Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 43–48
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Investigations on spectroscopic properties of Er3+-doped Li–Zn fluoroborate glass Sunil Thomas a, M.S. Sajna a, Rani George b, Sk. Nayab Rasool c, Cyriac Joseph a, N.V. Unnikrishnan a,⇑ a
School of Pure & Applied Physics, Mahatma Gandhi University, Kottayam, Kerala 686 560, India Department of Physics, St. Aloysius College, Edathua, Kerala 689 573, India c Department of Physics, Sri Venkateswara University, Tirupati, Andhra Pradesh 517 502, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3+
Er :Li–Zn fluoroborate glass was
fabricated for laser applications. Judd–Ofelt and radiative parameters were investigated. Broadband optical communications due to 1.5 lm emission. McCumber theory analysis for 1.5 lm region.
a r t i c l e
i n f o
Article history: Received 24 July 2014 Received in revised form 5 March 2015 Accepted 27 March 2015 Available online 2 April 2015 Keywords: Er3+ ions Borate glass Judd–Ofelt analysis Radiative properties Photoluminescence McCumber theory
a b s t r a c t Er3+-doped Li–Zn fluoroborate glass was synthesized via melt quenching technique. Optical properties of the glass were investigated by UV–Vis-NIR absorption and emission spectra. To evaluate the nature of Er3+-ligand bond in the glass network, nephelauxetic ratios and bonding parameter were calculated. Judd–Ofelt analysis and hence the radiative properties of the present glass system were evaluated for ascertaining the suitability of the glass for laser applications and compared those with the emission spectra. Absorption cross-sections have been calculated from the absorption spectrum and stimulated emission cross-sections were estimated using McCumber theory for 4I13/2 M 4I15/2 transitions. The results of the present glass were compared with those obtained for some other Er3+-doped glass systems. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Trivalent rare earth (RE3+) doped optical glasses are key materials for the development of optical fibers, waveguide lasers, bulk lasers and optical amplifiers [1,2]. Out of many RE3+ ions, Er3+-doped glasses have been extensively considered for laser applications due to their low loss in optical waveguide for 4 S3/2 ? 4I13/2 and 4I13/2 ? 4I15/2 transitions [3]. Mostly, green ⇑ Corresponding author. Tel.: +91 9745047850. E-mail address: [email protected] (N.V. Unnikrishnan). http://dx.doi.org/10.1016/j.saa.2015.03.118 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
emission by the 4S3/2 ? 4I15/2 transition of Er3+ ion has led to successful outcomes with fiber and bulk structures [4]. The selection of proper glass host for Er3+ ion is an important factor for attaining good laser performance, i.e., low optical losses, energy storage capacity and high gain. Among different glass matrices, borate glass is an appropriate optical material due to its low melting point, high thermal stability, good rare earth ion solubility and high transparency [5,6]. Fluoride glasses are appropriate for fiber amplifiers owing to its ability to move IR cut off edge to low frequencies and also due to its reactivity with OH group to produce hydrogen fluoride, which in turn decreases the OH absorption in
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the glass matrix [7,8]. Heavy metal oxides embedded borate glasses have wide range of applications in solid state laser materials, electro-optic modulators, electro-optic switches and nonlinear parametric converters [9,10]. The presence of Zn in the glass composition lowers the crystallization rate and enhances the glass forming ability [11]. Heavy metal fluoride (HMF) incorporated glasses possess some advantages over conventional borate, phosphate and silicate glasses. These advantages include low phonon energy, extended transparency from near-UV to mid-IR and capacity to incorporate large quantity of RE3+ ions [12,13]. The presence of ZnF2 (HMF) in the glass network produces more efficient radiative emissions due to its ability to lower phonon energies [13]. The purpose of this work is to study the spectroscopic properties of Er3+-doped Li–Zn fluoroborate glass through absorption and emission spectra. Judd–Ofelt [14,15] theory has been employed to investigate the radiative properties such as transition probabilities, branching ratios and lifetime for the various levels of Er3+ ions in the present glass system. The results are compared with other Er3+-doped glasses and the potential of the present glass system as a laser material is discussed. Fabrication and characterizations of the glass In this work, we have prepared an Er3+-doped Li–Zn fluoroborate glass, hereafter referred to as LBZnFEr, with a composition of 25Li2O + 64.9B2O3 + 10ZnF2 + 0.1Er2O3 in molar fraction via melt quenching technique. The systematic procedures for glass synthesis, polishing and measurements of the physical properties are similar to our earlier work [16]. The absorption spectrum of the glass was recorded on Varian Cary 5000 UV–Vis–NIR spectrophotometer in the wavelength range 200–2500 nm. Emission spectra of the LBZnFEr glass were taken from Horiba Scientific Fluoromax-4 spectrofluorometer at excitation wavelengths 378 and 521 nm. The emission spectrum in the NIR region of LBZnFEr glass was measured using Jobin Yvon Fluorolog - FL3–11 spectrofluorometer at an excitation wavelength of 973 nm. All these spectroscopic measurements were done at room temperature (RT) with a spectral resolution of 1 nm. The details of theory and calculations adopted for this work have been described elsewhere [17,18]. Some of the physical parameters of LBZnFEr glass have been measured and are presented in Table 1. Results and discussion Optical absorption studies
10 absorption transitions in the UV–Vis and 3 peaks in the NIR region. These inhomogeneously broadened bands at the wavelengths 356, 365, 378, 407, 442, 451, 488, 521, 543, 651, 797, 973 and 1529 nm are identified as transitions from the ground state 4I15/2 to the different excited levels (1) 2G7/2, (2) 4G9/2, (3) 4 G11/2, (4) (2G,4F,2H)9/2, (5) 4F3/2, (6) 4F5/2, (7) 4F7/2, (8) 2H11/2, (9) 4 S3/2, (10) 4F9/2, (11) 4I9/2, (12) 4I11/2 and (13) 4I13/2 due to the 4f– 4f interactions of Er3+ ions in the LBZnFEr glass, respectively (transitions are indicated in the figure with peak numbers labeled). Among these, the most intense 4I15/2 ? 4G11/2 (378 nm) and 4I15/2 ? 2H11/2 (521 nm) transitions follow the selection rules (|DS| = 0, |DL| 6 2 and |DJ| 6 2) for hypersensitive transitions (HSTs) and are found to be more sensitive to the environment. The absorption peaks, if any, could not be found below 350 nm due to the screening of higher energy levels of Er3+ ion in LBZnFEr glass by the upward sloping of Urbach edge of the glass matrix. To investigate the nature of Er3+-ligand bond in the LBZnFEr glass, nephelauxetic ratios (b) and the bonding parameter (d) have been calculated. It is observed from Fig. 2 that the nephelauxetic ratio of most of the absorption transitions have a value >1, which indicates the observed energy values in LBZnFEr glass (mc ) is higher than the corresponding energy values in aqua ion (ma ). This shift depends on the bonding nature of the ligand of Er3+ ion. The obtained value for bonding parameter (d) is 0.269 and this negative value is an indication of ionic nature of the Er3+-ligand bond in the present glass system. Similar ionic nature of Er3+-ligand bond was reported in some other glasses too [19,20]. Judd–Ofelt (JO) analysis of the LBZnFEr glass has been carried out from the absorption spectra. Fig. 3 shows the experimental and calculated oscillator strengths (f exp and f cal ) along with their difference (Df ) for 13 transitions observed in absorption spectra. The root mean square (r.m.s) deviation (r) of f exp and f cal is obtained as 0.649 106. It is concluded from Fig. 3 and the r.m.s deviation (r) that the experimental and calculated oscillator strengths agree very well and in turn leads to the accuracy of the JO analysis. It is observed that the oscillator strengths of the HSTs, 4 I15/2 ? 4G11/2 and 4I15/2 ? 2H11/2 are larger than those for other absorption transitions. Also, the oscillator strengths of these HSTs are larger than those reported for other Er3+-doped glasses [19,20], which indicates that the LBZnFEr glass possesses higher asymmetry around Er3+ ions. The JO intensity parameters, Xk (where k = 2, 4 and 6) were obtained by carrying out a least square fit for these f exp and f cal values and are presented in Table 2 along with JO parameters for other reported Er3+-doped glasses [20–22]. Comparatively higher values of Xk parameters for the present glass
Fig. 1 shows the absorption spectrum of 0.1 mol% Er3+ doped Li–Zn fluoroborate glass in the UV–Vis and NIR regions with the photograph of the prepared glass as inset. The spectrum shows Table 1 Physical parameters of 0.1 mol% Er3+-doped Li–Zn fluoroborate glass. Parameter
Value
Density (q) Optical path length (t) Refractive Index (n) Concentration of Er3+ (N) Polaron radius (r p ) Molar volume (V m ) Electronic polarizability (ae ) Molar refractivity (Rl ) Dielectric constant (e) Electric susceptibility (v) Interionic distance (r i ) Donor–acceptor distance (RDA ) Field strength (F) Reflection loss (R)
2.6175 g/cm3 5.0 mm 1.473 0.4276 1020 ions/cm3 1.1525 nm 24.2122 cm3/mol 2.6927 1024 6.7922 cm3 2.1697 0.0931 2.8597 nm 1.7740 108 cm 2.2588 1014 cm2 3.658%
Fig. 1. Absorption spectra of LBZnFEr glass in the UV–Vis and NIR regions.
S. Thomas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 43–48
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contrary to the result obtained from the bonding parameter calculation. This contradiction was already discussed in other Er3+-doped glasses [19,20]. For HSTs, the value of ||U(2)|| parameter is very high, resulting in a high oscillator strength. The X2 parameter is closely correlated to the HSTs and depends on the covalency. In this context, the presence of two HSTs with large oscillator strengths in the absorption spectra is attributed to the contrary results in the bonding nature. Also, the large value of X2 indicates higher asymmetry of the Er3+ sites. The spectroscopic quality factor (Q) for LBZnFEr glass, which is inversely proportional to the intensity of the laser transition, was obtained as 0.72 and it is evident from this low value that the glass is a good candidate for laser material [18]. Radiative properties and luminescence spectra
Fig. 2. Nephelauxetic ratios of all the observed absorption transitions.
Fig. 3. Experimental and calculated oscillator strengths of all the observed absorption transitions.
Table 2 Comparison of Xk (1020 cm2) parameters with previously reported Er3+-doped glasses. Glass code LBZnFEr 0.5ErBT [20] LBTAFEr10 [21] LiTFP [22] KTFP [22] NaTFP [22]
X2
X4
X6
(1020 cm2)
(1020 cm2)
(1020 cm2)
17.52 5.73 5.89 4.70 5.09 5.92
5.25 2.01 1.10 1.21 0.69 1.07
7.27 2.37 1.47 1.30 1.45 1.44
Q 0.72 0.85 0.75 0.93 0.48 0.74
system are due to the large oscillator strengths of HSTs transitions, which have a major influence on Xk parameter due to the high values of square reduced matrix elements (||U(k)||) [23]. The nature of JO intensity parameters for the LBZnFEr glass was found as X2 > X6 > X4 . The JO parameter X2 is a measure of the covalency of RE3+-ligand bond in the host. The large value of X2 indicates the covalent nature of Er3+-ligand bond in LBZnFEr glass, which is
It is established that the electric-dipole transitions are predominant in glasses and crystals incorporated with RE3+ ions. Only a few transitions of RE3+ ions follow the selection rules for magneticdipole transitions (DJ 6 1, DL = 0 and DS = 0). In particular, magnetic-dipole transitions can be ignored for Er3+ ions for the spectral range investigated in this study [14,15]. Radiative properties such as electric-dipole strength (Sed ), transition probability (A), radiative lifetime (sR ) and branching ratio (bR ) for various excited levels 4 G11/2, (2G,4F,2H)9/2, 4F3/2, 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2 of Er3+ ions in LBZnFEr glass have been calculated using JO intensity parameters and refractive index of the glass and are presented in Table 3. It can be noted from the table that the excited level 4I13/2 has the maximum lifetime (1.948 ms) and gives emission in the NIR region (1.53 lm). In optical amplifiers, it is essential to intensify the electric-dipole transition in order to get broadband emission spectra. For the 4I13/2 ? 4I15/2 transition, it is obvious that X6 is more significant for the value of electric-dipole transition owing to the higher value of its coefficient, ||U(6)|| (Table 3). Therefore the enhancement in the X6 value gives an increment in the bandwidth of 4I13/2 ? 4I15/2 transition. So the comparatively high value of X6 for LBZnFEr glass with other Er3+-doped glasses (Table 2) is adequate for broadband emission at 1.53 lm region. Considering the visible region emissions of Er3+ ions, 4S3/2 ? 4I15/2 is a characteristic peak, which shows a transition probability of 5235.38 s1 and lifetime of 129 ls for the 4S3/2 level. Since branching ratio describes the probability of achieving stimulated emission cross-section from any definite transition, it would be a critical parameter for laser designing. It is observed from Table 3 that all transitions from different excited levels to the ground level of Er3+ ions have the maximum transition probability and branching ratio except in the case of (2G,4F,2H)9/2 and the transitions (2G,4F,2H)9/2 ? 4I13/2 and 4I15/2 have about equal probability with equal branching ratio. Among all the transitions, the highest transition probability (61691.02 s1) was obtained for 4 G11/2 ? 4I15/2 emission and this 4G11/2 level has the shortest lifetime. It is noted that the transition probabilities for 4S3/2, 4F9/2 and 4I13/2 levels of Er3+ ions in LBZnFEr glass are considerably greater than those of other Er3+ doped glasses [24]. It is advantageous to attaining intense green, red and IR (1.53 lm) emissions under suitable excitation conditions. Fig. 4 shows the emission spectra of LBZnFEr glass at excitation wavelengths 378 and 521 nm. The spectra exhibit 10 emission peaks corresponding to the transitions from different excited levels to the 4I13/2 and 4I15/2 levels of Er3+ ions in LBZnFEr glass. Energy values corresponding to different transitions along with assigned peak numbers are presented in Table 4. Fig. 5 shows the NIR emission spectrum for LBZnFEr glass in 1.53 lm region at an excitation wavelength of 973 nm. The present glass system shows a broad and flat emission spectrum at 1.53 lm region and it can be considered to be a well known promising candidate for
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Table 3 Calculated radiative parameters for Er3+-doped LBZnFEr glass. SLJ
S0 L0 J0
Energy (cm1)
Matrix elements U
4
G11/2
(2G,4F,2H)9/2
F5/2
F7/2
H11/2
S3/2
7.87 7.99 10.38 83.22 88.23 63.88 1910.52 643.68 255.72 6849.71 61691.02
71,612
13
0.000 0.000 0.000 0.001 0.001 0.001 0.027 0.009 0.004 0.096 0.862
4
F3/2 F5/2 F7/2 2 H11/2 4 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2
2007 2409 4097 5400 6249 9424 12,131 14,376 18,029 24,631
0.0000 0.0118 0.1031 0.0310 0.0000 0.0054 0.0147 0.0422 0.0745 0.0000
0.0220 0.0254 0.0471 0.1809 0.0021 0.0311 0.0064 0.0809 0.1153 0.0181
0.0073 0.0061 0.0246 0.0643 0.0019 0.0372 0.0037 0.1120 0.3464 0.2222
16.85 38.44 223.24 195.99 2.48 52.82 31.80 197.80 442.81 171.00
0.28 1.11 32.38 64.37 1.25 127.83 117.60 1239.78 5345.33 5263.50
12,193
82
0.000 0.000 0.003 0.005 0.000 0.011 0.010 0.102 0.438 0.432
4
F5/2 F7/2 2 H11/2 4 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2
470 2512 3389 4290 7479 10,132 12,270 16,026 22,472
0.0609 0.0026 0.0000 0.0271 0.0000 0.0000 0.0000 0.0000 0.0000
0.0342 0.0583 0.0007 0.0000 0.0045 0.2270 0.0922 0.0000 0.0000
0.0000 0.0000 0.0039 0.0000 0.0589 0.0541 0.4869 0.0310 0.1320
124.65 35.15 3.20 47.48 45.17 158.45 402.27 22.53 95.94
0.07 2.87 0.64 19.31 97.32 848.76 3827.00 477.58 5607.02
10,881
92
0.000 0.000 0.000 0.002 0.009 0.078 0.352 0.044 0.515
4
F7/2 H11/2 4 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2
1688 2991 3840 7015 9722 11,967 15,620 22,222
0.0765 0.0000 0.0074 0.0005 0.0103 0.0000 0.0000 0.0000
0.0499 0.0597 0.0035 0.2378 0.0611 0.0983 0.1787 0.0000
0.1002 0.1866 0.0000 0.3520 0.1091 0.0027 0.3423 0.2236
233.05 166.96 14.80 381.52 129.41 53.55 342.57 162.52
4.55 15.34 3.35 452.15 408.24 315.09 4482.16 6122.54
11,803
84
0.000 0.001 0.000 0.038 0.035 0.027 0.380 0.519
2
H11/2 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2
1303 2152 5327 8034 10,279 13,932 20,534
0.1275 0.0001 0.0120 0.0159 0.0034 0.0000 0.0000
0.0169 0.0054 0.0367 0.0946 0.2659 0.3398 0.1466
0.4015 0.0000 0.0120 0.4315 0.1531 0.0002 0.6273
524.08 3.01 49.01 391.13 256.78 178.48 532.87
2.99 0.08 29.60 532.11 718.08 1242.76 11879.31
14,405
69
0.000 0.000 0.002 0.037 0.050 0.086 0.825
4
S3/2 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2
991 4092 6739 8881 12,642 19,084
0.0000 0.3512 0.2070 0.0299 0.0224 0.7084
0.1963 0.0198 0.0862 0.1766 0.0589 0.4108
0.0102 0.0040 0.3120 0.0433 0.0576 0.0949
110.43 628.64 634.68 176.54 112.02 1525.75
0.18 73.94 333.45 212.28 388.53 18204.45
19,213
52
0.000 0.004 0.017 0.011 0.020 0.948
4
F9/2 I9/2 4 I11/2 4 I13/2 4 I15/2
3175 5882 8127 11,780 18,382
0.0000 0.0000 0.0000 0.0000 0.0000
0.0002 0.0750 0.0050 0.0000 0.0000
0.0248 0.2542 0.0792 0.3428 0.2252
18.13 224.12 60.19 249.15 163.68
2.99 234.87 166.37 2097.39 5235.38
7737
129
0.000 0.030 0.022 0.271 0.677
4
I9/2 I11/2 I13/2 4 I15/2
2707 4952 8605 15,207
0.1239 0.0718 0.0109 0.0000
0.0060 0.0104 0.1538 0.5500
0.0230 1.2700 0.0802 0.4635
236.95 1054.31 158.11 625.53
11.64 268.70 207.51 4531.29
5019
199
0.002 0.054 0.041 0.903
4
I11/2 I13/2 4 I15/2
2245 5898 12,500
0.0025 0.0004 0.0000
0.0682 0.0096 0.1655
0.1403 0.7160 0.0080
142.15 526.14 92.67
4.36 222.36 372.84
600
1667
0.007 0.371 0.622
4
3653 10,255
0.0334 0.0288
0.1712 0.0003
1.0873 0.3962
938.63 338.58
85.82 626.80
713
1403
0.120 0.880
6602
0.0195
0.1172
1.4299
1134.94
513.22
513
1948
1.000
4
4
F9/2
4 4
4
I9/2
4
4
I11/2
I13/2 I15/2
4 4
I13/2
bR
663.91 78.38 75.87 225.45 116.92 68.92 766.59 135.80 34.58 501.79 1925.73
4
4
sR (ls)
0.1353 0.0916 0.0781 0.0151 0.0498 0.0037 0.0123 0.0243 0.0127 0.2611 0.1171
4
2
AT (s1)
0.1115 0.0225 0.0364 0.1259 0.1528 0.1262 0.4214 0.0121 0.0473 0.2620 0.5152
2
4
A (s1)
0.2894 0.0000 0.0000 0.0847 0.0003 0.0000 0.4214 0.0638 0.0003 0.0996 0.8962
4
4
U
Sed (1022) ð6Þ
1894 3901 4303 5991 7294 8143 11,318 14,025 16,270 19,923 26,525
4
F3/2
U
ð4Þ
(2G,4F,2H)9/2 4 F3/2 4 F5/2 4 F7/2 2 H11/2 4 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2 4
4
ð2Þ
4
I15/2
S. Thomas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 43–48
47
Fig. 4. Emission spectra of LBZnFEr glass at excitation wavelengths 378 and 521 nm. Fig. 6. Partial energy level diagram for LBZnFEr glass along with possible emission transitions. Table 4 Observed stimulated emission cross-sections and gain bandwidths for LBZnFEr glass. Sl No. 1 2 3 4 5 6 7 8 9 10 11
SLJ ? S0 L0 J0 4
F7/2 ? 4I15/2 2 H11/2 ? 4I15/2 4 S3/2 ? 4I15/2 4 F3/2 ? 4I13/2 4 F7/2 ? 4I13/2 4 F9/2 ? 4I15/2 4 I9/2 ? 4I15/2 2 H11/2 ? 4I13/2 4 S3/2 ? 4I13/2 4 I11/2 ? 4I15/2 4 I13/2 ? 4I15/2
Eexp (cm1)
Dkeff (nm)
re
21,413 19,342 18,416 16,077 13,870 14,684 13,245 12,579 12,092 10,267 6523
9.7 10.0 18.6 10.8 10.4 5.9 7.6 7.4 7.2 14.8 23.1
3.561 7.950 1.496 0.405 1.974 10.097 0.974 1.282 8.329 2.330 7.497
20
(10
2
cm )
DG (1027 cm3) 34.542 79.500 27.826 4.374 20.530 59.572 7.402 9.487 59.969 34.484 173.181
de-excitations to the 4I13/2 and 4I15/2 levels, Er3+ ions emit green– red wavelengths. Also the emission in the NIR region (1.53 lm) is due to the transition 4I13/2 ? 4I15/2. To evaluate the capability of the investigated glass for the laser applications, the stimulated emission cross-sections (re ) and gain bandwidths (DG) of LBZnFEr glass have been calculated and presented in Table 4 for experimentally observed emission transitions. The large value of re indicates the suitability of the glass system for applications in visible laser systems and fiber amplifiers. CIE chromaticity coordinates for LBZnFEr glass were calculated for the emissions at 378 and 521 excitation wavelengths and are presented in Fig. 7. CIE chromaticity diagram shows that the present glass system emits light in the near-white and red regions for 378 and 521 nm excitations, respectively.
Fig. 5. NIR emission spectrum for LBZnFEr glass at 973 nm excitation.
1.5 lm broadband optical amplifiers. Since the spectrum is within the telecommunication window, it is desirable to increase the information capacity of wavelength division multiplexing networks. Energy level diagram depicted in Fig. 6 shows, at the 378, 521 and 973 nm excitations, Er3+ ions from the ground state jump to the 4G11/2, 2H11/2 and 4I11/2 levels, respectively and then non-radiatively (NR) relax to their lower levels. From these levels,
Fig. 7. CIE chromaticity diagram for LBZnFEr glass for excitation wavelengths 378 and 521 nm.
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Conclusions In summary, Er3+-doped Li–Zn fluoroborate glass has been synthesized with good optical quality and transparency. The ionic nature of the Er3+-ligand bond in the glass network was confirmed by bonding parameter evaluation. Judd–Ofelt parameters Xt (t = 2, 4, 6) were found to be larger than those reported in the literature for different host materials. JO intensity analysis reveals the nature of JO parameters as X2 > X6 > X4 . In this study, the small value of spectroscopic quality factor and 1.53 lm broad emission with large emission cross-sections point out that the present glass system is an appropriate material for lasers and broadband optical amplifiers. Acknowledgements
Fig. 8. Absorption and emission cross-sections of LBZnFEr glass.
Absorption and emission cross-sections for 4I13/2 M 4I15/2 transitions Using absorption cross-section (ra ) of 4I15/2 ? 4I13/2 transition 4 4 (Fig. 1), the stimulated emission cross-section (rM e ) of I13/2 ? I15/2 (1.53 lm) has been evaluated using McCumber theory [25,26]. Absorption and emission cross-sections are expressed as:
ra ðmÞ ¼
2:303 AðmÞ Nt
ð1Þ
and
rMe ðmÞ ¼ ra ðmÞ exp
e hm kB T
ð2Þ
where m is the photon frequency, N is the concentration of Er3+ ions and t, the thickness of the glass. AðmÞ is the wavenumber dependent absorbance, e is the net free energy required for the transition 4 I15/2 ? 4I13/2 at temperature T, h is the Planck’s constant and kB , the Boltzmann’s constant. Using the method proposed by Miniscalco et al. [26], the energy e is estimated as 6540 cm1. The absorption and emission cross-sections for 4I13/2 M 4I15/2 (1.53 lm) transition are shown in Fig. 8. The emission cross-section (rM e ) is a significant parameter in an optical amplifier, realizing broadband high gain amplification. The maximum value of absorption and emission cross-sections for LBZnFEr glass is found to be 11.2 1021 cm2 at a wavelength of 1.53 lm. The integrated P P M re ) are absorption and emission cross-sections ( ra and 19 2 cm ), respectively. The broadening obtained as 8.0 and 6.3 (10 of the emission bandwidth is due to the self-absorption process, which causes the overlap of absorption and emission cross-section at 1.53 lm and this typically occurs in 3 level laser systems [27,28]. Also, self-absorption at 1.53 lm region is more efficient than that at other wavelengths [27].
The authors are thankful to UGC (Govt. of India) and DST (Govt. of India) for the financial assistance through SAP-DRS and DSTPURSE programs, respectively. Two of the authors, Sunil Thomas and M.S. Sajna are also thankful to UGC (for the award of RFSMS Fellowship) and CSIR, New Delhi (for the award of JRF), respectively. We are thankful to the reviewers for a critical reading of the manuscript and many useful suggestions. References [1] N. Rakov, F.E. Romas, G. Hirata, M. Xiao, Appl. Phys. Lett. 83 (2003) 272–274. [2] Y. Chen, Y. Huang, Z. Luo, Chem. Phys. Lett. 382 (2003) 481–488. [3] E. Snoeks, G.N. van den Hoven, A. Polnam, J. Quant. Electron. 32 (1996) 1680– 1684. [4] X.X. Zhang, P. Hong, M. Bass, B.H.T. Chai, Phys. Rev. B 51 (1995) 9298–9301. [5] S.M. Kaczmarek, Opt. Mater. 19 (2002) 189–194. [6] N. Soga, K. Hirao, M. Yoshimoto, H. Yamamoto, J. Appl. Phys. 63 (1988) 4451– 4454. [7] J.E. Marion, M.J. Weber, Eur. J. Solid State Inorg. Chem. 28 (1991) 271–287. [8] M.R. Sahar, A.K. Jehbu, M.M. Karim, J. Non-Cryst. Solids 213 (1997) 164–167. [9] L.S. Rao, M.S. Reddy, M.V.R. Reddy, N. Veeraiah, Phys. B 403 (2008) 2542–2556. [10] J.L. Piguet, J.E. Shelby, J. Am. Ceram. Soc. 68 (1985) 450–455. [11] P.G. Pavani, K. Sadhana, V.C. Mouli, Phys. B 406 (2011) 1242–1247. [12] B. Karmakar, J. Solid State Chem. 178 (2005) 2663–2672. [13] P.W. France, in: P.W. France, M.G. Drexhage, J.M. Parker, M.W. Moore, S.F. Carter, J.V. Wright (Eds.), Fluoride Glass Optical Fibers, Blackie, Glasgow, 1990. [14] B.R. Judd, Phys. Rev. 127 (1962) 750–761. [15] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511–520. [16] S. Thomas, Sk.N. Rasool, M. Rathaiah, V. Venkatramu, C. Joseph, N.V. Unnikrishnan, J. Non-Cryst. Solids 376 (2013) 106–116. [17] S. Thomas, R. George, Sk.N. Rasool, M. Rathaiah, V. Venkatramu, C. Joseph, N.V. Unnikrishnan, Opt. Mater. 36 (2013) 242–250. [18] H. Desirena, E. De la Rosa, V.H. Romero, J.F. Castillo, L.A. Diaz-Torres, J.R. Oliva, J. Lumin. 132 (2012) 391–397. [19] K. Selvaraju, K. Marimuthu, Phys. B 407 (2012) 1086–1093. [20] Z.A.S. Mahraz, M.R. Sahar, S.K. Ghoshal, M.R. Dousti, J. Lumin. 144 (2013) 139– 145. [21] B.C. Jamalaiah, T. Suhasini, L.R. Moorthy, K.J. Reddy, I.G. Kim, D.S. Yoo, K. Jang, Opt. Mater. 34 (2012) 861–867. [22] L.R. Moorthy, M. Jayasimhadri, S.A. Saleem, D.V.R. Murthy, J. Non-Cryst. Solids 353 (2007) 1392–1396. [23] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4424–4442. [24] H. Lin, K. Liu, E.Y.B. Pun, T.C. Ma, X. Peng, Q.D. An, J.Y. Yu, S.B. Jiang, Chem. Phys. Lett. 398 (2004) 146–150. [25] D.E. McCumber, Phys. Rev. 136 (1964) A954–A957. [26] W.J. Miniscalco, R.S. Quimby, Opt. Lett. 16 (1991) 258–260. [27] X. Feng, S. Tanabe, T. Hanada, J. Am. Ceram. Soc. 84 (2001) 165–171. [28] P.R. Ehrmann, J.H. Campbell, J. Am. Ceram. Soc. 85 (2002) 1061–1069.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Investigations on spectroscopic properties of Er3+-doped Li–Zn fluoroborate glass Sunil Thomas a, M.S. Sajna a, Rani George b, Sk. Nayab Rasool c, Cyriac Joseph a, N.V. Unnikrishnan a,⇑ a
School of Pure & Applied Physics, Mahatma Gandhi University, Kottayam, Kerala 686 560, India Department of Physics, St. Aloysius College, Edathua, Kerala 689 573, India c Department of Physics, Sri Venkateswara University, Tirupati, Andhra Pradesh 517 502, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3+
Er :Li–Zn fluoroborate glass was
fabricated for laser applications. Judd–Ofelt and radiative parameters were investigated. Broadband optical communications due to 1.5 lm emission. McCumber theory analysis for 1.5 lm region.
a r t i c l e
i n f o
Article history: Received 24 July 2014 Received in revised form 5 March 2015 Accepted 27 March 2015 Available online 2 April 2015 Keywords: Er3+ ions Borate glass Judd–Ofelt analysis Radiative properties Photoluminescence McCumber theory
a b s t r a c t Er3+-doped Li–Zn fluoroborate glass was synthesized via melt quenching technique. Optical properties of the glass were investigated by UV–Vis-NIR absorption and emission spectra. To evaluate the nature of Er3+-ligand bond in the glass network, nephelauxetic ratios and bonding parameter were calculated. Judd–Ofelt analysis and hence the radiative properties of the present glass system were evaluated for ascertaining the suitability of the glass for laser applications and compared those with the emission spectra. Absorption cross-sections have been calculated from the absorption spectrum and stimulated emission cross-sections were estimated using McCumber theory for 4I13/2 M 4I15/2 transitions. The results of the present glass were compared with those obtained for some other Er3+-doped glass systems. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Trivalent rare earth (RE3+) doped optical glasses are key materials for the development of optical fibers, waveguide lasers, bulk lasers and optical amplifiers [1,2]. Out of many RE3+ ions, Er3+-doped glasses have been extensively considered for laser applications due to their low loss in optical waveguide for 4 S3/2 ? 4I13/2 and 4I13/2 ? 4I15/2 transitions [3]. Mostly, green ⇑ Corresponding author. Tel.: +91 9745047850. E-mail address: [email protected] (N.V. Unnikrishnan). http://dx.doi.org/10.1016/j.saa.2015.03.118 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
emission by the 4S3/2 ? 4I15/2 transition of Er3+ ion has led to successful outcomes with fiber and bulk structures [4]. The selection of proper glass host for Er3+ ion is an important factor for attaining good laser performance, i.e., low optical losses, energy storage capacity and high gain. Among different glass matrices, borate glass is an appropriate optical material due to its low melting point, high thermal stability, good rare earth ion solubility and high transparency [5,6]. Fluoride glasses are appropriate for fiber amplifiers owing to its ability to move IR cut off edge to low frequencies and also due to its reactivity with OH group to produce hydrogen fluoride, which in turn decreases the OH absorption in
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S. Thomas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 43–48
the glass matrix [7,8]. Heavy metal oxides embedded borate glasses have wide range of applications in solid state laser materials, electro-optic modulators, electro-optic switches and nonlinear parametric converters [9,10]. The presence of Zn in the glass composition lowers the crystallization rate and enhances the glass forming ability [11]. Heavy metal fluoride (HMF) incorporated glasses possess some advantages over conventional borate, phosphate and silicate glasses. These advantages include low phonon energy, extended transparency from near-UV to mid-IR and capacity to incorporate large quantity of RE3+ ions [12,13]. The presence of ZnF2 (HMF) in the glass network produces more efficient radiative emissions due to its ability to lower phonon energies [13]. The purpose of this work is to study the spectroscopic properties of Er3+-doped Li–Zn fluoroborate glass through absorption and emission spectra. Judd–Ofelt [14,15] theory has been employed to investigate the radiative properties such as transition probabilities, branching ratios and lifetime for the various levels of Er3+ ions in the present glass system. The results are compared with other Er3+-doped glasses and the potential of the present glass system as a laser material is discussed. Fabrication and characterizations of the glass In this work, we have prepared an Er3+-doped Li–Zn fluoroborate glass, hereafter referred to as LBZnFEr, with a composition of 25Li2O + 64.9B2O3 + 10ZnF2 + 0.1Er2O3 in molar fraction via melt quenching technique. The systematic procedures for glass synthesis, polishing and measurements of the physical properties are similar to our earlier work [16]. The absorption spectrum of the glass was recorded on Varian Cary 5000 UV–Vis–NIR spectrophotometer in the wavelength range 200–2500 nm. Emission spectra of the LBZnFEr glass were taken from Horiba Scientific Fluoromax-4 spectrofluorometer at excitation wavelengths 378 and 521 nm. The emission spectrum in the NIR region of LBZnFEr glass was measured using Jobin Yvon Fluorolog - FL3–11 spectrofluorometer at an excitation wavelength of 973 nm. All these spectroscopic measurements were done at room temperature (RT) with a spectral resolution of 1 nm. The details of theory and calculations adopted for this work have been described elsewhere [17,18]. Some of the physical parameters of LBZnFEr glass have been measured and are presented in Table 1. Results and discussion Optical absorption studies
10 absorption transitions in the UV–Vis and 3 peaks in the NIR region. These inhomogeneously broadened bands at the wavelengths 356, 365, 378, 407, 442, 451, 488, 521, 543, 651, 797, 973 and 1529 nm are identified as transitions from the ground state 4I15/2 to the different excited levels (1) 2G7/2, (2) 4G9/2, (3) 4 G11/2, (4) (2G,4F,2H)9/2, (5) 4F3/2, (6) 4F5/2, (7) 4F7/2, (8) 2H11/2, (9) 4 S3/2, (10) 4F9/2, (11) 4I9/2, (12) 4I11/2 and (13) 4I13/2 due to the 4f– 4f interactions of Er3+ ions in the LBZnFEr glass, respectively (transitions are indicated in the figure with peak numbers labeled). Among these, the most intense 4I15/2 ? 4G11/2 (378 nm) and 4I15/2 ? 2H11/2 (521 nm) transitions follow the selection rules (|DS| = 0, |DL| 6 2 and |DJ| 6 2) for hypersensitive transitions (HSTs) and are found to be more sensitive to the environment. The absorption peaks, if any, could not be found below 350 nm due to the screening of higher energy levels of Er3+ ion in LBZnFEr glass by the upward sloping of Urbach edge of the glass matrix. To investigate the nature of Er3+-ligand bond in the LBZnFEr glass, nephelauxetic ratios (b) and the bonding parameter (d) have been calculated. It is observed from Fig. 2 that the nephelauxetic ratio of most of the absorption transitions have a value >1, which indicates the observed energy values in LBZnFEr glass (mc ) is higher than the corresponding energy values in aqua ion (ma ). This shift depends on the bonding nature of the ligand of Er3+ ion. The obtained value for bonding parameter (d) is 0.269 and this negative value is an indication of ionic nature of the Er3+-ligand bond in the present glass system. Similar ionic nature of Er3+-ligand bond was reported in some other glasses too [19,20]. Judd–Ofelt (JO) analysis of the LBZnFEr glass has been carried out from the absorption spectra. Fig. 3 shows the experimental and calculated oscillator strengths (f exp and f cal ) along with their difference (Df ) for 13 transitions observed in absorption spectra. The root mean square (r.m.s) deviation (r) of f exp and f cal is obtained as 0.649 106. It is concluded from Fig. 3 and the r.m.s deviation (r) that the experimental and calculated oscillator strengths agree very well and in turn leads to the accuracy of the JO analysis. It is observed that the oscillator strengths of the HSTs, 4 I15/2 ? 4G11/2 and 4I15/2 ? 2H11/2 are larger than those for other absorption transitions. Also, the oscillator strengths of these HSTs are larger than those reported for other Er3+-doped glasses [19,20], which indicates that the LBZnFEr glass possesses higher asymmetry around Er3+ ions. The JO intensity parameters, Xk (where k = 2, 4 and 6) were obtained by carrying out a least square fit for these f exp and f cal values and are presented in Table 2 along with JO parameters for other reported Er3+-doped glasses [20–22]. Comparatively higher values of Xk parameters for the present glass
Fig. 1 shows the absorption spectrum of 0.1 mol% Er3+ doped Li–Zn fluoroborate glass in the UV–Vis and NIR regions with the photograph of the prepared glass as inset. The spectrum shows Table 1 Physical parameters of 0.1 mol% Er3+-doped Li–Zn fluoroborate glass. Parameter
Value
Density (q) Optical path length (t) Refractive Index (n) Concentration of Er3+ (N) Polaron radius (r p ) Molar volume (V m ) Electronic polarizability (ae ) Molar refractivity (Rl ) Dielectric constant (e) Electric susceptibility (v) Interionic distance (r i ) Donor–acceptor distance (RDA ) Field strength (F) Reflection loss (R)
2.6175 g/cm3 5.0 mm 1.473 0.4276 1020 ions/cm3 1.1525 nm 24.2122 cm3/mol 2.6927 1024 6.7922 cm3 2.1697 0.0931 2.8597 nm 1.7740 108 cm 2.2588 1014 cm2 3.658%
Fig. 1. Absorption spectra of LBZnFEr glass in the UV–Vis and NIR regions.
S. Thomas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 43–48
45
contrary to the result obtained from the bonding parameter calculation. This contradiction was already discussed in other Er3+-doped glasses [19,20]. For HSTs, the value of ||U(2)|| parameter is very high, resulting in a high oscillator strength. The X2 parameter is closely correlated to the HSTs and depends on the covalency. In this context, the presence of two HSTs with large oscillator strengths in the absorption spectra is attributed to the contrary results in the bonding nature. Also, the large value of X2 indicates higher asymmetry of the Er3+ sites. The spectroscopic quality factor (Q) for LBZnFEr glass, which is inversely proportional to the intensity of the laser transition, was obtained as 0.72 and it is evident from this low value that the glass is a good candidate for laser material [18]. Radiative properties and luminescence spectra
Fig. 2. Nephelauxetic ratios of all the observed absorption transitions.
Fig. 3. Experimental and calculated oscillator strengths of all the observed absorption transitions.
Table 2 Comparison of Xk (1020 cm2) parameters with previously reported Er3+-doped glasses. Glass code LBZnFEr 0.5ErBT [20] LBTAFEr10 [21] LiTFP [22] KTFP [22] NaTFP [22]
X2
X4
X6
(1020 cm2)
(1020 cm2)
(1020 cm2)
17.52 5.73 5.89 4.70 5.09 5.92
5.25 2.01 1.10 1.21 0.69 1.07
7.27 2.37 1.47 1.30 1.45 1.44
Q 0.72 0.85 0.75 0.93 0.48 0.74
system are due to the large oscillator strengths of HSTs transitions, which have a major influence on Xk parameter due to the high values of square reduced matrix elements (||U(k)||) [23]. The nature of JO intensity parameters for the LBZnFEr glass was found as X2 > X6 > X4 . The JO parameter X2 is a measure of the covalency of RE3+-ligand bond in the host. The large value of X2 indicates the covalent nature of Er3+-ligand bond in LBZnFEr glass, which is
It is established that the electric-dipole transitions are predominant in glasses and crystals incorporated with RE3+ ions. Only a few transitions of RE3+ ions follow the selection rules for magneticdipole transitions (DJ 6 1, DL = 0 and DS = 0). In particular, magnetic-dipole transitions can be ignored for Er3+ ions for the spectral range investigated in this study [14,15]. Radiative properties such as electric-dipole strength (Sed ), transition probability (A), radiative lifetime (sR ) and branching ratio (bR ) for various excited levels 4 G11/2, (2G,4F,2H)9/2, 4F3/2, 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2 of Er3+ ions in LBZnFEr glass have been calculated using JO intensity parameters and refractive index of the glass and are presented in Table 3. It can be noted from the table that the excited level 4I13/2 has the maximum lifetime (1.948 ms) and gives emission in the NIR region (1.53 lm). In optical amplifiers, it is essential to intensify the electric-dipole transition in order to get broadband emission spectra. For the 4I13/2 ? 4I15/2 transition, it is obvious that X6 is more significant for the value of electric-dipole transition owing to the higher value of its coefficient, ||U(6)|| (Table 3). Therefore the enhancement in the X6 value gives an increment in the bandwidth of 4I13/2 ? 4I15/2 transition. So the comparatively high value of X6 for LBZnFEr glass with other Er3+-doped glasses (Table 2) is adequate for broadband emission at 1.53 lm region. Considering the visible region emissions of Er3+ ions, 4S3/2 ? 4I15/2 is a characteristic peak, which shows a transition probability of 5235.38 s1 and lifetime of 129 ls for the 4S3/2 level. Since branching ratio describes the probability of achieving stimulated emission cross-section from any definite transition, it would be a critical parameter for laser designing. It is observed from Table 3 that all transitions from different excited levels to the ground level of Er3+ ions have the maximum transition probability and branching ratio except in the case of (2G,4F,2H)9/2 and the transitions (2G,4F,2H)9/2 ? 4I13/2 and 4I15/2 have about equal probability with equal branching ratio. Among all the transitions, the highest transition probability (61691.02 s1) was obtained for 4 G11/2 ? 4I15/2 emission and this 4G11/2 level has the shortest lifetime. It is noted that the transition probabilities for 4S3/2, 4F9/2 and 4I13/2 levels of Er3+ ions in LBZnFEr glass are considerably greater than those of other Er3+ doped glasses [24]. It is advantageous to attaining intense green, red and IR (1.53 lm) emissions under suitable excitation conditions. Fig. 4 shows the emission spectra of LBZnFEr glass at excitation wavelengths 378 and 521 nm. The spectra exhibit 10 emission peaks corresponding to the transitions from different excited levels to the 4I13/2 and 4I15/2 levels of Er3+ ions in LBZnFEr glass. Energy values corresponding to different transitions along with assigned peak numbers are presented in Table 4. Fig. 5 shows the NIR emission spectrum for LBZnFEr glass in 1.53 lm region at an excitation wavelength of 973 nm. The present glass system shows a broad and flat emission spectrum at 1.53 lm region and it can be considered to be a well known promising candidate for
46
S. Thomas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 43–48
Table 3 Calculated radiative parameters for Er3+-doped LBZnFEr glass. SLJ
S0 L0 J0
Energy (cm1)
Matrix elements U
4
G11/2
(2G,4F,2H)9/2
F5/2
F7/2
H11/2
S3/2
7.87 7.99 10.38 83.22 88.23 63.88 1910.52 643.68 255.72 6849.71 61691.02
71,612
13
0.000 0.000 0.000 0.001 0.001 0.001 0.027 0.009 0.004 0.096 0.862
4
F3/2 F5/2 F7/2 2 H11/2 4 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2
2007 2409 4097 5400 6249 9424 12,131 14,376 18,029 24,631
0.0000 0.0118 0.1031 0.0310 0.0000 0.0054 0.0147 0.0422 0.0745 0.0000
0.0220 0.0254 0.0471 0.1809 0.0021 0.0311 0.0064 0.0809 0.1153 0.0181
0.0073 0.0061 0.0246 0.0643 0.0019 0.0372 0.0037 0.1120 0.3464 0.2222
16.85 38.44 223.24 195.99 2.48 52.82 31.80 197.80 442.81 171.00
0.28 1.11 32.38 64.37 1.25 127.83 117.60 1239.78 5345.33 5263.50
12,193
82
0.000 0.000 0.003 0.005 0.000 0.011 0.010 0.102 0.438 0.432
4
F5/2 F7/2 2 H11/2 4 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2
470 2512 3389 4290 7479 10,132 12,270 16,026 22,472
0.0609 0.0026 0.0000 0.0271 0.0000 0.0000 0.0000 0.0000 0.0000
0.0342 0.0583 0.0007 0.0000 0.0045 0.2270 0.0922 0.0000 0.0000
0.0000 0.0000 0.0039 0.0000 0.0589 0.0541 0.4869 0.0310 0.1320
124.65 35.15 3.20 47.48 45.17 158.45 402.27 22.53 95.94
0.07 2.87 0.64 19.31 97.32 848.76 3827.00 477.58 5607.02
10,881
92
0.000 0.000 0.000 0.002 0.009 0.078 0.352 0.044 0.515
4
F7/2 H11/2 4 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2
1688 2991 3840 7015 9722 11,967 15,620 22,222
0.0765 0.0000 0.0074 0.0005 0.0103 0.0000 0.0000 0.0000
0.0499 0.0597 0.0035 0.2378 0.0611 0.0983 0.1787 0.0000
0.1002 0.1866 0.0000 0.3520 0.1091 0.0027 0.3423 0.2236
233.05 166.96 14.80 381.52 129.41 53.55 342.57 162.52
4.55 15.34 3.35 452.15 408.24 315.09 4482.16 6122.54
11,803
84
0.000 0.001 0.000 0.038 0.035 0.027 0.380 0.519
2
H11/2 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2
1303 2152 5327 8034 10,279 13,932 20,534
0.1275 0.0001 0.0120 0.0159 0.0034 0.0000 0.0000
0.0169 0.0054 0.0367 0.0946 0.2659 0.3398 0.1466
0.4015 0.0000 0.0120 0.4315 0.1531 0.0002 0.6273
524.08 3.01 49.01 391.13 256.78 178.48 532.87
2.99 0.08 29.60 532.11 718.08 1242.76 11879.31
14,405
69
0.000 0.000 0.002 0.037 0.050 0.086 0.825
4
S3/2 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2
991 4092 6739 8881 12,642 19,084
0.0000 0.3512 0.2070 0.0299 0.0224 0.7084
0.1963 0.0198 0.0862 0.1766 0.0589 0.4108
0.0102 0.0040 0.3120 0.0433 0.0576 0.0949
110.43 628.64 634.68 176.54 112.02 1525.75
0.18 73.94 333.45 212.28 388.53 18204.45
19,213
52
0.000 0.004 0.017 0.011 0.020 0.948
4
F9/2 I9/2 4 I11/2 4 I13/2 4 I15/2
3175 5882 8127 11,780 18,382
0.0000 0.0000 0.0000 0.0000 0.0000
0.0002 0.0750 0.0050 0.0000 0.0000
0.0248 0.2542 0.0792 0.3428 0.2252
18.13 224.12 60.19 249.15 163.68
2.99 234.87 166.37 2097.39 5235.38
7737
129
0.000 0.030 0.022 0.271 0.677
4
I9/2 I11/2 I13/2 4 I15/2
2707 4952 8605 15,207
0.1239 0.0718 0.0109 0.0000
0.0060 0.0104 0.1538 0.5500
0.0230 1.2700 0.0802 0.4635
236.95 1054.31 158.11 625.53
11.64 268.70 207.51 4531.29
5019
199
0.002 0.054 0.041 0.903
4
I11/2 I13/2 4 I15/2
2245 5898 12,500
0.0025 0.0004 0.0000
0.0682 0.0096 0.1655
0.1403 0.7160 0.0080
142.15 526.14 92.67
4.36 222.36 372.84
600
1667
0.007 0.371 0.622
4
3653 10,255
0.0334 0.0288
0.1712 0.0003
1.0873 0.3962
938.63 338.58
85.82 626.80
713
1403
0.120 0.880
6602
0.0195
0.1172
1.4299
1134.94
513.22
513
1948
1.000
4
4
F9/2
4 4
4
I9/2
4
4
I11/2
I13/2 I15/2
4 4
I13/2
bR
663.91 78.38 75.87 225.45 116.92 68.92 766.59 135.80 34.58 501.79 1925.73
4
4
sR (ls)
0.1353 0.0916 0.0781 0.0151 0.0498 0.0037 0.0123 0.0243 0.0127 0.2611 0.1171
4
2
AT (s1)
0.1115 0.0225 0.0364 0.1259 0.1528 0.1262 0.4214 0.0121 0.0473 0.2620 0.5152
2
4
A (s1)
0.2894 0.0000 0.0000 0.0847 0.0003 0.0000 0.4214 0.0638 0.0003 0.0996 0.8962
4
4
U
Sed (1022) ð6Þ
1894 3901 4303 5991 7294 8143 11,318 14,025 16,270 19,923 26,525
4
F3/2
U
ð4Þ
(2G,4F,2H)9/2 4 F3/2 4 F5/2 4 F7/2 2 H11/2 4 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2 4
4
ð2Þ
4
I15/2
S. Thomas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 43–48
47
Fig. 4. Emission spectra of LBZnFEr glass at excitation wavelengths 378 and 521 nm. Fig. 6. Partial energy level diagram for LBZnFEr glass along with possible emission transitions. Table 4 Observed stimulated emission cross-sections and gain bandwidths for LBZnFEr glass. Sl No. 1 2 3 4 5 6 7 8 9 10 11
SLJ ? S0 L0 J0 4
F7/2 ? 4I15/2 2 H11/2 ? 4I15/2 4 S3/2 ? 4I15/2 4 F3/2 ? 4I13/2 4 F7/2 ? 4I13/2 4 F9/2 ? 4I15/2 4 I9/2 ? 4I15/2 2 H11/2 ? 4I13/2 4 S3/2 ? 4I13/2 4 I11/2 ? 4I15/2 4 I13/2 ? 4I15/2
Eexp (cm1)
Dkeff (nm)
re
21,413 19,342 18,416 16,077 13,870 14,684 13,245 12,579 12,092 10,267 6523
9.7 10.0 18.6 10.8 10.4 5.9 7.6 7.4 7.2 14.8 23.1
3.561 7.950 1.496 0.405 1.974 10.097 0.974 1.282 8.329 2.330 7.497
20
(10
2
cm )
DG (1027 cm3) 34.542 79.500 27.826 4.374 20.530 59.572 7.402 9.487 59.969 34.484 173.181
de-excitations to the 4I13/2 and 4I15/2 levels, Er3+ ions emit green– red wavelengths. Also the emission in the NIR region (1.53 lm) is due to the transition 4I13/2 ? 4I15/2. To evaluate the capability of the investigated glass for the laser applications, the stimulated emission cross-sections (re ) and gain bandwidths (DG) of LBZnFEr glass have been calculated and presented in Table 4 for experimentally observed emission transitions. The large value of re indicates the suitability of the glass system for applications in visible laser systems and fiber amplifiers. CIE chromaticity coordinates for LBZnFEr glass were calculated for the emissions at 378 and 521 excitation wavelengths and are presented in Fig. 7. CIE chromaticity diagram shows that the present glass system emits light in the near-white and red regions for 378 and 521 nm excitations, respectively.
Fig. 5. NIR emission spectrum for LBZnFEr glass at 973 nm excitation.
1.5 lm broadband optical amplifiers. Since the spectrum is within the telecommunication window, it is desirable to increase the information capacity of wavelength division multiplexing networks. Energy level diagram depicted in Fig. 6 shows, at the 378, 521 and 973 nm excitations, Er3+ ions from the ground state jump to the 4G11/2, 2H11/2 and 4I11/2 levels, respectively and then non-radiatively (NR) relax to their lower levels. From these levels,
Fig. 7. CIE chromaticity diagram for LBZnFEr glass for excitation wavelengths 378 and 521 nm.
48
S. Thomas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 43–48
Conclusions In summary, Er3+-doped Li–Zn fluoroborate glass has been synthesized with good optical quality and transparency. The ionic nature of the Er3+-ligand bond in the glass network was confirmed by bonding parameter evaluation. Judd–Ofelt parameters Xt (t = 2, 4, 6) were found to be larger than those reported in the literature for different host materials. JO intensity analysis reveals the nature of JO parameters as X2 > X6 > X4 . In this study, the small value of spectroscopic quality factor and 1.53 lm broad emission with large emission cross-sections point out that the present glass system is an appropriate material for lasers and broadband optical amplifiers. Acknowledgements
Fig. 8. Absorption and emission cross-sections of LBZnFEr glass.
Absorption and emission cross-sections for 4I13/2 M 4I15/2 transitions Using absorption cross-section (ra ) of 4I15/2 ? 4I13/2 transition 4 4 (Fig. 1), the stimulated emission cross-section (rM e ) of I13/2 ? I15/2 (1.53 lm) has been evaluated using McCumber theory [25,26]. Absorption and emission cross-sections are expressed as:
ra ðmÞ ¼
2:303 AðmÞ Nt
ð1Þ
and
rMe ðmÞ ¼ ra ðmÞ exp
e hm kB T
ð2Þ
where m is the photon frequency, N is the concentration of Er3+ ions and t, the thickness of the glass. AðmÞ is the wavenumber dependent absorbance, e is the net free energy required for the transition 4 I15/2 ? 4I13/2 at temperature T, h is the Planck’s constant and kB , the Boltzmann’s constant. Using the method proposed by Miniscalco et al. [26], the energy e is estimated as 6540 cm1. The absorption and emission cross-sections for 4I13/2 M 4I15/2 (1.53 lm) transition are shown in Fig. 8. The emission cross-section (rM e ) is a significant parameter in an optical amplifier, realizing broadband high gain amplification. The maximum value of absorption and emission cross-sections for LBZnFEr glass is found to be 11.2 1021 cm2 at a wavelength of 1.53 lm. The integrated P P M re ) are absorption and emission cross-sections ( ra and 19 2 cm ), respectively. The broadening obtained as 8.0 and 6.3 (10 of the emission bandwidth is due to the self-absorption process, which causes the overlap of absorption and emission cross-section at 1.53 lm and this typically occurs in 3 level laser systems [27,28]. Also, self-absorption at 1.53 lm region is more efficient than that at other wavelengths [27].
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