Infrared-to-visible CW frequency upconversion in Er3+-doped fluoroindate glasses

Article abstract of DOI:10.1063/1.116481

An effective CW pumped infrared-to-visible upconversion in Er³⁺-doped fluoroindate glass is demonstrated. InF₃ is obtained by fluoration in In₂O₃ at 400°C with NH₄H and HF in a platinum crucible. All fluoride components are then mixed and heated in a dry box under argon atmosphere at 700°C for melting and 800°C for finning. Next, the melt is poured and cooled into a preheated brass mold. Continuous-wave upconversion fluorescence measurements are performed using a diode laser emitting at 1.48μm as the excitation source.

Full text of DOI:10.1063/1.116481

Infraredtovisible CW frequency upconversion in Er3+doped fluoroindate glasses
Cid B. de Araújo, L. S. Menezes, G. S. Maciel, L. H. Acioli, A. S. L. Gomes, Y. Messaddeq, A. Florez, and M. A.
Aegerter
Citation: Applied Physics Letters 68, 602 (1996); doi: 10.1063/1.116481
View online: http://dx.doi.org/10.1063/1.116481
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3؉
Infrared-to-visible CW frequency upconversion in Er -doped fluoroindate
glasses
Cid B. de Ara u´ jo, L. S. Menezes, G. S. Maciel, L. H. Acioli, and A. S. L. Gomes
Departamento de F ´ı sica, Universidade Federal de Pernambuco, 50670-901 Recife PE, Brazil
a)
b)
Y. Messaddeq, A. Florez, and M. A. Aegerter
Instituto de F ´ı sica de S a˜ o Carlos, Universidade de S a˜ o Paulo, 13560-970 S a˜ Carlos SP, Brazil
͑
Received 22 May 1995; accepted for publication 21 November 1995͒
We report on efficient frequency upconversion in Er3 -doped fluoroindate glasses. The process is
observed under 1.48 ␮m laser diode excitation and results in the generation of strong blue ͑ϳ407
nm͒, green ͑ϳ550 nm͒, and red ͑ϳ670 nm͒ radiation. The main mechanism that allows for
upconversion appears to be the energy transfer among Er3 ions in excited states. The results
illustrate the large potential of this new class of glasses for photonic applications. © 1996
American Institute of Physics. ͓S0003-6951͑96͒02905-4͔
ϩ
ϩ
Presently there is a great interest in luminescent materi-
als for efficient frequency conversion of infrared radiation
into visible light. Among the mechanism that have been
offers useful information for the development of visible la-
sers pumped by infrared laser diodes.
5
The samples used have the following compositions:
(mol %) 39-x InF3; 20 ZnF2; 16 BaF2; 20 SrF2; 2 GdF3;
2 NaF; 1 GaF3; x ErF3 (xϭ1,2,3). They were prepared
following the procedure given in Ref. 10. Briefly, InF3 was
obtained by fluoration in In2O3 at 400 °C with NH4F and HF
in a platinum crucible. Then, all fluoride components were
mixed and heated in a dry box under argon atmosphere at
exploited for upconversion, stepwise excited state absorption
͑
ESA͒ and energy transfer ͑ET͒ involving rare earth ͑RE͒
1
–6
ions in a solid may present very large efficiencies. These
phenomena are useful for detection of infrared radiation by
changing this light to the visible range where detectors are
more efficient. Also, the operation of upconversion lasers
that emit in the blue-green region is another attractive appli-
cation to obtain short-wavelength sources that can be
pumped with diode lasers.
7
00 °C for melting and 800 °C for finning. After this process,
the melt was poured and cooled into a preheated brass mold.
The samples obtained have good optical quality and volumes
of a few cubic centimeters.
3
ϩ
The Er ion is perhaps the most studied RE ion due to
its laser transition, which is also exploited for optical ampli-
The absorption spectra show broad features that can be
identified with transitions from the ground state to the ex-
cited states of the Er3 ion. The positions of the bands and
7
fication at 1.55 ␮m. For this application, ESA and ET are
ϩ
deleterious effects that may affect the performance of the
amplifier. On the other hand, the studies of upconversion
their relative intensities agree with previous reports in other
materials.1
–6,8͑a͒
The spectra obtained for the three samples
3
ϩ
processes in Er -doped materials are of large interest be-
3
ϩ
studied are similar except for the bands intensities and their
linewidths, which are dependent on the Er3 concentration.
The bandwidths of several Angstroms observed are due to
the inhomogeneous broadening caused by the crystalline
field.
cause Er exhibits strong fluorescence in the visible and
infrared regions, and these transitions can be enhanced by a
ϩ
8
proper choice of the host material.
Among the new materials available to date, the fluoroin-
date glasses are emerging as a promising group of halide
9
–13
Continuous-wave upconversion fluorescence measure-
ments were performed using a diode laser emitting at 1.48
glasses for photonic applications.
These glasses present
high transparency from ϳ200 nm to 8 ␮m, are resistant to
␮m as the excitation source. The sample fluorescence was
atmospheric moisture, and are capable of incorporating large
concentrations of RE ions to the matrix. Recent studies1
1–13
dispersed by a 0.25 m grating spectrometer and detected by a
photomultiplier using either a lock-in or a digital oscillo-
scope. All measurements were performed at room tempera-
ture.
have shown their large potential to be used as optical con-
verters, laser hosts, or optical amplifiers, because of the
lower multiphonon emission rates and higher fluorescence
efficiencies for the doping RE ions than is observed when
they are doping other glasses.
Figure 1 shows the upconversion fluorescence spectrum
2
of one sample under 5.6 mW excitation (ϳ170 W/cm ).
The observed emissions correspond to transitions in the
In this letter, we report, for the first time to our knowl-
edge, an efficient CW pumped infrared-to-visible upconver-
3ϩ
Er ions from the excited states to the ground state. The
emissions in the visible range correspond to the following
3
ϩ
sion in Er -doped fluoroindate glass. The present study also
2
4
2
4
transitions:
(
(
H9/2→ I15/2(ϳ407 nm);
H
→ I
11/2
15/2
ϳ530 nm); 4S → I15/2(ϳ550 nm); and F9/2→ I
4
4
4
3/2
15/2
a͒Present address: Departamento de Qu ´ı mica, Universidade do Estado de
S a˜ o Paulo, 14800-900 Araraquara, SP, Brazil.
ϳ670 nm). The lines at ϳ550 and ϳ670 nm are readily
b͒
visible to the naked eye. Broad spectral features in the region
On leave from the Departamento de F ´ı sica, Universidade Industrial de
4
4
Santander, A.A.678. Bucaramanga, Colombia.
ϳ780 to ϳ900 nm corresponding to I9 → I
͑ϳ808 nm;
/2
15/2
602
Appl. Phys. Lett. 68 (5), 29 January 1996
0003-6951/96/68(5)/602/3/$10.00
© 1996 American Institute of Physics
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1

FIG. 1. Upconverted fluorescence in the visible spectrum. The excitation
wavelength was 1.48 ␮m in resonance with the transition 4I15/2→ I
4
13/2
͑sample with xϭ3͒. The intensities of ͑b͒ and ͑c͒ have been multiplied by
5
0 and 40, respectively.
ϳ827 nm͒ and 4S3/2→ I
4
͑ϳ854 nm͒ were also observed,
13/2
but their intensities are smaller than the visible transitions.
The spectra were analyzed with respect to their pump
power dependence and temporal behavior. To analyze the
results, we first note that for any unsaturated upconversion
mechanism, the fluorescence signal, I , will be proportional
s
to some power, n, of the excitation intensity such that I
ϰI , where nϭ2,3,4,... is the number of photons absorbed
per unconverted photon emitted. In our experiments, the de-
pendence of the fluorescence signal on the excitation inten-
sity was such that 3.6ϽnϽ3.9 for the emission at 407 nm;
FIG. 2. Fluorescence output intensity as a function of the input laser inten-
sity ͑sample with xϭ2͒. Straight lines with different slopes n are obtained
for each wavelength: ͑a͒ nϭ3.6 ͑407 nm͒; nϭ2.7 ͑547 nm͒; nϭ2.4 ͑662
nm͒; ͑b͒ nϭ1.8 ͑808 nm͒, nϭ1.8 ͑827 nm͒, and nϭ3.3 ͑854 nm͒.
s
n
2
.4ϽnϽ2.7 for the 550 and 670 nm bands; 1.7ϽnϽ2.0 for
because the non-radiative decay from higher lying states to
the fluorescent states may also contribute to the intensities of
the observed spectral lines.
the 808 and 872 nm bands, and 2.4ϽnϽ3.3 for the 854 nm
emission. The data for one of the samples are shown in Fig.
Figure 3 shows the relevant Er3 energy levels together
with two possible upconversion processes and the observed
fluorescence lines.
ϩ
2. From the intensity dependence observed and the wave-
length of the emitted radiations, we conclude that four laser
photons are involved in the 1.48 ␮m to 407 nm conversion,
three laser photons participate in the 1.48 ␮m to 550 nm,
Different processes may lead to the population of highly
excited Er3 states after excitation in the near infrared.
ϩ
1–6
1.48 ␮m to 670 nm, and 1.48 ␮m to 854 nm conversions,
and two incident photons produce the 1.48 ␮m-to-808 nm
and 1.48 ␮m to 827 nm upconversions. The deviations from
the exact n values are due to the strong absorption at 1.48
These processes rely either on multistep ESA or ET between
3ϩ
Er neighbors. The ET process in which an excited ion
nonradiatively transfers its energy to an already excited
neighbor is one of the most efficient mechanisms, and has
␮m and the absorption of the unconverted fluorescence, and
Appl. Phys. Lett., Vol. 68, No. 5, 29 January 1996
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88.227.189.67 On: Tue, 29 Apr 2014 09:28:45
Ara u´ jo et al.
603
1

In order to understand the observed behavior, we first
recall that the efficiency of upconversion depends on the
probability of ESA or ET between adjacent excited ions, as
well as the quantum efficiency of the emitting level. By ei-
ther process, the dynamics of the upconversion signals de-
pends on the lifetime of the intermediate excited states in-
volved. For the samples used, the lifetime of the states
2
4
4
4
H9/2
,
S3/2
,
F9/2 , and I
were reported in Ref. 12.
11/2
3ϩ
The values obtained for the same range of Er concentra-
tions were ␶( H )ϳ15–20 ␮s, ␶( S3/2)ϭ86–573 ␮s,
␶
lifetimes of the states I9 and I
on the basis of the results for other host materials,
2
4
9/2
4
4
( F )ϭ302–645 ␮s, and ␶( I11/2)ϭ10.6–9.4 ms. The
9/2
4
4
were not measured, but
13/2
/2
1
–6,8͑a͒
we
expect that they are of the same order of magnitude as
4
␶
( I11/2). Such long lifetimes are determined by the mul-
tiphonon relaxation rates, which are small because of the
small phonon energies associated with the fluoroindate ma-
4
4
trix. Thus, considering that the states I9 and I
are
13/2
/2
likely to participate in the upconversion processes, we con-
clude that the long values observed for ␶ and ␶ , provide a
r
d
favorable evidence for the relevance of the ET mechanism.
Although ESA may also contribute for the generation of
the upconverted radiation, its contribution is expected to be
very small in the present case. For samples with smaller
FIG. 3. Simplified energy levels scheme for Er3ϩ in the fluoroindate glass.
The downward arrows indicate the observed upconverted fluorescence and
the curved arrows on the right side represent energy transfer. The letters
beside the straight arrows correspond to the following spectral lines: A
3ϩ
Er concentration, both processes may become equally rel-
3ϩ
evant, and the selection of the proper Er concentration is
an important step for each kind of application.
(
ϳ530 nm); B(ϳ550 nm); C(ϳ670 nm); D (ϳ808 and ϳ827 nm͒;
1
D (ϳ854 nm); and E(ϳ407 nm).
2
This work was supported by the Brazilian Agencies Con-
selho Nacional de Desenvolvimento Cient ´ı fico e Tecnol o´ gico
been observed in a larger number of systems including flu-
͑CNPq͒, Financiadora Nacional de Estudos e Projetos
oroindate glasses.1
2,13
This mechanism can arise from elec-
͑FINEP͒, and Funda c¸ ao de Apoio a Pesquisa ͑FACEPE͒. We
˜ `
tric multipole or exchange interactions, and its rate constant
depends on the ion–ion separation. Here, we expect that ET
is the dominant process because of the large Er3 concentra-
tion in our samples, and because the intermediate ESA step
also thank Blenio J. P. da Silva for polishing the samples and
TELEBRAS for the loan of the diode laser used.
´
ϩ
4
2
(
I13/2→ H11/2) is a two-photon transition with small prob-
ability to occur due to the weak laser intensity used. There-
fore, the most relevant pathway for upconversion initiates
1
B. R. Reddy and P. Venkateswarlu, Appl. Phys. Lett. 64, 1327 ͑1994͒.
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4
4
with the transition I → I . Afterwards, ET between
2
15/2
13/2
3
ϩ
4
two excited Er ions at the I
level will take one ion to
3/2
Phys. Lett. 65, 1874 ͑1994͒.
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¨
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9
/2
4
3/2
4
A. S. L. Gomes, C. B. de Arau
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4
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3
H9/2
.
After nonradiative decay to the states
5
6
7
8
2H1
1/2
4S
3/2 9/2
4F9/2 , and I , radiative transitions to the
4
,
,
ground state give rise to the observed upconverted visible
fluorescence. The infrared emissions are due to transitions
E. Desurvire and J. R. Simpson, J. Lightwave Technol. 7, 835 ͑1989͒.
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4
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and S → I
.
15/2
3/2
13/2
9
To characterize the temporal evolution of the signals,
another series of experiments was performed. The laser beam
was chopped at 8 Hz and the fluorescence was observed with
a time resolution better than 1 ms. The signal corresponding
to the various upconverted emissions grew to their maximum
value in ␶ Ͻ15 ms and decay in ␶ Ͻ6 ms. In general, the
4
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0
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11
r
d
12
13
rise and decay times decrease for increasing concentrations,
and for the range of Er3 concentrations studied, ␶ and ␶
ϩ
d
r
L. E. E. de Araujo, A. S. L. Gomes, Cid B. de Araujo, Y. Messaddeq, A.
´
´
change up to ϳ50%.
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Appl. Phys. Lett., Vol. 68, No. 5, 29 January 1996
Ara u´ jo et al.
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1