Multi-photon polymerization of inorganic-organic hybrid ...

Multi-photon polymerization of inorganic-organic hybridpolymers using visible or IR ultra-fast laser pulses for optical

or (opto-)electronic devices

Sonke Steenhusena,b, Thomas Stichela,c, Ruth Houbertz*a and Gerhard Sextla,b

aFraunhofer ISC, Neunerplatz 2, 97082 Wurzburg, Germany;bJulius-Maximilians-Universitat Wurzburg, Chemische Technologie der Materialsynthese,

Rontgenring 11, 97070 Wurzburg, GermanycJulius-Maximilians-Universitat Wurzburg, Lehrstuhl fur Experimentelle Physik 5,

Am Hubland, Wurzburg, Germany

ABSTRACT

Integrated passive and active devices are the key components in current and future information technology.In order to fulfill requirements in miniaturization for (integrated) optical or electronic devices, nano-scaledmaterials with a good compatibility to high-resolution processing techniques are needed. According to theserequirements, multi-photon techniques attract much attention by providing a resolution far beyond the diffractionlimit. The patterning of the inorganic-organic hybrid polymers, which are synthesized by catalytically controlledhydrolysis/polycondensation reactions, will be discussed with respect to the underlying photochemical processes.Emphasis will be on the direct writing of structures using femtosecond laser pulses, making use of two- andthree-photon absorption (TPA/3PA) processes with visible or IR light, which also allows one to write arbitrary3D structures. Due to the very sharp threshold fluence for these processes and its non-linear behavior, featuresdown to 100 nm can be realized by choosing a suitable combination of material formulation and patterningparameters. Voxel arrays were written, whereas the resulting voxel sizes are compared to a growth model, andthe influence of radical diffusion and chain propagation is discussed. In order to determine the TPA cross-sectionand to estimate the role of the photoinitiator, a z-scan experiment was realized. The initiators’ cross-sectionswill be correlated to the resulting voxel sizes.

Keywords: two-photon polymerization, multi-photon polymerization, hybrid polymers, rapid prototyping

1. INTRODUCTION

The physical phenomenon of two-photon absorption (TPA) was considered as a scientific curiosity, when it wastheoretically postulated by Maria Goppert-Mayer in 1931.1 Due to the lack of highly irradiant electromagneticradiation, it took several decades for scientists to demonstrate the effect in the lab. With the invention ofthe laser in the seventies of the last century, especially ultrafast pulsed lasers, TPA made its leap into appliedscience.Besides the implementation of TPA in microscopy,2 this non-linear effect is used in the micromachining of avariety of materials. Two- or multi-photon polymerization (2PP or nPP) is a particular example, in whichfocussed ultrafast laser pulses are employed to trigger the photopolymerization in UV-sensitive polymers. Thistechnique makes use of the fact that TPA can only occur in the focal volume of the laser irradiation, whilethe rest of the photoresist has to be transparent for the laser pulses. Thus, the polymerization can be confinedstrongly, enabling the production of 3D microstructures in only one production step just by moving the focalvolume through the resist. Concerning the spatial resolution, 2PP prevails conventional UV lithography, becausethe diffraction limit can be overcome due to the non-linear absorption of photons and a special threshold effect.3

After its first demonstration in 1997,4 2PP has been employed for a wide range of applications. Among themare the fabrication of waveguides,5 photonic crystals,6 and phase masks7 as well as the production of three-dimensional structures for medical purposes.8 Due to the feasibility of arbitrary 3D micro- and macrostructures

*[email protected], Phone: +49-(0)931-4100-520, Fax: +49-(0)931-4100-559

Advanced Fabrication Technologies for Micro/Nano Optics and Photonics III, edited by Winston V. Schoenfeld,Jian Jim Wang, Marko Loncar, Thomas J. Suleski, Proc. of SPIE Vol. 7591, 759114 · © 2010 SPIE

CCC code: 0277-786X/10/$18 · doi: 10.1117/12.852324

Proc. of SPIE Vol. 7591 759114-1

Downloaded from SPIE Digital Library on 17 May 2010 to 153.96.216.2. Terms of Use: http://spiedl.org/terms

SHG

Laser

M3DL-

Steuer-

Einheit

PC

M3DL-

Steuer-

Einheit

PCPC

AOM

Move X/Y

La

se

r O

N/O

FF

Se

t P

ow

er

Laser

Control

Positioning, focussing,

“Fast-shutter“ 2 -Generationω

Power adjustmonitoring

AOM

Controller

τ

E

λ

= 325 fs

= 250 nJ

= 1030 nm= 515 nmλ

/2λ

Telescope

Control

Unit

Z

YSample

Objective

DM

Powermeter

Beam

Splitter

Camera

Mo

ve

Z

Stage illumination

X

Figure 1: Experimental setup used for nPP. For 2PP, infrared laser pulses are converted to visible light, andfinally focussed into the resist. The 3PP process requires the fundamental wavelength without SHG. For 3Dmovement, an air-bearing motion system is used. DM: dichroic mirror, AOM: acusto-optic modulator

(basically determined by the numerical aperture of the focussing optics), other future applications are conceivable.Up to now, the majority of 2PP structures was fabricated with Ti-sapphire lasers at wavelengths around 780to 820 nm, using mostly purely organic commercially available polymers (for example SU-8, polyurethane) asphotoresists (see, e.g., Ref. 9). Regarding the absorption spectra of typical photoinitiators and the diffractionlimit, an illumination with shorter wavelength light would be favorable, because the spatial resolution would beimproved, and the overlapping of the initiator spectra with the laser’s wavelength would increase, consequentlyincreasing the TPA efficiency. This can, for example, be accomplished by using additional optical parametricoscillators or amplifiers, and indeed some groups pioneered into 2PP with visible light.10, 11 Besides improvingthe irradiation strategy, inorganic-organic hybrid polymers (ORMOCER�s∗) should be used for the fabricationof devices, since their properties are superior to any other organic polymer material due to their outstandingchemical, thermal, and mechanical stability.12, 13

Currently, the limit for feature sizes is at approximately 100 nm for 2PP generated structures.14 However, thereare few reports about structures smaller than 100 nm.10, 11 Unfortunately, there is no comprehensive investigationon the impact of effects such as, for example, initiator diffusion or chain propagation, which affect the formationof a volume pixel (voxel). In this paper, a voxel size study of several combinations of hybrid polymers andphotoinitiators is presented, and the underlying processes are discussed. For the study, inorganic-organic hybridpolymers are used which were formulated with photoinitiators. The voxels were written by illumination withfemtosecond laser pulses at a wavelength of 515 nm. Furthermore, we report on the characterization of the two-photon absorption cross-sections of the employed photoinitiators, and correlate these findings to the resultingvoxel dimensions. In addition to 2PP experiments, we also show first results of photopolymerization with three-photon absorption (3PA) using ORMOCER�s.

2. MATERIALS AND METHODS

2.1 Multi-Photon Structuring

Figure 1 shows the experimental setup for the multi-photon patterning which was used to perform the experimentspresented in this work. In contrast to other groups, a diode-pumped low-cost Ytterbium femtosecond laser systemwith optional second harmonic generation (SHG) is used to trigger nPA in this setup. Pulses with a durationof 325 fs are emitted at 1030 nm. The repetition rate is 10.1 MHz, and the maximum pulse energy is 250 nJ.

∗ORMOCER� - Registered by the Fraunhofer-Gesellschaft zur Forderung der angewandten Forschung e.V.



75 mmf = 12.5 cm

BS

L2L1 CD1 Ap D2

515 nm

Figure 2: Setup for the z-scan. D1, D2: reference and transmission photodiodes; L1, L2: focussing lenses;C: quartz glass cuvette with initiator solution; BS: beam splitter; Ap: (optional) variable aperture.

A combination of polarizing beam splitter and half-wave plate mounted on a computer-controlled rotary stageallows one to define the desired pulse energy for the experiment automatically. Irradiation can be switched onand off by computer using an acusto-optic modulator (AOM). For the 2PP experiments, the 515 nm (SHG)irradiation is expanded by a factor of three before being focussed into the resist by a microscope objective witha numerical aperture of 1.4. For moving the sample in 3D, the setup is equipped with an Aerotech ABL-10000three axis system capable of 10 cm x 15 cm x 15 cm travel distance in x, y, and z direction, respectively. Forlarger structures, the system offers a maximum velocity of 300 mm/s, and a precision of few nanometers whencreating microstructures. The process can be monitored in-situ by illuminating the sample with red light, anddetecting the transmission with a CCD camera placed behind a dichroic mirror.

2.2 Ascending ScanVoxels can be characterized with respect to their diameter and length by the ascending scan-method describedin Ref. 15. The basic principle is to write arrays of voxels with constant illumination (exposure time and power)in each single voxel line. While creating lines of voxels, the height of the laser focus with respect to the substrateis varied, beginning inside the substrate and ending at a point, where the resulting voxel would be stripped offthe substrate’s surface during the development step. From row to row within the voxel arrays, one parameterwhich is either the exposure time or the average laser power is altered, keeping the other parameter constant.After development of the samples in a suitable solvent, the voxel arrays are investigated with a Zeiss Supra 25scanning electron microscope (SEM). The diameters of standing voxels are determined from top view images,whereas voxels which turned over, but still stick to the substrate, can be used for the determination of the voxellength. Voxel diameters were obtained from high resolution (3072 pixel x 1536 pixel) SEM images using anatomic force microscopy grain detection software.16 This allows one to more easily detect the individual voxels,and to obtain the equivalent disc diameter just by marking the voxel.

2.3 Z-ScanA z-scan experiment has been set up in order to characterize the non-linear properties of the employed pho-toinitiators. The basic principle is to move the sample across the focus of pulsed laser irradiation, and to logthe transmitted signal. The theory of the occurring non-linear effects, i.e. non-linear refraction and non-linearabsorption, is described in literature.17, 18 With an open-aperture scan,18 the transmitted signal only dependson the non-linear absorption, which is dominated by two-photon absorption. Since multi-photon absorption isdependent on the irradiance at the sample, the signal will have a dip, if the sample is directly located in thefocus. From the magnitude of this dip, the two-photon absorption cross-section σ2 can be calculated.19

With an additional aperture in front of the photodiode, non-linear refraction can be measured. As the beam isfirst focussed and then defocussed (or vice versa) while moving the sample through the focal spot, the part ofthe transmitted beam passing the aperture will increase or decrease, respectively.Figure 2 shows the setup for the characterization of photoinitiators. The incoming 515 nm pulses (see section2.1) are split up to provide a reference and a transmission signal. The transmission beam is then focussed anddetected with a 9.8 mm photodiode and an optional variable aperture. The initiator samples which were dis-solved in methylisobutylketone (MIBK) are located in a 1 mm quartz glass cuvette, which is moved along thebeam propagation path with a 75 mm linear travel stage. In order to improve the signal to noise ratio (SNR),multiple scans were performed for each measurement.



2.4 Hybrid Polymers and Photoinitiators

As cross-linkable resin materials for the patterning by multi-photon excitation, ORMOCER�s were used whichare inorganic-organic hybrid polymers. They have excellent chemical, thermal, and mechanical stability, and theoptical performance of selected ORMOCER� materials has been demonstrated in previous investigations.20 Atypical ORMOCER� consists of an inorganic [-Si-O-]n network which is organically modified by polymerizablemoieties like acrylic, methacrylic, styryl, or epoxy groups, and also by non-polymerizable groups. Physical pa-rameters such as refractive index, optical losses, or dielectric permittivity can be adjusted by suitable choice ofalkoxysilanes for synthesis.12 In this work, two different ORMOCER� systems were used which will be furtherabbreviated by OC followed by a roman number. Either methacrylic (system OC-I) or acrylic groups (systemOC-V) were used for the organic polymerization.Common photoinitiators are added to the ORMOCER�s. For the investigations, radical photoinitiators areemployed, which decay into radicals after multi-photon excitation. The radicals can then tackle the organiccross-linkable groups, and initiate the organic polymerization reaction. We here focus on commercially availablephotoinitiators of the Ciba Irgacure� series, because they match well with our choice of wavelength for the nPPprocess according to their linear absorption spectra (shown in appendix A). In addition to that, we investigatedthe performance of a specially designed photoinitiator (Ini3).21–23

3. RESULTS AND DISCUSSION

3.1 Z-Scan Measurement

The primary aim of the z-scan experiments is to obtain the TPA cross-section σ2, which is essential for anefficient 2PP process. However, for a good performance of an ORMOCER� in a 2PP process, the photoinitiatorshould not only have high absorption cross-section, but it should also exhibit a high chemical activity. Thus,before investigating the cross-linking by 2PP, the commercially available photoinitiators were screened by UV-DSC experiments in order to investigate their performance in the respective ORMOCER� systems. Althoughthis process is a 1PP process, it provides a good insight into the cross-linking behavior of the hybrid materialformulation upon light exposure. As a result, two initiators of the Irgacure� series were selected which arefurther labelled as Ini1 and Ini2.It has to be mentioned that it is important to determine whether the dip in the transmitted signal in the z-scanexperiment derives purely from a TPA process, or whether other non-linear effects might corrupt the experiment.Therefore, different excitation powers were used, and the magnitude of the resulting dip as a function of power wasplotted which is depicted in Figure 3a for the investigated initiators. From theory,19 the change in transmission

(a) (b)

Figure 3: Results of the z-scan experiments. (a) Magnitude of the transmission signal dip as a function ofexcitation power. (b) Open aperture trace for Ini3 (for illustration only every fifth point is shown).



ΔT (z) is given by,

ΔT (z) =α2 · I0

2√

2· 1 − exp(−α1L)

α1· 11 + z2/z2

R

, (1)

whereas zR is the Rayleigh length, z the position of the cuvette, α1 and α2 are the linear and non-linear absorptioncoefficients. For a calculation of σ2 from ΔT (z), the second factor of equation (1) can be approximated for smallvalues of the linear absorption coefficient α1 to be about the sample length L. The TPA coefficient α2 canbe linked to σ2 using the photon energy and the molecular density of the initiator in the quartz glass cuvette.Furthermore, the beam waist w0 is needed to get the incident on-axis irradiation I0. It was obtained using acustom-made USB camera beam profiler scanned along the beam path. For the chosen focussing conditions, abeam waist of 15.9 �m is found, which means that the thin sample approximation17 is still valid (zR > L).According to equation (1), a linear power dependence with zero offset is expected. To be sure that the signalis not dominated by non-linear refraction - if there is any - the transmitted beam was also focussed onto thedetector by using an additional lens. Furthermore, a variable aperture in front of the detector is used, and itwas tested for each incident power, whether closing the aperture step by step influences the signal. If this is notthe case, one can conclude that the detectors area is large enough, and that defocussing attributed to non-linearrefraction can be neglected.A typical z-scan curve is shown in Figure 3b. It was acquired using Ini3 and an average laser power of 243 mW.From the slope of the power dependence curves in Figure 3a, the TPA cross-sections can be calculated. It is betterto use the slope rather than the magnitudes of the dips at a certain power, because then more measurementscontribute to the determination of σ2 yielding better statistics. The results are given in Table 1. Errors areretrieved taking the maximum and the minimum slope measured for each initiator. Obviously, Ini3 has the bestTPA efficiency at 515 nm. From the commercial products, Ini2 shows best performance. It should be mentionedthat the resulting absorption cross-section of Ini1 is much lower compared to a value published in Ref. 24. Oneexplanation for that might be the different excitation wavelength. An alternative explanation might be that therewas some influence of non-linear refraction in the measurements of Ref. 24, leading to a higher cross-section.However, since the details are not published in Ref. 24, this cannot be proven. Probably, the most significantsource of error is the correct determination of w0. Due to the quadratic dependence of I0 from w0, little errors inw0 will impact the measured cross-section significantly. Thus, relative cross-section values are given preferablyfor being able to compare the different photoinitiators.

Table 1: Calculated TPA cross-sections for the employed photoinitiators.

Initiator Ini1 Ini2 Ini3

σ2 [m4· s] (6.74 ± 0.38)·10−59 (1.44 ± 0.26)·10−58 (3.18 ± 0.21)·10−56

σ2 (rel. to Ini1) (1 ± 0.06) (2.14 ± 0.38) (471.77 ± 31.82)

3.2 Fabrication with Two-Photon Polymerization

3.2.1 Writing arbitrary 3D structures

In order to demonstrate the performance of the experimental setup, three-dimensional microstructures wereproduced (cf., Figures 4a to 4d).

� Tooth: OC-I with 3 wt.-% Ini1; dimensions (w·l·h) = (32·37·55) �m3; velocity = 50 �m/s; averagepower = 500 �W

� Hollow ball with hexagonal holes (CAD model after Ref. 25): OC-V with 3 wt.-% Ini2; diameter = 75 �m;velocity = 100 �m/s; average power = 34 �W

� Knot (CAD model after Ref. 26): OC-V with 1 wt.-% Ini2; dimensions (w·l·h) = (90·90·50) �m3; veloc-ity = 50 �m/s; average power = 90 �W



10 µm

(a)

20 µm

(b)

20 µm

(c)

2 µm

(d)

4 µm

(e)

Figure 4: Examples for structures written with 2PP for different ORMOCER� formulations. (a) Tooth createdin OC-I/Ini1. (b) Hollow ball written in OC-V/Ini2. The diameter is 75 �m. (c) Knot created in OC-V/Ini2.(d) Photonic crystal with a period of 2 �m fabricated in OC-V/Ini1. (e) Typical voxel field created with theascending scan method.

� Photonic crystal: OC-V with 2 wt.-% Ini1; size = 50 �m; period = 2 �m; velocity = 60 �m/s; averagepower = 50 �W

All reported average powers in this section and in section 3.3 were determined at the exit of the focussingmicroscope objective.Finally Figure 4e shows a typical voxel field created with the ascending scan method as described in section 2.2.For this example, the average power at the sample was kept constant at 164 �W, while the exposure time wasscanned in intervals of 2.5 ms from left to right starting with 2.5 ms. From top to bottom of the voxel array, theheight of the laser focus was varied in 0.25 �m intervals. The distance between adjacent voxels is 2 �m.From the data, it is obvious that the fabrication of arbitrary structures can be routinely performed. The necessarypulse energy is in the order of 5 to 50 pJ, and thus much lower compared to former reports about 2PP with800 nm irradiation when assuming comparable focussing conditions and writing velocities.9, 27 This is consistentwith the broader overlap of laser pulse spectrum at 515 nm with the initiators absorption spectra compared tothe overlap at 800 nm28 (see also appendix A).

3.2.2 Investigation on voxel sizes

Table 2 shows the different combinations of materials which were structured using the ascending scan method.These combinations give the opportunity to investigate the effects on the voxel size with respect to the

� different crosslinking groups (i.e., acrylic or methacrylic) in the hybrid material,

� the choice of photoinitiator (for the same resists type), and

� the concentration of the photoinitiator.

Ascending scans were carried out with three different exposure times (50, 100, and 200 ms), and three differentpowers (120, 150, and 180 �W), respectively. Either the power was varied in a range from zero to 300 �W, or the



Table 2: Combinations of materials used for the investigations on the formation of voxels in 2PP.

Material OC-I OC-V

Initiator Ini3 Ini1 Ini2 Ini3 Ini1

Concentration [wt.-%] 0.01 2 1 0.01 0.25 0.5 1 2 3

exposure time was scanned ranging from 312.5 �s to 500 ms. According to Ref. 27, the voxel diameter d is given by

d(t, F0) = w0

√ln(

σ∗2F 2

0 tντ

ln(ρ0/(ρ0 − ρth))), (2)

with three unknown parameters: the beam waist w0, the effective TPA cross-section, σ∗2 , and the threshold

radical concentration necessary for 2PP, ρth. The incident photon flux is F0, and t, ν, and τ are the temporalparameters exposure time, repetition rate, and pulse duration, respectively. The initial concentration of thephotoinitiator is ρ0.First, the experiment of Ref. 27 was reproduced with OC-I and 2 wt.-% of Ini1 as photoinitiator. Assuming athreshold radical density of 0.25 wt.-%, a beam waist of about 359 nm and a TPA cross-section of 2.52·10−59 m4·sis obtained, which is four orders of magnitude higher than the one given in Ref. 27. In contrast to the parametersof 40 ms and 20 to 65 mW given in Ref. 27, the experiments presented here were carried out using an exposuretime of 50 ms and powers from 40 to 140 �W. This is attributed to the higher overlap of the employed laserwavelength with the initiator’s absorption spectrum.The dependence of the voxel sizes on the exposure time using different constant powers for OC-V which wasformulated with 3 wt.-% Ini1 is displayed in Figure 5a. As expected, less exposure time is needed for the samevoxel size, if a higher average laser power is used. By fitting these results with equation (2), an average beamwaist of (240.4 ± 5.8) nm is found.Before presenting the results for the acquired TPA cross-sections, it is necessary to discuss the impact of ρth

on the fits. From equation (2), it is obvious that ρth and σ2 can be exchanged by each other. Thus, insteadof a three-parameter fit just a two-parameter fit is performed. This, consequently, points out the necessity ofknowing the correct value for ρth in order to extract an absolute value for σ2 from the fits. Unfortunately, ρth

cannot be determined easily. Additional experiments have revealed that, dependent on the material system, anorganic polymerization can be initiated for initial concentrations below 0.25 wt.-%. If an initial concentrationof 0.25 wt.-% as described in Ref. 27 is used, σ2 turns out to be larger than the result derived with the z-scans.Since the exact threshold initiator concentration ρth is not known, relative values for σ2 are used. Table 3 shows

(a) (b)

Figure 5: Results of ascending scans. (a) Voxel size dependence on the applied exposure time for OC-V formulatedwith 3 wt.-% Ini1 for different powers. (b) Impact of the choice of crosslinkable moieties on the voxel size.



Table 3: Resulting relative TPA cross-sections for OC-V formulated with 3 wt.-% Ini1 for three different powervalues.

Power [μW] 90 120 150

σ2 rel. [a.u.] 1.00 1.42 1.94

the results for 90, 120, and 150 �W laser power. Since the material system, the initial initiator concentration,and the threshold concentration are identical, one would expect the same value of σ2 from all three fits. However,as obvious from Table 3, there are significant deviations which indicate that there are chemical influences likediffusion of radicals, which impact on the formation of voxels, and which are not yet covered by the theoreticalmodel given in Ref. 27.Figure 5b shows a comparison of the two different ORMOCER� systems formulated with 2 wt.-% Ini1. It canbe seen, that the acrylate system OC-V requires significant shorter exposure times than the methacrylate systemOC-I for the creation of a voxel of equivalent size. This can be attributed to the different cross-linking behavior ofthe individual moieties. Usually, acrylate-containing ORMOCER� systems react faster upon radical initiationthan methacrylate-containing systems. Unfortunately, the resulting σ2 for both systems cannot be compared,because a different ρth is expected. If the exact values of ρth for OC-I and OC-V would be known, the samevalue for σ2 would be expected since the same initiator was used. Deviations of σ2 might then again be due tochemical factors (e.g., diffusion of reactive species) which are not included in the model of Serbin et al.27

For OC-V, the effect of different initiator concentrations on the voxel formation was also studied. Therefore,material formulations with 1, 2, and 3 wt.-% were structured with an average power of 150 �W and exposuretimes between 0 and 20 ms. The resulting voxel sizes are displayed in Figure 6a. As expected from equation (2),a higher initiator concentration results in larger voxels for the same illumination parameters. Unfortunately, thefit for 1 wt.-% initiator intersects the fit curve for 2 wt.-% initiator, which is unexpected, but might be due tothe very difficult voxel size determination (e.g., due to voxel truncation). The fits again result in three differentTPA cross-sections which would not be expected for the same material system. This is attributed to the reactiondynamics of the complex ORMOCER�/initiator system which will be discussed elsewhere. Also interesting tonote is that the fits are not equidistant. This indicates that the influence of the initial initiator concentration onthe voxel sizes is not linear which is already given in the model. In addition, another reason might be that variousinitiator concentrations lead to different inorganic-organic networks in the final structure. This assumption issupported by the observation of an increasing refractive index with increasing initiator concentration.29

Finally, the impact of the initiator choice on the structuring results will be reported which is, in fact, not easyto perform, because in order to correlate resulting TPA cross-sections from different initiators to the results of

(a) (b)

Figure 6: Results of ascending scans. (a) Voxel growth for different initiator concentrations in OC-V. (b)Comparison of Ini1 and Ini2 in the same material system at 150 �W.



2 µm

(a)

2 µm

(b)

Figure 7: Examples for structures written with 3PP. 7a: High aspect ratio photonic crystal; 7b: Voxel field with2 μm voxel spacing.

the z-scan experiments, one has to take the initial initiator concentration and its threshold concentration intoaccount. To make the comparison more reliable, 1 wt.-% of Ini1 and Ini2 in OC-V, respectively, were chosen todemonstrate the resulting differences, under the assumption that the threshold concentration is not dependenton the type of initiator. The results for 150 �W average power are shown in Figure 6b. As obvious from thefigure, Ini2 requires less exposure time (or less photon flux) to create the same voxel size as observed for Ini1.This indicates a higher process efficiency, and corresponds to the results presented in Table 1. A fit of the voxeldimensions with ρth = 0.25 wt.-% yields relative cross-sections of 1.00 for Ini1 and 1.35 for Ini2. In this case,the deviation from the relative value of Table 1 may not only derive from reaction dynamics, but also fromdifferent quantum efficiencies of both photoinitiators. We also tried to compare Ini3 with the other initiators dueto its high TPA cross-section. Since the exact value of ρth is not known, a quantitative comparison is doubtful.However, as a proof for the outstanding performance of Ini3, it should be mentioned that voxels comparable tothe ones fabricated with Ini1 and Ini2 were written with a 200 times lower initial concentration of Ini3. Thepower necessary for that was only slightly higher (factor of 2.5).

3.3 Fabrication with Three-Photon Polymerization

Considering the linear UV-VIS absorption spectra of the photoinitiators given in appendix A, it can easily beconcluded, that they do not only offer the option for two-photon absorption at 515 nm, but may also allowthree-photon absorption at 1030 nm. Using the latter wavelength TPA cannot occur, because there is absolutelyno absorption in the spectra for 515 nm. Instead, excitation with three photons each having a wavelength of343.3 nm is likely depending on the three-photon absorption cross-sections.Figure 7a shows a preliminary result of a photonic crystal structure written in OC-V with 3 wt.-% Ini2 at1030 nm. Its total size is 50 �m, and the line spacing is 2 �m. It consists of 40 layers making the aspect ratioof the entire structure very high. For the fabrication, a writing velocity of 20 �m/s was used. Most interestingis the average power of 6 mW indicating a non-linear process of higher order compared to the results of section3.2. The factor of approximately 100 in the required power for creating the structure has to be a result of thelower efficiency of the 3PA relative to TPA process. Unfortunately, there was a tiny air bubble while structuringthe resist, leaving a huge free space inside the final crystal.The part of a 3PP-written voxel field depicted in Figure 7b was created in OC-I with 3 wt.-% Ini1. The exposuretime was kept constant at 500 ms, and the power was varied from about 4.4 to 5.8 mW in approximately 0.15 mWsteps starting on the left side of the image. The height was scanned from top to bottom. The voxel spacingis 2 �m, and the smallest voxel has a diameter of only 250 nm, which is far beyond the diffraction limit. Thenecessary power to produce voxels is again significantly higher at 1030 nm compared to voxels generated with515 nm (see section 3.2.2).In contrast to the results shown in section 3.2, the presented structures are intended as a proof of concept andderive from only few experiments. By varying the photon fluxes and writing velocities or exposure times, there



should be a vast potential for pushing the spatial resolution further down. A more comprehensive study of 3PPat 1030 nm is presently carried out and will be published elsewhere. Additionally, the z-scan experiments (seesection 3.1) will be also performed at a wavelength of 1030 nm in order to determine the three-photon absorptioncross-section of the employed photoinitiators.30

4. SUMMARY

From the results of the fabrication of arbitrary 3D microstructures, it is obvious that the chosen material systemsand wavelengths are appropriate for 2PP and for 3PP processes, respectively. In the case of 2PP, a very highprocess efficiency was demonstrated, which is mainly promoted by the better match between the employed initia-tors and the excitation wavelength. In addition, structures smaller than the diffraction limit could be fabricatedwith both processes.With the z-scan experiment it was demonstrated that the underlying absorption process at 515 nm is really aTPA process. The resulting TPA cross-sections could be correlated only indirectly to the voxel sizes. Since thecross-sections obtained from voxel size fits are only effective cross-sections (modified by the quantum efficiency)and, in addition, the polymerization threshold initiator concentration is not clear, it is not possible to comparethe absolute values of σ2. However, the effect of a higher σ2 measured with z-scan could be demonstrated: ahigher absorption cross-section allows one to decrease the photon flux or to formulate the ORMOCER� mate-rials with lower initiator concentrations. Especially, Ini3 shows an outstanding TPA performance.Besides TPA cross-sections, the ascending scan experiments also deliver important results for better understand-ing the reaction mechanism. As the theoretical model neglects radical diffusion and chain propagation, deviationsin the experiments from the model are indications for their impact on the voxel results. In this context, it wasshown that the illumination of one material with different constant powers reveals different σ2 if the theoreticalmodel of Ref.27 is used. Therefore, one can conclude that the reaction dynamics plays a significant role. Thesame applies to the experiments with different initiator concentrations, where also different σ2 were found. Fur-thermore, we demonstrated the influence of the choice of different polymerizable moieties on the resulting voxelsizes. As expected, acrylate groups require a lower photon flux than methacrylate groups. The impact of theinitiator kind on the voxel sizes was observed for OC-V, whereas a lower (relative) σ2 was evaluated for Ini1compared to Ini2, as predicted by the z-scan results.For a more reliable discussion of ascending scan results and a better correlation to z-scan experiments, a precisedetermination of ρth is necessary. With that it would be possible to compare resulting absolute values for σ2 fromz-scan and voxel size fits, and attribute deviations to quantum efficiency or effects not included in the model.In future work, the investigations will be also correlated to spectroscopic results on the degree of organic poly-merization of different material systems. Besides, a more comprehensive study on voxel sizes generated by 3PPwill be carried out.

Figure 8: Linear absorption of commercial photoinitiators. The wavelength range is cut off at 500 nm due toneglectable absorption at longer wavelengths.

Proc. of SPIE Vol. 7591 759114-10


APPENDIX A. LINEAR ABSORPTION SPECTRA

Spectra of commercial photoinitiators were taken with a Shimadsu UV-3100 spectrometer (Figure 8). As solvent,hexane was used. The measurement range was 200 - 1100 nm.

ACKNOWLEDGEMENTS

The authors would like to thank the German Science Foundation for funding the work with the ”sub-100-nmProgram” (grant: HO 2475/3-1). We would like to thank our project partners at the University of Hannoverand at the University of Jena (Germany) as well as our colleagues in Wurzburg for fruitful discussions. RobertLiska (Vienna University of Technology, Austria) and his colleagues are greatfully acknowledged for providingIni3.

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