High-speed two-photon polymerization 3D printing with a microchip laser at its fundamental wavelength.

High-resolution, high-speed 3D printing by two-photon polymerization (2PP) with a Nd:YVO4 Q-switched microchip laser at its fundamental wavelength of 1064 nm is demonstrated. Polymerization scan speeds of up to 20 mm/s and feature sizes of 250 nm are achieved using a high repetition rate Q-switched microchip laser with a semiconductor saturable absorber mirror (SESAM) and photoresist with a new photo-initiator bearing 6-dialkylaminobenzufuran as electron donor and indene-1,3-dione moiety as electron acceptor. The obtained results demonstrate the high potential of Q-switched microchip lasers for applications in 2PP 3D printing.

doped, high gain media, microchip lasers can be extremely compact, maintenance-free, and sufficiently cheap for mass-production.
The first attempts to utilize Q-switched microchip lasers for 2PP [16] reported results which were inferior to that achieved with femtosecond lasers. This is partially due to the characteristics of commercially available Q-switched microchip lasers that are not optimal for the 2PP process. Typically, microchip lasers generate sub-nanosecond pulses with the pulse energy of few microjoules and repetition rate below 50 kHz [17][18][19]. In addition, as the available photoresist-photoinitiator systems are not sufficiently sensitive to the radiation of Nd-doped Q-switched microchip lasers at the fundamental wavelength of 1064 nm, the second harmonic was applied for 2PP [18,19] resulting in additional complication of the laser system and reduced energy efficiency. Reliable photoresists for 2PP 3D printing at 1064 nm wavelength still have not been developed despite the reported attempts [20].
In this paper, significant improvements of the 2PP 3D printing quality and performance by using a Q-switched microchip laser are reported. This laser is based on a special microchip cavity with a semiconductor saturable absorber mirror (SESAM) bonded to a Nd:YVO 4 crystal [21]. This layout enabled the generation of picosecond laser pulses at the repetition rate of several hundred kHz. Thanks to the laser performance and high 2PP reactivity of the prepared photoresist directly at the fundamental wavelength of our microchip laser, 2PP 3D printing speeds of up to 20 mm/s are demonstrated and objects with feature sizes down to 250 nm are produced. These 2PP 3D printing characteristics are on par with femtosecond lasers.

Materials and methods
To improve the 2PP process, a recently synthesized photoinitiator based on an α,βunsaturated 1,3-diketone [22] was used ( Fig. 1(a)). The usage of ketones as initiators of both single-and two-photon polymerization is ubiquitous [23][24][25][26]. The carbonyl group being one of the strongest electron-withdrawing groups, offers obvious advantages in the design of organic dyes possessing large two-photon absorption. Moreover, conjugated 1,3-diketones possess even stronger electron-withdrawing effects [27,28]. In order to shift the one-photon absorption (and hence two-photon absorption) bathochromically the design of new photoinitiator implied the combination of 6-dialkylaminobenzofurane (one of the strongest, aromatic electron-donating moieties) and indene-1,3-dione (one of the strongest acceptors). As a result of this structural engineering the dye absorbs at 520 nm ( Fig. 1(b)), which given its dipolar D-π-A architecture should lead to strong two-photon absorption at ~1040 nm. Initial two photon absorption assessment of the synthesized photoinitiator is reported in [22]. The dye is highly transparent in the NIR with only background absorption (see inset in Fig. 1(b)). The additional crucial aspect of our design is the presence of long alkyl chains which secures very good solubility of the ketone in various media. Figure 2 shows the applied 2PP 3D printing setup. The microchip Q-switched laser was build using a Nd:YVO 4 microchip with attached SESAM that is commercially available from BATOP GmbH (Germany). A temperature stabilized laser diode pigtailed with a multimode optical fiber (50 µm core diameter, NA = 0.22) was used as a pump source at 808 nm. Pump radiation from the optical fiber was collimated and, after passing through a polarizer, focused on the dichroic mirror deposited directly on the back surface of Nd:YVO 4 microchip. A SESAM is attached to the front surface of the microchip and serves as a saturable absorber and output mirror. The size of the pump beam focal spot at the microchip surface is approximately 40 µm.
The microchip laser generates pulses at 1064 nm with a pulse energy of 95 nJ and pulse duration of 90 ps. The pulse repetition rate of 330 kHz, that was achieved at 300 mW pumping power, is used in all our polymerization experiments. Output of the microchip laser is collimated to a beam with the diameter of 4.5 mm. The microchip laser was installed into M4D Micro and Nano Structuring system (Laser nanoFab GmbH, Germany Polymerization of the Zr-based organic-inorganic hybrid material without adding the photoinitiator was also attempted. No microstructures could be produced in the accessible power range and scan speeds. This result indicates that under the relevant experimental conditions purely thermal polymerization was not possible and two-photon absorption by the photoinitiator was a primary mechanism of the polymerization. Examples of scanning electron microscope (SEM) images of the fabricated 3D woodpile structures are shown in Fig. 3. SEM images, illustrating the ultimate achieved feature size and fabrication speed are shown in Fig. 4 for the two scanning speeds of 5 mm/s and 20 mm/s with 14 mW and 27 mW average laser powers, respectively. The demonstrated smallest feature size of 250 nm is well beyond the diffraction limit and can be further improved by using a microscope objective with higher NA at 1 µm wavelength.
Relatively low NA of the used objective, which was chosen due to the lack of available alternatives optimized for 1064 nm, resulted in producing of narrow (~250 nm) and tall (several µm) polymeric rods (Fig. 3(b)). Correspondingly, due to the achieved high aspect ratio (> 10), some of the top layer polymeric rods tend to flex sideways as in Fig. 4.
In order to investigate the achievable feature sizes, widths of the polymeric rods forming the woodpile structures obtained at different scanning speeds and laser powers were measured. These measurements were carried out using magnified SEM images of the woodpiles.
The experimentally measured data of the polymeric rods width were fitted using the analytical expression for the 2PP voxel diameter [4]: The experimental results and the voxel diameter fitting curves obtained using [Eq. (1)] are shown in Fig. 5. The experimental results obtained with the SESAM Q-switched microchip laser are consistent with that reported for 2PP by a femtosecond laser [4]. Fitting of the data enables the determination of the polymerization threshold power for each scanning speed as well as the highest possible scanning speed for each of the examined powers.
The fitting applied to each experimental dependence measured at different experimental conditions is resulting in two fit parameters  Fig.  6. Although, the same value of the parameter 0 r could be expected for all dependencies in Fig. 5, it is actually different. Such a behavior was already reported in [30]. This discrepancy with theoretical model [4] is explained by the accumulation and diffusion of heat [30], light scattering and additional free radicals generation in the photoresist.

Conclusion
A high performance microchip laser, based on a Nd:YVO 4 SESAM Q-switched cavity at its fundamental wavelength of 1064 nm, was applied for two-photon polymerization of a hybrid photoresist using an indene-1,3-dione containing photoinitiator. Polymerized feature sizes of down to 250 nm and polymerization speeds of up to 20 mm/s have been achieved. Both, the feature sizes [4] and polymerization speeds [31] are on a par with 2PP fabrication by femtosecond lasers, with a significant reduction in the system footprint and cost.