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

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