Abstract

While the role of resin viscosity has been largely studied for stereolithography, where a very low viscosity material is preferred, an extensive study of its microscale counterpart, two-photon polymerization, is still lacking. In the present work, we tried to fill the gap by correlating directly the properties of the features produced by two-photon polymerization with the viscosity of four acrylate materials, prepared by mixing two monomers with very different viscosity, and a constant quantity of photoinitiator. Linewidth, polymerization and damage thresholds, dynamic range, and fabrication resolution have been object of investigation in our experiments.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction

Stereolithography (SL) and two-photon polymerization (TPP) are additive manufacturing processes employed in the fabrication of three-dimensional parts [1,2]. While the first one produces structures with dimensions that span from meters to centimeters and with feature sizes ranging from hundreds of microns to tens of microns, the latter one builds micro- and mesostructures with feature sizes as small as hundreds of nanometers. Although several advances have been implemented over the years to increase the throughput of these techniques [3,4], both SL and TPP are still considered slow manufacturing processes since they essentially make one part at a time. Therefore, SL is applied mostly in the production of objects that have complex geometries and require customization or in applications where prototyping is needed [5]. Alternatively, TPP has grown to be a unique and powerful microfabrication tool that enables researchers to investigate optical, mechanical, and biological phenomena at the microscale with unparalleled spatial accuracy [69].

Apart from their contrasting fabrication scales, the main difference between SL and TPP is the process they use to create three-dimensional structures. In SL, a photosensitive material (resin) is hardened when exposed to a focused beam of UV light. Since light absorption occurs in this case through a linear process, three-dimensional parts are formed in SL using a layer-by-layer approach. Specifically, a lifting platform is used to create a thin liquid film within the resin pool; laser scanning begins at the platform-resin interface and then continues over each polymerized layer as the platform is lifted until the entire three-dimensional part is formed. In TPP instead, a NIR ultra-short pulsed laser is used to initiate the photopolymerization of the resin through nonlinear optical absorption. In this case, light-matter interaction can be confined within sub-femtoliters volumes (voxels) allowing “true” three-dimensional writing. TPP microstructures are made directly in the volume of the resin by scanning the laser beam and/or moving the sample following three-dimensional patterns. Because of the microscopic scale of the parts made by TPP, fabrication typically begins or ends at the resin-substrate interface. In this way, the parts can be easily retrieved being anchored to the substrate after the development step. Recent studies have revealed that the order of optical nonlinearity in TPP depends greatly on the photoinitiator and the conditions used for light excitation [10]. Therefore, the acronym TPP is not formally correct since it describes only one of the multiphoton processes that can cause photopolymerization. Nonetheless, we will continue to refer to this high precision three-dimensional printing technology as two-photon polymerization for simplicity (some of the other names used to describe TPP are multiphoton lithography, 2-photon polymerization (2PP), multiphoton polymerization (MPP), and 3D direct laser writing).

One of the consequences of the different methods used in SL and TPP for creating three-dimensional parts is the viscosity of the resin. While low viscosity resins are preferred in SL, resins in TPP are often very viscous [11,12]. This asymmetry originates solely from practical reasons. In SL, the photopolymerization of a new layer begins with a recoating step. A thin film of the resin (which defines the axial resolution of SL) must be deposited on top of the last polymerized layer before photopolymerization can carry on. Low viscosity resins reduce the time needed to accomplish this step, hence decreasing fabrication time. Furthermore, low viscosity resins permit to obtain self-leveling layers. In TPP, samples are often placed on high-performance linear stages that move at high speed around a fixed laser beam. This is especially true when TPP is used to make smooth and continuous millimeter-sized parts. High viscosity resins are then preferable for avoiding adverse drag effects on the sample when making complex patterns because of the stages accelerations and decelerations at turning points. Additionally, highly viscous resins simplify the preparation of the sample which requires small quantities of material to be positioned close to or in contact with the front lens of a high numerical aperture objective. Popular organic-inorganic hybrid TPP resins are for example highly viscous or in a semi-solid state when used at room temperature [13,14].

Because of their fast curing speed, acrylic monomers and oligomers are commonly used in SL and TPP resins [15]. The ability of these molecules to create highly cross-linked networks is key for the fabrication of self-sustained and readily assembled three-dimensional structures [16]. Moreover, since acrylic monomers are used extensively in several industries, they are inexpensive, easily available, and can be found in a wide assortment of functionalities and sizes.

In order to be used successfully in SL, acrylic-based resins and their corresponding polymers must exhibit a series of physical and chemical properties. For example, the resin viscosity and wetting behavior are critical in SL [17]. The first one is preferable when it is less than few hundreds mPa·s for the reasons explained earlier, while the latter one requires low surface tension when in contact with the polymer surface in order to ensure proper coating. High tensile strength and low volume shrinkage are polymer requirements to produce a stiff material capable of supporting its shape and maintaining dimensional accuracy of the photocured product. The combination of these properties makes the preparation of a resin for SL challenging [18]. For instance, low molecular weight bifunctional acrylic monomers are excellent in promoting low volume shrinkage and decreasing the overall resin viscosity, but they tend to create polymers with low tensile strengths. The addition of small quantities of tri-, tetra-, and penta-acrylic monomers into the resin compensates this issue by producing dense and rigid polymer networks optimal for SL. Unfortunately, as their concentration is increased the resin becomes too viscous and the polymer too brittle to be used effectively. To solve these problems, SL resins typically consists of a mixture of low molecular weight diacrylate monomers, highly branched acrylate monomers, and high molecular weight acrylate oligomers based on urethane or epoxy backbones; a successful SL resin is obtained when these ingredients are mixed in the right proportions, a goal typically achieved by experimenting with different mixtures ratios and testing the resin and polymer properties [19].

Although TPP has been used with a variety of resins chemistry in order to obtain polymers for various applications, resins based on acrylates have received the most attention [13,2028]. Some of the first demonstrations of TPP were performed using commercially available acrylic resins or mixing commercially available acrylic monomers [29,30]. Subsequent works committed to improving the performance of TPP resins were centered around the synthesis and characterization of photoinitiators with large optical nonlinearities so to initiate radical polymerization of acrylates more efficiently than traditional UV photoinitiators [31]. Then, hybrid organic-inorganic resins have been developed that have improved the ease with which sturdy and precise three-dimensional microstructures can be fabricated by TPP with, among other things, minimal volume shrinkage [13,32]. The organic part of these hybrid resins still relies on acrylic moieties that are used to induce the material final cross-linking by means of radical polymerization.

SL was developed almost twenty years before TPP was first demonstrated. Furthermore, because of its manufacturing scale length, SL has found almost immediately applications that have demanded intense R&D efforts [33]. Therefore, it is not surprising to find a large number of published works on the design, preparation, and characterization of resins for SL [1,3437]. On the other hand, TPP research has been focused mostly on the development of methods for improving writing resolution, increasing writing speed and part overall size, and for studying a wide variety of natural phenomena at a length scale that would be otherwise impossible [6,8,3841]. Although these efforts have delivered impressive results, it has created also a lack of systematic investigation of the resins’ chemical and physical properties on the TPP process.

For example, in SL the viscosity of the resin is adjusted in a way to minimize recoating time between each polymerized layer. Accordingly, the UV curing of resins made by mixing varying concentrations of acrylic monomers with different viscosities has been studied extensively. Particularly, the photopolymerization kinetics of these mixtures have been investigated as a function of viscosity [4244]. Although it was found that the rate of polymerization for these systems depends on many factors such as crosslinking density, number of functionalities, reactivity, and hydrogen bonding, it was also found that a major influence comes from the resin viscosity. As the viscosity is increased the polymerization goes faster because termination reactions become diffusion-limited. At even higher viscosities, the rate of polymerization starts to decrease since propagation reactions become diffusion-limited as well. Thus, a most reactive composition is frequently obtained by mixing high and low viscosity monomers at specific molar ratios. Moreover, the writing characteristics of SL were studied as a function of the resin viscosity [45]. At low irradiation times, low viscosity resins produce the smallest polymerization depth. This is not the case anymore at higher irradiation times, indicating that in this exposure conditions other factors besides the resin viscosity must be taken into consideration for explaining the observed polymerization depth. It is noteworthy to point out that a similar trend was observed also for the polymerization width in SL.

By contrast, the effect of the resin viscosity in TPP has not yet been fully investigated. With the exception of the following studies indeed, the scientific literature on this topic is still lacking. By taking advantage of the scattering signal generated by the interaction of a probe beam with a single TPP voxel, Wegener and collaborators were able to monitor the polymerization kinetics of a series of acrylic-based resins with excellent spatial and temporal resolutions [46]. When compared to the kinetics of similar materials polymerized with UV light, this study has shown that there are substantial differences: the polymerization rate is several orders of magnitude faster in TPP than in UV curing, and radical quenching through oxygen plays a large effect on the termination processes of TPP. The study revealed that the viscosity of the resin contributes greatly to the kinetics of TPP, most probably by reducing oxygen diffusion. In a work by Kawata and collaborators, TPP was performed at different temperatures by placing the sample on a temperature-controlled block [47]. The temperature of the resin was varied from -60˚C to 80˚C, which consequently corresponds to a change in the resin viscosity. The width of the polymerized voxels was measured within this temperature range. It was found that the voxel becomes smaller as the temperature decreases (viscosity increases) and as the temperature increases (viscosity decreases) reaching the largest dimension at around room temperature. The magnitude of this effect was observed to depend on the exposure time. TPP writing in the presence of a radical quencher was studied by Farsari and collaborators [48]. The findings showed how both linewidth and writing resolution could be minimized if the range of action of the radicals generated in and around the voxel is made smaller. Thus, the diffusion of the quencher in the resin plays a fundamental role in scavenging radicals and in determining the fastest writing speed achievable in this process. The viscosity of the resin is evidently a major factor in the design of such an approach for performing high-resolution TPP writing. Finally, a recent work by Mendonça and collaborators has investigated the linewidths of TPP microstructures using acrylic-based resins made by mixing different concentrations of two monomers [49]. The authors of this study found that the writing linewidth decreased substantially as the concentration of one of the monomers increased, which coincided with an increase of the resin viscosity. This trend is in agreement with the observation made by Kawata and collaborators.

The effect of the resin’s viscosity on the photochemistry of SL and on several practical aspects involved in the fabrication of three-dimensional parts using this technique is well-known [17,50]. Considering that the materials used in TPP are often similar to the ones used in SL, and that various researches have already encountered the effect of the resin’s viscosity in TPP, it is then surprising to observe that a systematic study on this subject is not yet present in the TPP literature. Hence, in this work, we aim to fill this gap by reporting the results of a TPP study using a series of resins having viscosities varying over a range of three orders of magnitude. To be able to interpret the data as deriving mostly from the large variation of viscosity, the resins are prepared by mixing two acrylates in different molar proportions. Writing linewidth and fabrication dynamic range are measured and conspicuous differences are noted. A qualitative investigation on the effect of the resin viscosity on the writing resolution is provided as well.

As described elsewhere, the formation of the polymerized voxel in TPP can be thought as the result of two interaction volumes [32]. One, technical, which depends principally by the hardware employed in performing TPP (i.e. positioning system accuracy and repeatability, laser stability, quality of optics, and effective damping systems), and one chemical which is determined by a complex interplay of several factors such as the nature and concentration of the photoinitiator and the kinetics of the polymerization. The latter is influenced by the viscosity of the resin that controls, among other things, the diffusion of both radicals and radicals’ scavengers. Since the room temperature viscosity of the resins used in this study varies for more than three orders of magnitude, the viscosity of the resin is a dominant factor that influences not only the rate of polymerization but also the properties and sizes of the written microstructures. We believe this study complements our understanding of TPP by linking directly the effects of various TPP writing characteristics to the resin viscosity.

2. Methods and materials

TPP writing linewidths and thresholds experiments are performed using the output of a Ti:sapphire laser delivering 100 fs pulses at a repetition rate of 80 MHz (MaiTai, Spectra-Physics). The center wavelength of the excitation source chosen for these experiments is centered at 775 nm. The laser beam is focused on the sample by means of a 63x 1.4 NA oil immersion microscope objective (Zeiss, Plan-Apochromat). During microfabrication, the excitation focal point is kept fixed, while the sample is moved in predetermined geometries with the aid of a computer-controlled, motorized, three-axis translational stage assembly (XMS, Newport Corp.). Laser exposure is controlled by a mechanical shutter (Uniblitz); the action of the shutter is synchronized with the sample motion to avoid under and overexposures. The average laser power delivered to the sample is controlled by a polarizer and half-wave plate with the latter mounted on a manual rotational stage. TPP is monitored in real time by means of optical microscopy in transmission mode which is an imaging functionality added to the writing setup.

Test samples for measuring writing linewidths are made at the substrate-resin interface in the form of 15 µm long lines. Each line is written using a single laser pass. A series of parallel lines are written at different z-positions using a constant offset of 200 nm. In this series, there are lines that are truncated by the substrate for more than half of their lengths, while there are lines that are barely attached to the substrate and hence, are fallen over. The line used for extrapolating the width of the written feature is the one that appears just before the fallen line. We choose this method to minimize the contribution that volume shrinkage can add to the measurement of TPP linewidth [51]. Each line is written at a constant velocity of 50 µm/s and the laser energy (as measured before the microscope objective) is varied between 0.03 nJ/pulse and 0.45 nJ/pulse. At each laser energy, the line test is repeated at least five times, and the average linewidth is reported. Following the development step, the samples are characterized using an FE-SEM after being coated with a thin layer (∼ 5 nm) of Au.

Polymerization (Eth) and damage (Edamage) energy thresholds are determined by writing 20 µm long lines in the bulk of the resin at a constant speed of 50 µm/s and varying the laser energy per pulse. We define Eth as the lowest pulse energy that yields a visible change of the resin into a polymer by means of transmission light microscopy. Similarly, Edamage is defined as the highest energy per pulse that can be used before cavitation within the resin begins to occur. As the laser pulse energy surpasses Edamage, the resin boils forming bubbles around the focal point that are easy to spot by transmission light microscopy. Each reported value of Eth and Edamage is the average of twenty measurements.

TPP writing resolution experiments are made using a somewhat different system. The setup is based on the second harmonic of a femtosecond fiber laser (Toptica FemtoFiber Pro NIR), at 780 nm, with pulses of 100 fs and a repetition rate of 80 MHz. Laser average power is controlled through a polarizer and a half-wave plate mounted on a motorized rotation stage (Aerotech, MPS50GR). The laser beam is focused through a 100x 1.4 NA oil immersion microscope objective (Zeiss, Plan-Apochromat), which can be moved on the vertical axis by a counterbalanced linear stage (Aerotech, ANT130-LZS) to translate the laser focus along the beam direction. The substrate with the resin is moved on the horizontal plane by a 2-axis nanopositioning stage (Aerotech ANT95XY). The test samples used to characterize TPP writing resolution consist of arrays of woodpile microstructures. Woodpiles are made by stacking layers composed by parallel rods. Each layer is perpendicular to the previous one, and shifted laterally by half the rod separation distance from the nearest parallel layer. Each woodpile layer is separated vertically by 500 nm. Rod lateral separation was varied between 0.2 µm and 2.57 µm, at each pulse energy. Woodpiles are fabricated at a constant velocity of 100 µm/s, and with laser pulse energies ranging from 0.075 nJ to 0.375 nJ (as measured before the microscope objective). The samples are then developed and dried; subsequently, they are characterized by optical microscopy in transmission mode.

In this study we investigate the TPP writing behaviors of four resins A – D. They are made by mixing various concentrations of two acrylic monomers and the same amount of a photoinitiator. The two monomers are dipentaerythritol pentacrylate (DPEPA, Sartomer) and 1,3-butylene glycol diacrylate (BGDA, Sartomer). The photoinitiator is phenylbis (2,4,6-trimethyl benzoyl)phosphine oxide (BAPO, Sigma-Aldrich). The molecular structures of the monomers and photoinitiator are shown in Fig. 1. Besides presenting different functionalities (5 vs. 2), the two monomers appear quite different when handled since one is much thicker than the other. Specifically, the viscosities at room temperature of DPEPA and BGDA are 13,600 mPa·s and 6 mPa·s, respectively. The component settings for the resins are summarized in Table 1, alongside their corresponding viscosities measured at room temperature. The molar ratio between the carbon-carbon double bonds and the photoinitiator molecules in resins A, B, C, and D is constant. All materials are used as received without any further purification. Prior to use, the resins are mixed thoroughly until homogenous solutions are obtained. Samples are prepared by depositing a drop of resin in use on top of a 150 µm thick cover glass. The cover glass is then positioned on top of the motion system with the side opposite to the resin facing the incoming focused laser. After completion of the writing process, the unsolidified part of the sample assembly is washed away in an ethanol bath revealing the desired microstructure patterns on the glass substrate.

 

Fig. 1. Molecular structures of (a) monomer DPEPA, (b) monomer BGDA, and (c) photoinitiator BAPO used to make the four resins used in this study. (d) 2-BIT data for a resin containing 0.15 wt% BAPO. The dashed line is the result that would be expected for 2-photon absorption as a reference. The error bars are based on standard deviations from multiple measurements.

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Tables Icon

Table 1. Composition of the resins investigated in this study with the corresponding viscosity measured at room temperature.

All resins are heated for 1 minute in a 90 °C oven, and then they are blended at 1850 rpm for 5 minutes using a centrifugal mixer. This procedure is repeated until the resins appear homogeneous. Viscosity measurements are performed at 22.8 °C using a DV-2+ Brookfield viscometer. Because of the wide range of viscosity between the samples used in this study, the viscosity of Resin A is measured using an LV-4 spindle, while the viscosity of Resins B, C, and D are measured using an enhanced UL adapter. The errors in the viscosity measurements for Resins A, B, C, and D are 4%, 1%, 0.4%, and 0.1%, respectively.

The effective nonlinear absorption of a resin containing BAPO (0.15 wt% BAPO in SR399) is measured using the two-beam initiation threshold (2-BIT) technique, a detailed description of which is presented elsewhere [52]. In brief, this technique involves combining two interleaved, spatially overlapped pulse trains to expose the resin. The average power of one pulse train required to reach the polymerization threshold P1 is measured as a function of the average power of the other pulse train P2, and a plot of P1 versus P2 is used to determine the effective order of nonlinear absorption in the resin. For each set of powers, the polymerization thresholds were measured by creating sets of lines at a constant distance above the coverslip surface at a stage velocity of 20 µm/s. Representative 2-BIT data obtained for this system at a wavelength of 777.5 nm (chosen to be between the center wavelengths of the two laser systems used to obtain the other data in this manuscript) are shown in Fig. 1d. The average value of the exponent derived from multiple 2-BIT experiments is 2.01 ± 0.05, indicating that initiation at this wavelength is a 2-photon process.

3. Results and discussions

The exposure curves of resins employed in TPP are quite nonlinear, thus creating a light intensity threshold below which polymerization does not occur [53]. It is the presence of this intensity threshold that permits the formation of voxels with dimensions that are considerably smaller than the wavelength of light used for fabrication. For example, by adjusting the intensity of the excitation laser barely above the polymerization threshold, voxels with lateral dimensions smaller than 100 nm can be obtained with a typical exposure wavelength of 800 nm [54]. TPP resins display also a light intensity threshold above which boiling occurs [24]. While the polymerization threshold depends on the characteristics of the photoinitiator and its concentration, the damage threshold is an intrinsic property of the monomers and oligomers mixtures that constitute the bulk of the resin. The window between these two laser intensity thresholds defines the writing dynamic range available for making three-dimensional structures by TPP. Resins with large dynamic ranges are preferable for two reasons. The first reason is a consequence of the fact that such resins support the use of voxels with a wide variety of dimensions. This is obviously advantageous when structures with different overall sizes are demanded [55]. The second reason is that high-speed writing is facilitated when resins with large dynamic ranges are used [39].

In this study we are comparing the writing performances of four resins, hence we limit ourselves in interpreting the laser energy thresholds Eth and Edamage measured at a constant velocity instead of the absolute values for the resins’ laser intensities thresholds. The laser energy thresholds for resins A, B, C, and D are compiled in Table 2, where the dynamic ranges are added also by using the following expression $\left( {\frac{{{E_{damage}} - {E_{th}}}}{{{E_{damage}}}}} \right) \times 100.\; $It is significant to remember that the energy damage thresholds presented in this work do not take into consideration proximity effects. It is well-known that high feature densities in TPP writing produce energy damage thresholds that can be considerably lower than the energy damage thresholds found when writing isolated features [56].

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Table 2. Polymerization (Eth) and damage (Edamage) energy thresholds for the resins considered in this study. The dynamic range for each resin is listed as well.

Polymerization energy thresholds grow monotonically as the viscosity of the resins decreases. Eth goes from 0.04 nJ/pulse to 0.19 nJ/pulse as the viscosity of the resin is lowered from η = 10400 mPa·s to η = 15 mPa·s. At a laser repetition rate of 80 MHz, this corresponds to a change in average power for initiating TPP from 3.4 mW to 15.5 mW. By contrast, damage energy thresholds do not diverge much as the resin viscosity changes. Edamage remains constant at around 0.5 nJ/pulse (40 mW) across the four resins. Consequently, we observe the largest dynamic range in the resin with the highest viscosity. A plot of the measured dynamic ranges for A, B, C, and D as a function of the resin viscosity is shown in Fig. 2 and it illustrates clearly this point. As the viscosity of the resin becomes three order of magnitudes larger, the dynamic range increase from 64% to 92%. High-speed (mm/s to cm/s) TPP writing is typically performed by using a photoinitiator with a large nonlinear cross-section [39]. This has the effect of widening the dynamic range by essentially lowering Eth. The data in Fig. 2 suggests that an additional approach to performing safely high-speed TPP is to increase the resin viscosity.

 

Fig. 2. Dynamic ranges of the four investigated resins as a function of their viscosities. The blue, red, green, and orange colors represent resins A, B, C, and D, respectively.

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An example of the tests used to extrapolate TPP writing linewidths is shown in the FE-SEM image of Fig. 3(a). Here, five arrays of lines made under the same experimental conditions are visible. In each column, the focal spot axial position is gradually changing. The lines at the top are the most truncated by the glass substrate, while the lines at the bottom are barely attached to it and hence, they fall over during the development step. The lines that appears straight just before the ones fallen over are used for measuring linewidths.

 

Fig. 3. Scanning electron microscopy images of samples used to measure writing linewidths. (a) In this overview, five arrays of ascending lines are written using the same experimental condition in Resin D. The polymerized lines used for linewidth measurements are highlighted in yellow. (b) High magnification image of a line written in Resin A using an energy per pulse of 0.29 nJ at a velocity of 50 µm/s. (c) High magnification image of a line written in Resin D using an energy per pulse of 0.30 nJ at a velocity of 50 µm/s. The scale bars are 50 µm, 3 µm, and 2 µm, in (a), (b) and (c), respectively.

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The complete set of linewidth measurements versus laser pulse energies is shown in Fig. 4 for all four resins. Several striking differences are observed as the viscosity is decreased from Resin A to Resin D. This plot confirms the energy threshold results discussed earlier. The processing window of written TPP microstructures that survived with physical integrity the development step slowly decreases as the viscosity decreases from Resin A to Resin C, and then it becomes considerably smaller for the less viscous Resin D. The smallest measured linewidth differs only slightly among the four resins with an average value of around 360 nm. Contrarily, the largest measured linewidth falls within a wide range of values as the resin viscosity is changed. Maximum linewidths are indeed 2.98 µm, 2.31 µm, 1.42 µm, and 0.68 µm, for Resins A, B, C, and D, respectively. The large difference in linewidth written under the same experimental conditions observed as the resin viscosity is changed is clearly shown in Figs 3(b) and 3(c). Here, the SEM images of two lines are displayed. The line in 3(b) was made using Resin A at a writing speed of 50 µm/s and a laser pulse energy of 0.29 nJ. The line in 3(c) was made using Resin D at a writing speed of 50 µm/s and a laser pulse energy of 0.30 nJ. As the viscosity of the resin diminishes from 10,400 mPa·s to 15 mPa·s, the corresponding linewidth goes from a value of 2.3 µm to a value of 0.4 µm. Lastly, the rate of linewidth variation as the laser pulse energy is increased is rather different within the four resins. Specifically, a fixed change in laser pulse energy produces a larger linewidth change in the more viscous resin than in the less viscous one. Within the available writing processing window for example, linewidths created in Resin A change from 0.41 µm to 2.98 µm, a growth of almost 90%; linewidths created in Resin D instead change from 0.33 µm to 0.68 µm, a growth of roughly 50%. The rate of linewidth variation with laser pulse energy for the various resins can be quantified by performing a linear regression of the central data shown in Fig. 4. This analysis produces values of 7.3 µm/nJ, 5.9 µm/nJ, 3.3 µm/nJ, and 1.74 µm/nJ for Resins A, B, C, and D, respectively. The largest rate of linewidth change is observed for the resin with viscosity of 10.400 mPa·s, while the slowest rate of linewidth change is observed for the resin with viscosity of 15 mPa·s. It is significant to report that the results as described so far and shown in Fig. 2 and 4 have been reproduced using two different experimental setups. Specifically, when Resins A, B, C, and D were used in the system employed for studying writing resolution (see Section 2) the same trends were observed.

 

Fig. 4. Experimental values of TPP writing linewidths as a function of the laser energy per pulse. Data collected for Resins A, B, C, and D are presented together. All structures used for creating this plot were made at a writing speed of 50 µm/s.

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When using a simple threshold model for describing the growth of TPP features, a trend that follows the square root of the natural logarithm of the ratio between the laser intensity and the polymerization intensity threshold is found [57]. Noticeably, our data in Fig. 4 does not correlate well with this behavior but rather follows more a linear trend. We believe this discrepancy from the threshold model is due to the shape of the focal spot that at high laser pulse energy cannot be considered anymore as an ideal Gaussian [51]. Furthermore, it is likely that the smallest achievable linewidths could not be measured since the mechanical properties of the polymers produced at these low laser pulse energies might be too weak to survive intact the development step. This is especially true for the less viscous resins where the concentration of the cross-linker is low (Table 1).

The chemistry involved in the photopolymerization of resins made with multifunctional acrylic monomers is a complex interplay between several reactions: initiation, propagation, termination [58]. The kinetics of these reactions and the properties of the polymers produced correlate to many parameters such as monomer functionality/size/structure, type and concentration of photoinitiator, and, in resins made with more than one monomer, monomer molar and reactivity ratios [59]. The complexity of the photopolymerization of these materials originates also from the fact that the reactions involved in it occur almost simultaneously in a medium that is rapidly changing. In the case of negative tone resins employed in SL or TPP, that change is from liquid to solid and it is localized within small and well-defined volumes. Moreover, radical chain polymerizations are sensitive to the presence of inhibitors and retarders that have critical consequences on the polymerization kinetics [60]. Since the resins in our study are prepared and used in ambient pressure, they contained a certain amount of dissolved O2 (∼ 10−3 M) [61]. Oxygen is known to be a strong inhibitor as demonstrated by its large z values (ratio of the rate constants for inhibition and propagation) [62]. Specifically, oxygen interferes with the chain growth mechanism by forming peroxy radicals, which do not react rapidly with acrylates [63]. Thus, oxygen has the overall effect of reducing polymerization efficiency by scavenging radicals and effectively terminating them [64]. Hence its action cannot be ignored when analyzing results of TPP experiments such as the ones presented in this study.

For the reasons conveyed so far, comprehensive models for describing and predicting photopolymerization of multifunctional acrylic monomers are challenging to design. In TPP, this task is rendered even more arduous by the nonlinear order of light absorption and parasitic processes that can be induced when using high repetition-rate fs pulsed lasers. It is not our task to develop such model in this work; nonetheless we think that the results presented up to now can be attributed in great part to the effect the resin viscosity has on molecular diffusion.

Typical values for the propagation (kp) and termination (kt) rate constants of acrylic monomers are 103 L mol−1 s−1 and 107 L mol−1 s−1, respectively [65]. The rate constant of the radical scavenging by O2 (ko) is 108 L mol−1 s−1 [61]. A specific reaction becomes diffusion-limited when its coefficient rate is larger than the diffusion rate coefficient kdiff. The latter can be estimated in function of viscosity using the following equation

$${k_{diff}} = \frac{{8000 RT}}{{3\eta }}\; $$
where R is the gas constant (J mol−1 K−1), T is the temperature (K), and η is the viscosity (Pa·s) [43]. The constant in the numerator allows to get values of kdiff directly in units of L mol−1 s−1. Table 3 compiles kdiff for Resins A, B, C, and D. As the viscosity decreases, kdiff increases from 6.25·105 L mol−1 s−1 to 4.33·108 L mol−1 s−1. Thus, within this viscosity range, propagation is never diffusion-controlled while radical scavenging by O2 is always diffusion-controlled. Interestingly, termination is diffusion-controlled only for Resin A.

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Table 3. Diffusion rate coefficients calculated using Eq. (1) for the resin studied in this work. Viscosities of the resins in Pa·s are shown as well.

In light of these considerations, we can now attempt to explain the experimental results presented in this study. The observed variation in the dynamic range (Fig. 2) is caused by Eth becoming smaller as the resin viscosity increases (Table 2). For resin A, this is caused by the limited mobility of O2 in scavenging radicals and of radicals in finding pathways to terminate propagation. The result is that, under identical exposure time, the amount of laser pulse energy required to start and sustain polymerization by TPP is lower than in less viscous resins. As the viscosity decreases (Resins B, C, and D), termination processes are not diffusion-limited anymore. Radical scavenging by O2 becomes then the primary source of dynamic range variation as the viscosity changes. As the viscosity decreases, O2 motility is higher; thus, its ability to eliminate radicals from the polymer propagation is increased as well. Consequentially, Eth is larger in less viscous resins. More photons are needed to generate enough radicals that can sustain polymerization of TPP structures.

The results of TPP linewidths (Figs. 3 and 4) can be analyzed using similar arguments. Under the same dose, the most viscous resin produces the largest linewidth. The reduced action of O2 and the lower probability of termination events because of limited motility, allows for a bigger build-up of radicals that consequentially produce a larger polymer. Likewise, the same logic can describe the different magnitude of linewidth change that is observed as the laser pulse energy is increased.

So far, in the description of the results, we have neglected the possibility that the resin temperature in the focal volume increases during photopolymerization. Although under some experimental conditions localized heat accumulation may occur [66,67], it has been shown that the temperature change in the focal volume of a TPP experiment during laser irradiation amounts only to few degree Kelvins when excitation is performed with parameters almost identical to the ones used in this study (laser repetition rate, pulse width, wavelength, pulse energy, focusing lens, scan velocity) [68]. This observation together with the data published by Kawata and collaborators [47], give us confidence that the results presented in this work are due predominantly to the large variation in the resins’ viscosities.

In many applications, the ability to write features close together leaving a gap between them is of fundamental importance [69]. For example, many types of three-dimensional photonic crystals rely on this fabrication ability of TPP to work in different regions of the EM spectrum [70]. To evaluate how resin viscosity influences writing resolution in TPP, arrays of woodpile microstructures are written using Resin A and B. Images of these arrays recorded in reflection mode using an optical microscope are shown in Fig. 5. Laser energy per pulse is varied from 0.075 nJ to 0.375 nJ in incremental steps of 0.025 nJ, while line separation between the rods of the woodpile is increased between 0.4 µm and 2.6 µm using increments of around 200 nm. The same experimental conditions are used for both arrays. Depending on the linewidth of the rods and on separation chosen between rods, the woodpile microstructure displays different colors under white light illumination. We assigned the presence of colors in the reflectivity images to well-formed structures with separated rods. Woodpile microstructures that appear white have rods close together to a point of merging with each other. As the viscosity of the resin decreases from 10,400 mPa·s to 116 mPa·s, the number of colored microstructures increases. This observation is highlighted in Fig. 5 with a red outline. This result indicates that TPP writing resolution is favored in less viscous resins. This is in agreement with other reports where quenchers are added to the resin mixture to compete with photopolymerization, thus improving writing resolution [48,71]. We are achieving (qualitatively) the same result by lowering the resin viscosity so to increase the motility of O2. Writing speed determines the time that elapses between two adjacent rods; during this time, the scavenging action of O2 is fundamental in diminishing the probability of overlapping areas to convert interstitial resin into polymer. Under the experimental conditions employed in this study (100 µm/s), viscosity plays obviously an important role in defining writing resolution.

 

Fig. 5. Optical microscopy images of arrays of woodpile microstructures fabricated by TPP using Resin A and Resin B. Identical experimental conditions are used in both resins. The woodpile microstructures in each array have different lattice parameters and thus, exhibit different structural colors; this is achieved by varying the line separation (top to bottom) and the laser pulse energy (left to right). A red outline delineates the woodpile microstructures that present well-separated individual rods.

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As the resin viscosity decreases, the mechanical properties of the written structures are affected as well. Resins C and D for example, did not produced well-defined three-dimensional microstructures as the woodpiles shown in Fig. 5. This is most probably due to the higher percentages of the linear monomer BGDA in Resins C and D that result in polymers not as cross-linked as the ones produced by Resins A and B.

The image of the microstructure made with Resin A in Fig. 5 presents a large section of the processed substrate with burnt or exploded woodpiles. This observation depends on a well-known fact in TPP. That is, the damage threshold of TPP microstructures depend on the proximity between adjacent features. The Edamage of closely spaced or overlapping features are lower than the Edamage of the corresponding features when written spatially apart from each other. It has been demonstrated that this lowering of Edamage is caused by an increase in the single-photon absorptivity of the cured resin [56]. The geometry of the written microstructure and the writing speed both influence the proximity effect on Edamage. The result in Fig. 5 shows that viscosity plays an important role as well. Not only lower viscosity resins are preferable for writing microstructure with better resolution, but they diminish the possibility of destroying the microstructure itself because of Edamage.

4. Conclusions

In this paper, we showed how viscosity influences the formation of lines in two-photon polymerization, in four different acrylate resins with viscosity ranging from more than 10 Pa·s to 15 mPa·s. While in stereolithography low viscosity is a standard, in two-photon polymerization it is desirable in order to achieve higher resolution and to reduce the probability of damaged microstructures, even though it does not affect heavily the damage threshold for single lines. On the other hand, when a large dynamic range is needed, highly viscous resins allow for a wide variation of the usable pulse energy, thanks to the lower polymerization threshold, to control and increase line width with more freedom, to increase fabrication velocity, and to achieve a more robust fabrication process. These results can be understood considering the effects of the diffusion of quenchers in the different materials.

Funding

National Science Foundation (NSF) (CMMI-1449309); Horizon 2020 Framework Programme (H2020) (754467).

Acknowledgments

NL and JTF acknowledge support by the National Science Foundation, Grant CMMI-1449309. R.O. and T.Z. acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (ERC-PoC NICHOIDS - grant agreement No 754467). T.B. thanks Prof. Ewa Andrzejewska, and Prof Arnaud Bertsch for useful discussions during the preparation of the manuscript.

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References

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  1. S. C. Ligon, R. Liska, J. Stampfl, M. Gurr, and R. Mulhaupt, “Polymers for 3D Printing and Customized Additive Manufacturing,” Chem. Rev. 117(15), 10212–10290 (2017).
    [Crossref]
  2. C. N. LaFratta, J. T. Fourkas, T. Baldacchini, and R. A. Farrer, “Multiphoton fabrication,” Angew. Chem., Int. Ed. 46(33), 6238–6258 (2007).
    [Crossref]
  3. B. E. Kelly, I. Bhattacharya, H. Heidari, M. Shusteff, C. M. Spadaccini, and H. K. Taylor, “Volumetric additive manufacturing via tomographic reconstruction,” Science 363(6431), 1075–1079 (2019).
    [Crossref]
  4. C. N. LaFratta and L. J. Li, Making Two-Photon Polymerization Faster (William Andrew Inc, 2016).
  5. P. J. Bártolo, Stereolithography: Materials, Processes and Applications (Springer-Verlag Berlin, 2011).
  6. M. Malinauskas, A. Zukauskas, S. Hasegawa, Y. Hayasaki, V. Mizeikis, R. Buividas, and S. Juodkazis, “Ultrafast laser processing of materials: from science to industry,” Light: Sci. Appl. 5(8), e16133 (2016).
    [Crossref]
  7. M. Malinauskas, M. Farsari, A. Piskarskas, and S. Juodkazis, “Ultrafast laser nanostructuring of photopolymers: A decade of advances,” Phys. Rep. 533(1), 1–31 (2013).
    [Crossref]
  8. J. Fischer and M. Wegener, “Three-dimensional optical laser lithography beyond the diffraction limit,” Laser Photonics Rev. 7(1), 22–44 (2013).
    [Crossref]
  9. A. K. Nguyen and R. J. Narayan, “Two-photon polymerization for biological applications,” Mater. Today 20(6), 314–322 (2017).
    [Crossref]
  10. N. Liaros and J. T. Fourkas, “The Characterization of Absorptive Nonlinearities,” Laser Photonics Rev. 11(5), 1700106 (2017).
    [Crossref]
  11. J. Li, Y. H. Cui, K. Qin, J. C. Yu, C. Guo, J. Q. Yang, C. C. Zhang, D. D. Jiang, and X. Wang, “Synthesis and properties of a low-viscosity UV-curable oligomer for three-dimensional printing,” Polym. Bull. 73(2), 571–585 (2016).
    [Crossref]
  12. M. Malinauskas, A. Zukauskas, V. Purlys, K. Belazaras, A. Momot, D. Paipulas, R. Gadonas, A. Piskarskas, H. Gilbergs, A. Gaidukeviciute, I. Sakellari, M. Farsari, and S. Juodkazis, “Femtosecond laser polymerization of hybrid/integrated micro-optical elements and their characterization,” J. Opt. 12(12), 124010 (2010).
    [Crossref]
  13. A. Ovsianikov, J. Viertl, B. Chichkov, M. Oubaha, B. MacCraith, I. Sakellari, A. Giakoumaki, D. Gray, M. Vamvakaki, M. Farsari, and C. Fotakis, “Ultra-Low Shrinkage Hybrid Photosensitive Material for Two-Photon Polymerization Microfabrication,” Acs Nano 2(11), 2257–2262 (2008).
    [Crossref]
  14. F. Burmeister, S. Steenhusen, R. Houbertz, U. D. Zeitner, S. Nolte, and A. Tunnermann, “Materials and technologies for fabrication of three-dimensional microstructures with sub-100 nm feature sizes by two-photon polymerization,” J. Laser Appl. 24(4), 042014 (2012).
    [Crossref]
  15. C. Decker, T. N. T. Viet, D. Decker, and E. Weber-Koehl, “UV-radiation curing of acrylate/epoxide systems,” Polymer 42(13), 5531–5541 (2001).
    [Crossref]
  16. C. Decker and K. Moussa, “Photopolymerization of mutifunctional monomers in condensed phase,” J. Appl. Polym. Sci. 34(4), 1603–1618 (1987).
    [Crossref]
  17. X. Q. Zhang, Y. Xu, L. Li, B. Yan, J. J. Bao, and A. M. Zhang, “Acrylate-based photosensitive resin for stereolithographic three-dimensional printing,” J. Appl. Polym. Sci. 136(21), 47487 (2019).
    [Crossref]
  18. J. W. Stansbury and M. J. Idacavage, “3D printing with polymers: Challenges among expanding options and opportunities,” Dent. Mater. 32(1), 54–64 (2016).
    [Crossref]
  19. S. Corbel, O. Dufaud, and T. Roques-Carmes, Materials for Stereolithography (Springer-Verlag Berlin, 2011).
  20. W. H. Teh, U. Durig, U. Drechsler, C. G. Smith, and H. J. Guntherodt, “Effect of low numerical-aperture femtosecond two-photon absorption on (SU-8) resist for ultrahigh-aspect-ratio microstereolithography,” J. Appl. Phys. 97(5), 054907 (2005).
    [Crossref]
  21. C. A. Coenjarts and C. K. Ober, “Two-photon three-dimensional microfabrication of poly(dimethylsiloxane) elastomers,” Chem. Mater. 16(26), 5556–5558 (2004).
    [Crossref]
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2019 (2)

B. E. Kelly, I. Bhattacharya, H. Heidari, M. Shusteff, C. M. Spadaccini, and H. K. Taylor, “Volumetric additive manufacturing via tomographic reconstruction,” Science 363(6431), 1075–1079 (2019).
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X. Q. Zhang, Y. Xu, L. Li, B. Yan, J. J. Bao, and A. M. Zhang, “Acrylate-based photosensitive resin for stereolithographic three-dimensional printing,” J. Appl. Polym. Sci. 136(21), 47487 (2019).
[Crossref]

2018 (2)

L. Jonusauskas, S. Juodkazis, and M. Malinauskas, “Optical 3D printing: bridging the gaps in the mesoscale,” J. Opt. 20(5), 053001 (2018).
[Crossref]

P. M. Consoli, A. J. G. Otuka, D. T. Balogh, and C. R. Mendonca, “Feature size reduction in two-photon polymerization by optimizing resin composition,” J. Polym. Sci., Part B: Polym. Phys. 56(16), 1158–1163 (2018).
[Crossref]

2017 (5)

M. Y. Zakharina, V. B. Fedoseev, Y. V. Chechet, S. A. Chesnokov, and A. S. Shaplov, “Effect of Viscosity of Dimethacrylate Ester-Based Compositions on the Kinetics of Their Photopolymerization in Presence of o-Quinone Photoinitiators,” Polym. Sci., Ser. B 59(6), 665–673 (2017).
[Crossref]

S. K. Saha, C. Divin, J. A. Cuadra, and R. M. Panas, “Effect of Proximity of Features on the Damage Threshold During Submicron Additive Manufacturing Via Two-Photon Polymerization,” J. Micro- Nano-Manuf. 5(3), 031002 (2017).
[Crossref]

S. C. Ligon, R. Liska, J. Stampfl, M. Gurr, and R. Mulhaupt, “Polymers for 3D Printing and Customized Additive Manufacturing,” Chem. Rev. 117(15), 10212–10290 (2017).
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A. K. Nguyen and R. J. Narayan, “Two-photon polymerization for biological applications,” Mater. Today 20(6), 314–322 (2017).
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N. Liaros and J. T. Fourkas, “The Characterization of Absorptive Nonlinearities,” Laser Photonics Rev. 11(5), 1700106 (2017).
[Crossref]

2016 (5)

J. Li, Y. H. Cui, K. Qin, J. C. Yu, C. Guo, J. Q. Yang, C. C. Zhang, D. D. Jiang, and X. Wang, “Synthesis and properties of a low-viscosity UV-curable oligomer for three-dimensional printing,” Polym. Bull. 73(2), 571–585 (2016).
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M. Malinauskas, A. Zukauskas, S. Hasegawa, Y. Hayasaki, V. Mizeikis, R. Buividas, and S. Juodkazis, “Ultrafast laser processing of materials: from science to industry,” Light: Sci. Appl. 5(8), e16133 (2016).
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J. W. Stansbury and M. J. Idacavage, “3D printing with polymers: Challenges among expanding options and opportunities,” Dent. Mater. 32(1), 54–64 (2016).
[Crossref]

Z. Tomova, N. Liaros, S. A. G. Razo, S. M. Wolf, and J. T. Fourkas, “In situ measurement of the effective nonlinear absorption order in multiphoton photoresists,” Laser Photonics Rev. 10(5), 849–854 (2016).
[Crossref]

S. Rekstyte, T. Jonavicius, D. Gailevicius, M. Malinauskas, V. Mizeikis, E. G. Gamaly, and S. Juodkazis, “Nanoscale Precision of 3D Polymerization via Polarization Control,” Adv. Opt. Mater. 4(8), 1209–1214 (2016).
[Crossref]

2014 (2)

L. Jonusauskas, S. Rekstyte, and M. Malinauskas, “Augmentation of direct laser writing fabrication throughput for three-dimensional structures by varying focusing conditions,” Opt. Eng. 53(12), 125102 (2014).
[Crossref]

J. B. Mueller, J. Fischer, F. Mayer, M. Kadic, and M. Wegener, “Polymerization Kinetics in Three-Dimensional Direct Laser Writing,” Adv. Mater. 26(38), 6566–6571 (2014).
[Crossref]

2013 (6)

J. Fischer, J. B. Mueller, J. Kaschke, T. J. A. Wolf, A. N. Unterreiner, and M. Wegener, “Three-dimensional multi-photon direct laser writing with variable repetition rate,” Opt. Express 21(22), 26244–26260 (2013).
[Crossref]

J. B. Mueller, J. Fischer, Y. J. Mange, T. Nann, and M. Wegener, “In-situ local temperature measurement during three-dimensional direct laser writing,” Appl. Phys. Lett. 103(12), 123107 (2013).
[Crossref]

M. Malinauskas, M. Farsari, A. Piskarskas, and S. Juodkazis, “Ultrafast laser nanostructuring of photopolymers: A decade of advances,” Phys. Rep. 533(1), 1–31 (2013).
[Crossref]

J. Fischer and M. Wegener, “Three-dimensional optical laser lithography beyond the diffraction limit,” Laser Photonics Rev. 7(1), 22–44 (2013).
[Crossref]

J. Torgersen, X. H. Qin, Z. Q. Li, A. Ovsianikov, R. Liska, and J. Stampfl, “Hydrogels for Two-Photon Polymerization: A Toolbox for Mimicking the Extracellular Matrix,” Adv. Funct. Mater. 23(36), 4542–4554 (2013).
[Crossref]

Z. Q. Li, N. Pucher, K. Cicha, J. Torgersen, S. C. Ligon, A. Ajami, W. Husinsky, A. Rosspeintner, E. Vauthey, S. Naumov, T. Scherzer, J. Stampfl, and R. Liska, “A Straightforward Synthesis and Structure-Activity Relationship of Highly Efficient Initiators for Two-Photon Polymerization,” Macromolecules 46(2), 352–361 (2013).
[Crossref]

2012 (5)

D. S. Correa, M. R. Cardoso, V. Tribuzi, L. Misoguti, and C. R. Mendonca, “Femtosecond Laser in Polymeric Materials: Microfabrication of Doped Structures and Micromachining,” IEEE J. Sel. Top. Quantum Electron. 18(1), 176–186 (2012).
[Crossref]

V. Tribuzi, D. S. Correa, W. Avansi, C. Ribeiro, E. Longo, and C. R. Mendonca, “Indirect doping of microstructures fabricated by two-photon polymerization with gold nanoparticles,” Opt. Express 20(19), 21107–21113 (2012).
[Crossref]

F. Burmeister, S. Steenhusen, R. Houbertz, U. D. Zeitner, S. Nolte, and A. Tunnermann, “Materials and technologies for fabrication of three-dimensional microstructures with sub-100 nm feature sizes by two-photon polymerization,” J. Laser Appl. 24(4), 042014 (2012).
[Crossref]

T. Baldacchini, S. Snider, and R. Zadoyan, “Two-photon polymerization with variable repetition rate bursts of femtosecond laser pulses,” Opt. Express 20(28), 29890–29899 (2012).
[Crossref]

I. Sakellari, E. Kabouraki, D. Gray, V. Purlys, C. Fotakis, A. Pikulin, N. Bityurin, M. Vamvakaki, and M. Farsari, “Diffusion-Assisted High-Resolution Direct Femtosecond Laser Writing,” Acs Nano 6(3), 2302–2311 (2012).
[Crossref]

2011 (1)

W. E. Lu, X. Z. Dong, W. Q. Chen, Z. S. Zhao, and X. M. Duan, “Novel photoinitiator with a radical quenching moiety for confining radical diffusion in two-photon induced photopolymerization,” J. Mater. Chem. 21(15), 5650–5659 (2011).
[Crossref]

2010 (3)

A. Marcinkowska and E. Andrzejewska, “Viscosity Effects in the Photopolymerization of Two-Monomer Systems,” J. Appl. Polym. Sci. 116(1), 280–287 (2010).
[Crossref]

M. Malinauskas, A. Zukauskas, V. Purlys, K. Belazaras, A. Momot, D. Paipulas, R. Gadonas, A. Piskarskas, H. Gilbergs, A. Gaidukeviciute, I. Sakellari, M. Farsari, and S. Juodkazis, “Femtosecond laser polymerization of hybrid/integrated micro-optical elements and their characterization,” J. Opt. 12(12), 124010 (2010).
[Crossref]

F. P. W. Melchels, J. Feijen, and D. W. Grijpma, “A review on stereolithography and its applications in biomedical engineering,” Biomaterials 31(24), 6121–6130 (2010).
[Crossref]

2009 (1)

L. J. Li, R. R. Gattass, E. Gershgoren, H. Hwang, and J. T. Fourkas, “Achieving lambda/20 Resolution by One-Color Initiation and Deactivation of Polymerization,” Science 324(5929), 910–913 (2009).
[Crossref]

2008 (4)

M. Gurr, D. Hofmann, M. Ehm, Y. Thomann, R. Kubler, and R. Mulhaupt, “Acrylic nanocomposite resins for use in stereolithography and structural light modulation based rapid prototyping and rapid manufacturing technologies,” Adv. Funct. Mater. 18(16), 2390–2397 (2008).
[Crossref]

A. Ovsianikov, J. Viertl, B. Chichkov, M. Oubaha, B. MacCraith, I. Sakellari, A. Giakoumaki, D. Gray, M. Vamvakaki, M. Farsari, and C. Fotakis, “Ultra-Low Shrinkage Hybrid Photosensitive Material for Two-Photon Polymerization Microfabrication,” Acs Nano 2(11), 2257–2262 (2008).
[Crossref]

E. Andrzejewska and A. Marcinkowska, “New Aspects of Viscosity Effects on the Photopolymerization Kinetics of the 2,2-Bis 4-(2-hydroxymethacryloxypropoxy) pheny1 propane/Triethylene Glycol Dimethacrylate Monomer System,” J. Appl. Polym. Sci. 110(5), 2780–2786 (2008).
[Crossref]

K. Takada, K. Kaneko, Y. D. Li, S. Kawata, Q. D. Chen, and H. B. Sun, “Temperature effects on pinpoint photopolymerization and polymerized micronanostructures,” Appl. Phys. Lett. 92(4), 041902 (2008).
[Crossref]

2007 (2)

C. N. LaFratta, J. T. Fourkas, T. Baldacchini, and R. A. Farrer, “Multiphoton fabrication,” Angew. Chem., Int. Ed. 46(33), 6238–6258 (2007).
[Crossref]

R. Liska, M. Schuster, R. Infuhr, C. Tureeek, C. Fritscher, B. Seidl, V. Schmidt, L. Kuna, A. Haase, F. Varga, H. Lichtenegger, and J. Stampfl, “Photopolymers for rapid prototyping,” JCT Res. 4(4), 505–510 (2007).
[Crossref]

2006 (3)

C. N. LaFratta, L. J. Li, and J. T. Fourkas, “Soft-lithographic replication of 3D microstructures with closed loops,” Proc. Natl. Acad. Sci. U. S. A. 103(23), 8589–8594 (2006).
[Crossref]

B. Kaehr, N. Ertas, R. Nielson, R. Allen, R. T. Hill, M. Plenert, and J. B. Shear, “Direct-write fabrication of functional protein matrixes using a low-cost Q-switched laser,” Anal. Chem. 78(9), 3198–3202 (2006).
[Crossref]

A. K. O’Brien and C. N. Bowman, “Impact of oxygen on photopolymerization kinetics and polymer structure,” Macromolecules 39(7), 2501–2506 (2006).
[Crossref]

2005 (3)

S. Juodkazis, V. Mizeikis, K. K. Seet, M. Miwa, and H. Misawa, “Two-photon lithography of nanorods in SU-8 photoresist,” Nanotechnology 16(6), 846–849 (2005).
[Crossref]

L. H. Nguyen, M. Straub, and M. Gu, “Acrylate-based photopolymer for two-photon microfabrication and photonic applications,” Adv. Funct. Mater. 15(2), 209–216 (2005).
[Crossref]

W. H. Teh, U. Durig, U. Drechsler, C. G. Smith, and H. J. Guntherodt, “Effect of low numerical-aperture femtosecond two-photon absorption on (SU-8) resist for ultrahigh-aspect-ratio microstereolithography,” J. Appl. Phys. 97(5), 054907 (2005).
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2004 (4)

C. A. Coenjarts and C. K. Ober, “Two-photon three-dimensional microfabrication of poly(dimethylsiloxane) elastomers,” Chem. Mater. 16(26), 5556–5558 (2004).
[Crossref]

R. Hague, S. Mansour, N. Saleh, and R. Harris, “Materials analysis of stereolithography resins for use in Rapid Manufacturing,” J. Mater. Sci. 39(7), 2457–2464 (2004).
[Crossref]

T. Baldacchini, C. N. LaFratta, R. A. Farrer, M. C. Teich, B. E. A. Saleh, M. J. Naughton, and J. T. Fourkas, “Acrylic-based resin with favorable properties for three-dimensional two-photon polymerization,” J. Appl. Phys. 95(11), 6072–6076 (2004).
[Crossref]

X. M. Duan, H. B. Sun, K. Kaneko, and S. Kawata, “Two-photon polymerization of metal ions doped acrylate monomers and oligomers for three-dimensional structure fabrication,” Thin Solid Films 453-454, 518–521 (2004).
[Crossref]

2003 (2)

2002 (1)

S. Beuermann and M. Buback, “Rate coefficients of free-radical polymerization deduced from pulsed laser experiments,” Prog. Polym. Sci. 27(2), 191–254 (2002).
[Crossref]

2001 (3)

E. Andrzejewska, “Photopolymerization kinetics of multifunctional monomers,” Prog. Polym. Sci. 26(4), 605–665 (2001).
[Crossref]

S. Kawata, H. B. Sun, T. Tanaka, and K. Takada, “Finer features for functional microdevices - Micromachines can be created with higher resolution using two-photon absorption,” Nature 412(6848), 697–698 (2001).
[Crossref]

C. Decker, T. N. T. Viet, D. Decker, and E. Weber-Koehl, “UV-radiation curing of acrylate/epoxide systems,” Polymer 42(13), 5531–5541 (2001).
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1999 (2)

B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord-Maughon, J. Q. Qin, H. Rockel, M. Rumi, X. L. Wu, S. R. Marder, and J. W. Perry, “Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication,” Nature 398(6722), 51–54 (1999).
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H. B. Sun, S. Matsuo, and H. Misawa, “Three-dimensional photonic crystal structures achieved with two-photon-absorption photopolymerization of resin,” Appl. Phys. Lett. 74(6), 786–788 (1999).
[Crossref]

1997 (1)

1996 (3)

C. Decker, “Photoinitiated crosslinking polymerisation,” Prog. Polym. Sci. 21(4), 593–650 (1996).
[Crossref]

C. Decker, B. Elzaouk, and D. Decker, “Kinetic study of ultrafast photopolymerization reactions,” J. Macromol. Sci., Part A: Pure Appl.Chem. 33(2), 173–190 (1996).
[Crossref]

S. Zissi, A. Bertsch, J.-Y. Jezequel, S. Corbel, D. J. Lougnot, and J. C. Andre, “Stereolithography and microtechniques,” Microsyst. Technol. 2(2), 97–102 (1996).
[Crossref]

1989 (1)

F. Tudos and T. Foldesberezsnich, “Free-radical polymerization - inhibition and retardation,” Prog. Polym. Sci. 14(6), 717–761 (1989).
[Crossref]

1987 (1)

C. Decker and K. Moussa, “Photopolymerization of mutifunctional monomers in condensed phase,” J. Appl. Polym. Sci. 34(4), 1603–1618 (1987).
[Crossref]

1985 (1)

C. Decker and A. D. Jenkins, “Kinetic approach of O2 inhibition in ultraviolet-induced and laser-induced polymerizations,” Macromolecules 18(6), 1241–1244 (1985).
[Crossref]

Ajami, A.

Z. Q. Li, N. Pucher, K. Cicha, J. Torgersen, S. C. Ligon, A. Ajami, W. Husinsky, A. Rosspeintner, E. Vauthey, S. Naumov, T. Scherzer, J. Stampfl, and R. Liska, “A Straightforward Synthesis and Structure-Activity Relationship of Highly Efficient Initiators for Two-Photon Polymerization,” Macromolecules 46(2), 352–361 (2013).
[Crossref]

Allen, R.

B. Kaehr, N. Ertas, R. Nielson, R. Allen, R. T. Hill, M. Plenert, and J. B. Shear, “Direct-write fabrication of functional protein matrixes using a low-cost Q-switched laser,” Anal. Chem. 78(9), 3198–3202 (2006).
[Crossref]

Ananthavel, S. P.

B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord-Maughon, J. Q. Qin, H. Rockel, M. Rumi, X. L. Wu, S. R. Marder, and J. W. Perry, “Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication,” Nature 398(6722), 51–54 (1999).
[Crossref]

Andre, J. C.

S. Zissi, A. Bertsch, J.-Y. Jezequel, S. Corbel, D. J. Lougnot, and J. C. Andre, “Stereolithography and microtechniques,” Microsyst. Technol. 2(2), 97–102 (1996).
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Andrzejewska, E.

A. Marcinkowska and E. Andrzejewska, “Viscosity Effects in the Photopolymerization of Two-Monomer Systems,” J. Appl. Polym. Sci. 116(1), 280–287 (2010).
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E. Andrzejewska and A. Marcinkowska, “New Aspects of Viscosity Effects on the Photopolymerization Kinetics of the 2,2-Bis 4-(2-hydroxymethacryloxypropoxy) pheny1 propane/Triethylene Glycol Dimethacrylate Monomer System,” J. Appl. Polym. Sci. 110(5), 2780–2786 (2008).
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E. Andrzejewska, “Photopolymerization kinetics of multifunctional monomers,” Prog. Polym. Sci. 26(4), 605–665 (2001).
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Avansi, W.

Baldacchini, T.

T. Baldacchini, S. Snider, and R. Zadoyan, “Two-photon polymerization with variable repetition rate bursts of femtosecond laser pulses,” Opt. Express 20(28), 29890–29899 (2012).
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C. N. LaFratta, J. T. Fourkas, T. Baldacchini, and R. A. Farrer, “Multiphoton fabrication,” Angew. Chem., Int. Ed. 46(33), 6238–6258 (2007).
[Crossref]

T. Baldacchini, C. N. LaFratta, R. A. Farrer, M. C. Teich, B. E. A. Saleh, M. J. Naughton, and J. T. Fourkas, “Acrylic-based resin with favorable properties for three-dimensional two-photon polymerization,” J. Appl. Phys. 95(11), 6072–6076 (2004).
[Crossref]

Balogh, D. T.

P. M. Consoli, A. J. G. Otuka, D. T. Balogh, and C. R. Mendonca, “Feature size reduction in two-photon polymerization by optimizing resin composition,” J. Polym. Sci., Part B: Polym. Phys. 56(16), 1158–1163 (2018).
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Bao, J. J.

X. Q. Zhang, Y. Xu, L. Li, B. Yan, J. J. Bao, and A. M. Zhang, “Acrylate-based photosensitive resin for stereolithographic three-dimensional printing,” J. Appl. Polym. Sci. 136(21), 47487 (2019).
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Barlow, S.

B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord-Maughon, J. Q. Qin, H. Rockel, M. Rumi, X. L. Wu, S. R. Marder, and J. W. Perry, “Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication,” Nature 398(6722), 51–54 (1999).
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Bártolo, P. J.

P. J. Bártolo, Stereolithography: Materials, Processes and Applications (Springer-Verlag Berlin, 2011).

Beck, E.

K. Studer, C. Decker, E. Beck, and R. Schwalm, “Overcoming oxygen inhibition in UV-curing of acrylate coatings by carbon dioxide inerting, Part I,” Prog. Org. Coat. 48(1), 92–100 (2003).
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Belazaras, K.

M. Malinauskas, A. Zukauskas, V. Purlys, K. Belazaras, A. Momot, D. Paipulas, R. Gadonas, A. Piskarskas, H. Gilbergs, A. Gaidukeviciute, I. Sakellari, M. Farsari, and S. Juodkazis, “Femtosecond laser polymerization of hybrid/integrated micro-optical elements and their characterization,” J. Opt. 12(12), 124010 (2010).
[Crossref]

Bertsch, A.

S. Zissi, A. Bertsch, J.-Y. Jezequel, S. Corbel, D. J. Lougnot, and J. C. Andre, “Stereolithography and microtechniques,” Microsyst. Technol. 2(2), 97–102 (1996).
[Crossref]

Beuermann, S.

S. Beuermann and M. Buback, “Rate coefficients of free-radical polymerization deduced from pulsed laser experiments,” Prog. Polym. Sci. 27(2), 191–254 (2002).
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Bhattacharya, I.

B. E. Kelly, I. Bhattacharya, H. Heidari, M. Shusteff, C. M. Spadaccini, and H. K. Taylor, “Volumetric additive manufacturing via tomographic reconstruction,” Science 363(6431), 1075–1079 (2019).
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Bityurin, N.

I. Sakellari, E. Kabouraki, D. Gray, V. Purlys, C. Fotakis, A. Pikulin, N. Bityurin, M. Vamvakaki, and M. Farsari, “Diffusion-Assisted High-Resolution Direct Femtosecond Laser Writing,” Acs Nano 6(3), 2302–2311 (2012).
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Bowman, C. N.

A. K. O’Brien and C. N. Bowman, “Impact of oxygen on photopolymerization kinetics and polymer structure,” Macromolecules 39(7), 2501–2506 (2006).
[Crossref]

Buback, M.

S. Beuermann and M. Buback, “Rate coefficients of free-radical polymerization deduced from pulsed laser experiments,” Prog. Polym. Sci. 27(2), 191–254 (2002).
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Buividas, R.

M. Malinauskas, A. Zukauskas, S. Hasegawa, Y. Hayasaki, V. Mizeikis, R. Buividas, and S. Juodkazis, “Ultrafast laser processing of materials: from science to industry,” Light: Sci. Appl. 5(8), e16133 (2016).
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Burmeister, F.

F. Burmeister, S. Steenhusen, R. Houbertz, U. D. Zeitner, S. Nolte, and A. Tunnermann, “Materials and technologies for fabrication of three-dimensional microstructures with sub-100 nm feature sizes by two-photon polymerization,” J. Laser Appl. 24(4), 042014 (2012).
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Cardoso, M. R.

D. S. Correa, M. R. Cardoso, V. Tribuzi, L. Misoguti, and C. R. Mendonca, “Femtosecond Laser in Polymeric Materials: Microfabrication of Doped Structures and Micromachining,” IEEE J. Sel. Top. Quantum Electron. 18(1), 176–186 (2012).
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Chechet, Y. V.

M. Y. Zakharina, V. B. Fedoseev, Y. V. Chechet, S. A. Chesnokov, and A. S. Shaplov, “Effect of Viscosity of Dimethacrylate Ester-Based Compositions on the Kinetics of Their Photopolymerization in Presence of o-Quinone Photoinitiators,” Polym. Sci., Ser. B 59(6), 665–673 (2017).
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Chen, Q. D.

K. Takada, K. Kaneko, Y. D. Li, S. Kawata, Q. D. Chen, and H. B. Sun, “Temperature effects on pinpoint photopolymerization and polymerized micronanostructures,” Appl. Phys. Lett. 92(4), 041902 (2008).
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Chen, W. Q.

W. E. Lu, X. Z. Dong, W. Q. Chen, Z. S. Zhao, and X. M. Duan, “Novel photoinitiator with a radical quenching moiety for confining radical diffusion in two-photon induced photopolymerization,” J. Mater. Chem. 21(15), 5650–5659 (2011).
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Chesnokov, S. A.

M. Y. Zakharina, V. B. Fedoseev, Y. V. Chechet, S. A. Chesnokov, and A. S. Shaplov, “Effect of Viscosity of Dimethacrylate Ester-Based Compositions on the Kinetics of Their Photopolymerization in Presence of o-Quinone Photoinitiators,” Polym. Sci., Ser. B 59(6), 665–673 (2017).
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Chichkov, B.

A. Ovsianikov, J. Viertl, B. Chichkov, M. Oubaha, B. MacCraith, I. Sakellari, A. Giakoumaki, D. Gray, M. Vamvakaki, M. Farsari, and C. Fotakis, “Ultra-Low Shrinkage Hybrid Photosensitive Material for Two-Photon Polymerization Microfabrication,” Acs Nano 2(11), 2257–2262 (2008).
[Crossref]

Chichkov, B. N.

Cicha, K.

Z. Q. Li, N. Pucher, K. Cicha, J. Torgersen, S. C. Ligon, A. Ajami, W. Husinsky, A. Rosspeintner, E. Vauthey, S. Naumov, T. Scherzer, J. Stampfl, and R. Liska, “A Straightforward Synthesis and Structure-Activity Relationship of Highly Efficient Initiators for Two-Photon Polymerization,” Macromolecules 46(2), 352–361 (2013).
[Crossref]

Coenjarts, C. A.

C. A. Coenjarts and C. K. Ober, “Two-photon three-dimensional microfabrication of poly(dimethylsiloxane) elastomers,” Chem. Mater. 16(26), 5556–5558 (2004).
[Crossref]

Consoli, P. M.

P. M. Consoli, A. J. G. Otuka, D. T. Balogh, and C. R. Mendonca, “Feature size reduction in two-photon polymerization by optimizing resin composition,” J. Polym. Sci., Part B: Polym. Phys. 56(16), 1158–1163 (2018).
[Crossref]

Corbel, S.

S. Zissi, A. Bertsch, J.-Y. Jezequel, S. Corbel, D. J. Lougnot, and J. C. Andre, “Stereolithography and microtechniques,” Microsyst. Technol. 2(2), 97–102 (1996).
[Crossref]

S. Corbel, O. Dufaud, and T. Roques-Carmes, Materials for Stereolithography (Springer-Verlag Berlin, 2011).

Correa, D. S.

D. S. Correa, M. R. Cardoso, V. Tribuzi, L. Misoguti, and C. R. Mendonca, “Femtosecond Laser in Polymeric Materials: Microfabrication of Doped Structures and Micromachining,” IEEE J. Sel. Top. Quantum Electron. 18(1), 176–186 (2012).
[Crossref]

V. Tribuzi, D. S. Correa, W. Avansi, C. Ribeiro, E. Longo, and C. R. Mendonca, “Indirect doping of microstructures fabricated by two-photon polymerization with gold nanoparticles,” Opt. Express 20(19), 21107–21113 (2012).
[Crossref]

Cronauer, C.

Cuadra, J. A.

S. K. Saha, C. Divin, J. A. Cuadra, and R. M. Panas, “Effect of Proximity of Features on the Damage Threshold During Submicron Additive Manufacturing Via Two-Photon Polymerization,” J. Micro- Nano-Manuf. 5(3), 031002 (2017).
[Crossref]

Cui, Y. H.

J. Li, Y. H. Cui, K. Qin, J. C. Yu, C. Guo, J. Q. Yang, C. C. Zhang, D. D. Jiang, and X. Wang, “Synthesis and properties of a low-viscosity UV-curable oligomer for three-dimensional printing,” Polym. Bull. 73(2), 571–585 (2016).
[Crossref]

Cumpston, B. H.

B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord-Maughon, J. Q. Qin, H. Rockel, M. Rumi, X. L. Wu, S. R. Marder, and J. W. Perry, “Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication,” Nature 398(6722), 51–54 (1999).
[Crossref]

de Miguel, G.

G. de Miguel, G. Vicidomini, B. Harke, and A. Diaspro, Linewidth and Writing Resolution (William Andrew Inc, 2016).

Decker, C.

K. Studer, C. Decker, E. Beck, and R. Schwalm, “Overcoming oxygen inhibition in UV-curing of acrylate coatings by carbon dioxide inerting, Part I,” Prog. Org. Coat. 48(1), 92–100 (2003).
[Crossref]

C. Decker, T. N. T. Viet, D. Decker, and E. Weber-Koehl, “UV-radiation curing of acrylate/epoxide systems,” Polymer 42(13), 5531–5541 (2001).
[Crossref]

C. Decker, B. Elzaouk, and D. Decker, “Kinetic study of ultrafast photopolymerization reactions,” J. Macromol. Sci., Part A: Pure Appl.Chem. 33(2), 173–190 (1996).
[Crossref]

C. Decker, “Photoinitiated crosslinking polymerisation,” Prog. Polym. Sci. 21(4), 593–650 (1996).
[Crossref]

C. Decker and K. Moussa, “Photopolymerization of mutifunctional monomers in condensed phase,” J. Appl. Polym. Sci. 34(4), 1603–1618 (1987).
[Crossref]

C. Decker and A. D. Jenkins, “Kinetic approach of O2 inhibition in ultraviolet-induced and laser-induced polymerizations,” Macromolecules 18(6), 1241–1244 (1985).
[Crossref]

Decker, D.

C. Decker, T. N. T. Viet, D. Decker, and E. Weber-Koehl, “UV-radiation curing of acrylate/epoxide systems,” Polymer 42(13), 5531–5541 (2001).
[Crossref]

C. Decker, B. Elzaouk, and D. Decker, “Kinetic study of ultrafast photopolymerization reactions,” J. Macromol. Sci., Part A: Pure Appl.Chem. 33(2), 173–190 (1996).
[Crossref]

Diaspro, A.

G. de Miguel, G. Vicidomini, B. Harke, and A. Diaspro, Linewidth and Writing Resolution (William Andrew Inc, 2016).

Divin, C.

S. K. Saha, C. Divin, J. A. Cuadra, and R. M. Panas, “Effect of Proximity of Features on the Damage Threshold During Submicron Additive Manufacturing Via Two-Photon Polymerization,” J. Micro- Nano-Manuf. 5(3), 031002 (2017).
[Crossref]

Domann, G.

Dong, X. Z.

W. E. Lu, X. Z. Dong, W. Q. Chen, Z. S. Zhao, and X. M. Duan, “Novel photoinitiator with a radical quenching moiety for confining radical diffusion in two-photon induced photopolymerization,” J. Mater. Chem. 21(15), 5650–5659 (2011).
[Crossref]

Drechsler, U.

W. H. Teh, U. Durig, U. Drechsler, C. G. Smith, and H. J. Guntherodt, “Effect of low numerical-aperture femtosecond two-photon absorption on (SU-8) resist for ultrahigh-aspect-ratio microstereolithography,” J. Appl. Phys. 97(5), 054907 (2005).
[Crossref]

Duan, X. M.

W. E. Lu, X. Z. Dong, W. Q. Chen, Z. S. Zhao, and X. M. Duan, “Novel photoinitiator with a radical quenching moiety for confining radical diffusion in two-photon induced photopolymerization,” J. Mater. Chem. 21(15), 5650–5659 (2011).
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X. M. Duan, H. B. Sun, K. Kaneko, and S. Kawata, “Two-photon polymerization of metal ions doped acrylate monomers and oligomers for three-dimensional structure fabrication,” Thin Solid Films 453-454, 518–521 (2004).
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Dufaud, O.

S. Corbel, O. Dufaud, and T. Roques-Carmes, Materials for Stereolithography (Springer-Verlag Berlin, 2011).

Durig, U.

W. H. Teh, U. Durig, U. Drechsler, C. G. Smith, and H. J. Guntherodt, “Effect of low numerical-aperture femtosecond two-photon absorption on (SU-8) resist for ultrahigh-aspect-ratio microstereolithography,” J. Appl. Phys. 97(5), 054907 (2005).
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Dyer, D. L.

B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord-Maughon, J. Q. Qin, H. Rockel, M. Rumi, X. L. Wu, S. R. Marder, and J. W. Perry, “Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication,” Nature 398(6722), 51–54 (1999).
[Crossref]

Egbert, A.

Ehm, M.

M. Gurr, D. Hofmann, M. Ehm, Y. Thomann, R. Kubler, and R. Mulhaupt, “Acrylic nanocomposite resins for use in stereolithography and structural light modulation based rapid prototyping and rapid manufacturing technologies,” Adv. Funct. Mater. 18(16), 2390–2397 (2008).
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Ehrlich, J. E.

B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord-Maughon, J. Q. Qin, H. Rockel, M. Rumi, X. L. Wu, S. R. Marder, and J. W. Perry, “Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication,” Nature 398(6722), 51–54 (1999).
[Crossref]

Elzaouk, B.

C. Decker, B. Elzaouk, and D. Decker, “Kinetic study of ultrafast photopolymerization reactions,” J. Macromol. Sci., Part A: Pure Appl.Chem. 33(2), 173–190 (1996).
[Crossref]

Erskine, L. L.

B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord-Maughon, J. Q. Qin, H. Rockel, M. Rumi, X. L. Wu, S. R. Marder, and J. W. Perry, “Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication,” Nature 398(6722), 51–54 (1999).
[Crossref]

Ertas, N.

B. Kaehr, N. Ertas, R. Nielson, R. Allen, R. T. Hill, M. Plenert, and J. B. Shear, “Direct-write fabrication of functional protein matrixes using a low-cost Q-switched laser,” Anal. Chem. 78(9), 3198–3202 (2006).
[Crossref]

Farrer, R. A.

C. N. LaFratta, J. T. Fourkas, T. Baldacchini, and R. A. Farrer, “Multiphoton fabrication,” Angew. Chem., Int. Ed. 46(33), 6238–6258 (2007).
[Crossref]

T. Baldacchini, C. N. LaFratta, R. A. Farrer, M. C. Teich, B. E. A. Saleh, M. J. Naughton, and J. T. Fourkas, “Acrylic-based resin with favorable properties for three-dimensional two-photon polymerization,” J. Appl. Phys. 95(11), 6072–6076 (2004).
[Crossref]

Farsari, M.

M. Malinauskas, M. Farsari, A. Piskarskas, and S. Juodkazis, “Ultrafast laser nanostructuring of photopolymers: A decade of advances,” Phys. Rep. 533(1), 1–31 (2013).
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I. Sakellari, E. Kabouraki, D. Gray, V. Purlys, C. Fotakis, A. Pikulin, N. Bityurin, M. Vamvakaki, and M. Farsari, “Diffusion-Assisted High-Resolution Direct Femtosecond Laser Writing,” Acs Nano 6(3), 2302–2311 (2012).
[Crossref]

M. Malinauskas, A. Zukauskas, V. Purlys, K. Belazaras, A. Momot, D. Paipulas, R. Gadonas, A. Piskarskas, H. Gilbergs, A. Gaidukeviciute, I. Sakellari, M. Farsari, and S. Juodkazis, “Femtosecond laser polymerization of hybrid/integrated micro-optical elements and their characterization,” J. Opt. 12(12), 124010 (2010).
[Crossref]

A. Ovsianikov, J. Viertl, B. Chichkov, M. Oubaha, B. MacCraith, I. Sakellari, A. Giakoumaki, D. Gray, M. Vamvakaki, M. Farsari, and C. Fotakis, “Ultra-Low Shrinkage Hybrid Photosensitive Material for Two-Photon Polymerization Microfabrication,” Acs Nano 2(11), 2257–2262 (2008).
[Crossref]

Fedoseev, V. B.

M. Y. Zakharina, V. B. Fedoseev, Y. V. Chechet, S. A. Chesnokov, and A. S. Shaplov, “Effect of Viscosity of Dimethacrylate Ester-Based Compositions on the Kinetics of Their Photopolymerization in Presence of o-Quinone Photoinitiators,” Polym. Sci., Ser. B 59(6), 665–673 (2017).
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Feijen, J.

F. P. W. Melchels, J. Feijen, and D. W. Grijpma, “A review on stereolithography and its applications in biomedical engineering,” Biomaterials 31(24), 6121–6130 (2010).
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Fischer, J.

J. B. Mueller, J. Fischer, F. Mayer, M. Kadic, and M. Wegener, “Polymerization Kinetics in Three-Dimensional Direct Laser Writing,” Adv. Mater. 26(38), 6566–6571 (2014).
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J. Fischer, J. B. Mueller, J. Kaschke, T. J. A. Wolf, A. N. Unterreiner, and M. Wegener, “Three-dimensional multi-photon direct laser writing with variable repetition rate,” Opt. Express 21(22), 26244–26260 (2013).
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J. Fischer and M. Wegener, “Three-dimensional optical laser lithography beyond the diffraction limit,” Laser Photonics Rev. 7(1), 22–44 (2013).
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J. B. Mueller, J. Fischer, Y. J. Mange, T. Nann, and M. Wegener, “In-situ local temperature measurement during three-dimensional direct laser writing,” Appl. Phys. Lett. 103(12), 123107 (2013).
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Foldesberezsnich, T.

F. Tudos and T. Foldesberezsnich, “Free-radical polymerization - inhibition and retardation,” Prog. Polym. Sci. 14(6), 717–761 (1989).
[Crossref]

Fotakis, C.

I. Sakellari, E. Kabouraki, D. Gray, V. Purlys, C. Fotakis, A. Pikulin, N. Bityurin, M. Vamvakaki, and M. Farsari, “Diffusion-Assisted High-Resolution Direct Femtosecond Laser Writing,” Acs Nano 6(3), 2302–2311 (2012).
[Crossref]

A. Ovsianikov, J. Viertl, B. Chichkov, M. Oubaha, B. MacCraith, I. Sakellari, A. Giakoumaki, D. Gray, M. Vamvakaki, M. Farsari, and C. Fotakis, “Ultra-Low Shrinkage Hybrid Photosensitive Material for Two-Photon Polymerization Microfabrication,” Acs Nano 2(11), 2257–2262 (2008).
[Crossref]

Fourkas, J. T.

N. Liaros and J. T. Fourkas, “The Characterization of Absorptive Nonlinearities,” Laser Photonics Rev. 11(5), 1700106 (2017).
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Z. Tomova, N. Liaros, S. A. G. Razo, S. M. Wolf, and J. T. Fourkas, “In situ measurement of the effective nonlinear absorption order in multiphoton photoresists,” Laser Photonics Rev. 10(5), 849–854 (2016).
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L. J. Li, R. R. Gattass, E. Gershgoren, H. Hwang, and J. T. Fourkas, “Achieving lambda/20 Resolution by One-Color Initiation and Deactivation of Polymerization,” Science 324(5929), 910–913 (2009).
[Crossref]

C. N. LaFratta, J. T. Fourkas, T. Baldacchini, and R. A. Farrer, “Multiphoton fabrication,” Angew. Chem., Int. Ed. 46(33), 6238–6258 (2007).
[Crossref]

C. N. LaFratta, L. J. Li, and J. T. Fourkas, “Soft-lithographic replication of 3D microstructures with closed loops,” Proc. Natl. Acad. Sci. U. S. A. 103(23), 8589–8594 (2006).
[Crossref]

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L. Jonusauskas, S. Juodkazis, and M. Malinauskas, “Optical 3D printing: bridging the gaps in the mesoscale,” J. Opt. 20(5), 053001 (2018).
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Figures (5)

Fig. 1.
Fig. 1. Molecular structures of (a) monomer DPEPA, (b) monomer BGDA, and (c) photoinitiator BAPO used to make the four resins used in this study. (d) 2-BIT data for a resin containing 0.15 wt% BAPO. The dashed line is the result that would be expected for 2-photon absorption as a reference. The error bars are based on standard deviations from multiple measurements.
Fig. 2.
Fig. 2. Dynamic ranges of the four investigated resins as a function of their viscosities. The blue, red, green, and orange colors represent resins A, B, C, and D, respectively.
Fig. 3.
Fig. 3. Scanning electron microscopy images of samples used to measure writing linewidths. (a) In this overview, five arrays of ascending lines are written using the same experimental condition in Resin D. The polymerized lines used for linewidth measurements are highlighted in yellow. (b) High magnification image of a line written in Resin A using an energy per pulse of 0.29 nJ at a velocity of 50 µm/s. (c) High magnification image of a line written in Resin D using an energy per pulse of 0.30 nJ at a velocity of 50 µm/s. The scale bars are 50 µm, 3 µm, and 2 µm, in (a), (b) and (c), respectively.
Fig. 4.
Fig. 4. Experimental values of TPP writing linewidths as a function of the laser energy per pulse. Data collected for Resins A, B, C, and D are presented together. All structures used for creating this plot were made at a writing speed of 50 µm/s.
Fig. 5.
Fig. 5. Optical microscopy images of arrays of woodpile microstructures fabricated by TPP using Resin A and Resin B. Identical experimental conditions are used in both resins. The woodpile microstructures in each array have different lattice parameters and thus, exhibit different structural colors; this is achieved by varying the line separation (top to bottom) and the laser pulse energy (left to right). A red outline delineates the woodpile microstructures that present well-separated individual rods.

Tables (3)

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Table 1. Composition of the resins investigated in this study with the corresponding viscosity measured at room temperature.

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Table 2. Polymerization (Eth) and damage (Edamage) energy thresholds for the resins considered in this study. The dynamic range for each resin is listed as well.

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Table 3. Diffusion rate coefficients calculated using Eq. (1) for the resin studied in this work. Viscosities of the resins in Pa·s are shown as well.

Equations (1)

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k d i f f = 8000 R T 3 η

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