5d orbitals does not a!ect the f}f transition intensities. However, for Pr> ions, it is found that the energy gap between 4f and 5d orbitals is small enough (&15,000 cm\) [9] that 5d orbital in#uences the f}f transition intensities. Therefore, Kornienko et al. [13] considered the mixing of 4f-5d orbitals and modi"ed the JO theory. As per
Kornienko et al. [13], the modi"ed equation for electric dipole line strength that accounts the in#uence of excited con"guration of Pr> ion, neglecting electron correlation e!ects is S (J, J)"e [1#2 (E #E ( Y(Y H H !2E )](J ;H J) (3) D where are modi"ed JO parameters, is addiH tional parameter of about 10\ cm\ [13]. E and ( E are the energies of J and J levels, respecY(Y tively. E is the energy of the centre of gravity of the D 4f orbitals of Pr> ion that has been taken as 9940 cm\ [13,21]. In the present paper, the matrix elements have been modi"ed by multiplying with [1#2 (E #E + !2E )] and are used in least ( Y( D square "tting of oscillator strengths to obtain modi"ed JO parameters, . These are inturn H H used to calculate S using Eq. (3). 3.3. Radiative properties Radiative properties in the present paper have been calculated using the usual expressions as carried out for trivalent lanthanide ions [16,19}21].
4. Results 4.1. Optical absorption and emission spectra The optical absorption spectra of Pr> : LBP (LnBP, n"4}7 and LxFBP, x"2 and 5) measured at room temperature in the wavelength range of 400}2500 nm are shown in Fig. 1. All the bands originate from the ground state, H , to the excited levels shown in Fig. 1. Band assignments along with energy positions for the present and reported Pr> : systems are given in Table 1. The reported systems that are compared include Pr>: #uoroindate (InF1: 34 InF }20 ZnF }20 SrF }6 GaF } 2 NaF}17 BaF ) [9], #uorozirconate (ZBLAN2: 53 ZrF }20 BaF }4 LaF }3 AlF }20 NaF) [14], #uoroindate (InF2: 20 ZnF }20 SrF }20 BaF }40 InF ) [16], lead bismuth gallite (PBG: 57 PbO} 25 Bi O }18 Ga O ) [21], yttrium aluminium
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
329
Fig. 1. Absorption spectra of Pr> : LBP glasses: (a) L4BP; (b) L5BP; (c) L6BP; (d) L7BP; (e) L2FBP; (f) L5FBP.
garnet (YAG) [22], calcium boro-aluminate (CBA: CaO}B O }Al O ) [23], PAS (PBr /AlBr / SbBr ) [24] and Free ion (FI) [25]. Emission spectra of Pr> : L5FBP glass recorded both at room temperature and at 25 K in the energy range of 13 000}22 000 cm\ are shown in Fig. 2 along with assignment of the bands. The emission spectra has not been corrected for the instrument spectral sensitivity. 4.2. Judd}Ofelt intensity analysis The experimental oscillator strengths of the absorption bands of Pr> : LBP glasses are determined using Eq. (1) and are collected in Table 2 along with calculated oscillator strengths obtained from JO analysis. For Pr> : L5BP glass, the calculated oscillator strengths obtained with di!erent methods are also presented. The intensity parameters ( ) resulting from the JO analysis are H
also presented in Table 2. In order to evaluate the quality of the "t, JO parameters obtained with di!erent combination of levels and methods are also presented in Table 2 for Pr> : L5BP glass. Table 3 shows the comparison of the oscillator strengths for Pr> : L5FBP glass with some of the reported Pr> : systems of InF1 [9], #uorozirconate (ZBLAN1: ZrF }BaF }LaF }AlF }NaF) [12], InF2 [16], fuorozirconate (ZBLA: 57 ZrF }34 BaF }3 LaF }4 AlF ) [17], PbOF (PbO}PbF ) [18], PBG [21], YAG [22], PAS [24], Aquo-ion (Aquo) [26], zinc tellurite (ZnTe: 35 ZnO}65 TeO ) [27], sodium tellurite (NaTe: 20 Na O}80TeO ) [27]. Confusion may be avoided among three #uorozirconate glasses, ZBLAN1 [12], ZBLAN2 [14] and ZBLA [17], as these differs in composition and reported independently. The reduced matrix elements used in the present JO analysis and in the calculation of magnetic dipole radiative transition probabilities are
6855 9921 17 334 21 390 22 007 23 161 0.9737 0.9964 1.0027 0.9914
4.3. Radiative properties The intensity parameters obtained with modi"ed JO theory for absorption levels excluding H P P transition have been used to predict various radiative properties. The detailed results of electric (A ) and magnetic (A ) dipole radiative transition
probabilities, radiative (A) and total radiative (A ) transition probabilities, branching ratios ( ) 2 0 and lifetimes ( ) for all the excited states of 0 Pr> : L5FBP glass are presented in Table 4. The predicted for some of the important #uorescent 0 levels of Pr> systems are presented in Table 5. The predicted lifetimes for all the excited levels of Pr> : LBP glasses along with some of the reported systems are collected in Table 6. The reported Pr> : sytems that are compared in Tables 5 and 6 include ZnCdF (ZnF }CdF ) [8], ZBLAN1 [12],ZBLAN2 [14], InF2 [16], ZBLA [17], PbOF [18], PBG [21], CBA [23], ZnTe [27], NaTe [27], yttria-stabilized-zirconia (YSZ: ZrO }Y O ) [28]. Table 7 presents the luminescence properties such as emission peak wavelengths ( ), e!ective linewidths ( ), radiative transition probabilities (A), stimulated emission cross-sections ( ) for the observed emission bands from P and P levels of Pr >: L5FBP glass. Table 7 also compares the experimental and calculated branching ratios with di!erent sets of JO parameters for Pr> : L5FBP glass and reported Pr> : glasses of ZBLAN1 [12], InF2 [16], ZBLA [17] and PbOF [18]. 4.4. Nephelauxetic ewect
0.9905
0.9944
determined by using the appropriate eigenvalues and eigenvectors which are computed from intermediate coupling approximation for individual Pr> : LBP glasses [19], though they are almost host independent. The matrix elements for all the excited states to their lower levels of Pr> : L5FBP glass are presented in Table 4.
The average Nephelauxetic ratio (M ) between the rare earth ion and its environment in a host can be calculated with the following expression [29,30] as
M
6919 10 109 16 895 20 678 2 1204 22 512 F G D P P P
7029 * 16 892 20 665 21 178 22 497
6977 9989 16 883 20 648 21 155 22 434
0.9948
0.9907
0.9898
0.9992
0.9884
0.9911
1.0023
6714 9682 16 682 20 450 21 010 22 174 7143 9709 16 667 20 492 21 277 22 124 7063 * 16 983 20 790 21 321 22 421 9911 17 007 20 833 21 368 22 573 9862 17 007 20 661 21 277 22 727
6631 6562 6556
10 000 17 014 20 867 21 470 22 700 6958 9946 16 932 20 747 21 295 22 507 6906 9914 16 903 20 738 21 268 22 518
5185 6580 5184 6533 5191 6625 5203 6548 F F
5173 6557
5184 6567
7016 10 109 16 895 20 685 21 207 22 464
5131 6506
7018 9852 16 920 20 661 * *
4954 6272 5405 6667 5320 6639
4997 6415
4450 4320 * 4444
5000 * 4883 * 4234 H
4245
4245
4231
4259
4231
5094 *
PAS [24] YAG [22] CBA [23] PBG [21] InF2 [16] ZBLAN2 [14] InF1 [9] L5FBP L2FBP L7BP L6BP L5BP L4BP Energy level
Table 1 Observed band positions (cm\) and the average Nephelauxetic ratio (M ) for Pr> : LBP glasses and reported Pr> : systems
4389
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
FI [25]
330
M " /N ,
(4)
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
331
Fig. 2. Emission Spectra of Pr> : L5FBP glass at room temperature (RT) and at 25 K.
where " / , and are the energies of the corresponding transitions in the complex and freeion [25], respectively, and N refers the number of levels used to compute M values. The values of M are collected in Table 1 for present and reported Pr> : systems [9,14,16,21}24].
5. Discussion 5.1. Electronic structure of Pr3> The absorption spectra observed for Pr> : LBP glasses are shown in Fig. 1, which are similar to the spectra observed for Pr> : glasses [8,12,15,16]. The assignment of the observed excited levels (from the ground state, H ) has also been shown in Fig. 1 and their positions in wavenumbers are collected
in Table 1. As usual, out of 12 excited states of Pr> ion, only 9 are observed in the absorption spectra (except for Pr> : L4BP glass). The transitions which are not observed include G (too weak to locate), S (too high in energy, 46 500 cm\), I (spin forbidden and is also masked by the intense spin allowed H PP transitions) and H (too low in energy, NIR region). In Table 1, energy positions of Pr> ion is compared for glasses [9,14,16,21,23], crystal [22], solution [24] and free-ion [25] along with M . As seen from Table 1, all the energy positions for Pr> ions did not vary uniformly when environment is changed since the energy positions for high energy levels such as D and P multiplets are lower where as for low energy levels such as F and G are higher than those observed for free-ion. However, the average Nephelauxetic ratio (M ) calculated from
332
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
Table 2 Experimental ( f Level
) and calculated ( f ) oscillator strengths (;10\) and J}O parameters ( ;10\ cm) for Pr> : LBP glasses
L4BP f
H P H 0.01 F 1.47 F 4.09 F 1.32 G * D 0.92 P 1.87 P 2.52 P 5.28 P P F 4.44 H 1.94 N
L5BP
L6BP
L7BP
L2FBP
L5FBP
f
f
f
f
f
f
f
f
f
f
f
f
f
f
0.35 1.51 3.61 1.91 * 0.81 2.87 2.96 *
0.01 2.19 5.21 2.30 0.18 1.34 2.76 3.78 7.33
0.52 2.19 5.17 2.53 0.25 0.87 3.50 3.53 2.75
0.50 2.19 5.05 2.74 0.26 1.14 3.61 3.72 3.95
0.48 2.21 4.88 2.60 0.25 1.09 3.65 3.77 *
0.50 2.30 4.95 2.44 0.24 0.83 3.26 3.29 *
0.47 2.18 4.87 2.56 0.41 1.09 3.79 3.91 *
0.01 2.39 5.78 2.58 0.12 1.41 2.88 4.00 8.20
0.53 2.42 5.41 2.91 0.28 1.22 3.88 4.00 *
0.02 3.11 7.26 3.52 0.19 1.83 3.41 4.87 10.06
0.68 3.14 6.89 3.80 0.36 1.55 4.53 4.68 *
0.01 1.83 4.37 2.22 0.19 1.18 2.14 3.31 6.48
0.42 1.85 4.20 2.31 0.22 0.96 2.87 2.97 *
0.01 1.63 3.96 1.91 0.15 1.03 1.56 2.58 2.43
0.38 1.65 3.73 2.10 0.20 0.85 2.33 2.40 *
* * 7 0.52 0.13 4.61 3.92
4.59 5.06
* * 9 1.57 !0.56 5.64 3.76
* * 9 1.19 0.90 5.78 5.70
* * 8 0.40 0.98 5.85 5.36
0.28 6.68 10 1.49 0.04 5.26 3.62
5.75 10.69 10 1.86 0.72 6.06 5.26
1.84 3.76
* * 8 0.45 1.14 6.18 6.08
3.28 * 5.76 * 8 0.51 2.14 7.19 7.83
2.08 2.64
* * 8 0.34 0.98 4.67 4.96
1.04 2.88
* * 8 0.33 1.17 3.83 4.58
f
Absorption levels without P level*modi"ed JO theory. Absorption levels*standard JO theory. Absorption levels*modi"ed JO theory. Absorption levels without P level and emission levels*standard JO theory. Absorption levels without P level and emission levels*modi"ed JO theory.
Table 3 Experimental oscillator strengths (;10\) for Pr>ions in various hosts (all transitions are from H ) Level
Spectral region (cm\)
Present work L5FBP
H
4241
F F
5187 6566
1.63 3.96
F G D P P P
6969 10 010 16 892 20 704 21 231 22 472
1.91 0.15 1.03 1.56 2.58 5.53
Reported InF1 [9]
ZBLAN1 InF2 [12] [16]
1.43
3.30
0.01
6.49 0.25 1.77 0.96 2.19 6.29
8.30 0.20 1.89 2.50 4.32 9.15
ZBLA [17]
PbOF [18]
*
*
PBG [21]
ZnTe [27]
NaTe [27]
YAG [22]
0.93
1.56
2.06
1.00
PAS [24]
Aquo [26]
*
*
2.97 2.23 6.21
3.89 7.92
5.77 6.84
6.15 11.36
6.36 11.06
5.63 14.91
3.70 8.00
2.87 0.32 1.54 2.46 4.19 8.93
3.27 * 3.88 4.04 7.31 16.87
1.61 0.49 6.09 20.90 * *
3.92 0.35 3.13 5.66 9.16 16.43
3.41 0.51 3.28 5.98 8.75 15.60
5.68 0.50 6.35 6.83 15.58 27.56
3.94 0.71 2.68 5.79 2.66 15.06
8.71 0.27 2.07 2.30 4.73 10.14
* 12.79
For source of data as well as title of the system, see Section 4.2.
0.32 3.08 2.54 7.63 9.75
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
333
Table 4 Energies (v), matrix elements ( U ) and radiative properties (A , A , A, A , and ) for the excited states of Pr> : L5FBP glass 2 0 0 (cm\)
U
SLJ
SLJ
S
P 25 485 0.0700 I 26 617 0 P 26 697 0 P 27 245 0 D 31 060 0.4963 G 38 046 0 F 41 034 0 F 41 412 0 F 42 807 0.0045 H 43 761 0 H 45 925 0 H 47 992 0 A (S )"483 304 s\, (S )"2 s 2 0 I 1132 0 P 1212 0.4170 P 1760 0.1907 D 5575 0.0011 G 12 561 0.6746 F 15 549 0.4111 F 15 927 0.2554 F 17 322 0.0314 H 18 276 0 H 20 440 0 H 22 507 0 A (P )"24545 s\, (P )"40 s 2 0 P 80 0 P 628 0 D 4443 0.0000 G 11 429 0.2417 F 14 417 0.0886 F 14 795 0 F 16 190 0 H 17 144 0.0037 H 19 308 0.0003 H 21 375 0.0089 A (I )"5864 s\, (I )"170 s 2 0 P 548 0 D 4363 0.0825 G 11 349 0 F 14 337 0 F 14 715 0.5714 F 16 110 0.2683 H 17 064 0 H 19 228 0 H 21 295 0 A (P )"23596 s\, (P )"42 s 2 0 D 3815 0.0148 G 10 801 0 F 13 789 0 F 14 167 0 F 15 562 0.2951
P
I
P
P
U
U
A (s\)
A (s\)
0
0 0 0 0 0 0.3928 0.1696 0 0 0 0 0.0071
0 0.5669 0 0 0 0 0 0 0 0.0003 0 0
3310 118 947 0 0 42 446 201 095 108 931 0 1008 260 0 7275
3310 118 947 33 0 42 446 201 095 108 931 0 1008 260 0 7275
0.0068 0.2461 0.0001 0 0.0878 0.4161 0.2254 0 0.0021 0.0005 0 0.0151
0.0278 0 0 0.0778 0.0450 0.1044 0.3062 0.2930 0.4949 0.1886 0.0354
0.1559 0 0 0 0.0180 0.0102 0 0 0.0537 0.1323 0.1336
1 0.4 1 25 1008 1697 2889 2925 6348 5509 4140
0 0.08 0 2 0 0 0.54 0.04 0 0 0
1 1 1 27 1008 1697 2890 2925 6348 5509 4140
0 0 0 0.0011 0.0411 0.0691 0.1177 0.1192 0.2586 0.2244 0.1687
0 0 0.1555 1.3846 0.6345 0.0010 0.0055 0.0134 0.0007 0.0494
0.0004 0.0030 1.7002 0.6710 0.4295 0.0025 0.0333 0.0046 0.0019 0.0227
0 0 137 2415 2519 9 138 73 16 554
0 0 0 0 0 0 0 1.75 1.79 0
0 0 137 2415 2519 9 138 74 18 554
0 0 0.0234 0.4118 0.4296 0.0016 0.0234 0.0127 0.0030 0.0944
0 0 0.0843 0.2621 0.1964 0 0 0.2857 0.1714
0 0 0 0 0 0 0.1246 0.0893 0
0 7 382 2394 3672 1067 2297 8648 5129
0.01 0.60 0 0 0 0.07 0 0 0
0.01 7 382 2394 3672 1067 2297 8648 5129
0 0.0003 0.0162 0.1015 0.1556 0.0452 0.0973 0.3665 0.2174
0 0.0559 0.1075 0 0
0 0 0 0 0
2 655 2621 0 3175
0 0 0 0 0
2 655 2621 0 3175
0.0001 0.0269 0.1076 0 0.1303
A (s\)
0 0 33 0 0 0 0 0 0 0 0 0
334
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
Table 4. Continued. (cm\)
U
SLJ
SLJ
P
H 16 516 0 H 18 680 0 H 20 747 0 A (P )"24365 s\, (P )"41 s 2 0 G 6986 0.2937 F 9974 0.6077 F 10 352 0.0332 F 11 747 0.0144 H 12 701 0 H 14 865 0 H 16 932 0.0022 A (D ) "1955 s\, (D )"511 s 2 0 F 2988 0.0785 F 3366 0.0041 F 4761 0.0001 H 5715 0.2583 H 7879 0.0375 H 9946 0.0012 A (G )"493 s\, (G )"2027 s 2 0 F 378 0.0250 F 1773 0.0017 H 2727 0.5674 H 4891 0.0302 H 6958 0.0180 A (F )"353 s\, (F ) "2833 s 2 0 F 1395 0.0209 H 2349 0 H 4513 0.6286 H 6580 0.0656 A (F )"545 s\, (F ) "1835 s 2 0 H 954 0 H 3118 0 H 5185 0.5088 A (F )"243 s\, (F ) "4107 s 2 0 H 2164 0.1080 H 4231 0.0001 A (H )"19 s\, (H )"52050 s 2 0 H 2067 0.1094 A (H ) "8 s\, (H )"123 995 s 2 0
D
G
F
F
F
H H
U
U
A (s\)
A (s\)
A (s\)
0
0 0 0.1719
0.0726 0 0
3641 0 14271
0 0 0
3641 0 14271
0.1494 0 0.5857
0.0519 0.0000 0.0187 0.0891 0.0711 0.0022 0.0181
0.0760 0.0201 0 0 0.0069 0.0004 0.0540
148 389 60 282 303 16 753
0 0 3.40 2.63 0 0 0
148 389 63 285 303 16 753
0.0755 0.1988 0.0322 0.1455 0.1547 0.0082 0.3850
0.1448 0.0059 0.0154 0.2592 0.0997 0.0070
0.3513 0.0509 0.0063 0.2429 0.4169 0.0258
16 3 3 121 308 39
0.81 1.04 0 0 0.75 0.63
17 4 3 121 309 40
0.0346 0.0075 0.0052 0.2460 0.6266 0.0800
0.0718 0.0015 0.6115 0.3098 0.0503
0.0060 0.0898 0.4606 0.4457 0.4826
0 1 28 103 220
0 0 0 0.20 0.59
0.01 1 28 103 221
0 0.0018 0.0794 0.2926 0.6263
0.0510 0.3182 0.3468 0.3476
0 0.8459 0 0.6988
0.2 23 66 456
0.26 0 0 0.02
1 23 66 456
0.0008 0.0420 0.1209 0.8362
0.0164 0.2978 0.4032
0.3038 0.6590 0.1180
1 61 182
0 0 0
1 61 182
0.0025 0.2515 0.7459
0.2327 0.0327
0.6420 0.1403
8 11
0.85 0
8 11
0.4344 0.5656
0.2025
0.6101
7
0.85
8
1.0000
these energy positions (Table 1) using Eq. (4) shows variation from 0.9737 (PAS [24]) to 1.0027 (CBA [23]). Fig. 2 shows the emission spectra observed for Pr> : L5FBP glass which is similar to other Pr> : LBP glasses. The emission spectra has been recorded by exciting the P level with 457 nm of Ar> laser line. Due to thermalization e!ect, emission has been observed from both P and P
levels at room temperature. At low temperature (25 K) all the emissions are only from P level and the intensities are found to be stronger than those found at room temperature. This indicates the absence of thermalization e!ect between P and P levels at low temperatures. The P PH transition is found to be highly intense than the other emission levels which is in accordance with the Pr> : glasses [8,15,16].
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
335
Table 5 Predicted branching ratios of some important laser transitions of Pr>: LBP glasses and reported systems System transition
S PF S PG S PD S PI
P PH P PH P PH P PF D PH G PH
L4BP L5BP L6BP L7BP L2FBP L5FBP ZnCdF [8] ZBLAN1 [12] ZBLAN2 [14] InF2 [16] ZBLA [17] PbOF [18] PBG [21] CBA [23] ZnTe [27] NaTe [27] YSZ [28]
0.2675 0.2505 0.2468 0.2305 0.2406 0.2254 0.2158 0.2360 0.2070 0.2126 0.2448 0.2878 0.2794 0.2032 0.2350 0.2310 0.2917
0.4055 0.3819 0.3802 0.3672 0.3767 0.3665 0.3988 0.3629 0.3360 0.3940 0.4065 0.4044 0.3262 0.3351 0.3354 0.3250 0.3930
0.4920 0.4618 0.4489 0.4223 0.4431 0.4161 0.3984 0.4356 0.3821 0.3924 0.4519 0.5313 0.5158 0.3752 0.4339 0.4265 0.5386
0.0099 0.0539 0.0581 0.0879 0.0636 0.0878 0.0411 0.0894 0.1453 0.0498 0.0166 0.0009 0.1347 0.1483 0.1357 0.1549 0.0174
0.2111 0.2111 0.2233 0.2344 0.2296 0.2461 0.3253 0.2136 0.2364 0.3252 0.2680 0.1603 0.0376 0.2440 0.1655 0.1560 0.1307
5.2. Standard and modixed Judd}Ofelt analysis Table 2 gives the observed oscillator strengths ( f ) for the absorption bands from the ground state, H , to the excited states of Pr> : LBP glass es. The f values are found to increase uniformly with decrease in Li CO content and decrease with increase in LiF content for all the absorption transitions except for G transition. This is prob ably due to decrease and increase in site symmetry of the Pr> ion with decrease in Li CO and in crease in LiF contents, respectively. The inconsistent behaviour for G intensity may be due to its weak nature and hence more uncertainty in the experimental determination of its oscillator strength. Table 2 also shows the branching ratios observed for P PF and H transitions as oscillator strengths. As the emission intensities are relative, the emission intensity of P PH is nor malised to the absorption intensity of H PP and hence the intensities of other emission transitions have been corrected accordingly before using them in the "tting [8,16]. During the "tting of absorption and emission levels, the experimental branching ratios are uniformly modi"ed with a common factor to reduce RMS deviation as
0.7021 0.6487 0.6370 0.5988 0.6214 0.5857 0.5915 0.6033 0.5270 0.5810 0.6561 0.7344 0.6509 0.5190 0.5801 0.5648 0.7308
0.1264 0.1259 0.1332 0.1387 0.1400 0.1494 0.2084 0.1276 0.1406 0.2077 0.1679 0.0956 0.0205 0.1456 0.0954 0.0891 0.0765
0.0146 0.0786 0.0847 0.1283 0.0944 0.1303 0.0643 0.1305 0.2113 0.0778 0.0254 0.0014 0.1791 0.2161 0.1912 0.2162 0.0248
0.4323 0.3892 0.3944 0.3782 0.3951 0.3850 0.4854 0.3589 0.3350 0.4762 0.4709 0.3968 0.1704 0.3389 0.2873 0.2678 0.3498
0.6210 0.6186 0.6166 0.6186 0.6261 0.6266 0.6523 0.6158 0.6172 0.6513 0.6413 0.6063 0.4796 0.6205 0.5898 0.5817 0.5881
carried out by Quimby and Miniscalco [14]. Out of "ve emission transitions observed from P level, only two (P PF and H ) are included in the "t. The branching ratio of P P H is normalised to the oscillator strength of H P P and hence the former is excluded. Similarly, P PH and F transitions are excluded since the matrix elements for the former transition are zero and the for the latter is too weak to 0 measure. As seen from the literature, the oscillator strengths observed for Pr> ion have been analysed in di!erent ways to get reliable JO parameters with improved "ttings between experimental and calculated oscillator strengths. In order to test their reliability, all these di!erent methods have been applied to one (Pr> : L5BP) of our systems. In the "rst method, as usual the standard JO theory [5,6] has been applied for the observed absorption levels and found that is negative with poor "tting for P level (Footnote b of Table 2). In some cases the badly "tted level, P , has been excluded in the "tting and found to yield positive values for [11,17,18]. In the present analysis of Pr> : LBP glasses mixed trend has been noticed when P level is omitted in the "t, is positive for
582
1983
2800 1864 4260 52 377 125 568
D
G
F F F H H
2097 1340 2926 38 753 93 376
1485
384
27
1919 1213 2620 34 769 84 795
1250
346
25
26
1 25 107
1.14 6.18 6.08 1.598
L6BP
1419 911 2032 27 391 67 117
975
260
20
21
1 20 89
2.14 7.19 7.83 1.604
L7BP
Reported
2564 1636 3595 44 852 107 433
1811
459
34
35
1 34 140
0.98 4.67 4.96 1.569
2833 1835 4107 52 050 123 995
2027
511
41
42
1.17 3.83 4.58 1.553 2 40 170
1836 1273 3292 34 107 84 922
34 (40) 417 (245) 1356
1 32 124 (42) 34
0.72 4.80 7.93 1.53
38 (42) 36 (42) 478 (360) 2019 (100) 2855 1789 3814 52740 125 442
1 37 160
1.46 4.89 4.85 1.501
L2FBP L5FBP ZnCdF ZBLAN1 [8] [12]
The values shown in parenthesis are experimental lifetimes.
38
P
29
1 27 116
1 37 158
40
0.98 5.85 5.36 1.594
0.13 4.61 3.92 1.588
P
N States S P I
JO Present work parameters L4BP L5BP
2720 1724 3607 50 513 119 988
1914
430
37
38
1 37 165
2.44 4.41 5.52 1.48
ZBLAN2 [14]
2019 1398 3583 37 429 92 703
37 (46) 450 (400) 1488
38
1 35 137
0.80 4.33 7.26 1.527
InF2 [16]
2576 1724 4266 47 381 115 760
42 (13) 41 (44) 563 (55) 1886
2 39 156
0.24 4.50 5.40 1.527
ZBLA [17]
1470 881 1889 27232 65803
1043
265
15
16
&0 16 68
0.02 7.88 4.81 1.78
279 (53) 468 199 284 9059 19 601
36
2
2
&0 2 13
6.95 18.30 2.70 2.30
PbOF PBG [18] [21]
1116 711 1501 21 096 51 528
788
178
15
16
&0 15 68
4.84 8.42 11.08 1.58
CBA [23]
Table 6 Judd}Ofelt parameters ( ,;10\ cm), refractive index (n) and predicted radiative lifetimes ( , s) for excited states of Pr> : glasses H
765 446 841 14 423 34 082
523
103
8
8
&0 8 38
3.48 7.64 5.90 2.04
ZnTe [27]
2386 1364 2485 44749 103755
1610
300
23
25
1 25 118
3.68 6.97 5.16 1.51
NaTe [27]
892 508 1014 16513 39195
8 (1.8) 142 (180) 620
9
&0 9 38
0.34 7.25 3.56 2.17
YSZ [28]
336 P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
* * * 0.001 0.10 0 * 0.74 * * * 0.04 0.13 0.10 0.41 0.73 0.14 * * 0.03 0.16 0 * 0.65 0.08 * * 0.07 0.16 0.11 * 0.56 0.12 * * 0.08 0.32 0 * 0.46 0.05 * * 0.09 0.30 0.07 * 0.49 0.13 0.12 0.16 0.13 0.12 0 0.35 0.60 0.14 * 0.28 0.13 0.27 0.07 0.23 0.23 0.11 0.11 0.13 0.08 0.16 0 0.39 0.62 Absorption Absorption Absorption Absorption
levels*modi"ed JO theory. levels without P level*modi"ed JO theory. levels without P level and emission levels*standard JO theory. levels without P level and emission levels*modi"ed JO theory.
0.13 0.12 0.11 0.03 0.13 0 0.40 0.68 0.12 0.11 0.14 0.09 0.14 0 0.38 0.62 0.11 0.10 0.15 0.12 0.16 0 0.37 0.58 0.01 0.09 0.09 0.06 0.17 0.02 0.82 0.73 362 195 237 195 102 0 281 230 3338 3037 3956 2731 4053 0 10 511 17 983 14 21 20 13 30 10 16 24
( ) A
725 704 683 641 609 535 528 486 P PF P PF P PF P PF P PH P PH P PH P PH
Cal Exp Cal Exp Cal Exp
Cal
Cal
Cal
Cal
Cal Exp
Exp
PbOF [18] ZBLA [17] InF2 [16] ZBLAN1 [12] L5FBP
0 Level
Table 7 Emission peak wavelengths ( , nm), e!ective band widths ( , nm), total radiative transition probabilities (A, s\), stimulated emission cross-sections ( ( ), ;10\ cm), branching ratios ( ) of Pr> : L5FBP glass and reported systems 0
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
337
Pr> : L7BP and L5FBP glasses and negative for Pr> : L4BP, L5BP, L6BP and L2FBP glasses. In the second method, the modi"ed JO theory [13] has been used for all the absorption levels and found to yield positive value for and slightly decrease in RMS deviation, though the P level "ts badly (Footnote c of Table 2). Therefore in the third procedure, modi"ed JO theory has been applied to absorption levels, excluding P level and found that the "tting is very good with lowest RMS deviation of 0.40 for Pr> : L5BP glass (Footnote a of Table 2). In the fourth method, emission levels have been included along with absorption levels (without P level) and used standard JO theory and found that is positive and minimum in magnitude and poor "t for emission levels with RMS deviation of 1.49 (Footnote d of Table 2). Finally the modi"ed JO theory has been applied both for absorption (without P level) and emis sion levels (Footnote e of Table 2). Though this method yields positive value for but the emis sion levels "ts badly. Also this method gives RMS deviation of 1.86 which is maximum of all the methods adopted in the present study. Therefore the modi"ed JO theory for only absorption levels without P is found to be more appropriate as it gives low RMS deviation along with consistent JO parameters. Hence this method has been considered and applied to other Pr> : LBP glasses to derive more appropriate JO parameters which are presented in Table 2. The RMS deviations of the "ts are 0.52, 0.45, 0.51, 0.34 and 0.33 for Pr> : L4BP, L6BP, L7BP, L2FBP and L5FBP glasses, respectively. The JO parameters obtained in this method have been used in the present work for further analysis of predicting radiative properties. All these di!erent methods in#uences greatly the magnitude of where as moderate variation is found for and a little variation is found for value. As shown in Table 2, for Pr> : LnBP glasses, increase with decrease in Li CO conH tent where as in the case of LxFBP glasses, in creases, and decreases with increase in LiF content. The over all trend of for Pr> : LBP H glasses is found to be ( & . The oscillator strengths for absorption transitions of Pr> ion in various hosts have been
338
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
compared in Table 3 along with one (Pr> : L5FBP) of the present systems. The closely spaced levels such as F and H , F and F may not be well resolved in some environ ments, in such cases the combined oscillator strengths are reported [9,12,16]. However, in the present Pr> : LBP glasses, individual oscillator strengths are given for F and H , F and F levels, by considering the line shape of a level as Gaussian [21]. As seen from Table 3, the P transition possess highest and G possess lowest oscillator strengths, in all the environments. Pr> : Tellurite based glasses have relatively higher oscillator strengths than all the Pr> : glasses which are compared in Table 3. The oscillator strengths of Pr> : YAG [22] are relatively higher than those found in other systems. Also the oscillator strengths of Pr> : Aquo [26] are higher than those of Pr> : glasses [9,12, 16,17]. This trend suggests that the f-orbitals of Pr> ions interact relatively higher in YAG crystal than in the glass environments. 5.3. Radiative properties The intensity parameters obtained from modi"ed JO analyses have been used to predict radiative properties such as A , A , A, A , and for 2 0 0 Pr> : LBP glasses. The detailed quantitative values along with squared reduced matrix elements for all the excited states to lower levels of Pr> : L5FBP glass are presented in Table 4. The
UH and A values have been evaluated by using
appropriate eigenvalues and eigenvectors which are computed from the free-ion parameters of their respective Pr> : LBP glasses [19], though they are almost host independent. As seen from Table 4, the transition probabilities due to magnetic dipole is relatively small for all the allowed transitions except for S PP . Therefore the transition prob abilities for the levels shown in Table 4 are predominantly electric dipole in nature. The predominant nature of radiative magnetic dipole probability for S PP is mainly due to lower value of Jand higher energy di!erence since the former is inversely proportional and the later is proportional to A . As seen from Table 4, the radiative electric
dipole transition probabilities from S to lower levels are very high due to large energy di!erences
which inturn gives lower lifetime for S compared to other levels. Table 4 shows that the reduced matrix elements for I U D and I U G transitions are found to be 1.7002 and 1.3846, respectively, which are relatively higher than those of other transitions. Relatively higher for H and H are mainly due to higher 0 J values since A is inversely proportional to J values. The Pr> : systems is of particular interest since it contains several metastable multiplets, i.e., S , P , D and G and o!er the possibility of emission in UV (S PF , G , D and I , 240}410 nm [31]), Blue (P PH , 491 nm [32]), Green (P PH , 520 nm [31]), Orange (P PH , 605 nm, D PH , 590 nm [31]), Red (P PF , 635 nm [31]) and Infrared (G PH , 1.3 m [4,12,21]) wavelengths. The predicted branching ratios for the lasing transitions, S PI , D , G and F of Pr> : L5FBP in UV region are 24.6, 0.09, 41.6 and 22.5%, respectively, and are relatively higher than the remaining transitions from S . As seen from the emission probabilities of P metastable state, the predicted branching ratios for P PF , F , H , H and H transitions are found to be 10.8, 13.0, 14.9, &0 and 58.6%, respectively. Similarly Table 4 indicates that the predicted for D PF , F , F , H , 0 H and H found to be 19.9, 0.03, 14.6, 15.5, &0 and 38.5%, respectively. Experimentally emission for P PF , H , H and H and D PH , H and H [31}33] have been observed, though the JO theory did not predict branching ratio for P PH and D PH transitions. For other transitions of Pr> : L5FBP glass shown in Table 4, relatively stronger emission (*10% of ) channels that posses higher branch0 ing ratios are P PF , F , H , H and H , I PG , F and H , P PF , F , H , H and H , G PH and H , F PH and H , F PH and H , F PH and H , H PH and H and H PH . The experiments on Pr> : systems also con"rm that the emission has already been observed for P PH [31], and G PH [4,12,21] transitions and the other transitions may emit when the Pr> ion is situated in a suitable environment.
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
The can be used to predict the relative inten0 sities of all emission channels starting from a particular excited state. Special attention was paid to the emission channels that possesses relatively higher than those of other channels originating 0 from a given excited state as they may lead to a laser transitions. Table 5 compares the predicted branching ratios for the lasing transitions, S PF , G , D and I , P PH , P P H , H and F , D PH and G PH of Pr> : systems. The predicted for the transitions S PF , 0 S PG , P PH , P PH , D PH and G PH of Pr> : glasses under investiga tion (Table 5) indicates that it does not depend on environment as its value varies marginally whereas for the transitions S PD and P PF 0 depend on the environment as its magnitude varies greatly. For example, for P PH transition 0 varied from 0.5190 (Pr> : CBA) to 0.7344 (Pr> : PbOF) and for S PD varied from 0.0009 (Pr> : PbOF) to 0.1549 (Pr> : NaTe). The lowest for S PD and P PF 0 transitions of PbOF [18] is due to smallest value for and the matrix elements U and U are zero for these transitions. Similarly minimum for S PI , P PH , D PH and 0 G PH transitions of PBG [21] is due to smallest value for , and lower values for U and U . The parameters and refractive indices (n) H which are used to predict and are collected in 0 0 Table 6. The predicted lifetime variations (in s) found for all the 12 excited states of Pr> ion in di!erent environments are also presented in Table 6 for the sake of comparison. As seen from Table 6, the JO theory predicts shortest lifetime for the higher energy level, S , where as relatively longer lifetimes for the low lying energy levels, H and H . The for P , 0 P and P levels found to be more or less similar and comparatively shorter than those found for G , F , F and F by a factor of 50 or even more in some cases. The predicted for D is 0 found to be longer by a factor of 3 to 4 times than those found for I . The found for Pr> : LBP 0 glasses are comparable to ZnCdF [8], ZBLAN1 [12], ZBLAN2 [14], ZBLA [17] and NaTe [27]
339
glasses where as longer than those found for PBG [21], ZnTe [27], CBA [23] and PbOF [18] glasses. Among 16 Pr> : glasses compared in Table 6, Pr> ion exhibit shorter in PBG [21] environment 0 while longer in Zirconium based, L4BP and 0 L5FBP glasses. Slightly higher experimental than that of predicted for P level and 0 0 D level (Pr> : YSZ) and relatively lower experi mental than that of predicted for G may be 0 0 due to inadequacy of Judd}Ofelt theory for Pr> ions. The emission peak wavelengths ( ), e!ective band widths ( ), stimulated emission cross-sec tions ( ( )) and branching ratios ( ) for 0 Pr> : LBP glasses are determined from emission studies and are presented in Table 7 for Pr> : L5FBP glass. The calculated with di!er0 ent sets of JO parameters are found to be more or less similar. This indicates that is less sensitive to 0 JO parameters for Pr> ion, though the JO parameters vary signi"cantly with the method of calculation. A large deviation between experimental and calculated is noticed for P PH transition of 0 Pr> : LBP glasses. The of P PH for 0 Pr> : LBP glasses is found to be maximum of all the observed transitions from P level and is sim ilar to Pr> : PbOF [18] and higher than those found in ZBLAN [12], INF2 [16] and ZBLA [17] glasses. The closeness of experimental and calculated indicates that the JO theory still can be 0 used to predict radiative properties for Pr> ion within its limitations.
6. Conclusions The results of spectroscopic characterisation of Pr> ions in lithium borate and lithium #uoroborate glasses are presented. The modi"ed JO analysis, excluding P level in the present study yields rela tively better intensity parameters than those resulted from either standard JO analysis of absorption or combined absorption and emission levels. However, it is found that the predicted using di!erent 0 sets of JO parameters do not vary much. A detailed radiative properties have been predicted and compared for all the 12 excited states of Pr> ion in 17 di!erent environments. The
340
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
predicted lifetimes, using the modi"ed JO theory for Pr> : L5FBP glasses is comparable to ZBLAN1, ZBLAN2 and ZBLA glasses but are higher to that of Pr> : PbOF, PBG, ZnTe, CBA glasses. It is found that the radiative properties varies over a wide range for S PD and P PF transitions as they are sensitive to JO parameters and matrix elements, where as marginal variation for certain levels, S PF , S PG , P PH , P PH and G PH . In par ticular, it is found that the for the G PH 0 transition, useful for "bre ampli"ers, varies marginally with the environment. Acknowledgements C.K. Jayasankar is grateful to Sri Venkateswara University, Tirupati, for sanctioning the Minor Research Project, funded by University Grants Commission (UGC), New Delhi. P. Babu is also grateful to UGC, New Delhi for awarding the Teacher Fellowship under the Faculty Improvement Progamme. References [1] A.A. Kaminskii, Laser Crystals, 2nd Edition, Springer, Berlin, 1990. [2] D.B. Gatch, S.A. Holmstrom, W.M. Yen, J. Lumin. 83}84 (1999) 55. [3] D.M. Baney, L. Yang, J. Ratcli!, K.W. Chang, Electron. Lett. 31 (1995) 1842. [4] Y. Zhao, S. Fleming, IEEE J. Quant. Electron. 33 (1997) 905. [5] B.R. Judd, Phys. Rev. 127 (1962) 750. [6] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [7] R.D. Peacock, Struct. Bonding 22 (1975) 83. [8] M.A. Bunuel, R. Cases, M.A. Chamarro, R. Alcala, Phys. Chem. Glasses 33 (1992) 16. [9] A. Florez, O.L. Malta, Y. Messaddeq, M.A. Aegerter, J. Non-Cryst. Solids 213 & 214 (1997) 315.
[10] R.D. Peacock, Struct. Bonding 22 (1975) 113. [11] K. Binnemans, D. Verboven, C. Gorller-Walrand, J. Lucas, N. Duhamel-Henry, J.L. Adam, J. Alloys Compounds 250 (1997) 321. [12] A. Remillieux, B. Jacquier, C. Linares, C. Lesergent, S. Artigaud, D. Bayard, L. Hamon, J.L. Beylat, J. Phys. D: Appl. Phys. 29 (1996) 963. [13] A.A. Kornienko, A.A. Kaminskii, E.B. Dunina, Phys. Stat. Sol. A 157 (1990) 267. [14] R.S. Quimby, W.J. Miniscalco, J. Appl. Phys. 75 (1994) 613. [15] R. Alcala, R. Cases, Adv. Mater. 7 (1995) 190. [16] A.B. Arauzo, R. Cases, R. Alcala, Phys. Chem. Glasses 35 (1994) 202. [17] M. Eyal, E. Greenberg, R. Reisfeld, Chem. Phys. Lett. 117 (1985) 108. [18] P. Nachimuthu, R. Jagannathan, Phys. Chem. Glasses 36 (1995) 77. [19] C.K. Jayasankar, P. Babu, J. Alloys Compounds 275}277 (1998) 369. [20] C. Gorller-Walrand, K. Binnemans, Spectral intensities of f}f transitions, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare earths, vol. 25, North-Holland, Amsterdam, 1998 (Chapter 167). [21] Y.G. Choi, J. Heo, J. Non-Cryst. Solids 217 (1997) 199. [22] M. Malinowski, R. Wolski, W. Wolinski, Solid State Commun. 74 (1990) 17. [23] P. Nachimuthu, R. Jagannathan, Phys. Chem. Glasses 36 (1995) 194. [24] J.R. Sarkies, H.N. Rutt, Opt. Mater. 13 (1999) 231. [25] H.H. Crosswhite, G.H. Dieke, W.J. Carter, J. Chem. Phys. 43 (1965) 2047. [26] W.T. Carnall, P.R. Fields, B.G. Wybourne, J. Chem. Phys. 49 (1968) 4412. [27] J. Hormadaly, R. Reisfeld, J. Non-Cryst. Solids 30 (1979) 337. [28] B. Savoini, J.E. Munoz Santiuste, R. Gonzalez, Phys. Rev. B 56 (1997) 5856. [29] C.K. Jorgensen, Orbitals in Atoms and Molecules, Academic press, London, 1962. [30] S.P. Sinha, Complexes of the Rare Earths, Pergamon Press, Oxford, 1966. [31] A. Lezama, C.B. de Araujo, Phys. Rev. B 34 (1986) 126. [32] P. Xie, T.R. Gosnell, Opt. Lett. 20 (1995) 1014. [33] O.K. Moune, M.D. Faucher, C.K. Jayasankar, A.M. Lejus, J. Lumin. 85 (1999) 59.
Spectroscopy of Pr> ions in lithium borate and lithium #uoroborate glasses P. Babu, C.K. Jayasankar* Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Received 28 August 2000; received in revised form 8 November 2000
Abstract Optical absorption and emission spectra of 1 mol% Pr> doped lithium borate and lithium #uoroborate glasses have been recorded. The intensities of f}f transitions are parameterized in terms of Judd}Ofelt (JO) intensity parameters ("2, 4 and 6). The JO parameters obtained with modi"ed JO theory have been used to predict radiative properties H such as spontaneous emission probabilities (A), lifetimes ( ) and branching ratios ( ) for all the 12 excited states of these 0 0 Pr> doped glasses along with some of the reported Pr> : systems. The predicted are compared with the experimental 0 values for the emission from P and P levels. The stimulated emission cross-sections are also evaluated for all the observed emission transitions. The values for the potential laser transitions including the G PH (1.3 m) 0 transition useful for "bre ampli"er have been compared for 17 Pr> : systems. The e!ect of compositional changes of the glasses on the optical properties of Pr> ions have been discussed. 2001 Elsevier Science B.V. All rights reserved. PACS: 42.62.Fi; 76.30.Kg; 78.20; 78.40.Pg Keywords: Pr> ions; Optical properties; Borate glasses; Judd}Ofelt analysis; Laser spectroscopy
1. Introduction Praseodymium doped glasses "nd variety of practical applications such as UV-VIS-NIR lasers [1], up-converters [2], optical "bre lasers [3], "bre ampli"ers in 1.3 m region [4] etc. These applications can either be enhanced or optimized from systematic study of optical properties of Pr> ions in various environments. Generally the optical properties of rare earth ions in insulating matrices
* Corresponding author.Tel.:#91-8574-49666; fax:#918574-27499. E-mail address: [email protected] (C.K. Jayasankar).
are derived in the frame work of Judd}Ofelt theory [5,6]. However, it is known that the JO theory is less successful in the case of Pr> ion due to the small energy gap between the ground con"guration, 4f, and the "rst excited state con"guration, 4f5d [7]. The problems which arise due to the application of standard JO theory to the Pr> : systems are: (1) negative value for which has no physical signi"cance [8,9], (2) poor agreement between the calculated and measured oscillator strengths [10,11] and (3) inconsistency between predicted and measured radiative properties [8,12]. Kornienko et al. [13] tried to solve the problem of standard JO theory by modifying the equation for electric dipole line strength where the in#uence
0921-4526/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 2 3 9 - 3
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
of excited con"guration of Pr> ion has been taken into account. Later Quimby and Miniscalco [14] introduced the emission data along with the absorption data for the calculation of JO intensity parameters, . Alcala and Cases [15] applied the H modi"ed JO theory proposed by Kornienko et al. [13] to the absorption data and obtained positive value for with reasonable agreement between calculated and experimental oscillator strengths. They also considered the emission data along with absorption data for modi"ed JO theory and obtained positive value but the agreement between calculated and experimental oscillator strengths is little worse than the previous one. Arauzo et al. [16] also applied the modi"ed JO theory to the absorption data, absorption and emission data and found that the modi"ed JO theory yields positive values for and better agreement between the calculated and experimental values of oscillator strengths and branching ratios than the standard JO theory. In many cases, the experimental and calculated oscillator strengths for H PP "ts badly and therefore this transition is omitted in the standard JO analysis and found positive values for [11,17,18]. Florez et al. [9] used JO analysis for Pr> : #uoroindate glasses and obtained consistent set of JO parameters with the smallest root mean square (RMS) values of the "t when is elimi nated and odd rank parameters and have been introduced along with and besides suggestion for level assignment for Pr> ion. In the present paper, the JO intensity parameters have been derived by applying both the standard and modi"ed JO theories to the absorption data along with and without emission data. The relative merits of JO parameters obtained with these di!erent procedures have been discussed by taking in to account RMS deviations between experimental and calculated oscillator strengths along with trends of parameters. The JO parameters resultH ing from the best "t (smallest RMS deviation with consistent set of ), have been used to predict H radiative properties for all the 12 excited states of Pr> (4f) ion. Particular attention was given to lasing transitions and compared the values for 0 Pr> ion in various environments. This work will supplement the preliminary results of optical absorption studies in these glasses [19].
327
2. Experimental The molar compositions of Praseodymium doped lithium borate (LnBP, n"4}7) and lithium #uoroborate (LxFBP, x"2 and 5) glasses studied in the present paper are as follows: L4BP: 59.5Li CO #39.5H BO #1Pr O , L5BP: 49.5Li CO #49.5H BO #1Pr O , L6BP: 39.5Li CO #59.5H BO #1Pr O , L7BP: 29.5Li CO #69.5H BO #1Pr O , L2FBP: 24.75Li CO #24.75LiF #49.5H BO #1Pr O , L5FBP: 49.5LiF#49.5H BO #1Pr O . Above compositions (&5 g) are grinded homogeneously in an agate mortar and taken in a porcelain crucible and melted in an electric furnace in the temperature range of 900}9503C for 1 h. The melt is then air quenched to get a good optical quality glasses. The samples are annealed at 3503C for 5 h to remove thermal strains and then polished before measuring their optical properties. Absorption measurements were made on a Hitachi U-3400 spectrophotometer in the wavelength range of 400}2500 nm. Fluorescence spectra were recorded on a Hitachi 650}10 s #uorescence spectrometer at room and 25 K temperatures. Refractive indices, n, were measured on an Abbe refractometer at sodium wavelength (589.3 nm). Densities were measured by the Archimedes method using xylene as an immersion liquid.
3. Theoretical 3.1. Oscillator strengths*standard JO analysis 3.1.1. Absorption data The experimental oscillator strengths ( f ) of the f }f transitions in the absorption spectra are obtained by means of the usual expression [20],
f"4.318;10\ ()dv
(1)
328
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
where () is the molar absorptivity at a wave number . In the Judd}Ofelt f}f intensity model [5,6] of the static coupling between the radiation "eld and the electronic charge distribution of ligands, the oscillator strength, f, of an electric dipole f}f transition of trivalent rare earth ions from a level J to a level J is given by f(J, J)"[8mcv/3h(2J#1)][(n#2)/9n] ; (J ;H J) (2) H H where m is the mass of an electron, c is the velocity of light, is the wave number of the transition, h is the planck's constant, J and Jare the total angular momenta of the initial and "nal levels respectively, n is the refractive index, ("2, 4 and 6) are the H JO intensity parameters and ;H are the doubly reduced matrix elements evaluated in the intermediate coupling approximation for a transition JPJ. In general JO parameters can be obtained from the least square "t as carried out for Pr> : glasses [14}18]. 3.1.2. Both absorption and emission data The standard JO theory did not yield consistent JO parameters and shows discrepancy between experimental and calculated oscillator strengths for Pr> : systems [10,11]. Hence emission data have been included along with absorption data and obtained positive value as well as improved "t between experimental and calculated oscillator strengths [14}16]. In the present work, JO theory has been employed with and/or without emission data and obtained JO parameters. 3.2. Modixed Judd}Ofelt analysis The JO theory assumes that the energy gap between 4f and 5d orbitals is large and consequently the 5d orbitals does not a!ect the f}f transition intensities. However, for Pr> ions, it is found that the energy gap between 4f and 5d orbitals is small enough (&15,000 cm\) [9] that 5d orbital in#uences the f}f transition intensities. Therefore, Kornienko et al. [13] considered the mixing of 4f-5d orbitals and modi"ed the JO theory. As per
Kornienko et al. [13], the modi"ed equation for electric dipole line strength that accounts the in#uence of excited con"guration of Pr> ion, neglecting electron correlation e!ects is S (J, J)"e [1#2 (E #E ( Y(Y H H !2E )](J ;H J) (3) D where are modi"ed JO parameters, is addiH tional parameter of about 10\ cm\ [13]. E and ( E are the energies of J and J levels, respecY(Y tively. E is the energy of the centre of gravity of the D 4f orbitals of Pr> ion that has been taken as 9940 cm\ [13,21]. In the present paper, the matrix elements have been modi"ed by multiplying with [1#2 (E #E + !2E )] and are used in least ( Y( D square "tting of oscillator strengths to obtain modi"ed JO parameters, . These are inturn H H used to calculate S using Eq. (3). 3.3. Radiative properties Radiative properties in the present paper have been calculated using the usual expressions as carried out for trivalent lanthanide ions [16,19}21].
4. Results 4.1. Optical absorption and emission spectra The optical absorption spectra of Pr> : LBP (LnBP, n"4}7 and LxFBP, x"2 and 5) measured at room temperature in the wavelength range of 400}2500 nm are shown in Fig. 1. All the bands originate from the ground state, H , to the excited levels shown in Fig. 1. Band assignments along with energy positions for the present and reported Pr> : systems are given in Table 1. The reported systems that are compared include Pr>: #uoroindate (InF1: 34 InF }20 ZnF }20 SrF }6 GaF } 2 NaF}17 BaF ) [9], #uorozirconate (ZBLAN2: 53 ZrF }20 BaF }4 LaF }3 AlF }20 NaF) [14], #uoroindate (InF2: 20 ZnF }20 SrF }20 BaF }40 InF ) [16], lead bismuth gallite (PBG: 57 PbO} 25 Bi O }18 Ga O ) [21], yttrium aluminium
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
329
Fig. 1. Absorption spectra of Pr> : LBP glasses: (a) L4BP; (b) L5BP; (c) L6BP; (d) L7BP; (e) L2FBP; (f) L5FBP.
garnet (YAG) [22], calcium boro-aluminate (CBA: CaO}B O }Al O ) [23], PAS (PBr /AlBr / SbBr ) [24] and Free ion (FI) [25]. Emission spectra of Pr> : L5FBP glass recorded both at room temperature and at 25 K in the energy range of 13 000}22 000 cm\ are shown in Fig. 2 along with assignment of the bands. The emission spectra has not been corrected for the instrument spectral sensitivity. 4.2. Judd}Ofelt intensity analysis The experimental oscillator strengths of the absorption bands of Pr> : LBP glasses are determined using Eq. (1) and are collected in Table 2 along with calculated oscillator strengths obtained from JO analysis. For Pr> : L5BP glass, the calculated oscillator strengths obtained with di!erent methods are also presented. The intensity parameters ( ) resulting from the JO analysis are H
also presented in Table 2. In order to evaluate the quality of the "t, JO parameters obtained with di!erent combination of levels and methods are also presented in Table 2 for Pr> : L5BP glass. Table 3 shows the comparison of the oscillator strengths for Pr> : L5FBP glass with some of the reported Pr> : systems of InF1 [9], #uorozirconate (ZBLAN1: ZrF }BaF }LaF }AlF }NaF) [12], InF2 [16], fuorozirconate (ZBLA: 57 ZrF }34 BaF }3 LaF }4 AlF ) [17], PbOF (PbO}PbF ) [18], PBG [21], YAG [22], PAS [24], Aquo-ion (Aquo) [26], zinc tellurite (ZnTe: 35 ZnO}65 TeO ) [27], sodium tellurite (NaTe: 20 Na O}80TeO ) [27]. Confusion may be avoided among three #uorozirconate glasses, ZBLAN1 [12], ZBLAN2 [14] and ZBLA [17], as these differs in composition and reported independently. The reduced matrix elements used in the present JO analysis and in the calculation of magnetic dipole radiative transition probabilities are
6855 9921 17 334 21 390 22 007 23 161 0.9737 0.9964 1.0027 0.9914
4.3. Radiative properties The intensity parameters obtained with modi"ed JO theory for absorption levels excluding H P P transition have been used to predict various radiative properties. The detailed results of electric (A ) and magnetic (A ) dipole radiative transition
probabilities, radiative (A) and total radiative (A ) transition probabilities, branching ratios ( ) 2 0 and lifetimes ( ) for all the excited states of 0 Pr> : L5FBP glass are presented in Table 4. The predicted for some of the important #uorescent 0 levels of Pr> systems are presented in Table 5. The predicted lifetimes for all the excited levels of Pr> : LBP glasses along with some of the reported systems are collected in Table 6. The reported Pr> : sytems that are compared in Tables 5 and 6 include ZnCdF (ZnF }CdF ) [8], ZBLAN1 [12],ZBLAN2 [14], InF2 [16], ZBLA [17], PbOF [18], PBG [21], CBA [23], ZnTe [27], NaTe [27], yttria-stabilized-zirconia (YSZ: ZrO }Y O ) [28]. Table 7 presents the luminescence properties such as emission peak wavelengths ( ), e!ective linewidths ( ), radiative transition probabilities (A), stimulated emission cross-sections ( ) for the observed emission bands from P and P levels of Pr >: L5FBP glass. Table 7 also compares the experimental and calculated branching ratios with di!erent sets of JO parameters for Pr> : L5FBP glass and reported Pr> : glasses of ZBLAN1 [12], InF2 [16], ZBLA [17] and PbOF [18]. 4.4. Nephelauxetic ewect
0.9905
0.9944
determined by using the appropriate eigenvalues and eigenvectors which are computed from intermediate coupling approximation for individual Pr> : LBP glasses [19], though they are almost host independent. The matrix elements for all the excited states to their lower levels of Pr> : L5FBP glass are presented in Table 4.
The average Nephelauxetic ratio (M ) between the rare earth ion and its environment in a host can be calculated with the following expression [29,30] as
M
6919 10 109 16 895 20 678 2 1204 22 512 F G D P P P
7029 * 16 892 20 665 21 178 22 497
6977 9989 16 883 20 648 21 155 22 434
0.9948
0.9907
0.9898
0.9992
0.9884
0.9911
1.0023
6714 9682 16 682 20 450 21 010 22 174 7143 9709 16 667 20 492 21 277 22 124 7063 * 16 983 20 790 21 321 22 421 9911 17 007 20 833 21 368 22 573 9862 17 007 20 661 21 277 22 727
6631 6562 6556
10 000 17 014 20 867 21 470 22 700 6958 9946 16 932 20 747 21 295 22 507 6906 9914 16 903 20 738 21 268 22 518
5185 6580 5184 6533 5191 6625 5203 6548 F F
5173 6557
5184 6567
7016 10 109 16 895 20 685 21 207 22 464
5131 6506
7018 9852 16 920 20 661 * *
4954 6272 5405 6667 5320 6639
4997 6415
4450 4320 * 4444
5000 * 4883 * 4234 H
4245
4245
4231
4259
4231
5094 *
PAS [24] YAG [22] CBA [23] PBG [21] InF2 [16] ZBLAN2 [14] InF1 [9] L5FBP L2FBP L7BP L6BP L5BP L4BP Energy level
Table 1 Observed band positions (cm\) and the average Nephelauxetic ratio (M ) for Pr> : LBP glasses and reported Pr> : systems
4389
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FI [25]
330
M " /N ,
(4)
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
331
Fig. 2. Emission Spectra of Pr> : L5FBP glass at room temperature (RT) and at 25 K.
where " / , and are the energies of the corresponding transitions in the complex and freeion [25], respectively, and N refers the number of levels used to compute M values. The values of M are collected in Table 1 for present and reported Pr> : systems [9,14,16,21}24].
5. Discussion 5.1. Electronic structure of Pr3> The absorption spectra observed for Pr> : LBP glasses are shown in Fig. 1, which are similar to the spectra observed for Pr> : glasses [8,12,15,16]. The assignment of the observed excited levels (from the ground state, H ) has also been shown in Fig. 1 and their positions in wavenumbers are collected
in Table 1. As usual, out of 12 excited states of Pr> ion, only 9 are observed in the absorption spectra (except for Pr> : L4BP glass). The transitions which are not observed include G (too weak to locate), S (too high in energy, 46 500 cm\), I (spin forbidden and is also masked by the intense spin allowed H PP transitions) and H (too low in energy, NIR region). In Table 1, energy positions of Pr> ion is compared for glasses [9,14,16,21,23], crystal [22], solution [24] and free-ion [25] along with M . As seen from Table 1, all the energy positions for Pr> ions did not vary uniformly when environment is changed since the energy positions for high energy levels such as D and P multiplets are lower where as for low energy levels such as F and G are higher than those observed for free-ion. However, the average Nephelauxetic ratio (M ) calculated from
332
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
Table 2 Experimental ( f Level
) and calculated ( f ) oscillator strengths (;10\) and J}O parameters ( ;10\ cm) for Pr> : LBP glasses
L4BP f
H P H 0.01 F 1.47 F 4.09 F 1.32 G * D 0.92 P 1.87 P 2.52 P 5.28 P P F 4.44 H 1.94 N
L5BP
L6BP
L7BP
L2FBP
L5FBP
f
f
f
f
f
f
f
f
f
f
f
f
f
f
0.35 1.51 3.61 1.91 * 0.81 2.87 2.96 *
0.01 2.19 5.21 2.30 0.18 1.34 2.76 3.78 7.33
0.52 2.19 5.17 2.53 0.25 0.87 3.50 3.53 2.75
0.50 2.19 5.05 2.74 0.26 1.14 3.61 3.72 3.95
0.48 2.21 4.88 2.60 0.25 1.09 3.65 3.77 *
0.50 2.30 4.95 2.44 0.24 0.83 3.26 3.29 *
0.47 2.18 4.87 2.56 0.41 1.09 3.79 3.91 *
0.01 2.39 5.78 2.58 0.12 1.41 2.88 4.00 8.20
0.53 2.42 5.41 2.91 0.28 1.22 3.88 4.00 *
0.02 3.11 7.26 3.52 0.19 1.83 3.41 4.87 10.06
0.68 3.14 6.89 3.80 0.36 1.55 4.53 4.68 *
0.01 1.83 4.37 2.22 0.19 1.18 2.14 3.31 6.48
0.42 1.85 4.20 2.31 0.22 0.96 2.87 2.97 *
0.01 1.63 3.96 1.91 0.15 1.03 1.56 2.58 2.43
0.38 1.65 3.73 2.10 0.20 0.85 2.33 2.40 *
* * 7 0.52 0.13 4.61 3.92
4.59 5.06
* * 9 1.57 !0.56 5.64 3.76
* * 9 1.19 0.90 5.78 5.70
* * 8 0.40 0.98 5.85 5.36
0.28 6.68 10 1.49 0.04 5.26 3.62
5.75 10.69 10 1.86 0.72 6.06 5.26
1.84 3.76
* * 8 0.45 1.14 6.18 6.08
3.28 * 5.76 * 8 0.51 2.14 7.19 7.83
2.08 2.64
* * 8 0.34 0.98 4.67 4.96
1.04 2.88
* * 8 0.33 1.17 3.83 4.58
f
Absorption levels without P level*modi"ed JO theory. Absorption levels*standard JO theory. Absorption levels*modi"ed JO theory. Absorption levels without P level and emission levels*standard JO theory. Absorption levels without P level and emission levels*modi"ed JO theory.
Table 3 Experimental oscillator strengths (;10\) for Pr>ions in various hosts (all transitions are from H ) Level
Spectral region (cm\)
Present work L5FBP
H
4241
F F
5187 6566
1.63 3.96
F G D P P P
6969 10 010 16 892 20 704 21 231 22 472
1.91 0.15 1.03 1.56 2.58 5.53
Reported InF1 [9]
ZBLAN1 InF2 [12] [16]
1.43
3.30
0.01
6.49 0.25 1.77 0.96 2.19 6.29
8.30 0.20 1.89 2.50 4.32 9.15
ZBLA [17]
PbOF [18]
*
*
PBG [21]
ZnTe [27]
NaTe [27]
YAG [22]
0.93
1.56
2.06
1.00
PAS [24]
Aquo [26]
*
*
2.97 2.23 6.21
3.89 7.92
5.77 6.84
6.15 11.36
6.36 11.06
5.63 14.91
3.70 8.00
2.87 0.32 1.54 2.46 4.19 8.93
3.27 * 3.88 4.04 7.31 16.87
1.61 0.49 6.09 20.90 * *
3.92 0.35 3.13 5.66 9.16 16.43
3.41 0.51 3.28 5.98 8.75 15.60
5.68 0.50 6.35 6.83 15.58 27.56
3.94 0.71 2.68 5.79 2.66 15.06
8.71 0.27 2.07 2.30 4.73 10.14
* 12.79
For source of data as well as title of the system, see Section 4.2.
0.32 3.08 2.54 7.63 9.75
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
333
Table 4 Energies (v), matrix elements ( U ) and radiative properties (A , A , A, A , and ) for the excited states of Pr> : L5FBP glass 2 0 0 (cm\)
U
SLJ
SLJ
S
P 25 485 0.0700 I 26 617 0 P 26 697 0 P 27 245 0 D 31 060 0.4963 G 38 046 0 F 41 034 0 F 41 412 0 F 42 807 0.0045 H 43 761 0 H 45 925 0 H 47 992 0 A (S )"483 304 s\, (S )"2 s 2 0 I 1132 0 P 1212 0.4170 P 1760 0.1907 D 5575 0.0011 G 12 561 0.6746 F 15 549 0.4111 F 15 927 0.2554 F 17 322 0.0314 H 18 276 0 H 20 440 0 H 22 507 0 A (P )"24545 s\, (P )"40 s 2 0 P 80 0 P 628 0 D 4443 0.0000 G 11 429 0.2417 F 14 417 0.0886 F 14 795 0 F 16 190 0 H 17 144 0.0037 H 19 308 0.0003 H 21 375 0.0089 A (I )"5864 s\, (I )"170 s 2 0 P 548 0 D 4363 0.0825 G 11 349 0 F 14 337 0 F 14 715 0.5714 F 16 110 0.2683 H 17 064 0 H 19 228 0 H 21 295 0 A (P )"23596 s\, (P )"42 s 2 0 D 3815 0.0148 G 10 801 0 F 13 789 0 F 14 167 0 F 15 562 0.2951
P
I
P
P
U
U
A (s\)
A (s\)
0
0 0 0 0 0 0.3928 0.1696 0 0 0 0 0.0071
0 0.5669 0 0 0 0 0 0 0 0.0003 0 0
3310 118 947 0 0 42 446 201 095 108 931 0 1008 260 0 7275
3310 118 947 33 0 42 446 201 095 108 931 0 1008 260 0 7275
0.0068 0.2461 0.0001 0 0.0878 0.4161 0.2254 0 0.0021 0.0005 0 0.0151
0.0278 0 0 0.0778 0.0450 0.1044 0.3062 0.2930 0.4949 0.1886 0.0354
0.1559 0 0 0 0.0180 0.0102 0 0 0.0537 0.1323 0.1336
1 0.4 1 25 1008 1697 2889 2925 6348 5509 4140
0 0.08 0 2 0 0 0.54 0.04 0 0 0
1 1 1 27 1008 1697 2890 2925 6348 5509 4140
0 0 0 0.0011 0.0411 0.0691 0.1177 0.1192 0.2586 0.2244 0.1687
0 0 0.1555 1.3846 0.6345 0.0010 0.0055 0.0134 0.0007 0.0494
0.0004 0.0030 1.7002 0.6710 0.4295 0.0025 0.0333 0.0046 0.0019 0.0227
0 0 137 2415 2519 9 138 73 16 554
0 0 0 0 0 0 0 1.75 1.79 0
0 0 137 2415 2519 9 138 74 18 554
0 0 0.0234 0.4118 0.4296 0.0016 0.0234 0.0127 0.0030 0.0944
0 0 0.0843 0.2621 0.1964 0 0 0.2857 0.1714
0 0 0 0 0 0 0.1246 0.0893 0
0 7 382 2394 3672 1067 2297 8648 5129
0.01 0.60 0 0 0 0.07 0 0 0
0.01 7 382 2394 3672 1067 2297 8648 5129
0 0.0003 0.0162 0.1015 0.1556 0.0452 0.0973 0.3665 0.2174
0 0.0559 0.1075 0 0
0 0 0 0 0
2 655 2621 0 3175
0 0 0 0 0
2 655 2621 0 3175
0.0001 0.0269 0.1076 0 0.1303
A (s\)
0 0 33 0 0 0 0 0 0 0 0 0
334
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
Table 4. Continued. (cm\)
U
SLJ
SLJ
P
H 16 516 0 H 18 680 0 H 20 747 0 A (P )"24365 s\, (P )"41 s 2 0 G 6986 0.2937 F 9974 0.6077 F 10 352 0.0332 F 11 747 0.0144 H 12 701 0 H 14 865 0 H 16 932 0.0022 A (D ) "1955 s\, (D )"511 s 2 0 F 2988 0.0785 F 3366 0.0041 F 4761 0.0001 H 5715 0.2583 H 7879 0.0375 H 9946 0.0012 A (G )"493 s\, (G )"2027 s 2 0 F 378 0.0250 F 1773 0.0017 H 2727 0.5674 H 4891 0.0302 H 6958 0.0180 A (F )"353 s\, (F ) "2833 s 2 0 F 1395 0.0209 H 2349 0 H 4513 0.6286 H 6580 0.0656 A (F )"545 s\, (F ) "1835 s 2 0 H 954 0 H 3118 0 H 5185 0.5088 A (F )"243 s\, (F ) "4107 s 2 0 H 2164 0.1080 H 4231 0.0001 A (H )"19 s\, (H )"52050 s 2 0 H 2067 0.1094 A (H ) "8 s\, (H )"123 995 s 2 0
D
G
F
F
F
H H
U
U
A (s\)
A (s\)
A (s\)
0
0 0 0.1719
0.0726 0 0
3641 0 14271
0 0 0
3641 0 14271
0.1494 0 0.5857
0.0519 0.0000 0.0187 0.0891 0.0711 0.0022 0.0181
0.0760 0.0201 0 0 0.0069 0.0004 0.0540
148 389 60 282 303 16 753
0 0 3.40 2.63 0 0 0
148 389 63 285 303 16 753
0.0755 0.1988 0.0322 0.1455 0.1547 0.0082 0.3850
0.1448 0.0059 0.0154 0.2592 0.0997 0.0070
0.3513 0.0509 0.0063 0.2429 0.4169 0.0258
16 3 3 121 308 39
0.81 1.04 0 0 0.75 0.63
17 4 3 121 309 40
0.0346 0.0075 0.0052 0.2460 0.6266 0.0800
0.0718 0.0015 0.6115 0.3098 0.0503
0.0060 0.0898 0.4606 0.4457 0.4826
0 1 28 103 220
0 0 0 0.20 0.59
0.01 1 28 103 221
0 0.0018 0.0794 0.2926 0.6263
0.0510 0.3182 0.3468 0.3476
0 0.8459 0 0.6988
0.2 23 66 456
0.26 0 0 0.02
1 23 66 456
0.0008 0.0420 0.1209 0.8362
0.0164 0.2978 0.4032
0.3038 0.6590 0.1180
1 61 182
0 0 0
1 61 182
0.0025 0.2515 0.7459
0.2327 0.0327
0.6420 0.1403
8 11
0.85 0
8 11
0.4344 0.5656
0.2025
0.6101
7
0.85
8
1.0000
these energy positions (Table 1) using Eq. (4) shows variation from 0.9737 (PAS [24]) to 1.0027 (CBA [23]). Fig. 2 shows the emission spectra observed for Pr> : L5FBP glass which is similar to other Pr> : LBP glasses. The emission spectra has been recorded by exciting the P level with 457 nm of Ar> laser line. Due to thermalization e!ect, emission has been observed from both P and P
levels at room temperature. At low temperature (25 K) all the emissions are only from P level and the intensities are found to be stronger than those found at room temperature. This indicates the absence of thermalization e!ect between P and P levels at low temperatures. The P PH transition is found to be highly intense than the other emission levels which is in accordance with the Pr> : glasses [8,15,16].
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
335
Table 5 Predicted branching ratios of some important laser transitions of Pr>: LBP glasses and reported systems System transition
S PF S PG S PD S PI
P PH P PH P PH P PF D PH G PH
L4BP L5BP L6BP L7BP L2FBP L5FBP ZnCdF [8] ZBLAN1 [12] ZBLAN2 [14] InF2 [16] ZBLA [17] PbOF [18] PBG [21] CBA [23] ZnTe [27] NaTe [27] YSZ [28]
0.2675 0.2505 0.2468 0.2305 0.2406 0.2254 0.2158 0.2360 0.2070 0.2126 0.2448 0.2878 0.2794 0.2032 0.2350 0.2310 0.2917
0.4055 0.3819 0.3802 0.3672 0.3767 0.3665 0.3988 0.3629 0.3360 0.3940 0.4065 0.4044 0.3262 0.3351 0.3354 0.3250 0.3930
0.4920 0.4618 0.4489 0.4223 0.4431 0.4161 0.3984 0.4356 0.3821 0.3924 0.4519 0.5313 0.5158 0.3752 0.4339 0.4265 0.5386
0.0099 0.0539 0.0581 0.0879 0.0636 0.0878 0.0411 0.0894 0.1453 0.0498 0.0166 0.0009 0.1347 0.1483 0.1357 0.1549 0.0174
0.2111 0.2111 0.2233 0.2344 0.2296 0.2461 0.3253 0.2136 0.2364 0.3252 0.2680 0.1603 0.0376 0.2440 0.1655 0.1560 0.1307
5.2. Standard and modixed Judd}Ofelt analysis Table 2 gives the observed oscillator strengths ( f ) for the absorption bands from the ground state, H , to the excited states of Pr> : LBP glass es. The f values are found to increase uniformly with decrease in Li CO content and decrease with increase in LiF content for all the absorption transitions except for G transition. This is prob ably due to decrease and increase in site symmetry of the Pr> ion with decrease in Li CO and in crease in LiF contents, respectively. The inconsistent behaviour for G intensity may be due to its weak nature and hence more uncertainty in the experimental determination of its oscillator strength. Table 2 also shows the branching ratios observed for P PF and H transitions as oscillator strengths. As the emission intensities are relative, the emission intensity of P PH is nor malised to the absorption intensity of H PP and hence the intensities of other emission transitions have been corrected accordingly before using them in the "tting [8,16]. During the "tting of absorption and emission levels, the experimental branching ratios are uniformly modi"ed with a common factor to reduce RMS deviation as
0.7021 0.6487 0.6370 0.5988 0.6214 0.5857 0.5915 0.6033 0.5270 0.5810 0.6561 0.7344 0.6509 0.5190 0.5801 0.5648 0.7308
0.1264 0.1259 0.1332 0.1387 0.1400 0.1494 0.2084 0.1276 0.1406 0.2077 0.1679 0.0956 0.0205 0.1456 0.0954 0.0891 0.0765
0.0146 0.0786 0.0847 0.1283 0.0944 0.1303 0.0643 0.1305 0.2113 0.0778 0.0254 0.0014 0.1791 0.2161 0.1912 0.2162 0.0248
0.4323 0.3892 0.3944 0.3782 0.3951 0.3850 0.4854 0.3589 0.3350 0.4762 0.4709 0.3968 0.1704 0.3389 0.2873 0.2678 0.3498
0.6210 0.6186 0.6166 0.6186 0.6261 0.6266 0.6523 0.6158 0.6172 0.6513 0.6413 0.6063 0.4796 0.6205 0.5898 0.5817 0.5881
carried out by Quimby and Miniscalco [14]. Out of "ve emission transitions observed from P level, only two (P PF and H ) are included in the "t. The branching ratio of P P H is normalised to the oscillator strength of H P P and hence the former is excluded. Similarly, P PH and F transitions are excluded since the matrix elements for the former transition are zero and the for the latter is too weak to 0 measure. As seen from the literature, the oscillator strengths observed for Pr> ion have been analysed in di!erent ways to get reliable JO parameters with improved "ttings between experimental and calculated oscillator strengths. In order to test their reliability, all these di!erent methods have been applied to one (Pr> : L5BP) of our systems. In the "rst method, as usual the standard JO theory [5,6] has been applied for the observed absorption levels and found that is negative with poor "tting for P level (Footnote b of Table 2). In some cases the badly "tted level, P , has been excluded in the "tting and found to yield positive values for [11,17,18]. In the present analysis of Pr> : LBP glasses mixed trend has been noticed when P level is omitted in the "t, is positive for
582
1983
2800 1864 4260 52 377 125 568
D
G
F F F H H
2097 1340 2926 38 753 93 376
1485
384
27
1919 1213 2620 34 769 84 795
1250
346
25
26
1 25 107
1.14 6.18 6.08 1.598
L6BP
1419 911 2032 27 391 67 117
975
260
20
21
1 20 89
2.14 7.19 7.83 1.604
L7BP
Reported
2564 1636 3595 44 852 107 433
1811
459
34
35
1 34 140
0.98 4.67 4.96 1.569
2833 1835 4107 52 050 123 995
2027
511
41
42
1.17 3.83 4.58 1.553 2 40 170
1836 1273 3292 34 107 84 922
34 (40) 417 (245) 1356
1 32 124 (42) 34
0.72 4.80 7.93 1.53
38 (42) 36 (42) 478 (360) 2019 (100) 2855 1789 3814 52740 125 442
1 37 160
1.46 4.89 4.85 1.501
L2FBP L5FBP ZnCdF ZBLAN1 [8] [12]
The values shown in parenthesis are experimental lifetimes.
38
P
29
1 27 116
1 37 158
40
0.98 5.85 5.36 1.594
0.13 4.61 3.92 1.588
P
N States S P I
JO Present work parameters L4BP L5BP
2720 1724 3607 50 513 119 988
1914
430
37
38
1 37 165
2.44 4.41 5.52 1.48
ZBLAN2 [14]
2019 1398 3583 37 429 92 703
37 (46) 450 (400) 1488
38
1 35 137
0.80 4.33 7.26 1.527
InF2 [16]
2576 1724 4266 47 381 115 760
42 (13) 41 (44) 563 (55) 1886
2 39 156
0.24 4.50 5.40 1.527
ZBLA [17]
1470 881 1889 27232 65803
1043
265
15
16
&0 16 68
0.02 7.88 4.81 1.78
279 (53) 468 199 284 9059 19 601
36
2
2
&0 2 13
6.95 18.30 2.70 2.30
PbOF PBG [18] [21]
1116 711 1501 21 096 51 528
788
178
15
16
&0 15 68
4.84 8.42 11.08 1.58
CBA [23]
Table 6 Judd}Ofelt parameters ( ,;10\ cm), refractive index (n) and predicted radiative lifetimes ( , s) for excited states of Pr> : glasses H
765 446 841 14 423 34 082
523
103
8
8
&0 8 38
3.48 7.64 5.90 2.04
ZnTe [27]
2386 1364 2485 44749 103755
1610
300
23
25
1 25 118
3.68 6.97 5.16 1.51
NaTe [27]
892 508 1014 16513 39195
8 (1.8) 142 (180) 620
9
&0 9 38
0.34 7.25 3.56 2.17
YSZ [28]
336 P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
* * * 0.001 0.10 0 * 0.74 * * * 0.04 0.13 0.10 0.41 0.73 0.14 * * 0.03 0.16 0 * 0.65 0.08 * * 0.07 0.16 0.11 * 0.56 0.12 * * 0.08 0.32 0 * 0.46 0.05 * * 0.09 0.30 0.07 * 0.49 0.13 0.12 0.16 0.13 0.12 0 0.35 0.60 0.14 * 0.28 0.13 0.27 0.07 0.23 0.23 0.11 0.11 0.13 0.08 0.16 0 0.39 0.62 Absorption Absorption Absorption Absorption
levels*modi"ed JO theory. levels without P level*modi"ed JO theory. levels without P level and emission levels*standard JO theory. levels without P level and emission levels*modi"ed JO theory.
0.13 0.12 0.11 0.03 0.13 0 0.40 0.68 0.12 0.11 0.14 0.09 0.14 0 0.38 0.62 0.11 0.10 0.15 0.12 0.16 0 0.37 0.58 0.01 0.09 0.09 0.06 0.17 0.02 0.82 0.73 362 195 237 195 102 0 281 230 3338 3037 3956 2731 4053 0 10 511 17 983 14 21 20 13 30 10 16 24
( ) A
725 704 683 641 609 535 528 486 P PF P PF P PF P PF P PH P PH P PH P PH
Cal Exp Cal Exp Cal Exp
Cal
Cal
Cal
Cal
Cal Exp
Exp
PbOF [18] ZBLA [17] InF2 [16] ZBLAN1 [12] L5FBP
0 Level
Table 7 Emission peak wavelengths ( , nm), e!ective band widths ( , nm), total radiative transition probabilities (A, s\), stimulated emission cross-sections ( ( ), ;10\ cm), branching ratios ( ) of Pr> : L5FBP glass and reported systems 0
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
337
Pr> : L7BP and L5FBP glasses and negative for Pr> : L4BP, L5BP, L6BP and L2FBP glasses. In the second method, the modi"ed JO theory [13] has been used for all the absorption levels and found to yield positive value for and slightly decrease in RMS deviation, though the P level "ts badly (Footnote c of Table 2). Therefore in the third procedure, modi"ed JO theory has been applied to absorption levels, excluding P level and found that the "tting is very good with lowest RMS deviation of 0.40 for Pr> : L5BP glass (Footnote a of Table 2). In the fourth method, emission levels have been included along with absorption levels (without P level) and used standard JO theory and found that is positive and minimum in magnitude and poor "t for emission levels with RMS deviation of 1.49 (Footnote d of Table 2). Finally the modi"ed JO theory has been applied both for absorption (without P level) and emis sion levels (Footnote e of Table 2). Though this method yields positive value for but the emis sion levels "ts badly. Also this method gives RMS deviation of 1.86 which is maximum of all the methods adopted in the present study. Therefore the modi"ed JO theory for only absorption levels without P is found to be more appropriate as it gives low RMS deviation along with consistent JO parameters. Hence this method has been considered and applied to other Pr> : LBP glasses to derive more appropriate JO parameters which are presented in Table 2. The RMS deviations of the "ts are 0.52, 0.45, 0.51, 0.34 and 0.33 for Pr> : L4BP, L6BP, L7BP, L2FBP and L5FBP glasses, respectively. The JO parameters obtained in this method have been used in the present work for further analysis of predicting radiative properties. All these di!erent methods in#uences greatly the magnitude of where as moderate variation is found for and a little variation is found for value. As shown in Table 2, for Pr> : LnBP glasses, increase with decrease in Li CO conH tent where as in the case of LxFBP glasses, in creases, and decreases with increase in LiF content. The over all trend of for Pr> : LBP H glasses is found to be ( & . The oscillator strengths for absorption transitions of Pr> ion in various hosts have been
338
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
compared in Table 3 along with one (Pr> : L5FBP) of the present systems. The closely spaced levels such as F and H , F and F may not be well resolved in some environ ments, in such cases the combined oscillator strengths are reported [9,12,16]. However, in the present Pr> : LBP glasses, individual oscillator strengths are given for F and H , F and F levels, by considering the line shape of a level as Gaussian [21]. As seen from Table 3, the P transition possess highest and G possess lowest oscillator strengths, in all the environments. Pr> : Tellurite based glasses have relatively higher oscillator strengths than all the Pr> : glasses which are compared in Table 3. The oscillator strengths of Pr> : YAG [22] are relatively higher than those found in other systems. Also the oscillator strengths of Pr> : Aquo [26] are higher than those of Pr> : glasses [9,12, 16,17]. This trend suggests that the f-orbitals of Pr> ions interact relatively higher in YAG crystal than in the glass environments. 5.3. Radiative properties The intensity parameters obtained from modi"ed JO analyses have been used to predict radiative properties such as A , A , A, A , and for 2 0 0 Pr> : LBP glasses. The detailed quantitative values along with squared reduced matrix elements for all the excited states to lower levels of Pr> : L5FBP glass are presented in Table 4. The
UH and A values have been evaluated by using
appropriate eigenvalues and eigenvectors which are computed from the free-ion parameters of their respective Pr> : LBP glasses [19], though they are almost host independent. As seen from Table 4, the transition probabilities due to magnetic dipole is relatively small for all the allowed transitions except for S PP . Therefore the transition prob abilities for the levels shown in Table 4 are predominantly electric dipole in nature. The predominant nature of radiative magnetic dipole probability for S PP is mainly due to lower value of Jand higher energy di!erence since the former is inversely proportional and the later is proportional to A . As seen from Table 4, the radiative electric
dipole transition probabilities from S to lower levels are very high due to large energy di!erences
which inturn gives lower lifetime for S compared to other levels. Table 4 shows that the reduced matrix elements for I U D and I U G transitions are found to be 1.7002 and 1.3846, respectively, which are relatively higher than those of other transitions. Relatively higher for H and H are mainly due to higher 0 J values since A is inversely proportional to J values. The Pr> : systems is of particular interest since it contains several metastable multiplets, i.e., S , P , D and G and o!er the possibility of emission in UV (S PF , G , D and I , 240}410 nm [31]), Blue (P PH , 491 nm [32]), Green (P PH , 520 nm [31]), Orange (P PH , 605 nm, D PH , 590 nm [31]), Red (P PF , 635 nm [31]) and Infrared (G PH , 1.3 m [4,12,21]) wavelengths. The predicted branching ratios for the lasing transitions, S PI , D , G and F of Pr> : L5FBP in UV region are 24.6, 0.09, 41.6 and 22.5%, respectively, and are relatively higher than the remaining transitions from S . As seen from the emission probabilities of P metastable state, the predicted branching ratios for P PF , F , H , H and H transitions are found to be 10.8, 13.0, 14.9, &0 and 58.6%, respectively. Similarly Table 4 indicates that the predicted for D PF , F , F , H , 0 H and H found to be 19.9, 0.03, 14.6, 15.5, &0 and 38.5%, respectively. Experimentally emission for P PF , H , H and H and D PH , H and H [31}33] have been observed, though the JO theory did not predict branching ratio for P PH and D PH transitions. For other transitions of Pr> : L5FBP glass shown in Table 4, relatively stronger emission (*10% of ) channels that posses higher branch0 ing ratios are P PF , F , H , H and H , I PG , F and H , P PF , F , H , H and H , G PH and H , F PH and H , F PH and H , F PH and H , H PH and H and H PH . The experiments on Pr> : systems also con"rm that the emission has already been observed for P PH [31], and G PH [4,12,21] transitions and the other transitions may emit when the Pr> ion is situated in a suitable environment.
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
The can be used to predict the relative inten0 sities of all emission channels starting from a particular excited state. Special attention was paid to the emission channels that possesses relatively higher than those of other channels originating 0 from a given excited state as they may lead to a laser transitions. Table 5 compares the predicted branching ratios for the lasing transitions, S PF , G , D and I , P PH , P P H , H and F , D PH and G PH of Pr> : systems. The predicted for the transitions S PF , 0 S PG , P PH , P PH , D PH and G PH of Pr> : glasses under investiga tion (Table 5) indicates that it does not depend on environment as its value varies marginally whereas for the transitions S PD and P PF 0 depend on the environment as its magnitude varies greatly. For example, for P PH transition 0 varied from 0.5190 (Pr> : CBA) to 0.7344 (Pr> : PbOF) and for S PD varied from 0.0009 (Pr> : PbOF) to 0.1549 (Pr> : NaTe). The lowest for S PD and P PF 0 transitions of PbOF [18] is due to smallest value for and the matrix elements U and U are zero for these transitions. Similarly minimum for S PI , P PH , D PH and 0 G PH transitions of PBG [21] is due to smallest value for , and lower values for U and U . The parameters and refractive indices (n) H which are used to predict and are collected in 0 0 Table 6. The predicted lifetime variations (in s) found for all the 12 excited states of Pr> ion in di!erent environments are also presented in Table 6 for the sake of comparison. As seen from Table 6, the JO theory predicts shortest lifetime for the higher energy level, S , where as relatively longer lifetimes for the low lying energy levels, H and H . The for P , 0 P and P levels found to be more or less similar and comparatively shorter than those found for G , F , F and F by a factor of 50 or even more in some cases. The predicted for D is 0 found to be longer by a factor of 3 to 4 times than those found for I . The found for Pr> : LBP 0 glasses are comparable to ZnCdF [8], ZBLAN1 [12], ZBLAN2 [14], ZBLA [17] and NaTe [27]
339
glasses where as longer than those found for PBG [21], ZnTe [27], CBA [23] and PbOF [18] glasses. Among 16 Pr> : glasses compared in Table 6, Pr> ion exhibit shorter in PBG [21] environment 0 while longer in Zirconium based, L4BP and 0 L5FBP glasses. Slightly higher experimental than that of predicted for P level and 0 0 D level (Pr> : YSZ) and relatively lower experi mental than that of predicted for G may be 0 0 due to inadequacy of Judd}Ofelt theory for Pr> ions. The emission peak wavelengths ( ), e!ective band widths ( ), stimulated emission cross-sec tions ( ( )) and branching ratios ( ) for 0 Pr> : LBP glasses are determined from emission studies and are presented in Table 7 for Pr> : L5FBP glass. The calculated with di!er0 ent sets of JO parameters are found to be more or less similar. This indicates that is less sensitive to 0 JO parameters for Pr> ion, though the JO parameters vary signi"cantly with the method of calculation. A large deviation between experimental and calculated is noticed for P PH transition of 0 Pr> : LBP glasses. The of P PH for 0 Pr> : LBP glasses is found to be maximum of all the observed transitions from P level and is sim ilar to Pr> : PbOF [18] and higher than those found in ZBLAN [12], INF2 [16] and ZBLA [17] glasses. The closeness of experimental and calculated indicates that the JO theory still can be 0 used to predict radiative properties for Pr> ion within its limitations.
6. Conclusions The results of spectroscopic characterisation of Pr> ions in lithium borate and lithium #uoroborate glasses are presented. The modi"ed JO analysis, excluding P level in the present study yields rela tively better intensity parameters than those resulted from either standard JO analysis of absorption or combined absorption and emission levels. However, it is found that the predicted using di!erent 0 sets of JO parameters do not vary much. A detailed radiative properties have been predicted and compared for all the 12 excited states of Pr> ion in 17 di!erent environments. The
340
P. Babu, C.K. Jayasankar / Physica B 301 (2001) 326}340
predicted lifetimes, using the modi"ed JO theory for Pr> : L5FBP glasses is comparable to ZBLAN1, ZBLAN2 and ZBLA glasses but are higher to that of Pr> : PbOF, PBG, ZnTe, CBA glasses. It is found that the radiative properties varies over a wide range for S PD and P PF transitions as they are sensitive to JO parameters and matrix elements, where as marginal variation for certain levels, S PF , S PG , P PH , P PH and G PH . In par ticular, it is found that the for the G PH 0 transition, useful for "bre ampli"ers, varies marginally with the environment. Acknowledgements C.K. Jayasankar is grateful to Sri Venkateswara University, Tirupati, for sanctioning the Minor Research Project, funded by University Grants Commission (UGC), New Delhi. P. Babu is also grateful to UGC, New Delhi for awarding the Teacher Fellowship under the Faculty Improvement Progamme. References [1] A.A. Kaminskii, Laser Crystals, 2nd Edition, Springer, Berlin, 1990. [2] D.B. Gatch, S.A. Holmstrom, W.M. Yen, J. Lumin. 83}84 (1999) 55. [3] D.M. Baney, L. Yang, J. Ratcli!, K.W. Chang, Electron. Lett. 31 (1995) 1842. [4] Y. Zhao, S. Fleming, IEEE J. Quant. Electron. 33 (1997) 905. [5] B.R. Judd, Phys. Rev. 127 (1962) 750. [6] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [7] R.D. Peacock, Struct. Bonding 22 (1975) 83. [8] M.A. Bunuel, R. Cases, M.A. Chamarro, R. Alcala, Phys. Chem. Glasses 33 (1992) 16. [9] A. Florez, O.L. Malta, Y. Messaddeq, M.A. Aegerter, J. Non-Cryst. Solids 213 & 214 (1997) 315.
[10] R.D. Peacock, Struct. Bonding 22 (1975) 113. [11] K. Binnemans, D. Verboven, C. Gorller-Walrand, J. Lucas, N. Duhamel-Henry, J.L. Adam, J. Alloys Compounds 250 (1997) 321. [12] A. Remillieux, B. Jacquier, C. Linares, C. Lesergent, S. Artigaud, D. Bayard, L. Hamon, J.L. Beylat, J. Phys. D: Appl. Phys. 29 (1996) 963. [13] A.A. Kornienko, A.A. Kaminskii, E.B. Dunina, Phys. Stat. Sol. A 157 (1990) 267. [14] R.S. Quimby, W.J. Miniscalco, J. Appl. Phys. 75 (1994) 613. [15] R. Alcala, R. Cases, Adv. Mater. 7 (1995) 190. [16] A.B. Arauzo, R. Cases, R. Alcala, Phys. Chem. Glasses 35 (1994) 202. [17] M. Eyal, E. Greenberg, R. Reisfeld, Chem. Phys. Lett. 117 (1985) 108. [18] P. Nachimuthu, R. Jagannathan, Phys. Chem. Glasses 36 (1995) 77. [19] C.K. Jayasankar, P. Babu, J. Alloys Compounds 275}277 (1998) 369. [20] C. Gorller-Walrand, K. Binnemans, Spectral intensities of f}f transitions, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare earths, vol. 25, North-Holland, Amsterdam, 1998 (Chapter 167). [21] Y.G. Choi, J. Heo, J. Non-Cryst. Solids 217 (1997) 199. [22] M. Malinowski, R. Wolski, W. Wolinski, Solid State Commun. 74 (1990) 17. [23] P. Nachimuthu, R. Jagannathan, Phys. Chem. Glasses 36 (1995) 194. [24] J.R. Sarkies, H.N. Rutt, Opt. Mater. 13 (1999) 231. [25] H.H. Crosswhite, G.H. Dieke, W.J. Carter, J. Chem. Phys. 43 (1965) 2047. [26] W.T. Carnall, P.R. Fields, B.G. Wybourne, J. Chem. Phys. 49 (1968) 4412. [27] J. Hormadaly, R. Reisfeld, J. Non-Cryst. Solids 30 (1979) 337. [28] B. Savoini, J.E. Munoz Santiuste, R. Gonzalez, Phys. Rev. B 56 (1997) 5856. [29] C.K. Jorgensen, Orbitals in Atoms and Molecules, Academic press, London, 1962. [30] S.P. Sinha, Complexes of the Rare Earths, Pergamon Press, Oxford, 1966. [31] A. Lezama, C.B. de Araujo, Phys. Rev. B 34 (1986) 126. [32] P. Xie, T.R. Gosnell, Opt. Lett. 20 (1995) 1014. [33] O.K. Moune, M.D. Faucher, C.K. Jayasankar, A.M. Lejus, J. Lumin. 85 (1999) 59.