Femtosecond laser microfabrication of 3D photonic structures · The physical principles underlying...

Femtosecond laser microfabrication of 3D photonic structures

Kock Khuen Seet, Toshiaki Kondo, Vygantas Mizeikis,Vygandas Jarutis, Saulius Juodkazis, and Hiroaki Misawa

CREST-JST & Research Institute for Electronic Science, Hokkaido University, CR15 Bldg.,North 21 West 10, Sapporo 001-0021, Japan

ABSTRACTThe physical principles underlying the three-dimensional (3D) laser microstructuring technique are outlined, itsapplications for the fabrication of 3D photonic crystals are described. Structures recorded by direct laser writingand holographic recording techniques in SU-8 photoresist are presented along with their structural and opticalcharacterization.

Keywords: Photonic crystals, femtosecond laser microfabrication, holographic recording

1. INTRODUCTIONThree-dimensional (3D) laser nanolmicrofabrication using two-photon or multi-photon nonlinear absorption (ab-breviated as TPA and MPA, respectively) allows recording of structures containing features smaller than thediffraction-limited resolution achievable during the formation of light intensity patterns.' Use of femtosecondlaser pulses enables one to achieve high optical power densities required for nonlinear absorption. Moreover, itis easier to perform ultrafast photomodification of materials without significant influence of unwanted thermaleffects. In general, this approach allows 3D microstructuring of materials with resolution of 0.2-1 sam. Thesecharacteristics make femtosecond laser microfabrication a highly suitable technique for the fabrication of 3Dphotonic crystals and their templates.

Transient and permanent photomodification of materials, seeded by TPA or MPA of powerful ultrashortlaser pulses, can be induced at wavelengths to which the materials are perfectly transparent in the linear regime.Thus, it is possible to deliver radiation into the bulk of materials without linear losses. In sequential femtoseconddirect laser writing (DLW), laser beams are tightly focused, and nonlinear absorption is induced at the focalregion, which therefore can be treated as a tip of a very sharp pen. By translating the beam inside the material,photomodified patterns consisting of point-like or linear photomodified features can be recorded. Below we willdemonstrate that these regions have sub-micrometer dimensions. Using the DLW technique, extended photoniccrystal templates were recorded via the mechanism of dielectric breakdown in polymethylacrylate (PMMA). Innegative epoxy-based photoresist SU-8 the same approach was used to record extended periodic structures viaphotoinduced cross-linking of polymers. In both materials, various kinds of structures, for example those havingwoodpile and recently elaborated spiral column architecture were fabricated. Another approach discussed hereis simultaneous recording of microstructures by interference of multiple femtosecond pulses. The principle of thistechnique is the same as that of holography, and allows fabrication of periodic 1D, 2D and 3D dielectric structures.It will be shown that complexity and spatial symmetry of these patterns depend on the number of beams involvedand their parameters (incidence angle, amplitude, and phase). Advanced control of the patterns allows to obtainbody-centered cubic (bcc) lattices will be described by means of theoretical calculations. Experimentally, thestructures were recorded by interference in photoresist SU-8. Multiple laser pulses having zero mutual temporaldelay were obtained from a single laser beam simply and efficiently by a diffractive beam-splitter. As the featuresize of patterns becomes decreased, the issues of the structures' mechanical stability during their post-processing(e.g., wet development of SU-8 photoresist) become increasingly important. These aspects will be discussed aswell.

Further author information: (Send correspondence to S. J or H. M.)S. J.: E-mail: Saulius©es.hokudai.ac.jp;H. M.: E-mail: Misawa©es.hokudai.ac.jp, Telephone: (+81 11) 706 9358, Fax: (+81 11) 706 9359.

Int. Conference on Lasers, Applications, and Technologies 2005: Laser-Assisted Micro- and Nanotechnologies,edited by V. Konov, V. Panchenko, K. Sugioka, V. Veiko, Proc. of SPIE Vol. 6161, 616103,

(2006) · 0277-786X/06/$15 doi: 10.1117/12.674990

Proc. of SPIE Vol. 6161 616103-1

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(a) (b) (C) (d)

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Figure 1. The laser microfabrication procedure. The focused laser beam penetrates without absorption into the bulkof material until it reaches the focal region where it invokes TPA or MPA. This region is elongated in the beam'spropagation direction and is marked by the ellipse. Tight focusing and non-linear absorption helps to reduce the size ofthe photomodified region, which may become somewhat shorter than the wavelength of the fabricating laser, and in mostpractical cases is smaller than 1 jm (a). TPA or MPA modifies structural, physical, chemical or other properties of thematerial leading to the pronounced dielectric contrast (b). The smooth or step-like translation of the focal spot is used (c)to record extended dielectric features (solid or voids) or their arrays (d).

2. THE PRINCIPLES OF LASER MICROFABRICATIONMicrofabrication of materials using lasers is a field widely explored and increasingly applied in modern technolo-gies. Fabrication of photonic crystal structures is just one of the many applications existing within this field.The most important principles that underlie laser microfabrication of bulk dielectric materials are illustratedschematically in Fig. 1 with some emphasis on the DLW method (the interference, or holographic technique willbe described later). The typical sequence of laser microfabrication is depicted Fig. 1 . The method is based onthe irradiation of materials by intense optical waves that can be derived from both continuous-wave and pulsedlasers. Pulsed lasers providing ultrashort (picosecond or femtosecond) pulses have an advantage of high peakpower that helps achieve photomodification without undesired thermal effects. The high peak power density isneeded in order to induce TPA2 or MPA3 in materials that have negligible one-photon absorption at the laserwavelength. TPA and MPA are nonlinear processes that are dependent on the local power density. The laserbeam is focused by a positive lens into the bulk of the material and propagates toward the focal region withoutlosses due to the linear or nonlinear absorption Fig. 1 (a). With appropriately chosen laser intensity it is possibleto achieve conditions when non-linear absorption is induced in the focal region only, where the local radiationpower density exceeds some threshold value. The absorption leads to the generation of non-equilibrium chargecarriers in the focal spot. The optical modification of materials depend on their properties and characteristics oflhe laser beam or pulse (Fig. 1(b)). Usually some kind of physical or chemical process is induced which changesphysical properties or phase of the material. For the fabrication of photonic crystal structures, photomodifica-tion processes that lead to instantaneous or subsequent modification of the dielectric properties are the mostimportant.

The simplest example of an instantaneous permanent photomodification occurring under irradiation by laserpulses of various length is the light-induced damage,47 which obliterates the material through the dielectricbreakdown creating a void or rarefied center at the focal region. In such case, the contrast of refractive index, (theratio between the maximum and minimum values of the refractive index, n), is created at the void boundarybetween the non-damaged material (fl2 > 1) and the void (ni 1). The destruction of the material occursthrough the generation of electron-hole plasma, the breakdown, and leads to the mass-density modification.Generation of the free carrier plasma is usually treated as a consequence of the following processes: multi-photonionization causing the excitation of electrons into the conduction band, electron-electron collisional ionizationdue to Joule heating, and plasma energy transfer to the lattice. Femtosecond laser-induced breakdown in various



a)C',

000

Figure 2. Interference of two coherent plane waves with wave-vectors k1 and k2 represented by the beams 1 and . Inthe region of their spatial overlap the electric field intensity becomes spatially modulated as described by Eq. 3 along thedirection parallel to the difference vector k1 — k2.

Objective U SU-8Lens Resist

Figure 3. The experimental setup for the lithography by the multiple-beam interference.

glasses has been widely studied in the 5811 When the laser beam is focused into a small spot, thebreakdown occurs as a distinct "microexplosion" inside the material.9 It is often accompanied by the shockwave generation which may create a shell of densified material having a higher refractive index (compared tonon-damaged regions) surrounding the low refractive index void. Thus, periodic structures consisting of isolatedvoids or extended channels can be generated optically with little or no post-processing required. However, inthese circumstances shape of the focal region must be controlled as precisely as possible during the recording.Due to its explosive nature, laser-induced damage may also generate considerable amount of cracks and otherrandomly scattering unwanted micro- and nano-features. For example, damaging of vitreous silica, (Si02),which is mechanically very robust, may often produce significant randomness in the final structures 18 foran illustration). A somewhat less disruptive damage and smoother voids (possibly, with some role of thermalmelting) can be induced in softer organic materials, like polymethylmethacrylate (PMMA), which is widelyapplied in planar semiconductor processing technologies, and with some additives is also used in everyday life asorganic glass ("plexiglass").

An opposite example of instantaneous modification which leads to formation of hard particles in the irradiatedareas is photo-solidification of liquid photo-curing polymeric resins (see13' 14 for the examples of applications).The modified region of the initial material, which is a liquid, becomes solid, and after the removal of the unexposedliquid, the refractive index contrast between the solidified material (n1 > 1) and the surroundings (n2 1) iscreated. A similar modification can be induced by TPA or MPA in solid photoresists, where the modified regionsare initially latent. After the subsequent wet development, which removes the exposed (unexposed) regions frompositive (negative) photoresist, alternating arrays of voids and solid features are obtained with refractive indexcontrast between the photoresist (n > 1) and the ambient medium, usually air (n 1).

Large part of this work deals with photoresist SU-8 which turned out to be highly suitable for the laser mi-crofabrication of various microstructures. It is a negative epoxy-based photoresist, designed for the fabrication ofultra high-aspect-ratio micromechanical structures using linear exposure to ultraviolet irradiation by projection

DiffractiveBeam Beam

fs-pulse

Sample

Lens



Figure 4. Woodpile structure and its main parameters: Figure 5. Maxwell's scaling in PMMA structures. TheZd-distance between the rods, Lz-distance between the plots show reflection spectra of three samples which havelayers, m - number of layers (a), schematic side-view of lattice parameters proportionally scaled down (the pa-the woodpile structure consisting of rods having ellipti- rameters and scaling factors of 1, 0.83 and 0.66 are mdi-cal cross-sectional shapes, the ellipses are elongated in the cated in the plots) . The numbered dashed lines are guidesbeam focusing direction, their lateral and longitudinal di- to the eye that emphasize the scaling of the peaks' wave-ameters are and d, respectively (b). lengths. In the middle plot, the two peaks seen between

3.0 and 4.0 im actually originate from a single peaks splitat the center by the PMMA intrinsic absorption bandnear the 3.4im wavelength.

lithography. SU-8 has low intrinsic absorption at wavelengths longer than 360 nm, and is capable of resolution ofa few tens of nanometers. Photoexcitation of the photoinitiator results in the photoacid (Lewis acid) generation.The polymerization progresses with the ring-opening of the 1 2-epoxy group by the Lewis acid, and the subsequentregeneration of the Lewis acid that sustains the cross-linking. SU-8 has 8 epoxy groups making it a highly func-tional molecule. Polymerized SU-8 possess a dense network of cross-links and provides a high solubility contrastbetween the strongly and weakly exposed regions. P photopolymerization in SU-8 does not occur immediatelyafter the exposure; for this, the resist must be thermally baked after the exposure. SU-8 is currently commerciallyavailable from several companies worldwide, including Microchem Corp. A summary of the main properties andpotential applications of SU-8 can be found at their web site: http://www.microchem.com/products/su_eight.htm.

Fig. 1(c) illustrates another important element of the laser microfabrication process. The focal spot of thelaser is translated inside the material along the desired path with high precision (accuracy of several nanometersis achievable via use of piezo-electric transducer (PZT)-controlled translation stages) .Translation of the sampleor the laser beam thus facilitates drawing of complex-shaped patterns, including the periodic ones, inside thematerial. As a result, dielectric features, for example similar to those shown in Fig. 1(d) can be permanentlyrecorded. The method described in Fig. 1 also known as the direct laser writing (DLW), is used extensively inour laboratories for various studies involving fabrication of photonic crystal structures'0' 1522 and 3D opticalmemories.23

Besides DLW, spatially-modulated optical radiation patterns created by multiple-beam interference24 can beused to record an entire periodic structure at once. The periodic patterns produced by interference of multiplelaser beams depend on the mutual alignment of the beams' directions and their phases. These patterns canbe visualized and studied using numerical or analytical calculations. For a demonstration here we address thesimplest case of two interfering beams, denoted by numbers 1 and ,having the same angular frequency, w, but

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different wave-vectors, k1 and k2 and phases, q5i and çb2 as depicted in Fig. 2.

Electric field E1,2 of each beam can be to a first approximation described by a plane wave as follows:

Ei,2(ri,2, t) = E,2cos(ki,2 . r1,2—

w1,2t + q51,2) (1)

where index 1 or 2 is used to denote the particular wave. The electric field interference pattern is then obtainedby summing the electric fields of the form 1, and the intensity pattern is obtained by time-averaging the squaredsum-field E2 . For simplicity let us assume that both waves are scalar (i.e. , polarization is ignored) and the fieldamplitudes in both beams are equal to E0. Then, the field can be expressed as

E(r,t) = E1 +E2 = Eocos(ki . r —wt+çLi)+Eocos(k2 . r—wt+Ø2) (2)

(k1 +k2 qi +q2\ (k1 — k2 q5i —= 2E0cos2

r — wt + 2 ) X cos2 2

The intensity field 1(r) is proportional to the square of the electric field:

1(r) o (E(r, t)2) o E {1 + cos [(k1 —k2) . r + q — q]} (3)

The intensity pattern has periodic sinusoidal dependence on the coordinate along the direction defined by thevector k = k1 — k2 . Period of the pattern, A, can be expressed as:

A 71—

2sin[(2)] ki,2lsin [2]

' (

where A is the wavelength of the interfering waves. Hence, the period is inversely proportional to the gratingvector of the one-dimensional pattern Lk . It is easy to see, that assumption of unequal intensities of the twobeams would slightly alter the eqn. 3 which would then contain a spatially uniform "dc" background componentE? + E and an oscillating sinusoidal component with amplitude E1E2. The spatial modulation of the intensitypattern will therefore become weaker. Varying the phase difference of the two beams would result in the spatialshift of the periodic pattern. All these mechanisms also work with multiple beams and can be used to controlgeometry and lattice structure of 2D and 3D patterns they create. Their interference patterns can be calculatedusing an approach similar to that described above. Relation between the interfering beams' parameters and thestructure of the interference field is widely studied in the literature. In fact, it was demonstrated that in threedimensions interference patterns with symmetries of all fourteen Bravais point lattices can be generated by theinterference of four beams.25

The key element of a multi-beam interference setup is the method by which multiple coherent laser beams areobtained from a single laser beam. The simplest technique is to split the laser beam into multiple componentsusing transmission/reflection type beam-splitters. Then, the mutual temporal delays of the beams should beadjusted to be shorter than the laser coherence length or the pulse length , and all beams should be steeredtoward their common interference plane. Using this method all beams can be well separated in space, andit is easy to insert other elements into the beam's paths for the control of their intensities, polarizations andother desired properties. However, the experimental setup might become quite complex, especially when usingultrashort laser pulses. For example, the spatial length of a 100 fs pulse is only 30 ,am, an adjustment ofeach optical path length is needed with micrometer precision. Nevertheless such experimental setup enabled thestudies of recording and properties of 3D structures with fcc lattice.24'2729 An easier method to obtain multiplelaser beams is using multiple diffraction gratings. This technique employs the diffractive beam-splitter (DBS).3°Multiple diffraction of the initial beam on the DBS will result in an angular array of transmitted beams. In fact,a single plate with equivalent transmission function can be fabricated holographically. From the multiple beams,only those required for the creation of the desired interference pattern are selected using a transmission mask.The unmasked beams can then be converged to overlap in space using a system of lenses. DBS is particularlyconvenient for experiments with ultrafast lasers because it automatically ensures zero temporal delay between allindividual beams. Furthermore, due to the diffraction angle dependence on the laser wavelength the overlappingbeams create patterns whose period is wavelength-independent and is uniquely defined by the period of the DBSgratings. Thus, intensity patterns with well-defined periodicity can be obtained even with multicolor lasers.3'



#?

I S

Figure 6. Perspective view of square spiral photonic crystal with 2 interconnected L-shape line defect implemented.

3. EXPERIMENTALThe light source for the experiments was a Ti:Sapphire oscillator (Tsunami) with regenerative amplifier (Spitfire),from Spectra Physics, operating at the repetition rate of 1 kllz, which delivered pulses of about 150 fs durationcentered on the 800 nm wavelength. The fabrication by DLW technique was performed in an optical microscope(Olympus 1X71 ) equipped with oil-immersion objective lenses (magnification 60 x and 100x , numerical aperturesNA=1 .4 and 1 .35, respectively). With both objective lenses the diffraction-limited beam diameter at 1/e2 levelis d = 1.22t 72O nm). 3D drawing was accomplished by mounting the samples on a piezoelectric transducer-controlled 3D translation stage (Physik Instrumente PZ48E) which has a maximum positioning range of up to50 pm and accuracy of several nanometers. The stage motion is controlled by a custom-made software running ona personal computer. During the fabrication the samples were translated along the predefined path, along whichthe desired locations were sequentially irradiated by the tightly focused laser beam. The patterns of photonicstructures were designed using MatLab 6. 0 software. This method allowed to record periodic structures whichconsist both of isolated voxels or of extended (e.g., linear) features.

The experimental setup used in interference recording is shown schematically in Fig. 3. The input beam(from the same laser system as above) entered the DBS and was split into several components. The DBS wasplaced at the focal plane of a positive lens, which transformed the diverging beams into parallel ones. Thetransmission mask placed in their path selected the beams required for the interference pattern creation. Thesebeams were optionally passed through the phase-retarding unit, which consisted of variably-tilted glass plates.The relative phases of the beams can be adjusted by adjusting the tilt angles. For the assessment of the patternquality, a small fraction of the selected beams' intensity is reflected by a glass plate and imaged by a lens onthe CCD camera. The transmitted beams were then focused by a microscope lens with numerical aperture of0.75 into the sample, where they overlap at non-zero mutual angles, creating the desired intensity patterns.According to eqn. 4, the highest spatial frequency in the pattern is determined by the maximum mutual angleat which two (or more) beams overlap. The theoretical resolution limit is the half-wavelength, which is achievedfor counter-propagating beams. However, in most experiments based on the setup shown in Fig. 3, the beamsconverge on the sample at somewhat smaller angles and the resolution was lower.



. .... .(b) ::.:. . ... ..... . (cl::::

c 'iL1,'' , 4 S !fr —'4 — w

Figure 7. Spiral arms with large lateral size of about 370 nm (a), 270 nm (b) and formation of spiderweb-like weakcrosslinking. (c) Spiral arms with lateral size of about 230 nm. Spider-web like cross-linkings are not longer observed.Scale bars, 200 nm.

With both setups, the pulse duration at the focal spot where the fabrication took place was measured usingGRENOUILLE technique from Swamp Optics which allowed recording of the time x spectrum image of thepulse, and retrieval of the pulse duration using frequency-resolved optical gating (FROG) algorithm (FemtosecondTechnologies) as described 32

The SU-8 samples were films of a commerically available negative-tone photoresist SU-8 (from MicrochemCorp.) spin-coated to the thickness of 20 —40 1tm on the glass substrate. The PMMA samples were preparedsimilarly, but instead of spin-coating, droplets of liquid formulations of PMMA were deposited on the substrateand allowed to dry in a controlled environment. Structural characterization of the samples was performedusing Scanning Electron Microscopy (SEM) and Laser Scanning Microscopy (LSM). The assessment of photonicproperties of the structures (transmission and reflection measurements) were performed using a Fourier-TransformInfrared (FT-IR) spectroscope (Valor III, Jasco) coupled with infrared microscope (Micro 20, Jasco).

4. RESULTS AND DISCUSSION4.1. Direct laser writing of 3D structures in PMMAFor recording, the so-called woodpile architecture was chosen.33 Woodpile structure has a face-centered cubic(fcc) of face-centered tetragonal (fct) point lattice and an "atomic" basis consisting of two perpendicular dielectricrods, which results in a diamond-like structure, capable of opening a wide PBG. The woodpile architecture anddefinition of its parameters are shown in Fig. 4. Woodpiles can be formed conveniently by stacking layersof uniformly spaced dielectric rods and hence are easy to build in a layer-by-layer manner. In the case ofPMMA, the rods represent linear voids in a solid PMMA matrix. DLW of woodpile structures was performedby smoothly scanning the focal spot position along the channel lines in small steps, /.l = 0.05 — 0.2 tm, withsingle or multiple-shot exposure at each step. The neighboring exposed spots overlapped strongly leading to theformation of void channels. Figure 5 shows reflection spectra of samples with different periodicity measured alongthe z-axis direction (along the direction of recording). The most apparent feature of spectra is the presence ofreflection peak (actually, spectrally matched with the transmission dip; not shown here). The matching indicatesthat optical attenuation observed at this wavelength is due to the rejection of the incident radiation by thestructure and is not related to absorption. Such behavior is typical for PBG, however, since the magnitude ofthe transmission dip is quite low, photonic stop gap (a gap which exist only along a particular direction only)should be held responsible for this observation. Low magnitudes of the dips and peaks may also result from thefollowing factors: 1) low refractive index contrast, about 1.6 to 1, 2) scattering by random defects, 3) angularspreading and elimination of normally incident rays of the light beam in the infrared Cassegrainian microscopeobjective used for probing the sample, which therefore measures transmission and reflection properties alongmultiple directions.



Figure 8. Four non-coplanar beams can produce25: (left) Simple cubic (sc) calculated from the triclinic expression;(middle) body centered cubic (bcc) calculated from the body centered orthorhombic expression; (right) face centeredcubic (fcc) calculated from the body centered orthorhombic expression. Top row shows the single cells. A - lattice period.

4.2. Direct laser writing of 3D structures in SU-8SU-8 is a well suited material for DLW. The initial photomodification of SU-8 does not induce any observableoptical contrast changes, hence the focusing conditions are not altered by presence of the previously recordedpart of the pattern. This circumstance removes various limitations, like the choice of recording direction, depth,and order of recording. Several different patterns can be superimposed at the same location. Moreover, DLWcan be combined with the holographic recording to fabricate large-area periodic samples containing non-periodicdefects.

Previously we have reported successful recording of high quality woodpile structures in SU-8.21 As amore intriguing example, here we present the fabrication photonic crystals belonging to are relatively recentlyelaborated class of spiral structures.34' The spiral architecture consists of arrays of spirals winding up alongthe z-axis, and arranged on a square lattice with period a in the x — y plane. Other parameters of the spiralarchitecture is the spiral arm length, L, and length of its one period, c. Detailed definitions and explanationsof these parameters can be found in the references given above. Spiral structures were designed intended forfabrication by the glancing angle deposition (GLAD) method, which uses growth of silicon spirals on pre-patterned substrates. For silicon spiral structures in air, large theoretical photonic band gap of 24 28% ofthe gap center frequency (the so-called woodpile configuration is 18%) has been reported, close to the championstructure of the diamond network of 30%. However, fabrication of spiral structures is more difficult, as comparedto the more straightforward woodpile configuration.

Using DLW, we have fabricated squire spiral structure photonic crystals with lattice period 1.2 1.8im.36The SEM image of te structure 8 is shown in Fig. 6. This view illustrates that good periodicity has beenachieved. The mechanical stability of the photonic crystal is equally important as resolution, and is moreso when intentional defects are to be implemented within the periodic structure. Defect engineering is animportant application of photonic crystal, for creating functionalities such as waveguiding and microcavity.Defect engineering is demonstrated by implementing 2 interconnected L-shape line defect on the surface of asquare spiral photonic crystal shown in Fig. 6. These defects are non-functional, as they are constructed onthe surface of the structure for ease of inspection. It is evident that the defect is well sustained and the overallstructure have negligible distortion.

-0.5-1 -1 XQ)

00jN.

Y(?) -1 -i X(?)



Figure 9. (a) Geometry of holographic recording by 6 beamlets. (b) The corresponding calculated pattern by six planewave interference. SEM images of the recorded pattern (c) and its closeup view (d).

The expected photonic stop gap (a partial gap existing along one direction only) for these structures wereobserved to be between 2 6pm, the spectral occurrence of their reflectance peaks and transmission dipscorresponds to the lattice period as expected from Maxwell's scaling law.37 The feature size of the spiralarms that make up these periodic structures have lateral dimensions of 350 - 400nm, equivalent to half thediffraction limit (A/2). This resolution can be achieved by careful adjustment of the irradiation pulse energyduring exposure.

The two factors that affect the quality of photonic crystals most significantly are polymer shrinkage andthe photoacid diffusion. SU-8, like most polymers, shrinks during postbaking (after exposure), when the SU-8monomers move closer and join up to form a network (crosslinking) . Polymer shrinkage can introduce latticedistortion and internal stress, hence deforming or even destroying the intended structure. One way to minimizepolymer shrinkage is to have a near-solvent free SU-8 film for the recording (exposure) process. The SU-8monomers are then closer to each other initially with lesser solvent molecules in between them. However, thesolvent is still necessary as photoacid generated during the exposure requires certain mobility in order for thecrosslinking to progress. This mobility is provided by the solvent. In general, a high solvent content will causethe photoacid to have large diffusion length, more extended crosslinking network and hence poorer resolution,while a low solvent content curbs the diffusion length and gives better resolution. Too few crosslinking may notgive a sustainable structure after chemical development. The amount of photoacid that is generated, depends onthe exposure dose, or laser pulse energy in the case of DLW. Hence, to construct structures with high resolutionand are mechanically stable requires an SU-8 sample with appropriate solvent content, controlled exposure dose,and a careful chemical development to produce the latent image.

Fig. 7(a) shows a SEM image of a structure with excessive photoacid diffusion. The lateral diameter of therod are about 370 nm and appearance of spider-web like network are clearly visible due to undesirable diffusionand the formation of weak crosslinking. Such features are obtained when the irradiated points (voxels) arerecorded too close to each other resulting in excessive generation of photoacid in a localized area. For the samedesign parameter of the spiral periodic structure, by appropriate spacing of the irradiated points and controllingthe exposure dose, a much better resolution is obtained (see Fig. 7(b)) . The lateral size of the rods decreasedto about 270 nm. But tailoring the pre-processing condition of the SU-8 sample, and using extended soft-bakeduration several times the standard duration recommended by the maker, we obtained robust and fine features(lateral diameter 230 nm) as shown in Fig. 7(c). Notice that the spider-web like networks disappeared totally.These results illustrate that a combination of exposure dose and tailored pre-processing condition for the SU-8sample is effective in obtaining higher resolution recording. Rod size with lateral diameter 230 nm translate to



Figure 10. SEM image of a five beam hologram recoded in SU8 at different locations of the same 4OO-im-diameter spot.Four side beams made a 34 deg angle with the central beam. The intensity ratio of the beams was 1(side):4(central).

about 3A/1O, hence sub-diffraction limit recording has clearly been achieved.

Besides square spiral periodic structures, Noda et al. 38 proposed a more generic type of spiral architecturewhere instead of square spirals, circular spirals in which adjacent spiral columns are phase-shifted by half thespiral pitch are found to have large theoretical PBG ( 28% of the gap center frequency). We have successfullyimplemented this type of circular spiral photonic crystals for the first time. These circular spiral photoniccrystals also display photonic stop gaps similar to the square spirals. The lack of difference between the two canbe attributed to the low refractive index of SU-8 ( 1 .6 at visible wavelength).

4.3. Holographic recording of 3D structures in SU-8As was already mentioned earlier, interference of four non-coplanar beams can create light intensity distributionhaving symmetry of all 14 Bravais lattices.25 As an example, a 3D pattern having cubic point lattice has beencalculated by using formulae given in ref. [25]. Recording of such structures in SU-8 using the setup outlinedearlier is quite straightforward, provided that correct beam incidence angles were chosen.

The interference setup based on DBS,3° though limited from the point of view of achievable pattern ge-ometries, is very simple and can fabricate large areas (with cross-sectional dimensions of millimeters) withinseveral seconds of exposure. By supplementing this method with the phase control capability,39 even morecomplicated patterns can be recorded. Thus, it is possible to create lattices, which have no analogues in therealm of crystalline latices can be created, including Penrouse geometries. Such structures could find applicationas templates for the photonic crystals and plasmonic devices, and are potentially useful in the fields of nano-and micro-fluidics.

Figure 9 compares the theoretical and experimental geometries of a 2D structure. The structure is recordedusing six-beam interference; the beams are arranged as shown in Fig. 9 (a) . The resulting light intensity patternis displayed in Fig. 9 (b) where the iso-surfaces enclosing the shaded areas mark the regions where the lightpower density exceeds a certain threshold value. Figure 9 (c-d) shows the geometry of the experimental sample atvarious magnifications. Detailed comparison between the theoretical and experimental data shows that structurewas recorded in SU-8 with high fidelity. The smallest feature size in the pattern (the bridges between the circularspots) was approximately 100 nm.

A 3D structure recorded by the same approach is demonstrated in Fig. 10. It has a body-centered-tetragonalstructure. It was recorded in SU-8 using interference of five laser beams incident on the sample at a half. angleof 34°. The two images shown in Fig. 10 taken in different parts of the structure illustrate a quite commonproblem: the patterns appear to be different in the two areas. This is most likely due to the uncontrollablephase variations occurring in various regions of the cross-section of the interfering beams with diameters in therange of 50 — 200 .im). This example shows that for the best results, phases of the recording beams must be

Proc. of SPIE Vol. 6161 616103-10


controlled precisely. Numerical analysis also implies that an ideal bct structure obtainable without the phasecontrol is self-supporting, provided that suitably high exposure levels are used. However, the structures alsobecome impermeable, which inhibits their development and infiltration by other materials. At lower exposuresthe bct structure becomes not connected and dislodges during the development.

5. CONCLUSIONS

Micro- and nano-optics is becoming increasingly important in various technology-driven areas, like optoelectron-ics, communications, and data storage. Laser microstructuring of materials allows fast and low-cost fabricationof microstructures applicable in these fields. In this work we have demonstrated some of the possible applica-tions of two different laser microfabrication techniques, namely direct laser writing and holographic recording bymultiple-beam interference. These techniques, though still in need for further refinement, allow unprecedentedflexibility in microstructuring of materials and are likely to become indispensable part of photonic technologiesof the near future.

AcknowledgmentsWe are grateful to Drs. S. Matsuo and Y. Tanamura for many fruitful discussions regarding various aspect of SU-8microfabrication.

REFERENCES1. S. Juodkazis, V. Mizeikis, K. K. Seet, M. Miwa, and H. Misawa, "Two-photon lithography of nanorods in SU-8

photoresist," Nanotechnol. 16, pp. 846 — 849, 2005.2. M. Goppert-Mayer, "Uber Elementarakte mit zwei Quantensprungen," Ann. Phys. 9, pp. 273—294, 1931.3. N. Tanno, K. Ohkawara, and H. Inaba, "Coherent transient multi-photon scattering in a resonant two-level system,"

Phys. Rev. Lett. 46, pp. 1282—1285, 1981.4. B. Stuart, M. Feit, A. Rubenchik, B. Shore, and M. Perry, "Laser-induced damage in dielectrics with nanosecond to

subpicosecond pulses," Phys. Rev. Lett. 74, pp. 2248—2251, 1995.5. N. Bloembergen, "Laser-induced electric breakdown in solids," IEEE Journ. Quant. Electron. 10, pp. 375—386, 1974.6. B. Stuart, M. Feit, A. Rubenchik, S. Herman, B. Shore, and M. Perry, "Nanosecond-to-femtosecond laser induced

breakdown in dielectrics," Phys. Rev. B. 53, pp. 1749—1761, 1996.7. C. Carr, H. Radousky, and S. Demos, "Wavelength dependence of laser-induced damage: determining the damage

initiation mechanisms.," Phys Rev Lett 91(12), p. 127402, 2003.8. T. Apostolova and Y. Hahn, "Modelling of laser-induced breakdown in dielectrics with subpicosecond pulses," J.

Appi. Phys. 88, pp. 1024—1034, 2000.9. E. N. Glezer and E. Mazur, "Ultrafa.st-laser driven micro-explosions in transparent materials," Appi. Phys. Lett. 71,

pp. 882—884, 1997.10. H. Misawa, H. Sun, S. Juodkazis, M. Watanabe, and S. Matsuo, "Microfabrication by femtosecond laser irradiation,"

in Laser applications in microelectronic and optoelectronic manufacturing, H. Helvajian, K. Sugioka, M. C. Gower,and J. J. Dubowski, eds., pp. 246—260, SPIE, vol. 3933, 2000.

11. J. Natoli, L. Gallais, H. Akhouayri, and C. Amra, "Laser-induced damage of materials in bulk, thin-film, and liquidforms.," Appi Opt 41(16), pp. 3156—66, 2002.

12. H. B. Sun, Y. Liu, K. Sun, S. Juodkazis, M. Watanabe, S. Matsuo, H. Misawa, and J. Nishii, "Inlayed "atom-likethree-dimensional photonic crystal structures created with femtosecond laser microfabrication," Mat. Res. Soc. Symp.Proc. 605, pp. 85—90, 2000.

13. B. Cumpston, S. Ananthavel, S. Barlow, D. Dyer, J. Ehrlich, L. Erskine, A. Heikal, S. Kuebler, I.-Y. Lee, D. Mccord-Maughon, J. Qin, H. Rockel, M. Rumi, X.-L. Wu, S. Marder, and J. Perry, "Two-photon polymerization initiatorsfor three-dimensional optical data storage and microfabrication," Nature 398, pp. 51—54, 1999.

14. 5. Kawata, H-B. Sun, T. Tanaka, and K. Takada, "Finer features for functional microdevices," Nature 412, pp. 697—698, 2001.

15. H. Sun, S. Matsuo, and H. Misawa, "Three-dimensional photonic crystal structures achieved with two-photon-absorption photopolymerization of resin," Appi. Phys. Lett. 74, pp. 786—788, 1999.

16. H. B. Sun, Y. Xu, S. Matsuo, and H. Misawa, "Micro-fabrication and characteristics of two-dimensional photoniccrystal structures in vitreous silica," Opt. Rev. 6, pp. 396—398, 1999.

17. H. B. Sun, Y. Liu, S. Juodkazis, K. Sun, J. Nishii, Y. Suzuki, S. Matsuo, and H. Misawa, "Photonic lattices achievedwith high-power femtosecond laser microexplosion in transparent solid materials," in Proc. SPIE, e. a. X. Chen, ed.,3888, pp. 131—142, 2000.

Proc. of SPIE Vol. 6161 616103-11


18. H. B. Sun, Y. Liu, K. Sun, S. Juodkazis, M. Watanabe, S. Matsuo, H. Misawa, and J. Nishii, "Inlayed "atom"-likethree-dimensional photonic crystal structures created with femtosecond laser microfabrication," Mat. Res. Soc. Symp.Proc. 605, pp. 85—90, 2000.

19. H. Sun, S. Matsuo, and H. Misawa, "Three-dimensional photonic crystal structures achieved with two-photon-absorption photopolymerization of resin," Appi. Phys. Lett. 74, pp. 786—788, 1998.

20. H. Sun, V. Mizeikis, Y. Xu, S. Juodkazis, J.-Y. Ye, S. Matsuo, and H. Misawa, "Microcavities in polymeric photoniccrystals," Appi. Phys. Lett. 79, pp. 1—3, 2001.

21. V. Mizeikis, K. Seet, S. Juodkazis, and H. Misawa, "Three-dimensional woodpile photonic crystal templates for theinfrared spectral range.," Opt Lett 29(17), pp. 2061—3, 2004.

22. K. Seet, V. Mizeikis, S. Matsuo, S. Juodkazis, and H. Misawa, "Three-dimensional spiral-architecture photoniccrystals obtained by direct laser writing," Adv. Mater. 17, pp. 541—544, 2005.

23. M. Watanabe, S. Juodkazis, H-B. Sun, S. Matsuo, H. Misawa, M. Miwa, and R. Kaneko, "Transmission and photo-luminescence images of three-dimensional memory in vitreous silica," Appi. Phys. Lett. 74, pp. 3957—3959, 1999.

24. M. Campbell, D. Sharp, M. Harrison, R. Denning, and A. Thrberfleld, "Fabrication of photonic crystals for thevisible spectrum by holographic lithography," Nature 404(6773), pp. 53—6, 2000.

25. L. Cai, X. Yang, and Y. Wang, "All fourteen bravais lattices can be formed by interference of four noncoplanarbeams," Opt. Lett. 27, pp. 900—902, 2002.

26. X. Wang, J. Xu, H. Su, Z. Zeng, Y. Chen, H. Wang, Y. Pang, and W. Tam, "Three-dimensional photonic crystalsfabricated by visible light holographic lithography," Appi. Phys. Lett. 82, pp. 2212—2214, 2003.

27. C. Ullal, M. Maldovan, E. Thomas, G. Chen, Y.-J. Han, and S. Yang, "Photonic crystals through holographiclithography: Simple cubic, diamond-like, and gyroid-like structures.," Appi. Phys. Lett. 84, pp. 5434—5436, 2004.

28. D. Sharp, A. Thrberfield, and R. Denning, "Holographic photonic crystals with diamond symmetry," Phys. Rev.B. 68, p. 205102, 2003.

29. X. Wang, J. Xu, H. Su, Z. Zeng, Y. Chen, H. Wang, Y. Pang, and W. Tam, "Three-dimensional photonic crystalsfabricated by visible light holographic lithography," Appi. Phys. Lett. 82, pp. 2212—2214, 2003.

30. T. Kondo, S. Matsuo, S. Juodkazis, and H. Misawa, "A novel femtosecond laser interference technique with diffractivebeam splitter for fabrication of three-dimensional photonic crystals," Appi. Phys. Lett. . 79(6), pp. 725—727, 2001.

31. V. Berger, O.Gauthier-Lafaye, and E. Costard, "Photonic band gaps and holography," J. Appi. Phys. 82, pp. 60—64,1997.

32. S. Juodkazis, T. Kondo, S. Dubikovski, V. Mizeikis, S. Matsuo, and H. Misawa, "Three-dimensional holographicrecording in photo-thermo-refractive glass by femtosecond pulses," in mt. Conf. Advanced Laser Technologies, ALT-2002 (Sept. 15-20 2002, Adelboden, Switzerland) SPIE Proc. 5147, H. P. Weber, V. I. Konov, and T. Graf, eds.,pp. 226 — 235, 2003.

33. K. M. Ho, C. T. Chan, C. M. Soukoulis, R. Biswas, and M. Sigalas, "Photonic band gaps in three dimensions: Newlayer-by-layer periodic structures," Solid State Commun. 89, pp. 413—416, 1994.

34. 0. Toader and S. John, "Proposed square spiral microfabrication architecture for large three-dimensional photonicband gap crystals.," Science 292(5519), pp. 1133—5, 2001.

35. 0. Toader and S. John, "Square spiral photonic crystals: robust architecture for microfabrication of materials withlarge three-dimensional photonic band gaps.," Phys. Rev. E 66, p. 016610, 2002.

36. K. K. Seet, V. Mizeikis, S. Matsuo, S. Juodkazis, and H. Misawa, "Three-dimensional spiral - architecture photoniccrystals obtained by direct laser writing," Adv. Mat. 17(5), pp. 541 — 545, 2005 (DOT: 10.1002/adma.200401527).

37. J. D. Joannopoulos, R. D. Meade, and J. N. Winn, Photonic Crystals: Molding the Flow of Light, Princeton UniversityPress, Princeton, New Jersey, 1995.

38. 5. Noda, "Three-dimensional photonic crystals operating at optical wavelength region," Physica B 279, pp. 142—149,2000.

39. T. Kondo, S. Matsuo, S. Juodkazis, V. Mizeikis, and H. Misawa, "Three-dimensional recording by femtosecond pulsesin polymer materials," J. Photopolym. Sci. Tech. 16(3), pp. 427—432, 2003.

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