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

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