Optical characterization of Er3+and Yb3+ co-doped barium ...

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furnace at 750 1C for 2 h in platinum crucible and the molten glass was stirred intermittently to achieve homogeneity. The melt was then cast onto a preheated (250 1C) carbon mold. The glass disks thus obtained were annealed at 250 1C for 12 h to avoid undesirable thermal stresses. The samples were then polished to optical quality and only bubble and streak free samples of thickness 0.2 cm were taken for optical measurements. The density of the sample was measured by Archimedes’s principle and found to be 5.679 g/cm 3 and the calculated number density of the Er 3 þ ion in the glass were found to be 4.02 10 20 ions/cm 3 . 2.2. Optical measurements The refractive index were measured at five different wavelengths (473 nm, 532 nm, 632.8 nm, 1060 nm and 1552 nm) on Metricon M2010 Prism Coupler equipped with respective laser sources and the obtained values were fitted to Sellmeier’s dispersion equation to obtain wavelength dependent refractive index. The absorption spectra were measured in the 300–1700 nm range using a spectrophotometer (Cary, Model 14R) in the transmission mode. The NIR emission spectra and fluorescence decay kinetics of the samples were recorded on spectrofluorimeter (Model: Quantum Master enhanced NIR, PTI, USA) fitted with double monochromators on both excitation and emission channels. The instrument was equipped with liquid N 2 cooled gated NIR photo-multiplier tube (Model: NIR-PMT-R 1.7, Hamamatsu) as detector for acquiring both steady state spectra and phosphorescence decay. For decay measurements, a 60 W Xenon flash lamp was employed as an excitation source. The upconversion luminescence spectra of the glass were recorded in the above mentioned spectrofluorimeter using a 980 nm fiber pig-tailed diode laser with variable output power up to a maximum of 200 mW (Thorlab, USA) as an excitation source. For quantum yield measurements, the emission spectra of the samples were recorded by exciting the sample with 980 nm band of a Ti–S laser (Spectra Physics, Model 3900S) pumped by an Nd:YAG laser (Spectra Physics, Model Millennia) and 488 nm band of an Ar-ion laser (Spectra Physics, Model 2500). The upconversion and the visible emission from the sample were focused onto a photo multiplier tube (Model 1911, Horiba) and scanned using a 1200 grooves/mm diffraction grating blazed at 0.5 mm, with a spectral resolution of 0.01 nm. The detector signal was processed in a PC coupled to the data acquisition system through a lock in amplifier (Stanford Research System, Model SR510). The entire system was controlled through the data acquisition software Snerjy (Origin Lab, and Jobin Yvon). All the emissions and decay time measurements were performed at room temperature. The FTIR vibrational spectrum was measured on the solid sample in the transmission mode using the FTIR spectrometer (Bruker, Model vector 22). 3. Results 3.1. Judd–Ofelt (JO) parameters The room temperature UV–vis–NIR absorption spectra of the sample are shown in Fig. 1(a) and (b) with their spectral band assignments. In calculating the absorption coefficients, the scattering losses have been subtracted in all absorption measurements. The broadening of the absorption bands is a consequence of the absence of long range order in the host producing changes in the micro-symmetry around the Er 3 þ ion. It was found that all absorption bands of Er 3 þ in fluorotellurite glass is similar to those of previously reported tellurite glass hosts [19–21]. M. Pokhrel et al. / Journal of Luminescence 132 (2012) 1910–1916 1911 Abs. Coeff. (cm -1 ) Abs. Coeff. (cm -1 ) 6 3 0 3.0 1.5 0.0 4 G9/2+ 4 G 11/2 4 F5/2 4 F3/2 2 G9/2 4 F7/2 2 H11/2 4 S3/2 4 F9/2 400 450 500 550 600 650 700 Wavelength (nm) 4 I9/2 4 I11/2 (Er 3+ )+ 2 F 5/2 (Yb 3+ ) 800 1000 1200 1400 1600 Wavelength(nm) 4 I13/2 Fig. 1. Room temperature absorption spectra of Er 3þ in Er 3þ /Yb 3þ co-doped glass, 1(a) ranging from 350 to 700 nm, 1(b) ranging from 700 to 1650 nm including Yb 3þ ( 2 F7/2- 2 F5/2) transition at 980 nm. Table 1 Measured and calculated absorption line strengths of Er 3þ in Er 3 þ /Yb 3þ co-doped (Ba,La)-tellurite glass at 300 K. 4 I15/2- k (nm) S meas ed (10 –20 cm 2 ) S cal ed (10–20 cm 2 ) 4 I13/2 1532 2.0121 2.2836 4 I9/2 800 0.2452 0.2978 4 F9/2 654 1.4445 1.5151 4 S3/2 546 0.2088 0.3049 4 H11/2 522 5.2190 5.0589 4 F7/2 490 0.6235 1.1052 4 F5/2 452 0.1303 0.3078 4 F3/2 444 0.0138 0.1754 ( 2 G, 4 F, 2 H)9/2 408 0.1603 0.3422 4 G11/2 380 6.3243 6.5085 Twelve absorption bands were identified in the room temperature absorption spectra between 300 and 1600 nm, shown in Fig. 1(a) and (b) and are tabulated in Table 1. These absorption bands were chosen to determine the phenomenological Judd– Ofelt parameters [22–26] for Er 3 þ in Er 3 þ /Yb 3 þ doped fluorotellurite glass. The JO parameters obtained for the Er 3þ in the present glassy system are O 2¼5.97 10 20 70.3 cm 2 , O 4¼1.64 10 – 20 70.10 cm 2 ,andO6¼1.38 10 –20 70.067 cm 2 . These values are in excellent agreement with the values reported earlier for many tellurite glassy hosts [20,21,27,28] within the relative error of fit and falls at the lower end of the range 5–25%, as obtained by the application of the JO theory to other laser materials [22,23,29,30]. Using these phenomenological JO parameters, the emission line strengths corresponding to the transitions from the upper
1912 multiplet manifolds 2Sþ1 LJ to the corresponding lower lying multiplet manifolds 2S0 þ1 LJ’ of Er 3 þ in the present glass was calculated [11,22,23,26]. Using these line strengths, radiative transition probabilities from J to J 0 , radiative decay rates, and branching ratios were calculated and are tabulated in Table 2. 3.2. Infrared and upconversion luminescence spectra Fig. 2 shows the infrared emission of the glass composition, where peak emission was observed at 1571 nm with a spectral bandwidth of 210 nm and full width at half max (FWHM) of over 91 nm. Fig. 3 shows the upconversion luminescence spectra obtained for various pump powers under 980 nm excitation with an inset photograph of the green upconversion emission under 60 mW excitation. The emission bands were observed at 533 nm (green), 547 nm (green) and 670 nm (red), which are assigned to the 2 H11/2- 4 I15/2, 4 S3/2- 4 I15/2 and 4 F9/2- 4 I15/2 transitions respectively. The fluorescence branching ratios of the upconversion bands are 24% (533 nm), 64% (547 nm) and 13% (670 nm) respectively. Since the integrated intensity is highest for the 547 nm Table 2 Calculated electric dipole radiative transition probabilities (A ed), fluorescence branching ratios (b JJ 0), and radiative decay rates (t rad) of various excited states of Er 3 þ in Er 3 þ /Yb 3 þ co-doped (Ba,La)-tellurite glass at 300 K. Transitions k (nm) n S ed (10 –20 cm 2 ) A ed bJ-J 0 (%) s rad (ms) 4 I13/2-I15/2 1571 1.9741 2.3215 a 269.74 100 3.7072 4 I11/2-I15/2 988 1.9886 0.7058 260.02 88.79 3.4148 4 -I13/2 2747 1.9678 1.7404 32.81 11.20 4 F9/2-I15/2 656 2.0202 1.5690 2092.28 89.07 0.4257 4 -I13/2 1143 1.9825 0.4358 117.18 4.98 4 -I11/2 1957 1.9707 2.1547 131.78 5.01 4 -I9/2 3472 1.9666 0.5998 7.82 0.33 4 S3/2-I15/2 545 2.0465 0.3531 861.42 77.06 0.8946 4 -I13/2 842 1.9978 0.1088 75.27 6.73 4 -I11/2 1213 1.9805 0.4862 127.14 11.37 4 -I9/2 1645 1.9733 0.4347 53.96 4.83 2 H9/2-I15/2 408 2.1168 0.2052 1353.64 27.14 0.2005 4 -I13/2 554 2.0438 0.6366 1681.56 33.72 4 -I11/2 694 2.0141 0.7726 1147.26 23.00 4 -I9/2 821 1.9996 0.6192 649.06 13.01 4 F9/2 1076 1.9848 0.3434 155.61 3.12 - a 269.74 (Aed¼207.12þAmd¼62.62)—including the magnetic dipole contribution Amd. Cross section (cm 2 ) 6.0x10 -21 4.0x10 -21 2.0x10 -21 0.0 Absorption Emission 1450 1500 1550 1600 1650 Wavelength (nm) Fig. 2. NIR emission cross section spectrum of the glass under 980 nm laser excitation along with the overlap of the absorption corresponding to 4 I 15/2- 4 I 13/2. The Y-axis is in cross section normalized with respect to the absorption peak intensity. M. Pokhrel et al. / Journal of Luminescence 132 (2012) 1910–1916 Intensity (arb.units) 800 600 400 200 100 Laser Power (mW) 200 180 160 140 120 100 80 60 40 20 500 550 600 Wavelength (nm) 650 700 Upconversion Intensity (arb.units) 533nm - Slope-2.3 547nm - Slope-2.2 670nm - Slope-2.1 Laser Power (mW) Fig. 3. Upconversion emission spectrum under 980 nm excitation. Inset shows the dependence of the intensity of the upconversion bands under 980 nm excitation on pump power. Photograph in the inset is the green upconversion emission under 60 mw excitation power. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.) Energy 20 16 12 8 4 0 x (10 3 cm -1 ) 2 F5/2 980 nm 2 F7/2 Yb 3+ ET 980 nm ESA TPA 1529 nm 523 nm Er 3+ 4 F7/2 2 H11/2 4 S3/2 4 F9/2 4 I9/2 Er 3+ band, the color of the sample appears pure green as shown in inset of Fig. 3. The upconversion process in YbEr system is well understood in several materials [11,31] and is briefly explained for this sample using the following energy level diagram as shown in Fig. 4. Emission bands were observed at 533 nm, 547 nm, and 670 nm and are assigned to the 2 H11/2- 4 I15/2, 4 S3/2- 4 I15/2, and 4 F9/2- 4 I 15/2 transitions respectively. The appearance of both the green and red emission bands can be explained on the basis of various processes such as excited state absorption (ESA) and energy transfer (ET). When the 4 I11/2 level is excited under 980 nm directly through Er 3 þ or through Yb 3 þ excitation photons, part of the excitation energy at the 4 I11/2 level relaxes MP 546 nm 672 nm CR ET 4 I11/2 4 I13/2 4 I15/2 Fig. 4. Energy level diagram of Yb 3þ co-doped with Er 3þ and the possible excitation and de excitation mechanisms giving rise to the observed upconversion bands.
Page 1: Optical characterization of Er 3 þ
Page 5 and 6: 1914 Table 3 Fluorescence lifetime
Page 7: 1916 suppressed using moisture free

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