Journal of Non-Crystalline Solids 270 (2000) 265±268
www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Wavelength selective materials modi®cation of bulk As2S3 and
As2Se3 by free electron laser irradiation q
P. Hari a,*, C. Cheney a, G. Luepke a, S. Singh a, N. Tolk a, J.S. Sanghera b,
I.D. Aggarwal b
a
Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
b
Naval Research Laboratory, Code 5606, Washington, DC 20375, USA
Received 18 January 2000
Abstract
In this study we report ®rst measurements of wavelength-selective infrared-induced materials modi®cation of bulk
As2 S3 and As2 Se3 . These materials are currently being considered as candidate materials for infrared optical ®ber
transmission in the range of 1±10 lm. Our study is aimed at modifying oxygen, hydrogen and carbon impurities bound
to chalcogenide constituent elements in the materials to reduce absorption. Tunable infrared radiation from the W.M.
Keck Free Electron Laser (FEL) at Vanderbilt was used to excite speci®c vibrational modes, S±O±H and CHx modes in
bulk As2 S3 and Se±H, CHx and S±H2 modes in bulk As2 Se3 . Changes in vibrational mode amplitudes are monitored by
measuring the intensity of the Fourier transform infrared (FTIR) spectra before and after irradiation at appropriate
wavelengths. By tuning wavelengths to hydrogen vibrational modes, we ®nd evidence that hydrogen is released and/or
redistributed athermally. In particular, following irradiation at speci®c resonant wavelengths, vibrational mode amplitudes as monitored by FTIR associated with CHx are signi®cantly reduced in bulk As2 S3 and As2 Se3 samples. In
As2 S3 , the changes in CHx modes are reversed by heat treatment at 115°C for 35 min in nitrogen atmosphere. Ó 2000
Elsevier Science B.V. All rights reserved.
1. Introduction
Chalcogenide compounds of some elements
belonging to groups 4B and 5B in the Periodic
Table are excellent glass formers [1]. Fibers of
q
This paper was presented at the 18th International Conference on Amorphous and Microcrystalline Semiconductors ±
Science and Technology (ICAMS 18), Snowbird, UT, USA,
Aug. 1999.
*
Corresponding author. Present address: Department of
Physics, California State University, Fresno, CA 93740, USA.
Tel.: +1-559 278 7096; fax: +1-559 278 7741.
E-mail address: [email protected] (P. Hari).
chalcogenide glass core and cladding are being
developed presently by the Naval Research Laboratory (NRL) for chemical sensing purposes, and
for CO and CO2 laser power delivery [2]. In particular, As2 S3 and As2 Se3 ®bers are transparent at
infrared wavelengths (1±10 lm), and have excellent chemical durability and glass stability [2,3].
This capability has been enhanced by the recent
possibility that the elements As, S and Se have
been produced commercially with 99.9999% of the
elements for ®ber production [4]. However, the
surface of each element becomes oxidized or hydrated during handling [3]. These impurities will
contribute to transmission loss when bulk As2 S3
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 0 7 2 - 7
266
P. Hari et al. / Journal of Non-Crystalline Solids 270 (2000) 265±268
and As2 Se3 are formed and utilized to fabricate
®bers.
Our motivation in this work is to investigate the
possibility of non-thermally altering the material
in such a way as to reduce impurity complex
concentrations and consequently to reduce losses
due to absorption. Our approach is to irradiate
As2 S3 and As2 Se3 samples with infrared laser light
tuned to oxygen, hydrogen and carbon vibrational
modes which may stimulate impurity release and/
or redistribution due to localized, athermal wavelength-selective absorption processes.
2. Experimental
Samples of As2 S3 and As2 Se3 used in these experiments were produced by standard melt
quenching techniques (produced at the Naval
Research Laboratory (NRL), Washington, DC).
Details of sample preparation are well documented
[1] and will not be reproduced here. The samples
used in the measurements were cut into discs of
0.5 cm approximate thickness. The typical diameter
of discs used in the study was 1 cm. Samples were
cleaned using de-ionized water and polished prior
to irradiation. For irradiation and Fourier transform infrared (FTIR) measurements, the samples
were mounted on an aluminum platform. To ensure that the same area of the sample exposed to
irradiation was measured by FTIR techniques, a
circular aperture of 4 mm diameter was made on
the aluminum frame. All irradiation was limited to
the portion of the sample within the aperture.
The free electron laser (FEL) provides light up
to 10 MW, tunable from 1.2 to 0.12 eV. This laser
delivers a 30 Hz train of 2.5±5 ls macro pulses,
each of which is composed of a train of less than 1
ps micro pulses separated by 350 ps. In our studies,
the typical FEL power was of the order of 0.7 W.
For all the measurements reported in this work,
the typical exposure to irradiation was 45 min.
For bulk As2 S3 and As2 Se3 samples, wavelengths in the range of 2±6 lm were used for irradiation. To monitor changes in FEL power
during irradiation, a re¯ected beam was monitored
continuously during operation using an optical
pyrometer. To measure the heating due to the FEL
beam, a thermocouple was attached to the surface
of a bulk sample. During measurements, the FEL
beam line was purged by a steady ¯ow of dry nitrogen gas which was necessary to ensure that infrared absorption due to water vapor did not
occur during beam delivery. All FTIR measurements reported here were performed using a
commercial high resolution spectrometer.
3. Results
Our major results are summarized in Figs. 1±3.
In As2 S3 , irradiation at 2.88 lm resulted in an
increase of the amplitudes of the O±H and C±Hx
vibrational modes. A subsequent irradiation at 3.4
lm decreased the C±Hx modes and no changes in
the O±H modes was detected (Fig. 1). We have
also studied irradiation e€ects at 4 and 6 lm on the
amplitudes of impurity vibrational modes. After
an hour of irradiation no changes in the FTIR
Fig. 1. FTIR spectra of As2 S3 at selected irradiation are shown.
The arrow indicates the speci®c wavelength of irradiation: (a)
before irradiation; (b) after irradiation at 2.88 lm; (c) after irradiation at 3.4 lm.
P. Hari et al. / Journal of Non-Crystalline Solids 270 (2000) 265±268
Fig. 2. FTIR spectra of As2 Se3 at selected irradiation are
shown. The arrow indicates the speci®c wavelength of irradiation: (a) before irradiation; (b) after irradiation at 2.88 lm;
(c) after irradiation at 3.4 lm.
spectrum was detected at these wavelengths. Based
on these measurements we conclude that the
changes observed are wavelength-selective. The
changes in amplitudes of the vibrational modes
were also dependent on FEL power. We observed
no changes upon irradiation for output FEL
power levels less than 0.4 W. We increased the
FEL exposure time at such intensities to 110 min.
Measurements after this exposure did not produce
any detectable changes in the FTIR spectrum. We
monitored the temperature changes upon irradiation and, in every case studied, the temperature
changes were of the order of 1°C.
We also investigated the changes in CHx impurity vibrational modes in As2 Se3 at 2.88 lm. In
bulk As2 Se3 , irradiation at 2.88 lm increased the
C±Hx mode amplitude. Further irradiation at 3.4
lm resulted in a ®vefold increase in the C±Hx
vibrational mode amplitude (Fig. 2). The e€ects of
annealing on As2 S3 after these irradiations were
also investigated. For As2 S3 , irradiation at 3.5 lm
resulted in the reduction of C±Hx vibrational mode
267
Fig. 3. FTIR spectra of the e€ects of irradiation and annealing
on As2 S3 are shown. The arrow indicates the speci®c wavelength of irradiation: (a) before irradiation; (b) after irradiation
at 3.5 lm; (c) after annealing at 100°C for 35 min.
amplitudes (Fig. 3) at 3.52 and 3.4 lm. After annealing at 100°C for 35 min these modes were restored to their original amplitudes. We increased
the time of heat treatment to 45 and 65 min. FTIR
spectra of the bulk samples after such heat treatments did not yield any measurable change.
4. Discussion
The results presented here are the ®rst report, to
our knowledge, of wavelength-selective infrared
radiation induced materials alterations in chalcogenide glasses. Previous reports on FTIR studies
of chalcogenide glasses dealt only with characterizing the various vibrational modes associated with
hydrogen, oxygen and carbon impurities [5±8]. The
initial measurements reported here on bulk As2 S3
and As2 Se3 indicate that changes in the materials
are indeed wavelength dependent. This result along
with the observation that no temperature increase
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P. Hari et al. / Journal of Non-Crystalline Solids 270 (2000) 265±268
is measured by the thermocouple on the sample
upon irradiation is evidence that the changes observed are achieved athermally. Even after illumination at an intensity less than 0.4 W, there was no
observable change in absorption intensity. Recently a systematic increase in Si±H vibrational
modes upon illumination with light has been observed in hydrogenated amorphous silicon by Yiping et al. [9]. These observations performed on
various amorphous materials indicate that the underlying disordered structure may be responsible
for these large changes in the absorption amplitudes of these vibrational modes. One interesting
aspect of these changes is that the intensity of CHx
modes is more strongly a€ected than other modes.
Our preliminary study shows that such changes are
quite consistent with the changes we observed with
short time irradiation. The preliminary work described here provides motivation to undertake a
more detailed systematic study of changes in vibrational mode amplitudes associated with impurity
absorption in chalcogenide glasses.
5. Conclusions
We have demonstrated that materials alterations in chalcogenide glasses can be achieved by
irradiating at FEL wavelengths corresponding to
the wavelength of vibrational modes due to impurities. The changes observed in vibrational mode
amplitudes at particular wavelengths and upon
heat treatment we attribute to changes in the
metastable disordered structure. These studies
represent a novel and unique approach to altering
impurity complexes and consequently reducing
absorption in a non-thermal manner.
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