Optical properties and upconversions in Er3+ doped Li:TeO2 glass
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Transcript of Optical properties and upconversions in Er3+ doped Li:TeO2 glass
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Optical properties and upconversions in Er3C doped
Li:TeO2 glass
Anant Kumar Singh, S.B. Rai*, D.K. Rai
Laser and Spectroscopy Laboratory, Physics Department, Banaras Hindu University, Varanasi 221005, India
Received 11 May 2005; received in revised form 28 July 2005; accepted 8 August 2005 by A.K. Sood
Available online 25 August 2005
Abstract
A NIR excitation of Er3C doped Lithium modified tellurite (Li:TeO2) glass results in antistokes fluorescent emission near 380,
530, 551 and 654 nm (ultraviolet, green and red regions) in addition to NIR Stokes emission. The antistokes emissions are ascribed to
transition from the excited 4G11/2,4S3/2(2H11/2) and 4F9/2 levels in Er3C. The excitation involves three and two incident photons. On
excitation with the green laser line at 532 nm also leads to similar emissions. The mechanisms involved in these processes are
discussed on the basis of the known energy level diagram and the upconversion efficiency has been calculated. Lifetime of the 4S3/2
level has been measured. The temperature dependence of the upconversion process has also been investigated.
q 2005 Elsevier Ltd. All rights reserved.
PACS: 42.70 Ce
Keywords: A. LiTeO2:Er3C; D. ESA; D. ET; D. Fluorescence; D. Upconversion
1. Introduction
It is well known that glasses/crystals doped with certain
triply ionized rare earth ions viz. Pr3C, Er3C, Ho3C, Dy3C,
etc. can convert incident near infrared radiation into visible
or ultra violet light through upconversion. Er3C doped in
crystals or glasses with low phonon frequencies is
considered as one of the most promising system for
upconversion. Basically in this process two or three photons
of the incident light are absorbed by the rare earth ions
through low lying metastable excited states which serve as
intermediate states in the excitation of the final upper level
from which emission to the ground state results in the
form of visible/UV upconverted photons. Numerous studies
[1–15] have focused attention on Er3C doped glasses and
crystals. Tellurite based glasses have certain advantages
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2005.08.007
* Corresponding author. Tel.: C91 542 230 7308; fax: C91 542
368468.
E-mail addresses: [email protected] (A.K. Singh),
[email protected] (S.B. Rai).
over other oxide glasses. It has very low phonon frequency
with high refractive index. The chemical and thermal
durability and mechanical strength is also relatively high
and processing is easier due to its low melting point. Recent
studies on Er3C doped tellurite glasses by Vetrone et al. [1]
and by Rolli et al. [13] have shown upconversions in green
and red region when exciting radiation is in NIR region. On
the other hand if the exciting radiation is in the red region,
upconversion is seen in uv, green and red regions. We have
now observed upconversion in ultraviolet, green and red
regions and Stokes fluorescence in the NIR region in
Li:TeO2 glass when it is pumped with 780 and 532 nm
radiations. These studies are presented.
2. Experimental
The glasses studied have the following percent molar
composition
ð80KxÞTeO2 C20Li2CO3 CxEr2O3
ðx Z 0:5; 1:0; 1:5; 2:0Þ
Solid State Communications 136 (2005) 346–350
www.elsevier.com/locate/ssc
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A.K. Singh et al. / Solid State Communications 136 (2005) 346–350 347
The procedure used to obtain the samples of the doped
optical glass is identical to that used in our earlier work [16].
CW upconversion fluorescence measurements were
made using a Ti-Sapphire and a Nd-YAG laser emitting at
780 and 532 nm, respectively, as the excitation source. The
sample fluorescence was collected perpendicularly to the
direction of the incident laser beam and was dispersed using
a Spex 0.5 m grating spectrometer. The signal was sent to
chart recorder to plot it in the form of wavelengths versus
signal intensity. The signals were also measured at different
temperatures and different input laser power. We have also
measured the lifetime of the 4S3/2 level involved in green
fluorescence in both cases.
3. Results and discussion
The fluorescence spectra of Li:TeO2 glasses doped with
varying concentration of Er3C were recorded using 532 nm
line of a Nd-YAG laser (low power) as excitation source. It
was noted that the fluorescence yield is largest when Er3C
concentration is 1.5 mol%. This glass was used in further
studies.
3.1. Upconversion under 780 nm excitation
Fig. 1 shows the fluorescence (upconversion) spectrum
of the glass at different concentrations of the rare earth on
excitation by 780 nm line from a Ti-Sapphire laser at room
temperature. The photon energy 12,820 cmK1 is nearly
600 cmK1 larger than the excitation energy of the 4I9/2 level.
A tellurite lattice shows Raman shifts at 770, 650, 610 and
450 cmK1 [17] so this excess energy can be taken up by the
lattice resulting in the excitation of the 4I9/2 level revealed
through Stokes fluorescence at 798 nm. Emission lines are
observed at 380, 530, 551, and at 654 nm, which are
assigned to 4G11/2/4I15/2, 2H11/2/4I15/2, 4S3/2/4I15/2 and4F9/2/4I15/2 transitions, respectively. The green (530 and
551 nm) and red (657 nm) emissions are intense enough to
be seen by naked eyes even when the input power is only
Fig. 1. Upconversions observed in the spectrum of Er3C doped in
Li:TeO2 glass at different concentration of Er3C under 780 nm
excitation.
20 mW. The UV emission could be recorded only when the
input power was O150 mW.
In order to identify the excitation mechanism the
fluorescence spectra were recorded at different input laser
powers (P) and the fluorescence intensity (IF) of the various
bands were measured. A plot of log IF versus log P helped in
determining the number of photons involved in excitation of
the upper level of the upconversion process (Fig. 2). From
this plot it is seen that nw2.96 for the emission at 380 nm,
1.71 for the emission at 530 nm, 1.66 for emission at
551 nm, 1.94 for the emission at 654 nm and 0.91 for the
Stokes emission at 798 nm. It is concluded that three
incident laser photons are involved in the 380 nm
upconversion while only two laser photons participate in
the 530, 551 and 654 nm upconversions. The emission at
798 nm is a simple one photon fluorescence process.
The deviation of n from integral numbers is a
consequence of many factors, e.g. strong absorption at
780 nm, absorption of the upconverted fluorescence, and
involvement of non-radiative decays in populating the
fluorescing state. The upconversion peaks at 530 and
551 nm are almost two order of magnitude more intense
than the upconversion peak at 380 nm.
The upconversion measurements were made with glasses
with four concentrations of the rare earth. It is observed that
at Er3C concentration of 0.5 mol% in the glass, even though
the green fluorescence appears clearly, no UV fluorescence
is seen. At increased Er3C concentrations the UV
Fig. 2. Logarithmic plot of laser power versus upconversion
emission intensity for different lines under 532 and 780 nm
excitations.
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A.K. Singh et al. / Solid State Communications 136 (2005) 346–350348
fluorescence is also seen. The intensity of the upconversion
emissions is seen to increase with rare earth concentrations.
We observed that the upconversion emission intensity at
530 nm for the sample with xZ1.5 mol% is four times
larger than for the sample with 1 mol% and five times larger
than for the sample with xZ1 mol% (for red fluorescence).
The upconversion intensity decrease for higher concen-
tration of Er3 (R2 mol%).
3.2. Mechanism of upconversion
Different processes may populate the highly excited
Er3C states responsible for these different upconversion
transitions [18–21]. Since the upconversion is observed
even at low laser power, it cannot be due to simultaneous
multiphoton absorption. Therefore, these excitations must
be taking place either through multistep excited state
absorption (ESA) or by energy transfer (ET) between
different but close lying Er3C ions. Energy transfer may be
of importance in exciting the Er3C ions to higher levels as
one sees an increase in the intensity of the upconversion
peaks with increasing concentration of the rare earth. The
life times of 4I9/2, 4I11/2 and 4I13/2 levels is 9, 11 ms and
nearly 13 ms, respectively, and all three can serve as
intermediate state for ET. The transfer of energy from an ion
in the 4I13/2 level to another ion already excited to this level
will not be sufficient to take the second ion to an appropriate
level. However, energy transfer amongst the ions already
excited to 4I11/2 and/or 4I9/2 level will raise one of them to
Fig. 3. Energy level scheme for the upconversion processes in Er3C
doped Li:TeO2 glass.
Er3C level lying above 2H11/2 level (but below 2G11/2 level).
These excited ions relax to 2H11/2 and 4S3/2 levels and then
decay radiatively to yield the green fluorescence. Similar
mechanism has been observed in a large number of rare
earth systems including Er3C doped fluoroindate glasses
[21–28].
As regards the UV fluorescence at 380 nm we observe
that this is always accompanied by red fluorescence at
654 nm. The upper state of this 645 nm emission is 4F9/2
while the UV emission is from 4G11/2 level. We suggest the
following process for these simultaneous emissions.
Er3C�½4S3=2ð2H11=2Þ�CEr3C�½4S3=2ð
2H11=2Þ�
/Er3C�ð4G11=2ÞCEr3C�ð4F9=2Þ (1)
Er3C�ð4G11=2Þ/Er3Cð4I15=2ÞCUV fluorescence (2)
Er3C�ð4F9=2Þ/Er3Cð4I15=2ÞC red fluorescence (3)
Excitation of 4G11/2 level through direct excitation
involving excited state absorption from 4S3/2(2H11/2) or4F9/2 is also possible. The 4F9/2 level can also be populated
by non-radiative relaxation from the 4S3/3(2H11/2) level in
which case the red fluorescence will appear but UV
fluorescence would not. It is difficult to make a choice
between these alterative on the basis of the data available.
It is also possible that Er3C ions are excited to the2H11/2(4S3/2) level by absorption of an incident photon by an
Er3C ion in the 4I13/2 state. The increase in the fluorescence
intensity with rare earth concentration, however, favours the
ET process between excited ions. It may be that both these
processes partially contribute to the upconversion process.
3.3. Upconversion emission under 532 nm excitation
As mentioned earlier at lower pump power (!100 mW)
an excitation of the doped glass with 532 nm gives green
and weak red fluorescence. These emissions are due to
one photon fluorescence process. However, at higher powers
(O150 mW) ultraviolet emission at 380 nm is observed
along with the green and red fluorescence. Room
temperature upconverted and direct fluorescence spectra
under excitation by 532 nm laser beam is shown in Fig. 4.
The dominant emission is in the green region corresponding
to 4S3/2(2H11/2)/4I15/2 transition of Er3C and contributes
for more than 80% of the total emitted intensity. The
ultraviolet emission again is very weak compared to the
green and red emissions. The log I versus log P plot for
the different emissions in this case indicates that two
photons are involved in the UV fluorescence whereas the red
and the green fluorescence are one photon process.
Fig. 3 shows the relevant energy level scheme for the
Er3C ion together with possible upconversion pathways and
the observed fluorescence lines for both 780 and 532 nm
excitations (Fig. 4).
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A.K. Singh et al. / Solid State Communications 136 (2005) 346–350 349
3.4. Lifetime measurements
The lifetime of the 4S3/2 level responsible for the green
fluorescence on excitation with both 780 nm and with
532 nm radiations has been determined from the fluor-
escence decay curves (Fig. 5). From these curves the
lifetime under 780 nm excitation is found to be 567 ms,
while under 532 nm pumping it is measured as 589 ms. This
clearly indicates that in both a common upper state is
involved. The presently obtained lifetime of the 4S3/2 level
of Er3C is nearly the same as reported by other workers [29,
30].
Fig. 5. Fluorescence decay curves for 4S3/2 on 780 and 532 nm
excitations.
3.5. Radiative quantum yield and up conversion efficiency
The luminescence quantum yield hq at a particular
fluorescence wavelength is defined as
hq Zemitted light power
absorbed radiation power
This is conveniently approximated by wtexp
tR
(4)
where texp is the life time of the upper level measured
experimentally and tR is the theoretical radiative lifetime.
We have used tR calculated by the Judd Ofelt theory and
obtained hqw18.46% as the quantum yield.
The upconversion efficiency is defined as
hU ZPemitðvisÞ
PabsðNIRÞ(5)
where Pemit is the emitted light power in the visible region
and Pabs is the absorbed incident light power of the NIR
radiation.
Thus by comparing the upconversion luminescence
signal with the direct fluorescence signal, the upconversion
Fig. 4. Stokes fluorescence and upconversion emission observed in
the spectrum of Er3C doped in Li:TeO2 glass under 532 nm
excitation.
efficiency can be calculated. According to Vetrone et al. [1]
one can write
hU Z hq
PabsðvisÞ
PabsðNIRÞ
� �IemitðupconvertedÞ
IemitðdirectÞ
� �(6)
In the above formula, the absorbed light power Pabs(vis)
for direct excitation (532 nm pump) and Pabs(NIR) for
upconversion excitation (780 nm pump) were determined
from the measured incident light power. The luminescence
intensities Iemit(upconverted) and Iemit(direct) are the
emitted light at 553 nm corresponding to 4S3/2/4I15/2
transitions in both the cases measured under similar
experimental conditions. The upconversion efficiency hU
is found to be w2.1% which is consistent with the value
reported by Nii et al. [31].
3.6. Effect of temperature on luminecent properties of Er3C
The temperature variation in the range 300–473 K of the
NIR excited upconversion fluorescence intensity has been
studied (for the glass containing 1.5 mol% of Er3C). The
fluorescence signal versus temperature graphs are shown in
Fig. 6. It is clear that the fluorescence intensities at 380, 551
and 654 nm decrease with increasing temperature, the rate
of decrease being smallest for the peak at 645 nm. One can
conclude that relaxation rates for the corresponding upper
state of the different transitions change with temperature in
dissimilar fashion. Fig. 6 indicates that the effect of
temperature on the upper state of UV and green
(upconversions) is some what similar but the upper state4F9/2 causing the red upconverted emission is much less
affected. Further studies to understand this behavior are in
progress.
The intensities of the two close lying fluorescence lines
at w530 and w551 nm varies with temperature in similar
fashion indicating that the two excited states are efficiently
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Fig. 6. Variation of fluorescence signals with temperature for
different transitions.
A.K. Singh et al. / Solid State Communications 136 (2005) 346–350350
coupled thermally. Such levels have been used for
estimating the temperature of the glass [32,33].
4. Conclusions
Upconversions involving 2 and/or 3 photons have been
observed in Er3C doped lithium tellurite glass on pumping
with 780 and 532 nm radiations. Excitation by 780 nm yield
upconversion at 380 nm involving three photons while the
emissions at 530, 551 and 657 nm involve only two photons.
On 532 nm excitation upconversion at 380 nm is seen to
involve two photon while 530, 551 and 654 nm emissions
are one photon processes. The effect of temperature and
concentration on upconversion emission has also been
investigated.
Acknowledgements
Authors are grateful to Department of Science and
Technology Govt. of India, New Delhi for financial
assistance.
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