Photo-patterning of the quadratic optical properties of doped photopolymers

Article abstract of DOI:10.1016/j.cplett.2003.07.005

We present an original approach to structure the quadratic non-linear optical properties of doped polymers based on photopolymerizable resins. The orientation of the push-pull molecules incorporated in the material is locked by the polymerization process induced by an exposure to a visible light. The freeze of the orientation was investigated through second harmonic generation and infrared spectroscopy measurements. A mean relaxation time of over 4200 h was obtained with co-polymerizable chromophores. With the purely optical method we developed, one can durably photo-pattern the quadratic optical properties of photopolymers for applications in integrated optics.

Full text of DOI:10.1016/j.cplett.2003.07.005

Chemical Physics Letters 379 (2003) 203–208
www.elsevier.com/locate/cplett
Photo-patterning of the quadratic optical properties
of doped photopolymers
a
Fabrice Gillot , Loıc Mager , K.D. Dorkenoo , Stephane Mery ,
a,
a
b
*
€
Christiane Carre , Alain Fort
ꢀ
ꢀ
c
a
ꢀ
a
IPCMS, Groupe d’Optique Non Lineꢀaire et d’Optoeꢀlectronique, CNRS UMR 7504, 23 rue du Loess,
BP 43, 67034, Strasbourg Cedex 2, France
b
IPCMS, Groupe des Mateꢀriaux Organiques, CNRS UMR 7504, 23 rue du Loess, BP 43, 67034, Strasbourg Cedex 2, France
c
Deꢀpartement de Photochimie Geꢀneꢀrale, CNRS UMR 7525, 3 rue A. Werner, 68093, Mulhouse Cedex, France
Received 23 April 2003; in final form 25 July 2003
Published online: 10 September 2003
Abstract
We present an original approach to structure the quadratic non-linear optical properties of doped polymers based on
photopolymerizable resins. The orientation of the push–pull molecules incorporated in the material is locked by the
polymerization process induced by an exposure to a visible light. The freeze of the orientation was investigated through
second harmonic generation and infrared spectroscopy measurements. A mean relaxation time of over 4200 h was
obtained with co-polymerizable chromophores. With the purely optical method we developed, one can durably photo-
pattern the quadratic optical properties of photopolymers for applications in integrated optics.
Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction
quadratic optical properties of the medium depend
on the orientation of the NLO molecules [5],
waveguides [6,7] or periodical structures for quasi-
phase matching [8–10] can also be elaborated by
patterning the areas where the chromophores are
poled. Many techniques, such as contact poling
with patterned electrodes [6–8], corona discharge
through a grounded metallic mask [9], photother-
mal poling [10] or photoassisted poling [11,12],
have been proposed to control the spatial distri-
bution of the oriented areas.
Polymers doped with push–pull chromophores
are of special interest to elaborate organic non-
linear optical (NLO) devices [1,2]. In order to
make such optical devices, the doped polymers are
usually patterned using classical microlithography
techniques [3,4]. An alternative approach to build
up optical devices is the direct spatial control of
the orientation of the NLO chromophores em-
bedded in matrices. Indeed, since the linear and
In this Letter, we develop a low cost and easy
approach to pattern the orientation of chromo-
phores, based on photopolymerizable resins. Pre-
vious studies on such materials [13–22] have shown
* Corresponding author. Fax: +33-3-88-10-72-45.
E-mail address: mager@ipcms.u-strasbg.fr (L. Mager).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2003.07.005

204
F. Gillot et al. / Chemical Physics Letters 379 (2003) 203–208
their ability to freeze the orientation of NLO
molecules but, to our knowledge, no experiment
has been reported concerning the photopatterning
of the quadratic NLO properties. In this work, we
demonstrate that photopolymerizable resins allow
the creation of long lifetime molecular patterns at
a microscopic scale. The procedure freezes ori-
ented areas by appropriate illumination patterns
and consequently allows to create complex struc-
tures by self-implementation. After a brief de-
scription of the compounds used, a first part will
be devoted to the improvement of the length of
time over which the push–pull chromophores re-
main oriented. A second part will address the
possibility to create oriented NLO structures by
photopatterning the materials.
(Irgacure 784, Ciba), a photoiniator sensitive in the
blue/green part of the spectrum (see Fig. 1). The
polymerization of this formulation leads to a poly-
mer network with a high glass transition tempera-
ture (Tg ¼ 272 °C for a complete polymerization)
and to homogenous polymer films of good optical
quality.
Two push–pull chromophores, denominated,
respectively, EHOMONS and AcEONS, were
synthesized to dope the resin. In order not to in-
teract with the photopolymerization process, they
are transparent at the actinic wavelength used (514
nm), as shown in Fig. 1. The molecule EHO-
MONS is employed in a guest–host configuration.
The ethyl-hexoxy pendant on the left-hand side
of the molecule improves its solubility, allowing
more than 20 wt% doping of the photopolymer-
izable mixture. EHOMONS was prepared by us-
ing an etherification reaction (Williamson reaction)
between the tosylester of 2-ethyl-1hexanol and
2-methoxy-4-[(1E)-2-(4-nitrophenyl)ethenyl]phenol
(67% yield). The later compound was synthesized
by using a modified procedure condensing vanillin
with 4-nitrophenylacetic acid in DMF and sub-
sequent decarboxylation catalyzed by piperidine
2. Materials
The photopolymerizable formulation used in this
study combines a tri-functional monomer: tris(2-
hydroxy ethyl) isocyanurate triacrylate (SR368,
Cray Valley) and 2 wt% of bis(g5-cyclopentodienyl)
bis-[2,6-difluoro-3(1H-pyrrol-1-yl)phenyl]-titanium
Fig. 1. Chemical structure of the photoinitiator Irgacure 784 and of the chromophores EHOMONS and AcEONS as well as their
normalized absorption spectra.

F. Gillot et al. / Chemical Physics Letters 379 (2003) 203–208
205
where s and b are the KWW parameters. A mean
relaxation time of the chromophores was defined
by:
(84% yield). In the AcEONS molecule, an acrylate
function replaces the ethyl-hexoxy pendant to en-
able covalent bonding to the polymer backbone
during polymerization. However, the solubility of
this chromophore in the resin is limited to 1 wt%.
AcMONS was prepared in one step by using an
etherification reaction (Mitsunobu reaction) be-
tween the commercially available 40-hydroxy 4-
nitrostilbene and 2-hydroxyethyl acrylate (75%
yield). The quadratic optical non-linearities lb
(where l is the dipole moment and b the vector
part of the quadratic hyperpolarizability tensor) of
both molecules AcEONS and EHOMONS were
characterized with the conventional EFISH tech-
nique [23]. We found the static values lb0 ¼ 216
and lb0 ¼ 165 ꢀ 10ꢁ48 e.s.u. for EHOMONS and
AcEONS, respectively.
Z
1
sCð1=bÞ
b
srelax
¼
exp½ꢁðt=sÞ ꢂdt ¼
;
ð2Þ
b
0
where C is the gamma function.
The first studies were carried out using
SR368 + Irgacure (2 wt%) doped at 5 wt% with the
EHOMONS chromophore. The relaxation curve
of the NLO molecules in a sample irradiated at 40
°C, upon a 200 s exposure to 50 mW/cm2, is pre-
sented in Fig. 2a. The relaxation time of the
chromophores which was subsecond before illu-
mination, reached 14 h after polymerization. Us-
ing infra-red spectroscopy measurements and
comparing the 1620–1640 cmꢁ1 peak areas (C@C
stretch) before and after polymerization, we de-
termined that the monomer conversion degree was
only of 55%. Since higher conversion degrees
should involve a better stability of the chromo-
phore orientation, we investigated the role of both
actinic intensity and temperature on the photo-
polymerization process [22]. No influence of the
conversion degree on the actinic intensity was
found. In contrast, when the samples were heated,
polymerization progressed longer before the reac-
tive species were trapped in the growing polymeric
network [24]. Therefore, the conversion degree,
which was 55% at 40 °C, reached 83% when the
process was carried out at 120 °C. Heating the
samples enabled to obtain harder matrices but
refinements (pre-polymerization, simultaneous
poling and photopolymerization) should be used
to prevent electrical breakdowns during poling.
Furthermore, due to the dissociation of the
photoinitiator Irgacure 784, a thermally initiated
polymerization was observed above 100 °C, a
process which could possibly induce a loss of the
photopatterning properties of the materials.
Therefore, we limited the temperature at 80 °C
during the photopolymerization. Under these
conditions, the stability of the chromophore ori-
entation was improved to 80 h.
3. Photoinduced freeze of the chromophores
This part describes the ability of the photo-
polymerizable resin to freeze the chromophores in
a given orientation. For these experiments, the
doped photopolymerizable formulation was in-
troduced between two indium tin oxide (ITO)
coated glass slides. Calibrated glass beads were
used as spacers to obtain 100 lm thick films. The
cells were placed on top of a sample heater to
control their temperature, then poled by gradually
applying a static electric field and polymerized
under an homogeneous exposure to the 514 nm
expanded beam of an Arþ laser. After irradiation,
the samples were slowly cooled down to 25 °C
before the poling field was switched off. The re-
laxation dynamics of the chromophores were
characterized by recording the temporal evolution
of the second harmonic signal generated by the
samples. For these measurements, the samples
were placed in a thermally regulated oven to keep
the temperature at 25 °C and pumped with the
1064 nm, p-polarized beam of a Q-switched
Nd:YAG laser. In order to compare the stabilities
of the orientation in different samples, all decay
curves were fitted to a Kohlrauch–Williams–Watts
(KWW) law:
To further improve the stability of the NLO
properties, we have explored an other route based
on the use of the co-polymerizable chromophore
AcEONS. As expected, an appreciable improve-
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
b
ISHGðtÞ ¼ ISHGð0Þ exp½ꢁðt=sÞ ꢂ;
ð1Þ

206
F. Gillot et al. / Chemical Physics Letters 379 (2003) 203–208
Fig. 2. (a) Normalized relaxation behavior for a SR368 + Irgacure 784 2 wt% + EHOMONS 5 wt% mixture photopolymerized at 40 °C
(squares), 60 °C (circles) and 80 °C (triangles) upon a 50 mW/cm2, 200 s exposure. The solid lines correspond to the theoretical KWW
fits, leading to srelax ¼ 14, 60 and 80 h, respectively. (b) Normalized relaxation behavior for a SR368 + Irgacure 784 2 wt% + AcEONS 1
wt% mixture, photopolymerized at 40 °C. The KWW fit (solid line) leads to srelax ¼ 4200 h.
ment of the stability of the orientation was ob-
tained with this chromophore, compared to the
previous guest–host system (see Fig. 2b). Indeed, a
value of srelax ¼ 4200 h was deduced from the fit of
the experimental data recorded during the first 150
h. Moreover, we noticed that experimental results
deviated from the KWW law at long time scales
and the value of 4200 h was obviously under-
evaluated. The d33 quadratic susceptibility of the
AcEONS doped samples was evaluated using
Maker fringe experiments. Due to the poor solu-
bility and the weak lb0 non-linearity of this
chromophore, we obtained a low value for d33 of
the order of 0.1 pm/V. The improvement of the
chromophore solubility is then necessary. This will
be achieved by multiplying the acrylate groups on
the chromophore. This modification should also
improve the connection between the chromophore
and the network and lead to longer relaxation
time.

F. Gillot et al. / Chemical Physics Letters 379 (2003) 203–208
207
4. Patterning of the non-linear optical properties
abled to diminish the electrical conduction of the
samples and allowed to apply a higher electric field
in the second step of the procedure compared to a
reverse order protocol. The polymerization was
performed at 80 °C using the 514 nm light of an
Arþ laser with an intensity of 10 mW cmꢁ2. The
amplitude of the electric field promoting the ori-
entation of the chromophores was 20 V lmꢁ1. The
alternation of illuminated/non-illuminated areas
was achieved by imaging a Edmund Cr on glass
Ronchi ruling (10 lines/mm) on the sample with
unity magnification through a f-2f-f optical system.
The spatial distribution of the second harmonic
emission was tested by scrolling the sample in front
of the laser beam, s-polarized and at an incidence of
45° with respect to the sample. The source was a
Nd:Yag laser, delivering 10 ns pulses at 10 Hz
repetition rate and at a wavelength of 1064 nm. The
patterning of the quadratic NLO properties of the
sample is illustrated in Fig. 4, where the variation of
the second harmonic light intensity is plotted versus
the displacement of the sample. The periodic
structure of the emitted light observed in Fig. 4 is
the image of the square periodic modulation of the
chromophore orientation, probed with a 70 lm
FWHM gaussian beam. It is thus possible to create
periodic domains of linear/non-linear properties
with a sufficient accuracy for future quasi-phase
matched structures. Furthermore, it should be no-
ted that, theoretically, there is no hindrance to po-
lymerize periodic areas with an alternate orientation
The patterning of the quadratic NLO properties
was demonstrated by writing periodic sequences of
alternatively oriented and isotropic areas. This ar-
rangement is analogous to those used in quasi-
phase matched structures. The NLO properties of
the samples, 100 lm thick films of the resin
SR368 + Irgacure 784 (2 wt%) + AcEONS (1 wt%),
were structured through a two-step technique. First
(Fig. 3I, ), the cells were irradiated with a periodic
pattern of illumination. This inhomogeneous irra-
diation, obtained by imaging an amplitude grating
onto the samples, froze the chromophores in an
isotropic distribution in the irradiated areas. In the
second step (Fig. 3II), a static electric field was
applied on the whole sample. While the chromo-
phores remained randomly oriented in the previ-
ously polymerized areas, they aligned along the
poling field in the non-polymerized zones. A peri-
odic structure of oriented and non-oriented areas
was thus created. By removing the mask, the po-
lymerization of the whole sample was finally com-
pleted in order to stabilize the structure. The first
irradiation step, without applied electric field, en-
Fig. 3. Schematic procedure used for the patterning of qua-
dratic optical properties. In step I, the actinic light polymerizes
selected zones through the mask. In step II, the applied electric
field orientates the chromophores in the previous non-illumi-
nated areas. Then the light polymerizes and freezes the orien-
tation of the chromophores in these areas.
Fig. 4. Variation of the second harmonic generation versus the
spatial coordinate (in lm). The period of the oscillations cor-
responds to those of the illumination pattern.

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F. Gillot et al. / Chemical Physics Letters 379 (2003) 203–208
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An acrylate-based photopolymerizable formu-
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developed in order to elaborate photoprocessable
materials for quadratic optics. We have demon-
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to the photopolymerization process can be used to
freeze the doping chromophores in a given orien-
tation. Although the temporal stability of their
orientation still remains below industrial expecta-
tions, the tailoring of the photopolymerization
parameters and the use of a co-polymerizable
chromophore lead to samples with long time du-
ration of the photostructuration of their optical
quadratic properties. Combined with optical per-
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