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INTERACTION OF IR LASER RADIATION WITH POLYMER
PIEZOELECTRIC SUBSTRATES: GENERATION OF ACOUSTIC WAVES
WITH A LASER BEAM
Bormashenko Edward*, Pogreb Roman, Socol Yehoshua, Sutovski Semyon, Streltsov Vladimir,
Musina Albina, Bormashenko Yelena
The College of Judea and Samaria, The Research Institute, Ariel, 44837, Israel
1. INTRODUCTION
In our recent works we have shown already that IR laser radiation could be used for direct writing of microlenses on the
polymer piezoelectric substrates. Now we’ll discuss a possibility of acoustic waves generation in polymer piezoelectric
films with use of IR laser beam. Such a possibility makes possible realization of IR resonant waveguides. The infrared
(IR) waveguides were under intensive scientific research and technical development recently. The most commonly
encountered engineering decisions for IR lasers radiation transmission are AgCl(Br) fibers 1, chalcogenide glass IR
fibers2-3, and hollow plastic and glass waveguides4. A fresh approach to the IR radiation transmission was developed
recently by Temelkuran, Hart and Fink5. They designed and fabricated a hollow optical fiber lined with an interior
omnidirectional dielectric mirror. Thus confinement of light in the hollow core is provided by the large photonic
bandgaps established by the multiple alternating submicrometre-thick layers of a high-refractive-index glass and lowrefractive-index polymer6.
We propose a new approach for IR waveguides fabrication, based on the idea of resonant-grating waveguides. Guidedmoded resonances in planar dielectric layer systems were studied theoretically by Magnusson and Tamir 7-10. They have
shown that while an electromagnetic wave with a wavelength λ propagates along a dielectric waveguide with periodic
corrugated internal surface, strong resonance effects are predicted. Such resonance effects are known as Wood
anomalies. Similar effects are expected for refractive index periodic variations instead of surface corrugation, since the
governing formulas are nearly the same. Namely, in the former case the optical path varies due to change of distance, and
in the latter – due to change of wavelength. The resonance effects take place when λ ~ Λ where Λ is the corrugated
surface structure (or refractive index variation) period. In this case, leaky waves are supported by the relief
configuration7-10. The resonance width (FWHM) was estimated to vary from about 10 -3 down to about 10-5 under
different conditions 7-10.
Now we propose to produce the grating structure mentioned above as acousto-optical grating. The proposed scheme has
many advantages due to high tunability of the acousto-optical technique. Namely, exciting frequency (and therefore
grating period) can be adjusted with ppm accuracy to any value in the kHz – GHz region, enabling therefore tuning
(scaling) the given waveguide to many available lasers or other narrow-bandwidth sources. Alternatively, scanning
exciting frequency can be used in spectrometry applications.
Such waveguides may be based on polymer-(chalcogenide glass)-polymer sandwich composites developed, studied and
reported first by our group11-12. The advantage of this wavelength-scalable waveguide presented in this article lies in the
fact that polymer cladding (e.g., poly(vinyledene fluoride) (PVDF)) itself demonstrates strong piezoelectric properties.
Thus acousto-optical grating can be easily produced in such a way that resonance conditions for IR radiation propagation
are fulfilled.
Alternatively, single-layer PVDF light guides can be excited by electrical means in such a way that they will act as
resonant optical switches. Additionally it is possible to excite acousto-optical grating by means of periodic pulse laser
irradiation. In this case contactless switching can be obtained giving significant advantage in remote control and in
optical systems.
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Corresponding author. Phone: +972 3 9066134. E-mail address: [email protected]
2. EXPERIMENTAL
The proposed electrically controlled IR waveguide scheme is depicted in Fig. 1. The central layer made of chalcogenide
glass As20Se80 with a thickness of 100 μm is coated with PVDF facing layers with a thickness of 25 μm using the
pressing technology reported previously by our group11-12. Metal contacts are sputtered on the PVDF film (Fig. 1, other
contacts’ configurations discussed by Strashilov14 are also possible).
AC voltage
Gold contacts
Chalcogenide glass
PVDF layers
Fig. 1. Electrically controlled resonance IR waveguide.
The band of high transparency of As20Se80 glass is 1.5-15 μm. The dilatometrical softening temperature of the As20Se80
glass is 97 °C13. The heat deflection temperature of PVDF is established at 119 °C. This fact allows the use of pressing
technology with no change in PVDF physical properties. PVDF is a high-molecular-weight polymer of vinylidene
fluoride with the predominant repeating unit established as (-CH2-CF2-). Poled PVDF demonstrates extremely high
piezoelectric activity15. Mechanical and piezoelectric properties of PVDF are summarized in Table 1 14-16.
Table. 1 Mechanical, optical and piezoelectric properties of PVDF.
Elongation
160%
Elastic modulus
8.3 Gpa
Flexural modulus
5.9 Gpa
Continuous use temperature
130 °C
Specific gravity
1.78 g/cm3
Refractive index (λ=589.3 nm)
1.42
Piezoelectric modulus, d33
20-25 10-12 Q/N
Sound velocities
ct = 1.4 103 m/s, cl = 1. 9 103 m/s
Electromechanical coupling factor
0.24
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Lateral piezoelectric activity of PVDF films is much stronger than the longitudinal one 16, so piezoelectric modulus
d33>d31~d32 (see Fig. 2). The refractive index of PVDF is 1.42 when measured at λ = 589.3 nm (PVDF demonstrates
slight dispersion, so we believe that there is no dramatic change of the refractive index in the IR band), therefore it
provides satisfactory cladding for the chalcogenide glass core (refraction index 2.8). Cladding has to be sufficiently IRtransparent in order to avoid evanescent wave losses.
d33
d31
d32
Poled PVDF film
Fig. 2. dik - piezoelectric moduli of poled PVDF substrates. Ei 
d
ik  k
. Ei – components of the induced electric field, σk –
k
components of applied mechanical stress.
We studied the infrared absorption spectrum of PVDF film of 25 μm in thickness (supplied by Precision Acoustic LTD)
using a Bruker 22 FTIR spectrometer. The spectrum is presented in Fig. 3. There are three bands of satisfactory
transparency necessary for waveguide cladding: 1.5–6.5 μm, 12.8–15 μm and the narrow band, which lies in the nearest
vicinity of 10.6 μm (see Fig. 3). This band is of special importance in the context of possible applications connected with
CO2 laser radiation transmission.
Fig. 3. IR absorption spectrum of PVDF film (thickness of the sample 25 μm).
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When alternating voltage (at radio frequency) is applied to PVDF cladding (see Fig. 1), the surface acoustic wave (SAW)
is produced in the PVDF film14 (Fig. 4). Let us estimate SAW frequency that will bring to existence the resonance effects
described by Tamir and Magnusson7-10. The resonance propagation conditions are produced when the condition:
c
(1)
f
takes place, where λ is a wavelength of electromagnetic wave, Λ and f are wavelength and frequency of acoustic wave, c
– SAW velocity. SAW velocity c can be estimated roughly as 0.9 ct (see Table 1). So we obtain for a SAW frequency
estimation:
0.9ct
.
(2)
f 
λ
λ Λ
For the wavelength 1.5–6.5 μm we obtain f~0.2–1 GHz. Nagai and Nakamura reported high piezoelectric properties of
PVDF in the GHz acoustic band15. On the other hand, these frequencies are close to the limit of PVDF mechanical
endurance14. At GHz frequencies mechanical losses in PVDF are considerable, but for λ = 10.6 μm we obtain f ≈ 100
MHz, this frequency is usual for piezoelectric applications of PVDF films 14-16. And for the third transparency band we
obtain even lesser frequencies f ≈ 90 MHz necessary for providing resonance propagation conditions.
PVDF
Λ
IR radiation,
wavelength λ
Chalcogenide glass core
PVDF
Fig. 4. IR radiation propagation in PVDF coated chalcogenide glass when SAW is formed in PVDF cladding.
Let us discuss the second proposed scheme – refractive index periodic change by acoustic wave (Fig. 5). Fig. 5 presents
hollow PVDF waveguide, in which acoustic wave is generated by AV source. It is well known that refraction index is
stress-sensitive in many polymers17. E.g., for PVDF-HFA the value of dn/dp = 6.5 10-9 Pa-1 was reported18, and for poled
phase we can expect much stronger dependence. It can be shown that piezoelectric excitation can produce stress of about
105 Pa in 100-µm PVDF film if voltage of about 100 V is applied across the film. We obtain refraction index variation of
about 10-3 , which is a reasonable value for acousto-optical applications. It should be once more stressed that for poled
PVDF dn/dp may be much steeper.
PVDF film
~
Excitation
contacts
Compression
regions
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Rarefaction
regions
Fig. 5. Periodic change of a refractive index caused by an acoustic wave.
It is also possible to excite acousto-optical grating by periodic IR laser pulse radiation, as depicted on the Fig. 6. Pulse
sequence should have frequency of the desired acoustic wave. This can be achieved either by using laser in periodicpulse mode or in CW regime by appropriate chopper. Local instant heating leads to acoustic wave formation in the film.
Optical signal
transmitter
AO waveguide
AOG-exciting laser
Detector
Fig. 6. AOG with optical excitation (both fiber and planar waveguide configurations).
This effect was really obsereved using experimental set-up shown on the Fig. 7. PVDF films were irradiated with a pulse
CO2 IR laser (10.6 µm), peak power 1 MWatt, pulse energy 100 mJ, pulse width 50 nsec FWHM, repetition frequency 1
Hz, diameter of light spot focused with a ZnSe lens – 2 mm.
Poled
PVDF
ZnSe lens
CO2 laser (10.6 µm)
Contacts
Scope
Fig. 7. Experimental set-up for the observing acoustic wave formation in the PVDF film.
Fig. 8 presents the experimentally observed time dependence of the laser-induced stress, generated in a PVDF film with
25 µm thickness and dimensions 1212 mm during an irradiation with a CO2 laser pulse with 50 nsec FWHM, repetition
frequency 1 Hz.
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20
15
Amplitude (mV)
10
5
0
-5
-10
-15
-20
-5
0
5
10
15
20
25
Time (µs)
Fig. 8. Time dependence of the laser-induced stress, generated in a PVDF film.
The main problem of a proposed scheme is the necessity to achieve frequencies of hundreds MHz and higher as
mentioned above. Such frequencies cannot be obtained just by resonant acoustic oscillations of the optical switch itself.
Namely, for typical microelement size of 1 mm and sound velocity of 2000 m/s, we get eigenfrequency of about MHz
only. However, 100 MHz Nd-YAG lasers are now readily available, or alternatively chopping at high frequency may be
obtained by Kerr cells.
Therefore the possibility of electrically controlled IR waveguide was shown. Now we focus our efforts on the technical
realization of the proposed electrically controlled resonance IR waveguide.
3. CONCLUSIONS
Taking advantage of the polymer piezoelectric material PVDF as cladding over chalcogenide glass core, allows
fabrication of an electrically (acousto-optically) controlled IR wavegiude, which can act as fast and tunable optical
switch. Acousto-optical grating (AOG) excited at the waveguide edges makes the waveguide sharp-resonant (10-3 – 10-5
resonance width) and scalable in the broad middle and far-IR region. Alternatively, one-layer PVDF hollow waveguides
may be exploited. AOG may be excited as refractive index periodic change due to the acoustic wave pressure.
Additionally it is possible to excite AOG by irradiation of periodic-pulse laser. In this case optical switching becomes
fully contact-less. It was theoretically demonstrated that conditions for resonant scattering of the propagating IR wave
could be fulfilled in such a core/clad system.
ACKNOWLEDGEMENTS
The work was supported by the Israeli Ministry of Absorption.
REFERENCES
1.
2.
3.
L. Nagli, D. Bunimovich, N. Kristianpoller, A. Katzir, “Optical properties of mixed silver halide crystals and
fibers”, J. Appl. Phys., 74 (9), 5737-5741, Nov. 1993.
P. Klocek, M. Roth, R.D. Rock, “Chalcogenide glass optical fibers and bundles: properties and applications”,
Opt. Eng. 26 (2), 88-95, Febr. 1987.
J.S. Sanghera, I.D. Aggarwal, “Active and passive chalcogenide glass optical fibers for IR applications: a
review”, J. Non-Cryst. Solids, 257, 6-16, 1999.
4 - 42
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
A. Inberg, M.B. David, M. Oksman, A. Katzir, Croitoru N., “Theoretical model and experimental studies of
infrared radiation propagation in hollow plastic and glass waveguides”, Opt. Eng., 39 (5), 1316-1320, May
2000.
S.D. Hart, G. R. Maskaly, B. Temelkuran, P.H. Prideaux, J.D. Joannopoulos, Y. Fink, “External reflection from
omnidirectional dielectric mirror fibers”, Science, 296, 510-513, April, 2002.
B. Temelkuran, S. D. Hart, G. Benoit, J.D. Joannopoulos, Y. Fink, “Wavelength-scalable hollow optical fibers
with large photonic bandgaps for CO2 laser transmission”, Nature, 420, 650-653, December, 2002.
S. Tibuleac, R. Magnusson, “Reflection and transmission guided-mode resonance filters”, J. Opt. Soc. Am., 14
(7), 1617-1626, 1997.
S.S. Wang, R. Magnusson, J.S. Bagby, M.G. Moharam, “Guided-mode resonances in planar dielectric-layer
diffraction gratings”, J. Opt. Soc. Am., 7 (8), 1470-1474, 1990.
T. Tamir, Sh. Zhang, “Resonant scattering by multilayered dielectric gratings”, J. Opt. Soc. Am., 14 (7), 16071616, 1997.
R. Magnusson, S.S. Wang “New principle for optical filters”, Appl. Phys. Lett., 61 (9), 1022-1024, 1992.
E. Bormashenko, R. Pogreb, Z. Pogreb, S. Sutovski, “Development of new near-infrared filters based on the
“sandwich” polymer-chalcogenide glass-polymer composites”, Opt. Eng., 40, 661-662, 2001.
E. Bormashenko, R. Pogreb, S. Sutovski, “Development of a novel composite based on thermoplastic polymers
and low melting thermoplastic chalcogenide glasses”, Journal of Thermoplastic Composite Materials, 12, 511523, November 2002.
V. Kokorina, Glasses for Infrared Optics, CRC Press, 1996.
V.L. Strashilov, “Efficiency of PVDF thin films for excitation of surface acoustic waves”, Journal of Applied
Physics, 88 (6), 3582-3586, 2000.
M. Nagai, K. Nakamura, H. Uehara, T. Kanamoto, Y. Takahashi, T. Furukawa, “Enhanced electrical properties
of highly oriented PVDF films prepared by solid-state coextrusion”, Journal of Polymer Science B: Polymer
Physics, 37, 2549-2566, 1999.
V. Piefort, “Finite element modelling of piezoelectric active structures”, Doctoral thesis, Universite Libre de
Bruxelles, 2001.
M. Born, E. Wolf. Principles of Optics. Pergamon Press, Oxford, 1964.
Y. Kano, M. Okamoto, T. Kotaka, “Elongational flow birefringence of poly(methyl
methacrylate)/poly(vinilidene fluoride-co-hexafluoro acetone) blends”, Polymer 40 (1999) 2459–2463, 1999.
4 - 43