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: anantksingh@gmail.com (A.K. Singh),

sbrai49@yahoo.co.in (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

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.

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).

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

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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