INTERACTION OF IR LASER RADIATION WITH
POLYVINYLIDENE FLUORIDE FILMS
Edward Bormashenko, Roman Pogreb, Yehoshua Socol, Mordechai Hakham Itzhaq,
Vladimir Streltsov, Semion Sutovski, Avigdor Sheshnev, Yelena Bormashenko
The College of Judea and Samaria, The Research Institute, 44837, Ariel, Israel
E-mail: Edward@ycariel.yosh.ac.il
PACS: 42.70Jk; 42.70Km, 42.82Cr; 42.79Bh
Abstract
The suitability of polyvinylidene fluoride films for IR integrated optics applications
was demonstrated. Polyvinylidene fluoride is a piezoelectric polymer, which
demonstrates high transparency in the middle-IR and has several transparent windows
in the far-IR. The fabrication of cylindrical microlenses and microlens arrays by CO 2
laser irradiation of polyvinylidene fluoride substrates has been demonstrated. Pseudospherical microlenses were also fabricated by direct laser writing. Strong piezoelectric
properties, high chemical resistance, stability to UV radiation and high continuous-use
temperature make PVDF suitable for various IR integrated optics applications.
Keywords: polyvinylidene fluoride, integrated optics, infrared, piezoelectric, CO2
laser, direct writing, microlens arrays.
1. Introduction
Integrated infrared optics devices are currently under intensive development
[1-3]. At the same time, selection of materials suitable for IR integrated optics
applications remains problematic. It is required of such material to be transparent in a
particular area of the IR band of the spectrum. Additionally, it faces well-known
difficulties in fabrication of integrated optics elements [1-4]. Arrays of microlenses
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for IR integrated optics applications are produced currently from chalcogenide glasses
[4-6]. But chalcogenide glasses suffer from inherent disadvantages that limit the
feasibility of their use; they are toxic, expensive and fragile materials. Chalcogenide
optical elements also suffer from Fresnel losses, due to their high refractive index.
We developed a process allowing use of IR-transparent high-grade polymer polyvinylidene fluoride (PVDF) for fabrication of cylindrical and pseudo-spherical
microlenses. Different groups have recently used polymer materials for the fabrication
of spherical microlenses recently [7-10]. The novelty of our approach lies in the fact
that PVDF demonstrates strong piezoelectric properties [11-12]. Thus applications of
the PVDF microlenses in various integrated optics schemes are possible, including
board-to-board or chip-to-chip optical interconnects in digital systems [13].
Cylindrical microlenses are applied for simultaneous irradiation or parallel frequency
readout of microsensor arrays [14-16]. All above-mentioned applications demand
precise positioning of the microlenses; thus there are clear advantages in piezoelectric
substrates allowing precise displacement of the microlens.
Microlenses are currently produced by different technologies: micromolding
[17], local melting of glass by CO2 laser [18], microplastic embossing process [1920], argon ion-beam etching [21] or direct laser writing [7]. We established that
PVDF is highly suitable for direct IR laser writing of cylindrical and pseudo-spherical
microlenses.
2. Experimental
2. 1. Physical properties of polyvinylidene fluoride
PVDF is a high-molecular-weight polymer of vinylidene fluoride with the
predominant repeating unit established as (-CH2-CF2-). It is a crystalline material with
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a melting point of 338 °C. Polyvinylidene fluoride is distinguished by its excellent
mechanical properties, summarized in Table 1.
Table 1. Mechanical, piezoelectric and optical properties of PVDF
Tensile strength
0.84 GPa
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
The lateral piezoelectric activity of PVDF films is much stronger than the
longitudinal activity [12], so piezoelectric modulus d33>d31~d32 (see Fig. 1). PVDF is
distinguished by its excellent chemical resistance, high stability to UV radiation, and
high continuous-use temperature. It is important to note that after poling in a high
electric field (50 MV/m), PVDF is pyroelectric as well as piezoelectric [11-12]. We
performed our experiments with non-poled PVDF films (trademark Kynar) supplied
by Westlake Plastics Inc, and poled PVDF supplied by Precision Acoustics Ltd. Both
kinds of polymers allow production of microlenses, thus enabling fabrication of
passive integrated optics elements using the non-poled substrates and active ones with
the use of poled films. Mechanical properties of poled and non-poled films are
generally the same [12].
Transmittance spectra of the non-poled and poled PVDF films with a thickness
of 25 µm were studied in the UV, visible and broad IR bands using spectrometers:
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Varian Cary 500 (UV, visible and near-IR) and FTIR Bruker 22 (middle and far-IR
bands).
2.2. Direct laser writing of microlenses and study of their properties
PVDF microlenses were obtained by irradiation of PVDF substrates with IR
radiation generated by a Sharplan 791&791 He/CO2 laser. Fabricated microlenses
were studied by optical and SEM microscopy (scanning electron microscope JSM35CF). Testing of the microlenses at a wavelength of 1.55 µm was performed with an
Agilent IR 81689A tunable laser. Visualization was carried out with a Newport IR
conversion screen.
3. Results and discussions
3. Transmittance spectra of poled and non-poled PVDF
We established that there is no difference in the visible spectra of poled and
non-poled PVDF substrates. Thus it will suffice to present the spectrum of one kind of
substrates. Fig. 2 presents the UV, visible and near-IR absorption spectrum of nonpoled PVDF with a thickness of 25 µm. It could be recognized that PVDF
demonstrates the transparency satisfactory for integrated optical applications starting
from the 1.1 µm wavelength.
Fig. 3 presents the middle and far-IR absorption spectrum of the non-poled
(curve A) and poled (curve B) PVDF film with a thickness of 25 µm. Both polymers
demonstrate two bands of sufficient transparency necessary for IR integrated optics
applications: 1.1-3 and 3.5-6.3 µm, and the narrow band, which lies in the nearest
vicinity of 10.6 μm (see Fig. 3). The latter is of special importance in the context of
possible microlens applications related to CO2 laser radiation. Beulieu and Lessard
[19] showed that PVDF films are suitable for infrared to visible image conversion at
the wavelength 10.6 μm. Transparency at the 1.55 µm wavelength is also important
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from the viewpoint of future applications of microlenses for optical communications
purposes.
Poled PVDF demonstrates one more relatively broad band of transparency:
12.8 - 16 μm, which makes possible far-IR applications of the material. Non-poled
PVDF presents a much more narrow band of transparency in the far IR: 14-15.5 μm.
Distinctions between the far-IR spectra of the poled and non-poled PVDF were
studied recently by Salimi and Yousefi [23].
First attempts to use piezoelectric PVDF substrates for the purposes of
adaptive optics were undertaken in the early 1990s by Sato [24-26]. However, there
was little progress in optical applications of PVDF because of its poor transparency in
the visible band, illustrated in Fig. 2. PVDF films become satisfactorily transparent at
wavelengths above 1.1 μm only, so we believe that PVDF films have a significant
potential for IR optics applications only.
3.2. Fabrication of cylindrical microlenses on PVDF substrates
In order to obtain cylindrical microlenses, we irradiated PVDF samples with
continuous radiation produced by IR laser, using the optical bench depicted in Fig. 4.
IR radiation generated by the CO2 laser was directed toward the polymer film through
an iris diaphragm (diameter 1.6 mm) and ZnSe lens. PVDF film was located in the
focal plane of the ZnSe lens. At first we established the energy densities necessary for
the thermal deformation of polymer films. We started with small power densities,
which didn’t cause any visible changes in polymer structure. Then we gradually
increased the intensity of the incident IR radiation. When the average power density
of the incident laser beam achieved 30 mW/mm2, thermal deformation of the samples
took place, and “craters” were formed on the surface of the substrate (Fig. 5). Power
densities necessary for the crater formation are still far from the ablation threshold,
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typical for polyolefines; hence, formation of the crater is due to the melting of the
substrate [27]. Temperatures produced in thin polymer films when irradiated with IR
laser radiation were previously studied by the authors and could be estimated as 230250 °C [28].
After establishing the parameters of IR irradiation necessary for the crater
formation, we pulled films exposed to the established threshold values with the
constant speed V = 100 - 150 mm/min in the plane normal to the laser beam. Thus
cylindrical microlenses with diameters of 300-100 µm respectively were obtained, as
shown in Fig. 6 (top). We also demonstrated the possibility of cylindrical microlens
array fabrication (Fig. 6 bottom), a fact that significantly expands the scientific and
technological value of the proposed method.
Profiles of the resulting microlenses were studied using the following
experimental technique: irradiated films were imbedded into acrylic polymer in an
upright position. After polymer consolidation, the sample was polished and a crosssectional view of the sample was studied with an optical microscope equipped with a
CCD camera. A typical cross-section of a cylindrical microlens is presented in Fig. 7.
It could be seen that the microlenses have an imperfect cylindrical shape. Shrink
necks were observed, formed as a result of the thermal deformation of PVDF (see Fig.
7). The microlens shape depends strongly on the parameters of the fabrication
process: pulling speed and intensity of the incident laser beam. Now we will
concentrate our efforts on obtaining the perfectly shaped microlenses. The roughness
of fabricated microlenses, studied by SEM microscopy (see Fig. 8), was established as
0.2-0.5 μm, which seems to be satisfactory for IR optics applications.
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We tested the ability of the fabricated PVDF cylindrical microlens to shape an
IR laser beam. Fig. 9 depicts conversion of an IR laser beam (wavelength 1.55 μm)
normally incident on the microlens into a straight line.
2.3. Direct IR laser writing of pseudo-spherical microlenses on PVDF substrates
We found that the direct laser writing process also allows fabrication of
pseudo-spherical microlenses on PVDF substrates, such as the example presented in
Fig.10. The microlens presented in Fig. 10 was produced by continuous irradiation of
immobile PVDF Kynar film by IR laser beam with the intensity 0.5 mW/mm2 during
1.5 s.
2.4. Precise positioning of microlenses, taking advantage of the piezoelectric
properties of PVDF substrates
The piezoelectric properties of PVDF make possible integrated optics
applications for the resulting cylindrical microlenses [13-16]. Precise positioning of
the microlens by use of DC voltage is possible. Let us estimate achievable
deformation: suppose that only one piezoelectric modulus is non-zero (this is of
course not exact, but it nevertheless gives the correct order of magnitude). Then,
starting from basic formulae, we come to the following relation between the relative
deformation (strain) u and the electric field strength E [29-30]:
u
where d31 ≈ 2 ·10
–11
d
E
4
Q/N is the piezoelectric modulus of PVDF, and ε=12 is its
dielectric constant.
6
For the electric field E  10 V
m
(which is e.g. 100 V across 100 μm-thick
film), we obtain u ~ 2 ·10–5. Electrical strength of PVDF is as high as

6
E  3.7  10 V
m
, so the discussed electric field is still far from the electrical
4 - 50
breakdown threshold [12]. So for a realistic linear dimension of the microlens array
a~1 cm, we obtain for the possible displacement of the microlens δ~au~200 nm. This
simple realistic calculation confirms that precise positioning of microlenses by
applying voltage is possible. Additionally, by using attached plates with opposite
directions of the piezoelectric modulus, the achievable deformation (in this case,
bending) can be increased by a factor of about a/t, where t is the element’s thickness.
Thus, the possible relative displacement u can reach values of u~ 10–3.
3. Conclusions
The possibility of direct writing of mid-infrared cylindrical microlenses on PVDF
substrates was demonstrated. PVDF is distinguished by its high thermal stability,
good mechanical and strong piezoelectric properties. UV, visible and IR transmittance
spectra of PVDF were studied. Microlenses fabricated according to the proposed
technology could be used for IR optics purposes in the bands: 1.1-3 and 3.5-6.3 µm.
PVDF also demonstrates good transparency in the nearest vicinity of 10.6 µm
wavelength; thus applications connected with CO2 lasers use are possible. The
proposed technology allows fabrication of cylindrical microlens arrays as well. The
profiles and roughness of the microlenses were studied. The piezoelectric properties
of PVDF substrates make possible integrated optics applications of the microlenses.
The possibility of pseudo-spherical microlens fabrication was also demonstrated.
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d33
d31
d32
poled PVDF film
Fig. 1
4 - 54
100
95
90
Transmittance, %
85
80
75
70
65
60
55
50
45
40
0.2 0.3 0.4 0.5
0.6 0.7 0.8 0.9 1
Wavelength, m
Fig. 2
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1.1 1.2 1.3 1.4 1.5
Fig. 3
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ZnSe lens
pulling
device
CO2 laser
diaphragm
polymer substrate
Fig. 4
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Fig. 5
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Fig. 6.
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microlens/substrate
contact line
shrink neck
microlens
cross-section
Fig. 7.
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2 μm
Fig. 8.
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incident IR laser beam
Fig. 9.
4 - 62
Fig. 10
4 - 63
Fig. 1. dik - piezoelectric moduli of poled PVDF substrates. Ei   d ik k .
k
Ei – components of the induced electric field, σk – components of applied
mechanical stress.
Fig. 2. UV, visible and near-IR transmittance spectrum of the poled
PVDF film (thickness of the sample: 25 µm).
Fig. 3. IR transmittance spectrum of the non-poled (curve A) and poled
(curve B) PVDF film (thickness of the samples: 25 µm).
Fig. 4. Optical bench used for the microlens fabrication.
Fig. 5. Thermal deformation of the PVDF film irradiated by CO2 laser
beam (beam intensity - 30 mW/mm2 ).
Fig. 6. Cylindrical microlens (top) and array of microlenses (bottom)
obtained by CO2 laser irradiation of PVDF films.
Fig. 7. Cross-section of cylindrical microlens.
Fig. 8. SEM image of the cylindrical microlens surface.
Fig. 9. Conversion of near-IR laser beam (wavelength 1.55 μm) into
straight line with cylindrical microlens.
Fig. 10. Pseudo-spherical microlens obtained by irradiation of PVDF
Kynar (beam intensity – 0.5 mW/mm2, irradiation time 1.5 s).
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