T he rapid technical develop-ment of ultrashort laser sys-tems is creating exciting pos-
sibilities for very precise local-ization of laser energy in time andspace. These achievements havetriggered novel laser applicationsbased on nonlinear interactionprocesses.
A promising three-dimensional microfabrication method thathas recently attracted consider-able attention is based on two-photon polymerization with ul-
trashort laser pulses.1-5 When focused into the volume of a photosensitive material (or pho-toresist), the pulses initiate two-photon polymerization viatwo-photon absorption and sub-sequent polymerization. After illumination of the desired struc-tures inside the photoresist vol-ume and subsequent develop-ment — e.g., washing out thenonilluminated regions — thepolymerized material remains inthe prescribed 3-D form. This al-
lows fabrication of any com-puter-generated 3-D structureby direct laser “recording” intothe volume of a photosensitivematerial (Figure 1).
Because of the threshold be-havior and nonlinear nature ofthe process, a resolution beyondthe diffraction limit can be real-ized by controlling the laser pulseenergy and the number of appliedpulses. As a result, the techniqueprovides much better structuralresolution and quality than the
Reprinted from the October 2006 issue of PHOTONICS SPECTRA © Laurin Publishing
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Two-Photon Polymerization: A New Approach to
MicromachiningFemtosecond lasers enable microfabrication with resolution
beyond the diffraction limit.
by Andreas Ostendorf and Boris N. Chichkov, Laser Zentrum Hannover eV
Figure 1. These SEM images show a micro-scale dragon (left) and a movable windmill (right) fabricated by two-photon polymerization in organically modified ceramics.
well-known stereolithographymethod.
Three-dimensional micro-structuring of photosensitive ma-terials by two-photon polymer-ization is effective for the fabri-cation of 3-D structures having aresolution of 100 nm or better.For two-photon polymerizationand 3-D materials processing,computer-controlled positioningsystems are combined with alight source that is typically anear-infrared Ti:sapphire fem-tosecond laser oscillator emittingat ∼800 nm.
To benefit from the high res-olution inherent in the two-pho-ton polymerization process,highly accurate positioning sys-tems — e.g., piezoelectric stagesand/or scanners — are re-quired. However, piezoelectricstages have a traveling range ofonly a few hundred microns ineach direction. Alternatively, op-tical scanning systems can beused to move the laser beam,but they must deflect the writ-ing laser beam through theouter edges of the focusing op-tics, which can cause distor-tions in the outer parts of theimage and a subsequent loss ofintensity and structural homo-geneity.
3-D microstructuring systemTo overcome these limitations,
Laser Zentrum Hannover eV hasdeveloped an autonomous, mov-able system for the generation ofmicro- and nanoscale 3-D struc-tures. The system integrates afemtosecond laser, a scanner forfast writing of small-area struc-tures and a motor-driven linearpositioning system from AerotechGmbH that uses air bearings. Thelaser is a compact Ti:sapphiresystem from High Q LaserProduction GmbH, with 200-mWof average power, an 800-nmwavelength, a pulse durationbelow 100 fs and a repetition rateof 73 MHz.
Figure 2. Front view of the 3-D microstructuring system shows a CAD drawing(top) and a photograph (bottom).
Two-Photon Polymerization
The positioning system, withthree axes, provided a travelingrange of 10 cm in each direction.Now commercially available, the3-D system is equipped with arotational axis that allows curvedcylindrical structures to be cre-ated by two-photon polymeriza-tion (Figure 2).
For two-photon polymerizationmicrostructuring, an X-Y galvo
scanner deflected the expandedlaser beam through the high-nu-merical-aperture immersion-oilobjective, which focused the fem-tosecond pulses into the photo-sensitive material, or resin (Figure3). The scanner was mounted onthe Z-axis of the large-range X-Ypositioning system. The CCDcamera enabled real-time processmonitoring. The sample was
mounted on a 2-D translationalstage.
By using a scanner and trans-lational stages to move the tinybeam waist three-dimensionallyinside the resin, one can writecomplex 3-D structures. The ac-curacy of the scanner-based writ-ing is ∼100 nm, while the posi-tioning accuracy over the com-plete travel range is better than400 nm.
Negative- and positive-tonephotoresists are the two types ofphotosensitive materials that canbe structured by two-photonpolymerization (Figure 4). Withnegative-tone photoresists, two-photon exposure results in cross-linking of polymer chains, allow-ing the unexposed resist to bewashed out. With positive-toneresists, light exposure leads tochain scission, creating shorterunits that can be dissolved andwashed away in the developmentprocess. Most hollow structurescan be machined by removingonly a small fraction of the totalmaterial in the original workpiece;in which case, positive-tone pho-toresists are more efficient.
Negative-tone resist materialscan be divided into solid and liq-
Figure 3. This is the principal setup for 3-D writing and structuring.
PhotosensitiveMaterials
Positive Resists
Negative Resists
LiquidSolid
Epoxy-BasedAcrylate-Based
AZPhotonics,
Lithography
S1800Templates,
Liftoff
SU 8Photonics,
Lithography
CAR44Photonics,
Lithography,Templates
OrmocerBroad Range
of Applications
PEGdmaBiology,
Medicine
SR499MEMS,
Medicine
Figure 4. Photosensitive materials are studied at Laser Zentrum Hannover eV.
Two-Photon Polymerization
uid subgroups. The solid mate-rials shown in Figure 4 are epoxy-based cationic photoresists. Incationic systems (such as thecommercially available SU 8 pho-toresist) an acid is generatedupon laser irradiation. In thiscase, polymerization does nottake place during laser irradia-tion (only after a postexposurebake). This is an important prop-
erty of cationic photoresists be-cause the difference in the re-fractive index of exposed and un-exposed areas is negligible, whichallows for a flexible irradiationstrategy and for combining directlaser beam writing with holo-graphic exposure.
Real-time monitoringLiquid materials, except or-
ganically modified ceramics(Ormocers), are acrylate-based,and the polymerization reactionis triggered by radically reactingphotoinitiators during laser irra-diation. This allows real-time op-tical monitoring of the polymer-ization reaction.
We often use inorganic/organichybrid polymers (Ormocers) fromMicro Resist Technology GmbH
Figure 5. SEM images of micro-optical elements are fabricated by two-photon polymerization.
Figure 6. Bends and splitter structures are fabricated by two-photon polymerization on metal surfaces for plasmonic applications.
Two-Photon Polymerization
in our work. They are synthesizedby sol-gel processing,6 wherebyinorganic units are connected toorganic moieties on a molecularlevel.7 The polymers are hybrids,combining the properties of or-ganic polymers (low-temperatureprocessing, functionality andtoughness) with those of glasslikematerials (hardness, chemicaland thermal stability, and trans-parency).8 This allows one to
achieve material properties notaccessible with composite or poly-mer materials.
Applications in photonicsBecause of their unique opti-
cal properties, these polymers areeffective for the fabrication ofmicro-optical components anddevices such as microprism ar-rays and diffractive optical ele-ments (Figure 5).
Further miniaturization of op-tical elements to the subwave-length size requires new tech-niques. The use of surface plas-mon polaritons on metal surfacesas information carriers in a kindof “optical circuit” is one suchmethod (see “Nanophotonics withSurface Plasmons,” PhotonicsSpectra, January 2006, p. 56).Surface plasmon polaritons areelectromagnetic excitations prop-agating along and bound to aninterface between a metal and adielectric. They can carry infor-mation along metallic waveguideson a dielectric substrate or alongdielectric structures on metalsurfaces such as bends or split-ters (Figure 6). These structureshave been fabricated by two-pho-ton polymerization of Ormocerson a gold surface.9
Two-photon polymerization israpidly developing as an enablingtechnology for the fabrication of3-D photonic crystals and pho-tonic-crystal templates. In par-ticular, it allows the introductionof defects at any location in asubstrate, which is crucial forpractical applications. Photoniccrystals are periodic structuresconsisting of spatially alternat-
Figure 7. SEMimages of photonic crystalsare created in SU 8 (aboveleft), S1813(above right) andOrmocers (left).
Two-Photon Polymerization
ing regions with different dielec-tric constants.
Propagation of light inside acertain frequency range — thephotonic bandgap — is precludedin such structures. If the dielec-tric constant periodicity occursin all directions, then the struc-ture is a 3-D photonic crystal.
Depending on the topology anddielectric-constant contrast ofphotonic crystals, their opticalproperties can be tailored. Sincethe concept of a 3-D photoniccrystal was introduced in 1987by Eli Yablonovitch10 and SajeevJohn,11 photonic crystals havebeen the subject of intense re-
search. Despite this, fabricationof photonic crystals with a fully3-D bandgap in the visible rangeremains a challenge.
To realize photonic crystalswith a full photonic bandgap re-quires 3-D microstructuring ofhigh-refractive-index materials.The most attractive option is toinfiltrate fabricated templateswith a high-refractive-index ma-terial and then remove the tem-plate.5 Using most negative-tonephotoresists to fabricate tem-plates is rather complicated be-cause the structures fabricatedin these materials are stable andnot readily soluble. An example ofa photonic crystal template fab-ricated in SU 8 is shown in Figure7. In the case of positive-tonephotoresists, the polymer isweakened and is usually moresoluble in developing solutions.This is advantageous for the fab-rication of 3-D templates. InFigure 7 (top right photo), a scan-ning electron microscope image ofthe photonic crystal template fab-ricated in Shipley S1813 pho-toresist is shown.
Another way to fabricate pho-
Figure 8. SEM images of scaffold structures (left) and a free-standing Lego-type structure for cell growth experiments (right)are produced by two-photon polymerization of Ormocers.
Figure 9. ThisSEM image showsmicroneedles fordrug delivery fabricated by two-photon polymerization ofOrmocers. The microneedles exhibit appropriatemechanical properties and can penetrate skinwithout fracturing.
Two-Photon Polymerization
tonic crystals is to use inor-ganic/organic photosensitive ma-terials with a high degree of in-organic content, as depicted inFigure 7 (left).12 Using thismethod, it is possible to skip thereplication steps and to fabricate3-D inorganic structures directly.By an appropriate thermal treat-ment of inorganic/organic hybridmaterials, one can remove or-ganic components from the laser-fabricated 3-D structures andleave the purely inorganic part.In this way, the two-photon poly-merization technique (or, in gen-eral, two-photon-activated pro-cessing) and thermal posttreat-ment can be used to fabricateinorganic 3-D photonic crystalstructures.
The two-photon polymerizationtechnique shows promise for bi-ological applications, includingtissue engineering, drug deliv-ery, medical implants and med-ical sensors. For tissue engi-neering, the ability to producean arbitrary 3-D scaffold struc-ture is appealing. Scaffolds arerequired to artificially fabricateliving tissue that can integratewith host tissue inside the body,a challenging undertaking. Two-photon polymerization, in com-bination with the right materi-als, allows precise control overthe 3-D geometry of the scaffold,enabling modeling and repro-duction of cellular microenvi-ronments (Figure 8). Further-more, the high resolution of thetechnique can provide controlover the cell organization insidethe scaffold and, consequently,over cell interactions. Anotheradvantage is that the near-IRlaser radiation used for two-pho-ton polymerization is not harm-ful to cells at the applied inten-sities and also could be used forthe manipulation and encapsu-lation of cells.
For applications in biomedi-cine, Ormocers are interesting
materials. Biocompatibility of thepolymers has recently been stud-ied, and the results have demon-strated good adherence of celltypes to this material and agrowth rate comparable to bioac-tive materials such as extracel-lular matrix.13
MicroneedlesTwo-photon polymerization also
can be utilized for the fabricationof drug delivery devices; e.g.,microneedle arrays.13 These de-vices enable transdermal deliv-ery of a wide diversity of phar-macologic agents. Application ofmicroneedle arrays may over-come many of the issues associ-ated with conventional intra-venous drug administration, in-cluding pain to the patient,trauma at the injection site anddifficulty in providing sustainedrelease of a pharmacologic agent.
Moreover, the flexibility of two-photon polymerization enablesarbitrary changes to the needledesign and, therefore, compari-son of the effect of geometry onmechanical and puncturing prop-erties (Figure 9). Investigationsof microneedle arrays for drugdelivery are in progress. o
AcknowledgmentThe authors acknowledge help
from Carsten Reinhardt, SvenPassinger and Alexander Ovsiani-kov of Laser Zentrum HannovereV in preparing this article, andcooperation from Ruth Houbertzfrom Fraunhofer Institut fürSilicatforschung. They also grate-fully acknowledge technical sup-port from Daniel Kopf and JörgSmolenski of High Q LaserProduction GmbH for providingtheir laser systems for experiments.
Meet the authorsAndreas Ostendorf is executive director ofLaser Zentrum Hannover eV, an inde-pendent organization carrying out ap-plied research in the field of lasers and op-
tics; e-mail: [email protected]. Boris N. Chichkov is a professor in
the department of physics at LeibnizUniversität Hannover and head of thenanotechnology department at laserZentrum Hannover eV; e-mail: [email protected].
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Two-Photon Polymerization
