# Two-Photon Polymerization of Hybrid Sol-Gel Materials for Photonics Applications

Authors: A. Ovsianikov; A. Gaidukeviciute; B. N. Chichkov; M. Oubaha; B. D. MacCraith; I. Sakellari; A. Giakoumaki; D. Gray; M. Vamvakaki; M. Farsari; C. Fotakis
Journal: Laser Chemistry
 (2008)

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## Abstract

Two-photon polymerization of photosensitive materials has emerged as a very promising technique for the fabrication of photonic crystals and devices. We present our investigations into the structuring by two-photon polymerization of a new class of photosensitive sol-gel composites exhibiting ultra-low shrinkage. We particularly focus on two composites, the first containing a zirconium alkoxide and the second a nonlinear optical chromophore. The three-dimensional photonic crystal structures fabricated using these materials demonstrate high resolution and clear bandstops in the near IR region.


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## Body

## 1. Introduction

Nonlinear optical stereolithography
is a laser technique which allows the direct-writing of high-resolution three-dimensional
(3D) structures. The technique is based on the two-photon polymerization (2PP)
of photosensitive materials; when the beam of a femtosecond infrared laser is
tightly focused within the volume of such a material, the polymerization
process can be initiated by nonlinear absorption within the focal volume. By
moving the laser focus through the resin in the three dimensions, 3D structures
can be fabricated. A variety of acrylate
[1–7] and epoxy [8, 9] materials have been used to make components and
devices such as photonic crystals templates [10],
mechanical devices [11, 12],
plasmonic structures [13, 14],
biomolecule scaffolds [15, 16],
and microscopic models [17, 18]. The highest resolution reported to
date is 65 nm [19].The
first materials employed in 2PP were acrylic photopolymers and the negative
photoresist SU8 [8, 9, 18]; more recently, photosensitive sol-gel hybrid
materials [20] such as the commercially
available ORMOCER [21, 22] have been used. These
materials benefit from straight-forward preparation, modification, and
processing and in combination with their high optical quality, postprocessing chemical and
electrochemical inertness, and good mechanical and chemical stability, they are
emerging as a very useful class of materials for multiphoton polymerization
[23]. The process is based on the phase
transformation of a sol obtained from metallic oxide or alkoxide precursors.
This sol is first hydrolyzed and condensed at a low temperature to form a wet
gel. It is subsequently polymerized through radical photopolymerization to give
a product similar to glass.In this paper, we report our
investigations into the structuring by two photon polymerization of two composite
sol-gel materials. The first material is a zirconium/silicon composite; we show
that by changing the zirconium/silicon ratio, we can tune the refractive index
of the material. The second material is a copolymer of a photosensitive silicon
alkoxide with a nonlinear optical (NLO) alkoxysilane; this is used to make 3D photonic
crystals containing an NLO chromophore. These materials share
the characteristic of minimal shrinkage during photopolymerization.


## 2. Experimental

### 2.1. Materials
Synthesis and Preparation

A photosensitive sol-gel
process usually involves the catalytic hydrolysis of sol-gel precursors and the
heat-activated polycondensation of the hydrolyzed products followed by the
photopolymerization of the organic moieties to form a macromolecular network
structure of hybrid sol-gel materials. The material is generally formed through
a 4-step process.(1)
The first step is the hydrolysis and condensation in which
precursors or monomers such as metal oxides or metal alkoxides are mixed
with water and then undergo hydrolysis and condensation to form a porous
interconnected cluster structure. Either an acid such as HCl or a base
like NH3 can be employed as a catalyst.(2)
The second step includes gelation, where the solvent is
removed and a gel is formed by heating at low temperature. Hydrolysis and
condensation do not stop with gelation; it is at this stage that solvents
are removed and any significant volume loss occurs.(3)
Thirdly, the process moves to photopolymerization. Because
of the presence of the double bonds and provided that a photoinitiator
has been added
to the gel, the photoinduced radicals will cause polymerization only in
the area in which
they are present. At this step, there is no material removal and no volume
loss; the reaction that occurs is the cleavage of the pendant
carbon-carbon double bonds by a free-radical process to form the organic polymer backbone.(4)
Finally comes the development step; the sol-gel is immersed in an
appropriate solvent and the area of the sol-gel that is not
photopolymerized is removed.
In this work, the sol-gel process has been used to prepare two
different photosensitive composites, a zirconium containing hybrid and an NLO
hybrid. The synthesis of these materials is described below.


### 2.2. Zirconium
Composite Synthesis

The
material was fabricated from methacryloxypropyltri-methoxysilane (MAPTMS,
Polysciences Inc.) and methacrylic acid (MAA, Sigma-Aldrich), both of which
possess photopolymerizable methacrylate moieties. Zirconium n-propoxide Zr(OPr)4, (ZPO, 70% solution in 1-propanol, Sigma-Aldrich) was used as an inorganic
network former. The molar ratio of MAPTMS to ZPO was varied from 10 : 0 to 5 : 5,
in order to investigate
the processability and the refractive index variation of the resulting
copolymer.MAPTMS was firstly hydrolyzed by
adding HCl, (concentration 0.01 M) at a 1 : 0.75 ratio
and the mixture was stirred for 30 minutes. ZPO was chelated by adding MAA
(molar ratio ZPO:MAA, 1 : 1), an equal volume of 1-PrOH was then added, and the
sol was stirred for 30 minutes. The MAPTMS sol was added drop-wise to the
stirred ZPO sol. Following
another 45 minutes, water was added to this
mixture with a  final 2.5 : 5 MAPTMS:H2O molar ratio. 4,4′-Bis(diethylamino)benzophenone
(Sigma-Aldrich), 1% to
the final product was used as a photoinitiator (PI). The photoinitiator
Irgacure 369 was obtained from Ciba Speciality Chemicals. After stirring for 24
hours, the materials were filtered using 0.22 μm filters.The
samples were prepared by spin-coating or drop-casting onto glass substrates,
and the resultant films were dried on a hotplate at100°C for 1 hour before
the photopolymerization. The heating process resulted in the condensation of
the hydroxy-mineral moieties and the formation of the inorganic matrix. In a
subsequent processing step, the material, which was not exposed to the laser
radiation, was removed by developing in 1-propanol (Sigma-Aldrich).The transmission spectra of thin
films of this material were measured using a UV-Vis (Perkin-Elmer)
spectrometer. Thin films were prepared by spin-coating and drying on a hotplate
at100°C for 1 hour. Figure 1 shows the difference in the absorption spectra
induced by the addition of ZPO and photoinitiator. It can be seen that in the
presence of photoinitiator even the addition of only 1% ZPO causes the
absorption region to extend from 420 nm to 500 nm. A further increase in the
amount of ZPO also causes an increase of the absorption around 470 nm; however,
as also reported by Bhuian et al. [23], the extension of the absorption band
disappears when no PI is added to the sol-gel composite; it also disappears when
the composite is in solution. This behavior indicates an interaction between the
ZPO and the PI which requires molecular proximity; a possible explanation is a
charge transfer between the two composites.Figure 1
Transmission spectra for the material with varying ZPO and
photoinitiator (PI) content. PI alone results in absorption around 360 nm.
Addition of PI to ZPO containing material extends the absorption region from
420 nm to 500 nm.


### 2.3. NLO Hybrid Synthesis

The second material prepared consisted of the second-order NLO chromophore DR1
(Disperse Red 1, Sigma-Aldrich), which was firstly reacted with
(3-isocyanatopropyl) triethoxysilane to form a functionalized silicon alkoxide
precursor (SGDR1); SGDR1 was subsequently mixed with MAPTMS and the PI.(3-isocyanatopropyl)
trimethoxysilane and dibutyltin dilaurate (DBTDL) were purchased from
Sigma-Aldrich and used without further purification. DR1 was recrystallized
twice from ethanol before use. The synthesis of the NLO-active triethoxysilane
was carried out according to
[24]. (3-isocyanatopropyl)
trimethoxysilane and DR1 at a 2 : 1 mole ratio were dissolved in anhydrous
tetrahydrofuran (THF) under a nitrogen atmosphere. Next, 1 wt% DBTDL was added
to the reaction flask and the solution was refluxed for 4 hours. The resultant
solution was reduced to about half its initial volume under vacuum, followed by
precipitation of the product as a red solid in hexane. The final product
(SGDR1) was dried in a vacuum oven at 60°C for 24 hours and subsequently
stored under vacuum until use. The product was characterized by H1
NMR spectroscopy which verified the successful synthesis of SGDR1.To obtain the photosensitive gel,
SGDR1 was first dissolved in toluene and stirred for 24 hours. Then MAPTMS was
added to the SGDR1 solution (SGDR1 up to 40% w/w) and the mixture was
hydrolyzed by the addition of HCl(pH=1). After stirring for 1 hour, Irgacure
369 (up to 3.3 wt% to MAPTMS) was added and the mixture was stirred for a
further 1 hour. To remove any aggregates, the solution was
filtered through a 0.22 μm pore size Millipore syringe filter. After
filtration, the solution remained clear without any sign of further particle
aggregation for several weeks. Thermogravimetric analysis
of the composite showed that it was stable up to 250°C, above which temperature it started to
decompose.The absorption spectra of both DR1
and SGDR1 are shown in Figure2. Due to the presence of the DR1 azobenzene rings, the
composite material absorbs very strongly in the spectral region 400–550 nm, but it is
completely transparent at 600–800 nm and has a
window of transparency in the spectral region 300–400 nm. These
properties make SGDR1 ideal for two-photon polymerization using a Ti:Sapphire
laser. The IR transparency allows focusing the laser within the volume of the material, while
the relatively high UV transparency means that there will be two-photon
absorption mostly by the PI, and not by the NLO chromophore.Figure 2
Absorption spectrum of DR1 and SGDR1 in THF.Films were prepared by drop-casting
the above mixture on 100μm thick glass substrates, and the resultant films
were baked at 100°C for 1 hour, to condense the silanol moieties and remove
any residual solvent. After the completion of the photopolymerization process,
the sample was developed for three minutes in THF and
rinsed in isopropanol.


## 2.1. Materials
Synthesis and Preparation

A photosensitive sol-gel
process usually involves the catalytic hydrolysis of sol-gel precursors and the
heat-activated polycondensation of the hydrolyzed products followed by the
photopolymerization of the organic moieties to form a macromolecular network
structure of hybrid sol-gel materials. The material is generally formed through
a 4-step process.(1)
The first step is the hydrolysis and condensation in which
precursors or monomers such as metal oxides or metal alkoxides are mixed
with water and then undergo hydrolysis and condensation to form a porous
interconnected cluster structure. Either an acid such as HCl or a base
like NH3 can be employed as a catalyst.(2)
The second step includes gelation, where the solvent is
removed and a gel is formed by heating at low temperature. Hydrolysis and
condensation do not stop with gelation; it is at this stage that solvents
are removed and any significant volume loss occurs.(3)
Thirdly, the process moves to photopolymerization. Because
of the presence of the double bonds and provided that a photoinitiator
has been added
to the gel, the photoinduced radicals will cause polymerization only in
the area in which
they are present. At this step, there is no material removal and no volume
loss; the reaction that occurs is the cleavage of the pendant
carbon-carbon double bonds by a free-radical process to form the organic polymer backbone.(4)
Finally comes the development step; the sol-gel is immersed in an
appropriate solvent and the area of the sol-gel that is not
photopolymerized is removed.
In this work, the sol-gel process has been used to prepare two
different photosensitive composites, a zirconium containing hybrid and an NLO
hybrid. The synthesis of these materials is described below.


## 2.2. Zirconium
Composite Synthesis

The
material was fabricated from methacryloxypropyltri-methoxysilane (MAPTMS,
Polysciences Inc.) and methacrylic acid (MAA, Sigma-Aldrich), both of which
possess photopolymerizable methacrylate moieties. Zirconium n-propoxide Zr(OPr)4, (ZPO, 70% solution in 1-propanol, Sigma-Aldrich) was used as an inorganic
network former. The molar ratio of MAPTMS to ZPO was varied from 10 : 0 to 5 : 5,
in order to investigate
the processability and the refractive index variation of the resulting
copolymer.MAPTMS was firstly hydrolyzed by
adding HCl, (concentration 0.01 M) at a 1 : 0.75 ratio
and the mixture was stirred for 30 minutes. ZPO was chelated by adding MAA
(molar ratio ZPO:MAA, 1 : 1), an equal volume of 1-PrOH was then added, and the
sol was stirred for 30 minutes. The MAPTMS sol was added drop-wise to the
stirred ZPO sol. Following
another 45 minutes, water was added to this
mixture with a  final 2.5 : 5 MAPTMS:H2O molar ratio. 4,4′-Bis(diethylamino)benzophenone
(Sigma-Aldrich), 1% to
the final product was used as a photoinitiator (PI). The photoinitiator
Irgacure 369 was obtained from Ciba Speciality Chemicals. After stirring for 24
hours, the materials were filtered using 0.22 μm filters.The
samples were prepared by spin-coating or drop-casting onto glass substrates,
and the resultant films were dried on a hotplate at100°C for 1 hour before
the photopolymerization. The heating process resulted in the condensation of
the hydroxy-mineral moieties and the formation of the inorganic matrix. In a
subsequent processing step, the material, which was not exposed to the laser
radiation, was removed by developing in 1-propanol (Sigma-Aldrich).The transmission spectra of thin
films of this material were measured using a UV-Vis (Perkin-Elmer)
spectrometer. Thin films were prepared by spin-coating and drying on a hotplate
at100°C for 1 hour. Figure 1 shows the difference in the absorption spectra
induced by the addition of ZPO and photoinitiator. It can be seen that in the
presence of photoinitiator even the addition of only 1% ZPO causes the
absorption region to extend from 420 nm to 500 nm. A further increase in the
amount of ZPO also causes an increase of the absorption around 470 nm; however,
as also reported by Bhuian et al. [23], the extension of the absorption band
disappears when no PI is added to the sol-gel composite; it also disappears when
the composite is in solution. This behavior indicates an interaction between the
ZPO and the PI which requires molecular proximity; a possible explanation is a
charge transfer between the two composites.Figure 1
Transmission spectra for the material with varying ZPO and
photoinitiator (PI) content. PI alone results in absorption around 360 nm.
Addition of PI to ZPO containing material extends the absorption region from
420 nm to 500 nm.


## 2.3. NLO Hybrid Synthesis

The second material prepared consisted of the second-order NLO chromophore DR1
(Disperse Red 1, Sigma-Aldrich), which was firstly reacted with
(3-isocyanatopropyl) triethoxysilane to form a functionalized silicon alkoxide
precursor (SGDR1); SGDR1 was subsequently mixed with MAPTMS and the PI.(3-isocyanatopropyl)
trimethoxysilane and dibutyltin dilaurate (DBTDL) were purchased from
Sigma-Aldrich and used without further purification. DR1 was recrystallized
twice from ethanol before use. The synthesis of the NLO-active triethoxysilane
was carried out according to
[24]. (3-isocyanatopropyl)
trimethoxysilane and DR1 at a 2 : 1 mole ratio were dissolved in anhydrous
tetrahydrofuran (THF) under a nitrogen atmosphere. Next, 1 wt% DBTDL was added
to the reaction flask and the solution was refluxed for 4 hours. The resultant
solution was reduced to about half its initial volume under vacuum, followed by
precipitation of the product as a red solid in hexane. The final product
(SGDR1) was dried in a vacuum oven at 60°C for 24 hours and subsequently
stored under vacuum until use. The product was characterized by H1
NMR spectroscopy which verified the successful synthesis of SGDR1.To obtain the photosensitive gel,
SGDR1 was first dissolved in toluene and stirred for 24 hours. Then MAPTMS was
added to the SGDR1 solution (SGDR1 up to 40% w/w) and the mixture was
hydrolyzed by the addition of HCl(pH=1). After stirring for 1 hour, Irgacure
369 (up to 3.3 wt% to MAPTMS) was added and the mixture was stirred for a
further 1 hour. To remove any aggregates, the solution was
filtered through a 0.22 μm pore size Millipore syringe filter. After
filtration, the solution remained clear without any sign of further particle
aggregation for several weeks. Thermogravimetric analysis
of the composite showed that it was stable up to 250°C, above which temperature it started to
decompose.The absorption spectra of both DR1
and SGDR1 are shown in Figure2. Due to the presence of the DR1 azobenzene rings, the
composite material absorbs very strongly in the spectral region 400–550 nm, but it is
completely transparent at 600–800 nm and has a
window of transparency in the spectral region 300–400 nm. These
properties make SGDR1 ideal for two-photon polymerization using a Ti:Sapphire
laser. The IR transparency allows focusing the laser within the volume of the material, while
the relatively high UV transparency means that there will be two-photon
absorption mostly by the PI, and not by the NLO chromophore.Figure 2
Absorption spectrum of DR1 and SGDR1 in THF.Films were prepared by drop-casting
the above mixture on 100μm thick glass substrates, and the resultant films
were baked at 100°C for 1 hour, to condense the silanol moieties and remove
any residual solvent. After the completion of the photopolymerization process,
the sample was developed for three minutes in THF and
rinsed in isopropanol.


## 3. Experimental Techniques

Refractive Index Measurements
The
refractive index of the sol-gel films at 632.8 nm was determined from an m-line
prism coupling experiment [25],
using He-Ne laser. A thin film of the material was first made by spin-coating
and subsequent curing under a UV lamp. A Schott Glass SF6 prism, as a higher
refractive index medium, was used to couple light into the TE modes of the
material waveguide; the sample was then mounted on a high-resolution rotation
stage. The laser beam was directed toward the sample, and the transmitted light
was detected with a photodiode. The coupling angles were then determined, and
from the prism angle and refractive index the mode could then be determined.
From the crossing point of the modes supported by the thin film, both the
thickness and the refractive index of the film could be then calculated, using
the mode equation [16, 26].


## 4. 3D Microfabrication by Two-Photon Polymerization

The experimental setup for the fabrication of three-dimensional
microstructures by two-photon polymerization is shown in
Figure3. In the present work, two different Ti:Sapphire
femtosecond lasers were used; for the processing of the MAPTMS:SGDR1
composite, the laser characteristics were 60 fs, 90 MHz, <450 mW, 780 nm, while
for the MAPTMS:ZPO composite, it was 140 fs, 80 MHz, 780 nm. A 100X microscope objective lens (Zeiss, Plan
Apochromat, N.A. = 1.4) was used to focus the laser beam into the volume of the
photosensitive material. The photopolymerized structure was generated in a
layer-by-layer format, by
using an x-y galvanometric mirror scanner. Movement on the z-axis was achieved
using a high resolution linear stage.Figure 3
Experimental setup for two-photon
polymerization.For the online monitoring of the photopolymerization process, a CCD
camera was mounted behind a dichroic mirror. As the refractive index of the
photopolymer changes during polymerization, the illuminated structure becomes
visible during the building process.


## 5. Results

Refractive
Index Measurements
In
the case of the MAPTMS:SGDR1 composite, the refractive index of the material was
measured for 10% w/w SGDR1 content and was found to ben=1.497±0.006.
In
the case of the zirconium-containing sol-gel, by varying the molar ratio of MAPTMS
and ZPO, the refractive index of the composite could be modified
(Figure4). It can be seen that as the ZPO content increases,
so does the material’s refractive index. The fact that this increase is linear
greatly simplifies the material design criteria, as typically such increases
are saturating so that the doping concentration becomes very critical. However,
in this concentration range, no such limitation is apparent.Figure 4
Refractive index variation of the MAPTMS:ZPO complex.


## 6. Two-Photon Fabrication of Photonic Crystals

For the fabrication of the photonic crystals, the woodpile geometry was
chosen. It consists of layers of one-dimensional rods with a stacking sequence
that repeats itself every four layers. The distance between four adjacent layers is “a” and
within each layer, the axes of the rods are parallel to each other with a
distance “d” between them. The adjacent layers are rotated by 90°. Between every other layer, the rods are
shifted relative to each other by “d/2.”  For the case of  “a/d" =2, the lattice can be derived from a face-centred-cubic (fcc) unit cell
with a basis of two rods. A constant “a/d” ratio of 1.34 was applied for all the woodpile
structures presented here.SGDR1 Photonic Crystals
Photonic crystals are considered to be an optical equivalent of
semiconductors, since they modify the properties of light in the same way as
semiconductors do for electrons. However, in contrast to electrons, photon
energy cannot be easily tuned. Therefore, fabrication of photonic crystals made
of nonlinear materials, whose optical response depends on propagating light
intensity, is important.
Figure5(a) shows a microscope image of an array of photonic
crystals; they have the bright red color of Disperse Red 1. Figure 5(b)
shows a
scanning electron microscope (SEM) image of a DR1-containing photonic crystal
fabricated by the 2PP method using a laser fluence of 44 mJ/cm2 and
a beam scanning speed of 20 μm/s. As it can be seen, SGDR1 can be structured
very accurately and without
defects. The smallest lateral feature size that was experimentally obtained
with this material is 250 nm. The refractive index of the material was found to
be n=1.497±0.006 (10% w/w SGDR1 content).Optical microscope (a) and SEM (b) images of
photonic crystals by MAPTMS:SGDR1.
(a)(b)ZPO Photonic Crystals
Figure6 shows one of the fabricated woodpile photonic crystal structures
exhibiting a bandstop in the near-IR region; in this case, the employed
material had an 8 : 2 MAPTMS:ZPO molar ratio. As the material exhibits negligible
distortion due to photopolymerization, no additional efforts such as precompensation
or mechanical stabilization to avoid structural distortions are necessary.
FTIR measurements (Equinox, Bruker
Optics) of the reflection and the transmission spectra of woodpile structures
with rod distances between 1.2μm and 1.8 μm indicate clear bandstops with the
central frequency shifting to shorter wavelengths as the rod distance is
reduced (see Figure
7). In addition, the spectra show the appearance of
higher order bandstops in all samples, indicating the high quality of the
fabricated structures. The observed bandstop positions are blue shifted when
compared to the values estimated from Bragg’s condition or obtained from
theoretical simulations. There are two additional bands at 3 μm and 3.4 μm
whose origin is the absorption of the material, as confirmed by measurements of
transmission through flat, unstructured layers. Also in this case, material
with 8 : 2 MAPTMS:ZPO molar ratio was used. The material has no natural
absorption in the spectral region 550–2700 nm, which
makes it suitable for structuring by 2PP using a 780 nm laser and for making
photonic crystal structures at telecommunication wavelengths.
The observed splitting of the
absorption and transmission peaks as well as blue shift of the bandstops
central position can be explained by taking a closer look at the experimental
setup used for the FTIR transmission measurements. In order to focus the beam on
the size of the fabricated photonic crystal, a Cassegrain reflective optical
assembly is used. In contrast to the ideal case, when the measuring beam is
perpendicular to the surface of the structure, this assembly provides
illumination of the structure with a hollow light cone having an acceptance
angle between15° and 30°. Previous studies on 3D photonic crystal systems have
shown that scattering of the measuring beam entering the photonic crystal at a
large angle leads to the reflection peak splitting and a blue shift of its
central position [27–29] Theoretical simulations have also confirmed these
observations [30].
An approach taken by many groups is
the templating of
photonic crystals by inversion, infiltrating them with another, higher
refractive index material [8, 18]. The materials described here also can be used for
template fabrication. Like most hybrids, their organic components decompose at
relatively low temperatures (approx. 220°C). The inorganic components, however,
which are dominant after condensation in the materials described here, are very
resistant to high temperatures. It should be possible to remove them by HF acid,
in order to obtain the inverse structures, as has been demonstrated in similar
material systems [31].Figure 6
SEM image of Zr-containing photonic crystal structure (applied
material MAPTMS:ZPO ratio is 8 : 2).Figure 7
FTIR spectra of zirconium-containing photonic
crystal structures with rod spacings 1.2μm–1.8 μm (applied material MAPTMS:ZPO ratio is 8 : 2).


## 7. Conclusions

We have presented our investigations
into two sol-gel hybrid photosensitive materials which
can be structured accurately by two-photon polymerization. The first is a
zirconium containing sol-gel; by varying its zirconium content, we have shown
it is possible to “tune” its refractive index. The second material contains the
nonlinear optical chromophore Disperse Red 1; it is a first step towards the
fabrication of nonlinear three-dimensional photonic crystal devices using the
two-photon polymerization technique.As neither materials shrinks during
photopolymerization, it was possible to fabricate photonic crystals with bandstops in the near IR
region without any precompensation for structure
distortions induced by the material shrinkage, or any support structures
for their mechanical stabilization.



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*Source: 493059-2008-10-28.xml*