1348 OPTICS LETTERS / Vol. 27, No. 15 / August 1, 2002

Three-dimensional microfabrication by two-photon-initiatedpolymerization with a low-cost microlaser

Irène Wang, Michel Bouriau, and Patrice L. Baldeck

Laboratoire de Spectrométrie Physique, Université J. Fourier, Centre National de la Recherche Scientifique,Unité Mixte de Recherche 5588, 38402 Saint Martin d’Hères Cedex, France

Cécile Martineau and Chantal Andraud

Ecole Normale Supérieure de Lyon, Laboratoire de Stéréochimie et Interactions Moléculaires, Unité Mixte de Recherche 5532,46 Allée d’Italie, 69364 Lyon Cedex 07, France

Received February 26, 2002

Fabrication of submicrometer structures by two-photon-initiated polymerization is performed with an inexpen-sive and low-power microlaser. This is made possible by the design of photoinitiators with strong two-photonabsorption cross sections. We analyze the inf luence of both material properties and irradiation conditions onthe two-photon polymerization rate and show that resins based on our highly sensitive two-photon photoini-tiator can be solidified with microlaser excitation, whereas commercial UV photoresins require ultrashort andintense laser pulses. © 2002 Optical Society of America

OCIS codes: 220.4000, 190.4710, 160.4330, 160.5470, 350.3390.

Photopolymerization initiated by two-photon absorp-tivity (TPA) is an interesting method for the fabricationof complex micrometer-size three-dimensional (3D)structures.1 – 9 When a chromophore acting as a pho-toinitiator is excited by the simultaneous absorption oftwo photons, it can chemically activate polymerizationreactions, leading to the formation of solid structures.Since the absorption rate depends on the square ofthe incident laser intensity, TPA polymerization bene-fits from a large penetration depth and tight confine-ment of the excitation volume about the focal point.Thus, TPA polymerization provides a very simple andstraightforward way to produce intricate 3D structureswith high resolution. The wide range of microstruc-tures of arbitrary complexity that can be produced bythis technique should f ind applications in microf lu-idics, microelectromechanical systems, 3D integratedoptical components, and biomedical microdevices(e.g., for drug delivery, manipulation of biologicalobjects).

Recently, two-photon-induced polymerization wasused to produce integrated optical devices,3 opticalmemories,4 photonic bandgaps,4 – 6 and microma-chines,7,8 However, an important limitation to thewidespread application of this attractive approacharises from the weak sensitivity of commercial UVphotopolymers to TPA excitation: They require highlaser intensities and substantial exposure times. Thenecessity to employ lasers with ultrashort pulses,high peak powers, and large repetition rates is notonly onerous but also may damage the material, sothat, in practice, only a small intensity window canbe used for safe fabrication. In this context it seemsinteresting to identify molecules that display goodinitiation efficiency together with a strong TPA. Suchan approach would make it possible to reduce boththe laser intensity and the exposure time. Up tonow, only a few studies in this direction have beenundertaken.4,9 – 11

We recently reported the design and synthesisof new chromophores that exhibit high TPA in the

0146-9592/02/151348-03$15.00/0 ©

visible spectral range (450–550 nm) and are ableto photochemically initiate the polymerization ofacrylate monomers.11 These new chromophores arebased on a D P D structure (where D is an aminodonor and P is a weak conjugated bridge, biphenyl orf luorene).4,12 Our results show that resins based onthese photoinitiators are orders of magnitude moresensitive to TPA excitation than are conventional UVphotoresins.

In this Letter we demonstrate the possibility ofusing low-power microlaser TPA-initiated polymer-ization. First, a simple model is proposed to relatethe progression of the photopolymerization reactionto material and laser irradiation characteristics. Onthe basis of this model, we can compare our materialwith commercial UV systems, showing that laserswith moderate intensities and low repetition rates areable to induce TPA polymerization of resins based onour photoinitiator. An example of 3D microstructurefabricated with a low-cost microlaser is given.

For our experimental conditions, we consider thebasic chemical processes involved in two-photon poly-merization to be those shown Fig. 1. Under laserirradiation, the photoinitiator dye can be excited bysimultaneous absorption of two photons. Then, it un-dergoes an energy-transfer reaction with an acrylatemonomer, resulting in the creation of radicals that areable to initiate the chain polymerization reactions.The growth of chain radicals is restrained by termi-nation reactions. Below, we assume that bimolecular

Fig. 1. Basic steps of polymerization.

2002 Optical Society of America

August 1, 2002 / Vol. 27, No. 15 / OPTICS LETTERS 1349

recombination between propagating radicals is thepredominant termination process.13,14

The polymer structure that is fabricated has toresist the developing stage when unexposed resinis washed away. It should also possess adequatemechanical strength. Therefore, a suff icient degreeof cross linking has to be achieved. The progressionof the cross-linking reactions under two-photon ex-citation for given exposure conditions and materialproperties is derived below.

The degree of polymerization can be estimatedthrough the quantity of monomer that is convertedto polymer. The rate of monomer conversion can beconsidered equal to the rate of propagation and isgiven by Eq. (1), where kp is the rate constant forpropagation, �M� is the monomer concentration, and�M≤� is the total concentration in chain radicals:

2d�M��dt � kp�M≤� �M � . (1)

The evolution of the concentration in propagating radi-cals �M≤� results from a competition between the ini-tiation and termination rates, as shown in Eq. (2), inwhich Ri stands for the initiation rate and kt is therate constant for termination:

d�M≤��dt � Ri 2 2kt�M≤�2. (2)

We consider Eqs. (1) and (2) to describe completely thekinetics of the polymerization process. In this modelthe inf luence of scavengers such as oxygen is over-looked. Diffusion and other nonlocal phenomena arenot taken into account either.

Although pulsed lasers are used to generate the radi-cal species, the initiation rate is assumed to be con-stant in time, since the cycle time of lasers is usuallyfast compared with the lifetime of growing radi-cals.13 Equation (3) gives an expression of the av-erage initiation rate, which depends on the rate ofproduction of excited photoinitiators by TPA and theefficiency of the energy-transfer process leading to theformation of radicals:

Ri � FsTPA�A�2�h̄v�2

µZpulse

I2 dt∂F . (3)

In Eq. (3), F is the quantum yield for radical genera-tion, i.e., the probability that an excited photoinitiatorwill give rise to a radical; sTPA is the TPA cross sec-tion of the initiator dye; �A� is the concentration in thephotoinitiator; h̄v is the photon energy; �

Rpulse I2 dt�

is the square of the light intensity integrated over onepulse; and F is the repetition rate of the laser.

Solving Eqs. (1) and (2) yields the progression ofthe polymerization reaction: The evolution of the con-centration of radicals is expressed in Eq. (4), and thedensity of monomer converted to polymer is given byEq. (5). The laser is switched on at t � 0. During ex-posure, �M≤� increases until it reaches its steady-statevalue, �M≤�s � �Ri��2kt��1�2:

�M≤� � �M≤�s tanh�2kt�M≤�st� , (4)

Mconsumed � �M�in1 2 �cosh�2kt�M≤�st��2kp�2kt

o. (5)

�M�i is the initial concentration of monomer.A close approximation of Eq. (5) is given by

Mconsumed � �M �i�1 2 exp�2kp�M≤�st�� when exposure

times are long compared with 1�kt�M≤�s. For therange of exposure times that we use experimen-tally ��10 ms�, the quantity 1�t � kp�M≤�s can beconsidered the rate of polymerization. The inf lu-ence of laser and material parameters on this rateis detailed in Eq. (6), where Ki � FsTPA�A� andKl � �

Rpulse I2 dt�F��2�h̄v�2�:

1t

�kpp2kt

K1�2i K1�2

l . (6)

In Eq. (6), there are contributions from the reactiv-ity of the monomer and propagating radicals (kp andkt), the properties of the photoinitiator �Ki�, and thecharacteristics of the laser source and exposure geom-etry, i.e., intensity, pulse duration, and repetition rate�Kl�. Therefore, Eq. (6) can be used to compare thepolymerization speed obtained with different resinsand irradiation conditions.

Below, we show that, because of the high sTPAvalues of our photoinitiator, a nanosecond microlaserleads to the same polymerization speed as that ob-tained with femtosecond lasers and commercial UVresins. As kinetic parameters, kp and kt, are of thesame order of magnitude for most usual monomersin radical chain polymerization,13 our comparison isbased only on Ki and Kl. As an example of TPAfabrication using a commercial UV initiator anda femtosecond laser, we consider the experimentalconditions employed by Kawata and co-workers.8,15

For our photopolymerization experiments, the lightsource was a frequency-doubled Nd:YAG microlaserfrom Nanolase JDS Uniphase delivering 0.5-ns pulseswith a 6.5-kHz repetition rate at the wavelength of532 nm. An average power of 1.2 mW was used,and the laser was focused into a 1.8-mm-waist focalspot.11 In our case, the factor Kl yields a value ofKl � 6 3 1058 photons2 m24 s22 at the center of thefocal spot. The photopolymerization experimentsby Kawata and co-workers were carried out with amode-locked Ti:sapphire laser with a 150-fs pulseduration, a 76-MHz repetition rate, and 140-pJ pulseenergy focused through a 1.4-N.A. objective lens. Anapproximate value of Kl � 1 3 1064 photons2 m24 s22

is obtained in these irradiation conditions. Therefore,by using a microlaser and weak focusing, we lose afactor of 105 on Kl.

Let us now compare the values of Ki. Ourresin consisted of 3% photoinitiator �N4,N40-bis-�4-methoxyphenyl�-N4,N40-diphenyl-2, 7-diamino-9, 9-diethylf luorene�, 70% monomer [tris(2-hydroxyethyl)isocyanurate triacrylate with a radical inhibitor levelof 100 parts in 106] and 27% polymer binder[poly(styrene-co-acrylonitrile) �75�25�]. The synthe-sis, TPA cross section, electrochemical potential, andinitiation properties of our photoinitiator werereported previously.11 Its TPA cross section at532 nm was found to be sTPA � 8 3 10249 cm4 sphoton21. The quantum yield F of the energytransfer between the excited photoinitiator and themonomer can be deduced from biomolecular quenchingvalues already reported for molecules similar to ourphotoinitiator in presence of acrylates.16 Consideringthat the typical excited-state lifetime was 1 ns, we

1350 OPTICS LETTERS / Vol. 27, No. 15 / August 1, 2002

Fig. 2. Scanning electron microscope images of a 1-Eurocoin, fabricated with a microlaser: (a) overview, (b) mag-nif ied detail.

found an approximate value of F � 1023. In the caseof commercial UV photoinitiators, they exhibitweak sTPA values. As an example, a value of3.8 3 10256 cm4 s photon21 was measured by Miwaet al. on a commercial photoresin.17 This is 7 ordersof magnitude lower than our photoinitiator’s TPAcross section. However, the radical generationquantum yield of commercial photoinitiators isexpected to be better than 1021. Therefore, theoverall value of Ki is approximately 5 orders ofmagnitude higher for our resin than for commercialUV resins. This higher magnitude compensates forthe lower value of Kl resulting from our irradiationsource. Hence, the polymerization speed obtained isof the same order of magnitude in both experiments.

The threshold exposure for polymerization of ourresin was determined to be 95 J�cm2,11 correspondingto a minimum exposure time of 4 ms to induce poly-merization at the center of the beam. This time is ofthe same order of magnitude as the exposure times re-ported by Tanaka et al.15 In accordance with our pre-dictions, polymerization speeds are comparable whenone is using either our optimized photoinitiator andlow-intensity exposure or a commercial UV initiatorand tightly focused ultrashort laser pulses.

We created polymer microstructures with our mi-crolaser to illustrate the large range of possibilitiesoffered by the two-photon polymerization technique.Figure 2 displays scanning electron microscope im-ages of a 1-Euro coin. The images were fabricated bypoint-by-point irradiation with a high-N.A. (1.3-N.A.)objective lens, allowing a focal spot of �0.2-mm radialsize with an average power of �3 mW to form. Thedistance between adjacent points was 200 nm. Afterthe resin film was irradiated, we immersed it in ace-tone to dissolve the unexposed material, leaving onlythe polymer structure on the glass plate. The 1-Eurocoin had a diameter of 20 mm, and features as smallas 160 nm could be depicted on the micrograph. Webelieve that better spatial resolution can be attainedthrough optimization of the irradiation conditions withrespect to the resin photosensitivity. This aspect isunder study.

In conclusion, we have shown that two-photon-initiated polymerization could be performed with a

common and inexpensive laser source, owing to thedesign of photoinitiators with large TPA. This resultshould encourage the widespread use of this versatiletechnique, which makes it possible to fabricate 3D mi-crostructures of arbitrary complexity. We proposed asimple model to relate the progression of the polymer-ization process to the photopolymer and irradiationcharacteristics, providing a basis on which differentexperimental conditions can be compared. Usingthis model, we can clarify why our photoresin can besolidified by exposure to low intensities, at speedsequivalent to those obtained in other microfabricationexperiments that employed femtosecond lasers.5,8,17

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