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

Optical characterization of Er 3 þ and Yb 3 þ co-doped barium
fluorotellurite glass
Madhab Pokhrel a , G.A Kumar a , Sathravada Balaji b , Radhaballabh Debnath c , Dhiraj K. Sardar a,n
a
Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX 78249-0697, USA
b
Central Glass and Ceramic Research Institute, Kolkata 700032, India
c
School of Laser Science and Engineering, Jadavpur University, Kolkata 700032, India
article info
Article history:
Received 26 August 2011
Received in revised form
30 December 2011
Accepted 17 February 2012
Available online 9 March 2012
Keywords:
Tellurite glass
Absorption spectra
Fluorescence spectra
Upconversion
Radiative
Energy transfer
1. Introduction
abstract
Trivalent Erbium (Er 3 þ ) doped materials play a critical role in
many photonic devices including fiber amplifiers, lasers, display,
data storage, biomedical imaging, scintillators, security marking
etc. [1–8]. Especially, in the fiber optical communication network,
due to the demand of high transmission capacity wavelength
division multiplexing (WDM), Er 3 þ doped tellurite glasses and
fiber amplifiers have attracted considerable attention [9,10].
Tellurite glass owing to its high density, chemical durability,
and wide transparency has been considered as suitable host for
rare earth [11]. It has the lowest maximum phonon energy of
590 cm 1 among oxide glasses and the largest refractive index
values, both of which are beneficial for high radiative transition
rates of rare earth ions [12,13]. In addition, tellurite glass has the
ability to dissolve high concentrations of lanthanide ions without
clustering and thereby increasing the fluorescence lifetime and
quantum efficiency, which are important spectroscopic requirements
for a good luminescent material. In the past, different
groups have reported that the incorporation of halides and
barium oxide has shown the improvement not only in the glass
forming ability, stability, and chemical resistance [14,15], but also
n Corresponding author. Tel.: þ1 210 458 6905.
E-mail address: Dhiraj.Sardar@utsa.edu (D.K. Sardar).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jlumin.2012.02.041
Journal of Luminescence 132 (2012) 1910–1916
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence
journal homepage: www.elsevier.com/locate/jlumin
Spectroscopic studies of Er 3 þ /Yb 3 þ co-doped (Ba,La)-fluorotellurite glass composition have been
carried out using standard experimental and theoretical methods. Quantitative analyses of the room
temperature absorption and emission spectra as well as the emission lifetimes yield various important
spectroscopic parameters such as the radiative decay rates, fluorescence branching ratios, and
emission/absorption cross sections. In addition, internal radiative quantum yields have been determined
for the infrared emission at 1571 nm and for the upconversion emission at 547 nm. The
influence of various non-radiative properties such as multiphonon relaxation, concentration quenching,
and quenching by hydroxyl radicals have also been quantitatively estimated and correlated with the
observed spectral properties. The comparative studies with the other composition of tellurite and
different glasses showed that present glass composition could be a potential candidate for the
broadband amplifier.
& 2012 Elsevier B.V. All rights reserved.
reduces the phonon frequencies resulting in an improvement of
internal quantum efficiency [14–18].
This paper reports the characterization of room temperature
spectroscopic properties of Yb 3 þ /Er 3 þ co-doped (Ba,La) fluorotellurite
glass and compares those with TeO 2 glasses with different
compositions. The experimental and theoretical methods have
been used to analyze the radiative spectral properties of the
system and to measure the internal quantum yield of the
1571 nm emission band. Upconversion processes have been
analyzed and the efficiency of the green emission at 547 nm has
been estimated in a quantitative way. The superior spectroscopic
properties with its chemical and thermal stability make the
present glass system as an attractive host for many practical
photonic device applications.
2. Experimental
2.1. Sample preparation
The glass compositions studied was 80TeO 2þ15(BaF 2þBaO)þ
3La2O3þ1Er2O3þ1Yb2O3. A glass doped with only 1Yb2O3 and
a base glass were also prepared to use as the reference. All
samples were prepared from the starting chemical constituents
TeO 2, BaF 2, BaCO 3,Al 2O 3,Yb 2O 3, and Er 2O 3 (all from Sigma Aldrich
with 99.99% purity). A 20 g batch of the glass composition in each
case was mixed in an agate mortar and melted in an electric

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.

non-radiatively to the 4 I13/2 level which give rise to the transition
within the manifold 4 I13/2- 4 I15/2 corresponds to emission at
1571nm. At the same time, the populated 4 I11/2 level is further
excited to the 4 F7/2 state by ESA of a second photon. From 4 F7/2 level
another 980 nm photon can be absorbed to an equally similar excited
level. As explained earlier these excited state processes can equally
happen under high photon density through nonlinear absorption
processes such as two photon or three photon absorption. After these
excited state processes, de-excitation accumulates electron in different
excited states and the intensity of emission depends on the
electron density at that particular level as well as the nonradiative
contribution to the emission band. The two photon processes that
populate the 4 F7/2 level is so efficient that it can give very efficient
green emission transition from 2 H 11/2 due to the fast multiphonon
non-radiative decay of 4 F7/2 level to 2 H11/2. Because of several closely
spaced levels in between 2 H 11/2 and 4 S 3/2 multiphonon relaxation
results in decaying the 2 H11/2 level to 4 S3/2 yielding the strongest
emission band at 547 nm. Further, since the energy gap between
4 S3/2 and 4 F9/2 is 3080 cm 1 multiphonon relaxation is unlikely to
happen and hence the 4 S 3/2- 4 F 9/2 transition could be radiative. Part
of the population decaying to level 4 F9/2 can result in the red emission
at 670 nm through the 4 F 9/2- 4 I 15/2 transition. The other possible
major process that can populate the 4 F9/2 level of Er 3þ is
the energy transfer process from Yb according to the equation
2 F5/2(Yb 3þ )þ 4 I13/2(Er 3þ )- 2 F7/2(Yb 3þ )þ 4 F9/2(Er 3þ ). In order to
establish the presence of these nonlinear processes, the pump power
dependence of the emission bands on the emission intensity was
measured and the relationship obtained is shown in the inset of
Fig. 3. The slope obtained for 533, 547 and 670 nm emission bands
are respectively 2.3, 2.2 and 2.1, which shows that two photon
processes contribute to these emissions.
Fig. 5 presents the NIR emission spectra of Yb 3 þ ions singly
doped and Yb 3 þ /Er 3 þ ions co-doped glasses. A 2 fold decrease
in the peak emission intensity of Yb 3 þ : 2 F5/2- 2 F7/2 transition at
1006 nm in the co-doped sample clearly indicates the occurrence
of energy transfer. The left inset of Fig. 5 shows the decay profiles
for the 1006 nm emission of Yb 3 þ ions in the singly doped and codoped
sample fitted by single exponential function with a
Emission Intensity (a.u)
10.0 x10 6
8.0 x10 6
6.0 x10 6
4.0 x10 6
2.0 x10 6
0
M. Pokhrel et al. / Journal of Luminescence 132 (2012) 1910–1916 1913
Yb: 2 F 5/2-> 2 F 7/2
Log Photon Counts (a.u)
λemi = 1006nm
YbEr; τf = 152μs
Yb; τf = 634μs
correlation factor of 0.99998, and the right inset shows the decay
profile for 1571 nm of Er 3þ ions in the co-doped sample also fitted
by single exponential function with a correlation factor of 0.99999.
From the measured lifetimes of Yb 3þ transitions at 1006 nm in the
singly doped (Yb 3þ )andco-dopedglass(Yb 3þ /Er 3þ ), the energy
transfer rate and efficiency can be calculated using the following
expressions [32]:
WET ¼ 1 1
ð1Þ
t
Z ET ¼ 1
t0
t
, ð2Þ
t0
where t is the lifetime of donor, Yb 3 þ ions in the presence of
acceptor, Er 3 þ ions and t 0 is the lifetime of donor, Yb 3 þ ions in
the absence of acceptor, Er 3 þ ions. The energy transfer rate from
(Yb 3 þ ) 2 F 5/2-(Er 3 þ ) 4 I 11/2 in the present glass is found to be
5001.6 s 1 showing an energy transfer efficiency of 76%.
3.3. Cross section and gain coefficient
Emission cross-section (se) and the FWHM are important
parameters in an optical amplifier’s achieving broadband and
high-gain amplification. The bandwidth properties of the optical
amplifier can be evaluated from the product FWHM s e. The
larger the product is, the better are the performance of the
amplifier. Tables 3 and 4 list the parameters corresponding to
1571 nm emission for comparison such as FWHM, se, and
FWHM s e of Er 3 þ in various Er 3 þ /Yb 3 þ doped tellurite and
oxide glasses. It is seen that FWHM se in present glass composition
80TeO 2þ15(BaF 2þBaO)þ3La 2O 3 is larger than in other tellurite
glasses reported in the literature [17,27], but less than the
reported in the literature [33,34] system, which indicates that
present glass composition is better among the composition based
on TeO 2þBaF 2, TeO 2þZnO, germinate [35], and silicate [36] for a
broadband amplifier.
A long upper state lifetime in a gain medium enables a
significant population inversion with a relatively low pump
400 800 120016002000 1200 2400 3600 4800
Time (micro sec)
Yb
YbEr
λ emi = 1571nm
YbEr;τ f = 2693 μs
1000 1125 1250 1375 1500 1625
Wavelength (nm)
Fig. 5. Emission spectra of Yb 3þ ion singly doped and Yb 3þ /Er 3þ ion co-doped tellurite glasses. Insets show the decay profiles for the emissions.
Er: 4 I 13/2-> 4 I 15/2

1914
Table 3
Fluorescence lifetime (tf), emission cross-section (se), and full width at half max (FWHM) for the emission at 1571 nm in tellurite glass with different compositions.
Sample (mol%) t f (ms) s e (10 21 cm 2 ) FWHM (nm) s e FWHM (10 28 cm 3 )
80TeO2þ 15(BaF2þBaO)þ3La2O3þ1Er2O3þ1Yb2O3 a
2.693 6.82 91 621
75TeO2þ20ZnOþ5Na2Oþ0.5Er2O3þ1Yb2O3[21] 3.15 8.8 56 493
70TeO2þ20ZnOþ5Li2Oþ5La2O3þ0.5Er2O3þ0.5Yb2O3[27] 4.02 8.5 65 553
70TeO2þ10ZnOþ5BaF2þ15PbCl2þ0.5Er2O3þ1Yb2O3[17] 4.16 8.15 52 424
70TeO2þ5BaF2þ10ZnBr2þ15PbF2þ0.5Er2O3þ1Yb2O3[17] 4.05 8.3 56 465
70TeO2þ5BaF2þ10ZnBr2þ15PbCl2þ0.5Er2O3þ1Yb2O3[17] 3.85 8.45 55 465
75TeO2þ15WO3þ10La2O3þ1Er2O3þ5Yb2O3[33] 3.0 10 77 770
65TeO2þ15B2O3þ20SiO2 þ0.5Er2O3þ2.5Yb2O3[37] 2.38 7.49 71 532
70TeO2þ20ZnOþ10PbOþ1Er2O3þ5Yb2O3[34] 2.69 8.77 72 631
a Sample under study.
Table 4
Fluorescence lifetime (t f), emission cross-section (s e), and full width at half max (FWHM) for the emission at 1571 nm in different
glasses.
Glass type t f (ms) s e (10 21 cm 2 ) FWHM (nm) s e FWHM (10 28 cm 3 ) s e t f (10 23 cm 2 s)
Tellurite [21] 3.15 8.8 56 493 2.77
Silicate [36] 3.81 5.5 40 220 2.09
Bismuth [38] 4.5 8.5 75 637 3.82
Germanate [35] 9.55 5.7 53 302 5.44
Oxyfluoride silicate [39] 9.4 7.4 60 444 6.95
((Ba,La)-tellurite) a
2.693 6.82 91 621 1.84
a Sample under study.
power. A higher excited state population with high emission cross
section enables to achieve high optical gain. Thus in the design of
high gain amplifiers the product of emission cross section and
excited state lifetime (s et) is very important and is found to be
threshold pump power [40,41], The set value obtained for the
present glass composition is listed in Table 4 along with other
glass compositions, indicating that high gain could be achieved in
the present glass.
In analyzing the amplifier performance for broadband applications
gain cross section is an important parameter. By measuring
the absorption and emission cross sections using standard procedure
[11] one can simulate the gain spectrum using the
expression
sg ¼ Nse ð1 NÞsa, ð3Þ
where sa is the absorption cross section, se is the emission cross
section and N is the fractional population inversion in the
emitting level. The calculated gain spectra for different population
inversions shown in Fig. 6 indicate that the gain increases with
the population inversion and positive gain in the system can be
achieved only for a population inversion of 20% and above. It is
interesting to notice from Fig. 6 that even for a 20% of population
a flat gain was observed in the L-band side and further increase in
the population 50–60% a better gain was observed both in the S
and L band regions. Moreover, the obtained gain spectrum spread
from 1440 nm to 1650 nm with a spectral width of over 210 nm.
3.4. Non-radiative processes and internal quantum yield
A quantitative way of measuring the internal quantum yield
for a particular emission band is done through the fluorescence
lifetime measurements. Non-radiative processes such as multiphonon
relaxation, vibrational losses by hydroxyl and other
functional groups and energy transfer interaction quench the
fluorescence intensity and the efficiency. The observed lifetime
M. Pokhrel et al. / Journal of Luminescence 132 (2012) 1910–1916
Gain corss section, σ g (10 -20 cm 2 )
0.5
0.0
-0.5
1450 1500 1550 1600 1650
Wavelength (nm)
of the emission can be written as [42]
Ν =0
Ν =10
Ν =20
Ν =30
Ν =40
Ν =50
Ν =60
Ν =70
Ν =80
Ν =90
Ν =100
Fig. 6. Gain cross section spectrum for the 4 I13/2- 4 I15/2 transition showing the
possible spectral width and peak gain cross section. The spectrum was simulated
for different population inversion P.
1
¼ AradþWmp þWOH þWET, ð4Þ
tlum where A rad, W mp, W HO, W ET are the radiative transition rate,
transition rate from multiphonon relaxation, hydroxyl groups
and energy transfer interactions, respectively. The contribution
to non-radiative decay comes from multiphonon relaxation from
the host and energy transfer interaction between nearby ions. The
transition probability by energy transfer depends on the distance
between the donor (D) and acceptor (A) ions, R DA, and hence
depends on the concentration of the ions. When the ions are
homogeneously distributed in the matrix, the energy transfer

elaxation rate WET can be written as [43]
WETp 1
R 6
pN
DA
2 , ð5Þ
where N is the ionic concentration. Since the Er 3 þ 3 þ
and Yb
concentration on the present sample is too low compared to that
reported in the literature [24,44], ET does not have appreciable
influence on the non-radiative losses. The values of WOH can be
determined from the quantitative measurements of the OH
content and the low frequency vibrational modes in the sample
from the infrared vibrational spectrum shown in Fig. 7. The FTIR
spectrum shows the characteristic Te–O symmetric vibration at
640 cm 1 , which is usually present in the tellurite glass [45].
Using this frequency, the transition rate for multiphonon vibrational
loss (Wmp) of the 1571 nm emission band was estimated to
be 0.00282 s 1 [42], which is negligibly small.
So the main contribution is coming from water content, which is
confirmed by the absorption band at 2933 cm 1 in the IR spectrum.
In the tellurite glasses, these absorption bands originate from the
stretching vibration of OH groups, which are incorporated as Te(OH) 6
and H2TeO6 [45–47], and the 2283 cm 1 absorption are ascribed to
the strongly hydrogen-bonded OH-groups. Because of the various
sites of OH-groups in the vitreous host and the highly disordered
hydrogen-bonded OH-groups made the absorption bands quite
broad. The absolute measurement of OH content in glasses is
difficult, particularly at concentration below 1000 ppm. However, a
relative estimate of the water content can be obtained from the
infrared absorption measurements corresponding to the OH band.
The amount of water content in the glass samples can be obtained
from the measurement of the absorption coefficient at the OH peak
of the IR spectrum and is estimated to be nearly 2.4 cm 1 .AnOH
concentration of 3607100 ppm gives absorption of 2.8 cm 1 [48].
By comparison, the corresponding water content was estimated to be
308 ppm (5.5 10 19 molecules/cm 3 ). Since the second order vibrational
frequency of OH is almost in resonance with the 4 113/2- 4 I15/2
transition in Er 3þ , they are powerful quenchers of the Er 3þ emission
at 1571 nm. The quenching rate due to this water content is found to
be proportional to the absorption coefficient of the OH radicals [49].
ThemajorsourcesofOHcontentintheglassarefromthestarting
chemicals as well as from the synthesis atmosphere. Higher level of
water content can be suppressed using moisture free chemicals
melted in water free environment as well as by re-melting of the
glasses.
The internal quantum efficiency, (ZInt) can be evaluated from
the ratio of the fluorescence to radiative decay time [26]. The
Absorbance
4
3
2
1
0
Te-O
O-H
1000 2000 3000 4000
Frequency (cm -1 )
Te(OH) 6
Fig. 7. Infrared vibration spectrum of glass composition showing the presence of
OH radicals associated with the tellurite network.
M. Pokhrel et al. / Journal of Luminescence 132 (2012) 1910–1916 1915
Upconversion efficiency (%)
0.300
0.225
0.150
0.075
lifetime obtained for the 1571 nm emission in the glass composition
was 2.693 ms and this together with the calculated radiative
decay time of 3.707 ms yields a radiative internal quantum
efficiency of near 73%. It should be noted that this internal
quantum yield is smaller compared to similar reported tellurite
composition [17,21,27,33]. However the influence of OH radicals
and other non-radiative interactions from Er 3 þ ions detracts the
efficiency less than 100% as noticed from our calculations.
The upconversion efficiency (Zup) can be calculated by comparing
the upconversion luminescence signal with the directly
excited signal intensity using the expression [50]
P
Z ¼ Z
absð488nmÞI550ð980nmÞ
q
Pabsð980nmÞI550ð488nmÞ where Pabs(l) represent the power absorbed by the glass sample
at the indicated wavelength, and I 550(l) denotes the relative
intensity generated in the green when the sample is photo excited
at the indicated pump wavelength, and Zq represents the internal
radiative quantum yield of the 547 nm emission. For the 4 S3/2
state of Er 3 þ , tflu and trad were determined to be 26 ms and
0.260 ms respectively yielding an internal radiative quantum
yield of 10% for the green upconversion emission. The values of
P abs(l) in Eq. (6) were calculated from the measured power
incident on the sample, and the known absorption coefficient at
the excitation wavelength. On the basis of Eq. (6), the 980 nm-
547 nm upconversion efficiency in glass has been calculated from
experimental measurements made for several values of excitation
power at 980 nm, 488 nm and the results are illustrated in Fig. 8.
For all the pump powers, Z increases approximately linearly with
an efficiency of 0.3% at 60 mW excitation. The saturation of the
detector limits the efficiency measurements for higher values of
the excitation power.
4. Conclusion
10 20 30 40 50 60
Excitation Power (mW)
Fig. 8. Green upconversion efficiency dependence under 980 nm excitation power.
An in depth spectroscopic analysis of Yb 3 þ /Er 3 þ co-doped
(Ba,La) tellurite glass host has been performed following the
theoretical and experimental methods. The investigations on the
fluorescence spectral properties of Yb 3 þ /Er 3 þ co-doped (Ba,La)
tellurite glass composition shows better optical performance in
terms of gain cross section and full width at half max (FWHM) of
over 91 nm compared to other glass system. Although, the
internal quantum efficiency of the present system approaches
near 73% that is comparable to similar compositions, the influence
of non-radiative interactions from OH radicals can be
ð6Þ

1916
suppressed using moisture free chemical melting in water free
environment or by re-melting the glass and increase the internal
efficiency close to100%. The internal radiative quantum yield of
10% was obtained for the green upconversion emission at 547 nm.
The low upconversion efficiency offers additional possibility of
enhancing the infrared emission quantum yield by optimizing the
dopant concentration as well as synthesizing the sample in
moisture free environment. More detailed investigations on range
of Yb 3 þ /Er 3 þ dopant concentrations are under way to understand
the details of the energy transfer and other non-radiative process.
Acknowledgments
This research was supported by the National Science Foundation
Partnership for Research and Education in Materials (NSF-PREM)
Grant no. DMR-0934218. One of the authors, Balaji, also acknowledges
the support from the Director, CSIR-CGCRI, Kolkata.
References
[1] A. Brenier, Chem. Phys. Lett. 290 (1998) 329.
[2] Y. Chen, Y. Huang, Z. Luo, Chem. Phys. Lett. 382 (2003) 481.
[3] N. Chiodini, A. Paleari, G. Brambilla, E.R. Taylor, Appl. Phys. Lett. 80 (2002)
4449.
[4] D.F. de Sousa, L.F.C. Zonetti, M.J.V. Bell, R. Lebullenger, A.C. Hernandes,
L.A.O. Nunes, J. Appl. Phys. 85 (1999) 2502.
[5] D. Jaque, J. Capmany, F. Molero, Z.D. Luo, J. García Sole, Opt. Mater. 10 (1998)
211.
[6] N. Rakov, F.E. Ramos, G. Hirata, M. Xiao, Appl. Phys. Lett. 83 (2003) 272.
[7] T.J. Whitley, C.A. Millar, R. Wyatt, M.C. Brierley, D. Szebesta, Electron. Lett. 27
(1991) 1785.
[8] Y.C. Yan, A.J. Faber, H. de Waal, P.G. Kik, A. Polman, Appl. Phys. Lett. 71 (1997)
2922.
[9] Y. Hu, S. Jiang, G. Sorbello, T. Luo, Y. Ding, B.-C. Hwang, J.-H. Kim, H.-J. Seo,
N. Peyghambarian, J. Opt. Soc. Am. A 18 (2001) 1928.
[10] H. Lin, G. Meredith, S. Jiang, X. Peng, T. Luo, N. Peyghambarian, E.Y.-B. Pun,
J. Appl. Phys. 93 (2003) 186.
[11] P.C. Becker, N.A. Olsson, J.R. Simpson, Erbium-Doped Fiber Amplifiers
Fundamentals and Technology, Academic Press, San Diego, 1999.
[12] J.C. Michel, D. Morin, F. Auzel, Rev. Phys. Appl. 13 (1978) 859.
[13] Z. Pan, S.H. Morgan, K. Dyer, A. Ueda, H. Liu, J. Appl. Phys. 79 (1996) 8906.
[14] K.J. Pluciński, W. Gruhn, J. Wasylak, J. Ebothe, D. Dorosz, J. Kucharski,
I.V. Kityk, Opt. Mater. 22 (2003) 13.
[15] H. Sun, S. Xu, S. Dai, J. Zhang, L. Hu, Z. Jiang, Solid State Commun. 132 (2004)
193.
[16] X. Wang, H. Lin, D. Yang, L. Lin, E.Y.-B. Pun, J. Appl. Phys. 101 (2007) 113535.
[17] J. Yang, L. Zhang, L. Wen, S. Dai, L. Hu, Z. Jiang, J. Appl. Phys. 95 (2004) 3020.
M. Pokhrel et al. / Journal of Luminescence 132 (2012) 1910–1916
[18] M.A.P. Silva, Y. Messaddeq, V. Briois, M. Poulain, F. Villain, S.J.L. Ribeiro,
J. Phys. Chem. Solids 63 (2002) 605.
[19] J. Lu, M. Prabhu, J. Song, C. Li, J. Xu, K. Ueda, A.A. Kaminskii, H. Yagi,
T. Yanagitani, Appl. Phys. B Lasers Opt. 71 (2000) 469.
[20] A. Ghosh, R. Debnath, Opt. Mater. 31 (2009) 604.
[21] I. Jlassi, H. Elhouichet, M. Ferid, C. Barthou, J. Lumin. 130 (2010) 2394.
[22] B.R. Judd, Phys. Rev. 127 (1962) 750.
[23] G. Ofelt, J. Chem. Phys. 37 (1962) 511.
[24] M. Pokhrel, G.A. Kumar, P. Samuel, K.I. Ueda, T. Yanagitani, H. Yagi, D.K. Sardar,
Opt. Mater. Express 1 (2011) 1272.
[25] A.A. Kaminskii, Laser Crystals: Physics and Properties, Springer-Verlag, Berlin,
New York, 1979.
[26] A.A. Kaminskii, Laser Crystals: Their Physics and Properties, Springer-Verlag,
Berlin, New York, 1990.
[27] H. Desirena, E.D.l. Rosa, A. Shulzgen, S. Shabet, N. Peyghambarian, J. Phys. D
41 (2008) 095102.
[28] D.K. Sardar, D.M. Dee, K.L. Nash, R.M. Yow, J.B. Gruber, R. Debnath, J. Appl.
Phys. 102 (2007) 083105.
[29] A. Kaminskii, K. Ueda, A. Konstantinova, H. Yagi, T. Yanagitani, A. Butashin,
V. Orekhova, J. Lu, K. Takaichi, T. Uematsu, M. Musha, A. Shirokava, Crystallogr.
Rep. 48 (2003) 868.
[30] W.F. Krupke, M.D. Shinn, J.E. Marion, J.A. Caird, S.E. Stokowski, J. Opt. Soc. Am.
B 3 (1986) 102.
[31] M.J.F. Digonnet, Society of Photo-optical Instrumentation, Selected Papers on
Rare-Earth-Doped Fiber Laser Sources and Amplifiers, SPIE Optical Engineering
Press, Bellingham, Washington, 1992.
[32] L.M.S. El-Deen, M.S.A. Salhi, M.M. Elkholy, J. Alloys Compd. 465 (2008) 333.
[33] X. Shen, Q. Nie, T. Xu, Y. Gao, Spectrochim. Acta A 61 (2005) 2189.
[34] Z. Jin, Q. Nie, T. Xu, S. Dai, X. Shen, X. Zhang, Mater. Chem. Phys. 104 (2007)
62.
[35] H. Lin, E.Y.B. Pun, S.Q. Man, X.R. Liu, J. Opt. Soc. Am. B 18 (2001) 602.
[36] X. Feng, S. Tanabe, T. Hanada, J. Am. Ceram. Soc. 84 (2001) 165.
[37] T. Xu, X. Zhang, S. Dai, Q. Nie, X. Shen, X. Zhang, Physica B 389 (2007) 242.
[38] J. Yang, S. Dai, N. Dai, S. Xu, L. Wen, L. Hu, Z. Jiang, J. Opt. Soc. Am. B 20 (2003)
810.
[39] S. Xu, Z. Yang, S. Dai, J. Yang, L. Hu, Z. Jiang, J. Alloys Compd. 361 (2003) 313.
[40] H. Kühn, S.T. Fredrich-Thornton, C. Kränkel, R. Peters, K. Petermann, Opt. Lett.
32 (2007) 1908.
[41] D.S. Sumida, T.Y. Fan, Opt. Lett. 19 (1994) 1343.
[42] B. Di Bartolo, G. Armagan, International School of Molecular, Spectroscopy of
Solid-State Laser-Type Materials. Plenum Press, New York, 1987.
[43] D. Dexter, J. Chem. Phys. 21 (1953) 836.
[44] W.Q. Shi, M. Bass, M. Birnbaum, J. Opt. Soc. Am. B 7 (1990) 1456.
[45] G. Liao, Q. Chen, J. Xing, H. Gebavi, D. Milanese, M. Fokine, M. Ferraris, J. Non-
Cryst. Solids 355 (2009) 447.
[46] G.A. Hebbink, L. Grave, L.A. Woldering, D.N. Reinhoudt, F.C.J.M. van Veggel,
J. Phys. Chem. A 107 (2003) 2483.
[47] J. Massera, A. Haldeman, J. Jackson, C. Rivero-Baleine, L. Petit, K. Richardson,
J. Am. Ceram. Soc 94 (2010) 130.
[48] D.E. Day, J.M. Stevels, J. Non-Cryst. Solids 14 (1974) 165.
[49] S.A. Payne, M.L. Elder, J.H. Campbell, G.D. Wilke, M.J. Weber, J. Am. Ceram.
Soc. 28 (1991) 253.
[50] G.A. Kumar, A. Martinez, E. Mejia, J.G. Eden, J. Alloys Compd. 365 (2004) 117.