Hindawi Publishing CorporationLaser ChemistryVolume 2008, Article ID 493059, 7 pagesdoi:10.1155/2008/493059

Research Article

Two-Photon Polymerization of Hybrid Sol-Gel Materials forPhotonics Applications

A. Ovsianikov,1 A. Gaidukeviciute,1 B. N. Chichkov,1 M. Oubaha,2 B. D. MacCraith,2

I. Sakellari,3, 4 A. Giakoumaki,3, 5 D. Gray,3 M. Vamvakaki,3, 5 M. Farsari,3 and C. Fotakis3, 4

1 The Laser Zentrum Hannover e.V., 30419 Hannover, Germany2 The Optical Sensors Laboratory, National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland3 Institute of Electronic Structure and Laser, Foundation for Research and Technology - Hellas, P.O. Box 1527, 711 10 Heraklion, Greece4 Department of Physics, University of Crete, 71003 Heraklion, Crete, Greece5 Department of Materials Science and Technology, University of Crete, 710 03 Heraklion, Crete, Greece

Correspondence should be addressed to B. N. Chichkov, b.chichkov@lzh.de

Received 24 April 2008; Accepted 12 July 2008

Recommended by Saulius Juodkazis

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

Copyright © 2008 A. Ovsianikov et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Nonlinear optical stereolithography is a laser techniquewhich allows the direct-writing of high-resolution three-dimensional (3D) structures. The technique is based onthe two-photon polymerization (2PP) of photosensitivematerials; when the beam of a femtosecond infrared laseris tightly focused within the volume of such a material, thepolymerization process can be initiated by nonlinear absorp-tion within the focal volume. By moving the laser focusthrough the resin in the three dimensions, 3D structurescan be fabricated. A variety of acrylate [1–7] and epoxy[8, 9] materials have been used to make components anddevices such as photonic crystals templates [10], mechanicaldevices [11, 12], plasmonic structures [13, 14], biomoleculescaffolds [15, 16], and microscopic models [17, 18]. Thehighest resolution reported to date is 65 nm [19].

The first materials employed in 2PP were acrylic pho-topolymers 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-forwardpreparation, modification, and processing and in combina-tion with their high optical quality, postprocessing chemicaland electrochemical inertness, and good mechanical andchemical stability, they are emerging as a very useful class ofmaterials for multiphoton polymerization [23]. The processis based on the phase transformation of a sol obtainedfrom metallic oxide or alkoxide precursors. This sol is firsthydrolyzed and condensed at a low temperature to forma wet gel. It is subsequently polymerized through radicalphotopolymerization to give a product similar to glass.

In this paper, we report our investigations into thestructuring by two photon polymerization of two compositesol-gel materials. The first material is a zirconium/siliconcomposite; we show that by changing the zirconium/siliconratio, we can tune the refractive index of the material. Thesecond material is a copolymer of a photosensitive siliconalkoxide with a nonlinear optical (NLO) alkoxysilane; thisis used to make 3D photonic crystals containing an NLOchromophore. These materials share the characteristic ofminimal shrinkage during photopolymerization.

2 Laser Chemistry

2. Experimental

2.1. Materials Synthesis and Preparation

A photosensitive sol-gel process usually involves the cat-alytic hydrolysis of sol-gel precursors and the heat-activatedpolycondensation of the hydrolyzed products followed bythe photopolymerization of the organic moieties to form amacromolecular network structure of hybrid sol-gel mate-rials. The material is generally formed through a 4-stepprocess.

(1) The first step is the hydrolysis and condensationin which precursors or monomers such as metaloxides or metal alkoxides are mixed with water andthen undergo hydrolysis and condensation to forma porous interconnected cluster structure. Either anacid such as HCl or a base like NH3 can be employedas a catalyst.

(2) The second step includes gelation, where the solventis removed and a gel is formed by heating at lowtemperature. Hydrolysis and condensation do notstop with gelation; it is at this stage that solvents areremoved and any significant volume loss occurs.

(3) Thirdly, the process moves to photopolymerization.Because of the presence of the double bonds andprovided that a photoinitiator has been added to thegel, the photoinduced radicals will cause polymer-ization only in the area in which they are present.At this step, there is no material removal and novolume loss; the reaction that occurs is the cleavageof the pendant carbon-carbon double bonds by afree-radical process to form the organic polymerbackbone.

(4) Finally comes the development step; the sol-gel isimmersed in an appropriate solvent and the area ofthe sol-gel that is not photopolymerized is removed.

In this work, the sol-gel process has been used to prepare twodifferent photosensitive composites, a zirconium containinghybrid and an NLO hybrid. The synthesis of these materialsis described below.

2.2. Zirconium Composite Synthesis

The material was fabricated from methacryloxypropyltri-methoxysilane (MAPTMS, Polysciences Inc.) and meth-acrylic acid (MAA, Sigma-Aldrich), both of which possessphotopolymerizable 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 therefractive index variation of the resulting copolymer.

MAPTMS was firstly hydrolyzed by adding HCl, (con-centration 0.01 M) at a 1 : 0.75 ratio and the mixturewas stirred for 30 minutes. ZPO was chelated by addingMAA (molar ratio ZPO:MAA, 1 : 1), an equal volume of

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Figure 1: Transmission spectra for the material with varying ZPOand photoinitiator (PI) content. PI alone results in absorptionaround 360 nm. Addition of PI to ZPO containing material extendsthe absorption region from 420 nm to 500 nm.

1-PrOH was then added, and the sol was stirred for 30minutes. The MAPTMS sol was added drop-wise to thestirred ZPO sol. Following another 45 minutes, water wasadded to this mixture with a final 2.5 : 5 MAPTMS:H2Omolar 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 fromCiba Speciality Chemicals. After stirring for 24 hours, thematerials were filtered using 0.22 μm filters.

The samples were prepared by spin-coating or drop-casting onto glass substrates, and the resultant films weredried on a hotplate at 100◦C for 1 hour before the photopoly-merization. The heating process resulted in the condensationof the hydroxy-mineral moieties and the formation of theinorganic matrix. In a subsequent processing step, thematerial, which was not exposed to the laser radiation, wasremoved by developing in 1-propanol (Sigma-Aldrich).

The transmission spectra of thin films of this materialwere measured using a UV-Vis (Perkin-Elmer) spectrometer.Thin films were prepared by spin-coating and drying on ahotplate at 100◦C for 1 hour. Figure 1 shows the differencein the absorption spectra induced by the addition of ZPOand photoinitiator. It can be seen that in the presence ofphotoinitiator even the addition of only 1% ZPO causesthe absorption region to extend from 420 nm to 500 nm.A further increase in the amount of ZPO also causes anincrease of the absorption around 470 nm; however, asalso reported by Bhuian et al. [23], the extension of theabsorption band disappears when no PI is added to thesol-gel composite; it also disappears when the composite isin solution. This behavior indicates an interaction betweenthe ZPO and the PI which requires molecular proximity; apossible explanation is a charge transfer between the twocomposites.

Laser Chemistry 3

DR1 and SGDR1 in THF

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Figure 2: Absorption spectrum of DR1 and SGDR1 in THF.

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 alkoxideprecursor (SGDR1); SGDR1 was subsequently mixed withMAPTMS and the PI.

(3-isocyanatopropyl) trimethoxysilane and dibutyltindilaurate (DBTDL) were purchased from Sigma-Aldrich andused without further purification. DR1 was recrystallizedtwice 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 moleratio were dissolved in anhydrous tetrahydrofuran (THF)under a nitrogen atmosphere. Next, 1 wt% DBTDL wasadded to the reaction flask and the solution was refluxed for4 hours. The resultant solution was reduced to about halfits initial volume under vacuum, followed by precipitationof the product as a red solid in hexane. The final product(SGDR1) was dried in a vacuum oven at 60◦C for 24hours and subsequently stored under vacuum until use. Theproduct was characterized by 1H NMR spectroscopy whichverified the successful synthesis of SGDR1.

To obtain the photosensitive gel, SGDR1 was firstdissolved in toluene and stirred for 24 hours. Then MAPTMSwas 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 3.3wt% to MAPTMS) was added and the mixture was stirredfor a further 1 hour. To remove any aggregates, the solutionwas filtered through a 0.22 μm pore size Millipore syringefilter. After filtration, the solution remained clear withoutany 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 startedto decompose.

The absorption spectra of both DR1 and SGDR1 areshown in Figure 2. Due to the presence of the DR1 azoben-zene rings, the composite material absorbs very stronglyin the spectral region 400–550 nm, but it is completelytransparent at 600–800 nm and has a window of trans-parency in the spectral region 300–400 nm. These propertiesmake SGDR1 ideal for two-photon polymerization usinga Ti:Sapphire laser. The IR transparency allows focusingthe laser within the volume of the material, while therelatively high UV transparency means that there will be two-photon absorption mostly by the PI, and not by the NLOchromophore.

Films were prepared by drop-casting the above mixtureon 100 μm thick glass substrates, and the resultant films werebaked at 100◦C for 1 hour, to condense the silanol moietiesand remove any residual solvent. After the completion of thephotopolymerization process, the sample was developed forthree 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 wasdetermined from an m-line prism coupling experiment [25],using He-Ne laser. A thin film of the material was firstmade by spin-coating and subsequent curing under a UVlamp. A Schott Glass SF6 prism, as a higher refractiveindex medium, was used to couple light into the TE modesof the material waveguide; the sample was then mountedon a high-resolution rotation stage. The laser beam wasdirected toward the sample, and the transmitted light wasdetected with a photodiode. The coupling angles werethen determined, and from the prism angle and refractiveindex the mode could then be determined. From thecrossing point of the modes supported by the thin film,both the thickness and the refractive index of the filmcould be then calculated, using the mode equation [16,26].

4. 3D Microfabrication by Two-PhotonPolymerization

The experimental setup for the fabrication of three-dimensional microstructures by two-photon polymerizationis shown in Figure 3. In the present work, two differentTi:Sapphire femtosecond lasers were used; for the processingof the MAPTMS:SGDR1 composite, the laser characteris-tics were 60 fs, 90 MHz, <450 mW, 780 nm, while for theMAPTMS: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 volumeof the photosensitive material. The photopolymerized struc-ture was generated in a layer-by-layer format, by using an x-ygalvanometric mirror scanner. Movement on the z-axis wasachieved using a high resolution linear stage.

4 Laser Chemistry

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Figure 3: Experimental setup for two-photon polymerization.

For the online monitoring of the photopolymerizationprocess, a CCD camera was mounted behind a dichroicmirror. As the refractive index of the photopolymer changesduring polymerization, the illuminated structure becomesvisible during the building process.

5. Results

Refractive Index Measurements

In the case of the MAPTMS:SGDR1 composite, the refractiveindex of the material was measured for 10% w/w SGDR1content and was found to be n = 1.497± 0.006.

In the case of the zirconium-containing sol-gel, byvarying the molar ratio of MAPTMS and ZPO, the refractiveindex of the composite could be modified (Figure 4). Itcan be seen that as the ZPO content increases, so does thematerial’s refractive index. The fact that this increase is lineargreatly simplifies the material design criteria, as typicallysuch increases are saturating so that the doping concentra-tion becomes very critical. However, in this concentrationrange, no such limitation is apparent.

6. Two-Photon Fabrication of Photonic Crystals

For the fabrication of the photonic crystals, the woodpilegeometry was chosen. It consists of layers of one-dimensionalrods with a stacking sequence that repeats itself every fourlayers. The distance between four adjacent layers is “a” andwithin each layer, the axes of the rods are parallel to eachother with a distance “d” between them. The adjacent layersare rotated by 90◦. Between every other layer, the rods areshifted 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” ratioof 1.34 was applied for all the woodpile structures presentedhere.

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Figure 4: Refractive index variation of the MAPTMS:ZPO complex.

SGDR1 Photonic Crystals

Photonic crystals are considered to be an optical equivalentof semiconductors, since they modify the properties oflight in the same way as semiconductors do for electrons.However, in contrast to electrons, photon energy cannot beeasily tuned. Therefore, fabrication of photonic crystals madeof nonlinear materials, whose optical response depends onpropagating light intensity, is important.

Figure 5(a) shows a microscope image of an array of pho-tonic crystals; they have the bright red color of Disperse Red1. Figure 5(b) shows a scanning electron microscope (SEM)image of a DR1-containing photonic crystal fabricated by the2PP method using a laser fluence of 44 mJ/cm2 and a beamscanning speed of 20 μm/s. As it can be seen, SGDR1 can bestructured very accurately and without defects. The smallestlateral feature size that was experimentally obtained with thismaterial is 250 nm. The refractive index of the material wasfound to be n = 1.497± 0.006 (10% w/w SGDR1 content).

ZPO Photonic Crystals

Figure 6 shows one of the fabricated woodpile photonic crys-tal structures exhibiting a bandstop in the near-IR region; inthis case, the employed material had an 8 : 2 MAPTMS:ZPOmolar ratio. As the material exhibits negligible distortiondue to photopolymerization, no additional efforts suchas precompensation or mechanical stabilization to avoidstructural distortions are necessary.

FTIR measurements (Equinox, Bruker Optics) of thereflection and the transmission spectra of woodpile struc-tures with rod distances between 1.2 μm and 1.8 μm indicateclear bandstops with the central frequency shifting to shorterwavelengths as the rod distance is reduced (see Figure 7). Inaddition, the spectra show the appearance of higher orderbandstops in all samples, indicating the high quality of thefabricated structures. The observed bandstop positions areblue shifted when compared to the values estimated from

Laser Chemistry 5

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Figure 5: Optical microscope (a) and SEM (b) images of photonic crystals by MAPTMS:SGDR1.

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Figure 6: SEM image of Zr-containing photonic crystal structure(applied material MAPTMS:ZPO ratio is 8 : 2).

Bragg’s condition or obtained from theoretical simulations.There are two additional bands at 3 μm and 3.4 μm whoseorigin is the absorption of the material, as confirmed bymeasurements of transmission through flat, unstructuredlayers. Also in this case, material with 8 : 2 MAPTMS:ZPOmolar ratio was used. The material has no natural absorptionin the spectral region 550–2700 nm, which makes it suitablefor structuring by 2PP using a 780 nm laser and formaking photonic crystal structures at telecommunicationwavelengths.

The observed splitting of the absorption and transmis-sion peaks as well as blue shift of the bandstops centralposition can be explained by taking a closer look atthe experimental setup used for the FTIR transmissionmeasurements. In order to focus the beam on the sizeof the fabricated photonic crystal, a Cassegrain reflectiveoptical assembly is used. In contrast to the ideal case,when the measuring beam is perpendicular to the surfaceof the structure, this assembly provides illumination of thestructure with a hollow light cone having an acceptance anglebetween 15◦ and 30◦. Previous studies on 3D photonic crystalsystems have shown that scattering of the measuring beamentering the photonic crystal at a large angle leads to the

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Figure 7: FTIR spectra of zirconium-containing photonic crystalstructures with rod spacings 1.2 μm–1.8 μm (applied materialMAPTMS:ZPO ratio is 8 : 2).

reflection peak splitting and a blue shift of its central position[27–29] Theoretical simulations have also confirmed theseobservations [30].

An approach taken by many groups is the templatingof photonic crystals by inversion, infiltrating them withanother, higher refractive index material [8, 18]. Thematerials described here also can be used for templatefabrication. Like most hybrids, their organic componentsdecompose at relatively low temperatures (approx. 220◦C).The inorganic components, however, which are dominantafter condensation in the materials described here, are veryresistant to high temperatures. It should be possible toremove them by HF acid, in order to obtain the inversestructures, as has been demonstrated in similar materialsystems [31].

6 Laser Chemistry

7. Conclusions

We have presented our investigations into two sol-gel hybridphotosensitive materials which can be structured accuratelyby two-photon polymerization. The first is a zirconiumcontaining sol-gel; by varying its zirconium content, wehave shown it is possible to “tune” its refractive index. Thesecond material contains the nonlinear optical chromophoreDisperse Red 1; it is a first step towards the fabrication ofnonlinear three-dimensional photonic crystal devices usingthe two-photon polymerization technique.

As neither materials shrinks during photopolymeriza-tion, it was possible to fabricate photonic crystals with band-stops in the near IR region without any precompensation forstructure distortions induced by the material shrinkage, orany support structures for their mechanical stabilization.

Acknowledgments

This work was supported by the Marie Curie Transfer ofKnowledge project “NOLIMBA” (MTKD-CT-2005-029194)and by the UV Laser Facility operating at IESL-FORTHunder the European Commission “Improving HumanResearch Potential” program (RII3-CT-2003-506350). Theauthors would like to thank group of Professor MartinWegener for valuable assistance in optical characterisationof photonic crystals. Travel between Crete, Greece andHannover, Germany, was supported by an IKYDA/DAADtravel grant.

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