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Abstract
The limited throughput of nano-scale laser lithography has been the bottleneck for its industrial applications. Although using multiple laser foci to parallelize the lithography process is an effective and straightforward strategy to improve rate, most conventional multi-focus methods are plagued by non-uniform laser intensity distribution due to the lack of individual control for each focus, which greatly hinders the nano-scale precision. In this paper, we present a highly uniform parallel two-photon lithography method based on a digital mirror device (DMD) and microlens array (MLA), which allows the generation of thousands of femtosecond (fs) laser foci with individual on-off switching and intensity-tuning capability. In the experiments, we generated a 1,600-laser focus array for parallel fabrication. Notably, the intensity uniformity of the focus array reached 97.7%, where the intensity-tuning precision for each focus reached 0.83%. A uniform dot array structure was fabricated to demonstrate parallel fabrication of sub-diffraction limit features, i.e., below 1/4 λ or 200 nm. The multi-focus lithography method has the potential of realizing rapid fabrication of sub-diffraction, arbitrarily complex, and large-scale 3D structures with three orders of magnitude higher fabrication rate.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Miniaturized fabrication capability is highly desirable in various emerging fields such as photonic chips, micro-sensors, on-chip artificial intelligence, etc. However, the products required by these fields are yet to be produced at scale via conventional two-dimensional (2D) lithography or electron beam lithography, due to the lack of rapid fabrication capability for three-dimensional (3D) nanostructures. A technology with high-precision, high-throughput, and large-scale 3D nanofabrication capability is eagerly required. Two-photon lithography (TPL) based on femtosecond (fs) laser can provide an effective approach to the above 3D nanofabrication demands for its advantage of high resolution [1], low heat impact [2], diversity of processable materials [3–7], 3D processing capacity [8], low environmental requirements [9], etc. TPL has always been a research hotspot and been widely used for fabricating metamaterials [10], microfluidic channels [11], diffractive optical elements (DOEs) [12], optical probes [13], microlens arrays (MLA) [14], 3D microscaffolds for cell culture [15], etc. However, current TPL techniques still face the issue of low processing accuracy and efficiency. Recently, the resolution of TPL has broken through the diffraction limit and is continuously improved by effort of researchers [16,17]. To significantly improve the throughput of TPL would be the next big step, which is the key of bringing TPL-based 3D nanofabrication into the industry.
Single-beam writing based on scanning devices is most commonly used in TPL system. Its writing efficiency strongly depends on the rate of scanning devices. Currently, the mostly-used scanning device like galvo [18] and polygon laser scanner [19,20] can provide a speed of several meters per second. However, it still takes several hours to fabricate a three-dimensional structure with hundred micrometer scale.
Parallelization via multi-foci generation is an effective approach to improve the manufacturing efficiency of direct laser writing. The reported multi-foci parallel nanofabrication mainly based on MLA, multi-beam interference, DOE, spatial light modulator (SLM) and digital micromirror device (DMD). In these methods, MLA [21,22], multi-beam interference [23] and DOE [24,25] are the most straight-forward approaches to generate multi-foci patterns. However, the geometry of the foci arrays generated by these methods are pre-defined, which will result in periodic structure manufacturing. Moreover, as each focus cannot be individually controlled, the intensity and quality of each focus can vary significantly, leading to deteriorated foci array and inferior fabrication structure. The multi-foci generation based on SLM [26–30] and DMD [31] enable the amplitude and phase control of each laser foci, which substantially improve the system flexibility, i.e., capability to fabricate complex and aperiodic structures. However, limited by the power of fs laser, the low diffraction efficiency, and etc., the foci number generated by SLM and DMD is still reported to be tens of laser beams, which is insufficient for industrial application. Besides, the quality of the as-generated foci array is also limited. A simulation showed that the uniformity of less than 100 foci generated by DMD can be optimized to ∼95%, but would deteriorate to ∼80% with 1000 foci [32].
It can be seen from above that small-scale foci array can often regulate each focus with more flexibility. Laser beams with large parallelism basically lack of individually controlling ability, leading to the foci pattern with periodical distribution and poor uniformity, which will result in simple, repetitive and nonuniform structure fabrication. This work provides a method for the generation of thousands of femtosecond laser foci based on DMD and MLA. With individual on-off switch and intensity tuning for each focus, complex foci pattern and ∼97.7% uniformity of 1, 600 laser foci can be achieved respectively. And the features of fabricated 40 × 40 dots are all below 1/4 λ (200 nm), showing a uniformity of 88%. The result verifies that this system can simultaneously realize (1) thousandfold throughput improvement, (2) sub-200 nm resolution for each focus, (3) high uniformity in large-scale structure fabrication, and (4) high processing flexibility with arbitrary-designed multi-foci pattern.
2. Optical design
Figure 1 presents the optical design for the high-throughput parallel nanofabrication system based on DMD and MLA. To provide enough power for generating thousands of writing foci, a Ti:sapphire fs laser amplifier (800 nm, 35 fs, 7 W, Solstice-ACE, Spectra-Physics) is adopted. Firstly, the power of femtosecond laser is adjusted by the combination of 1/2 waveplate and polarizing beam splitting cube. Next, through the beam shaper (Pishaper 12_12_TiS_HP, AdlOptica), the intensity of input laser beam is homogenized roughly. To fully utilize the pixels on DMD (DLP9500, 10.8 µm, 1920 × 1080 pixels, Texas Instruments), lens L1 and L2 form a 4-f system and expand the beam diameter to be larger than DMD aperture. A blazed transmission grating (T-1400-800-24 × 24-94, LightSmyth) is inserted in system to compensate the angular dispersion generated by DMD [31]. And DMD is conjugated with the front plane of MLA via another 4-f system combined by two laser scanning tube lens TL1 and TL2 (TTL200MP, thorlabs). Finally, the multi-foci array generated at back focal plane of MLA (Customized, pitch 150 µm, lens size 140 µm, square, f = 7.71 mm) is conjugated to the focal plane for lithography via a 4-f arrangement of scanning tube lens TL3 (TTL200MP, thorlabs) and objective (60x/1.35 OIL, Olympus & 100x/1. 40 OIL, Leica). By inserting a 100: 1 beam splitter after TL3, a small portion of laser is guided into the multi-foci detecting path, composing of the tube lens TL4 (TTL200MP, thorlabs) which forms a 4-f system with TL3 and a CMOS camera located at the focal plane of TL4. To monitor the fabricating process, a microscopic imaging system is built in conjunction with the fabrication setup. As shown in Fig. 1, the microscope shares the objective with the fabrication system. A LED light source is coupled into the system through a 1: 1 beam splitter for sample illumination. The CMOS camera after the dichroic mirror (DM) is placed at the conjugating plane of the objective focal plane via a 4-f arrangement of L3 and the objective. The sample is mounted on a motorized precision XYZ stage. The resin used in TPL experiments is IP-Dip (Nanoscribe GmbH).
Fig. 1. Optical configuration of the multi-foci nanofabrication system based on DMD and MLA. 1/2λ: 1/2 plate; PBS: polarizing beam splitter; Pishaper: beam shaper, transforming Gaussian to flat-top profile; DM, dichroic mirror; BS, beam splitter; OB: objective; L1-L3: lenses (fL1, fL2, fL3 = 100, 300, 200 mm, respectively); TL1-TL4: tube lenses (fTL1, fTL2, fTL3, fTL4 = 200 mm)
3. Method
3.1 Matching DMD subdistricts with MLA
To achieve individual control of each focus, the first step is to realize alignment of each DMD subdistrict with each microlens in the MLA. As present in Fig. 2(a), DMD is conjugated with the front plane of MLA via 1:1 4-f system. The pitch and size of the square microlens of the customized MLA is 150 µm and 140 µm respectively. As the pitch of DMD is 10.8 µm, the width of each subdistrict must be less than 13 pixels (< 140 µm /10.8 µm). The DMD subdistricts are configured to maximize the intensity tuning capability for each focus while not exceeding the edge of the microlens, as such, each subdistrict is configurated to have 11 × 11 micromirrors on the DMD. After pattern generation of the DMD (as shown in Fig. 2) and the precise alignment for the DMD and MLA, each DMD subdistrict would be mapped to the corresponding microlens, generating a high-quality focus.
Fig. 2. Matching scheme of DMD subdistricts with MLA. (a) Imaging system from DMD to MLA. The left sketch illustrates the design of each DMD subdistrict. (b) Generation of DMD pattern with discrete subdistricts precisely matching with MLA.
Figure 2(b) illustrates the DMD pattern generation steps: (1) Search the centre pixel of each DMD subdistrict: As the DMD subdistricts and MLA are not perfect aligned in space, it is needed to find the nearest centre pixels as the first step. A 2D impulse array is generated according to the MLA microlens distribution (i.e., 150 µm pitch). It is then pixelized by the size of the DMD pixel (10.8 µm), i.e., the index for the centre pixel should be $round(N \times 150/10.8)$, $N$ is an integer. The as-generated 2D impulse array represents the coordinates of the centre pixels in corresponding DMD subdistricts. (2) Define the pattern geometry: A pattern to be fabricated is defined in the form of a binary image, e.g., a triangle pattern shown in Fig. 2(b). Then, the pattern is multiplied to the 2D impulse array, after which only the array elements within the desired pattern are remained. (3) Lastly, the remaining impulse array is convolved with a DMD subdistrict of 11 × 11 pixels. And the designed DMD pattern with discrete subdistricts well-aligned to the MLA is generated.
3.2 Individual on-off switch and intensity tuning of each focus
Individually on-off switching of each focus in the generated multi-foci array can be realized by turning all the micromirrors in corresponding DMD subdistrict to ‘off’ state. This can be verified via generating foci array with custom patterns, where the unwanted foci are turned ‘off’. As shown in Fig. 3(a), DMD subdistricts with ‘A’-shaped distribution can be obtained by designing a binary image ‘A’ in step (2) of Fig. 2(b). As each subdistrict corresponds a focus, the ‘A’-shaped foci pattern will be generated, which can be finally imaged to the objective focal plane in photoresist for parallel fabrication.
Fig. 3. Individual control of every focus in multi-foci array based on DMD. (a) Generation of ‘A’-shaped multi-foci pattern by independent switch of each focus. (b) Focal intensity decreases from 100% to 90.08% by symmetrically switching the peripheral 12 micromirrors in corresponding DMD subdistrict to ‘off’ state.
In addition, intensity of each focus can also be finely tuned by adjusting the quantity of ‘on’ micromirrors in corresponding DMD subdistrict. Figure 3(b) illustrates the correspondence between single focus intensity and quantity of ‘on’ pixels in subdistrict. Specifically, 11 × 11 micromirrors with all ‘on’ state correspond to 100% intensity, permitting an intensity tuning step of 0.83%. If inverting 12 of them to ‘off’ state, the focal intensity will be reduced by 12/121, namely 90.08% remained. The adjustable minimum intensity step relies on the energy a single micromirror regulates. Notably, the 12 ‘off’ micromirrors are symmetrically distributed in the subdistrict periphery for minimizing the impact on focus quality.
3.3 Multi-foci intensity uniformity optimization
Figure 4(a) illustrates the 40 × 40 subdistricts obtained based on the DMD pattern generation method in section 3.1. The corresponding 40 × 40 foci array detected by CMOS camera is shown in Fig. 4 (b), subplot of which is the enlarged view of one single focus. Firstly, the intensity of each focus must be calculated. As presented in the subplot of Fig. 4(b), a single focus nearly maps to 10 × 10 pixels on the camera. After taking an image of the foci array, we can calculate the intensity of each focus by summing up the grey level of the corresponding area of the image, after which the overall uniformity can be derived by the following equation:
Where U is the multi-foci intensity uniformity; I is the intensity of the focus.Fig. 4. Multi-foci intensity uniformity calculation and optimization. (a) Designed DMD pattern including 40 × 40 subdistricts. (b) 40 × 40 foci array detected by CMOS camera. Subplot illustrates a single focus enlarge view. (c) Flow chart of iteration algorithm for uniformity optimization. (d) Schematic diagram of iteration process. Each blue scatter dot corresponds a focus. Red dashed line indicates the multi-foci average intensity Iavgi in each iteration.
Then, a one-time uniformity optimization process is performed to obtain evenly distributed intensities of the foci array. The principle is to adjust the focus intensity by changing the number of ‘on’ micromirrors in each DMD subdistrict, which makes the foci with higher intensity approach to the lowest focal intensity. However, it is found in our experiment that the detection for the lowest focal intensity can be troublesome as it might be affected by several factors, e.g., the flicker of low-cost CMOS camera, the dust on optical elements, vibration, etc. We propose a robust iteration method to achieve high uniformity, as described in Fig. 4(c). (1) Firstly, load the designed DMD pattern ($Patter{n_i}$) as presented in Fig. 4(a) and detect the foci array (). (2) Then calculate the intensity of each focus ${I_i}$, the average intensity ${I_{avgi}}$ of all foci, and the $Foc{i_i}$ minimum intensity tuning step ${I_{avgi}}$/121 regulated by single DMD pixel (e.g., 1 × 102 in this paper). (3) Find the foci with intensity higher than ${I_{avgi}}$. Comparing intensity of these foci with ${I_{avgi}}$ respectively, calculate the quantity of micromirrors that should be switched off in corresponding DMD subdistrict for each focus to make their intensity approach to ${I_{avgi}}$. (4) Symmetrically switch off the micromirrors in periphery of DMD subdistricts and set the newly generated DMD pattern as $Patter{n_{i + 1}}$, (5) load DMD $Patter{n_{i + 1}}$, detect the foci array $Foc{i_{i + 1}}$, and calculate the multi-foci average intensity ${I_{avgi + 1}}$. If |${I_{avgi}}$-${I_{avgi + 1}}$|<${I_{Thresh}}$ (e.g., ${I_{Thresh}}$=1 × 102 in this paper), terminate the iteration. If not, repeat the step (3)-step (5) until the optimized foci array satisfy our predefined criterion. Noticeably, the ${I_{Thresh}}$ can be varied in different experiments. In our paper, the intensity integral of all pixels covered by one single focus in our camera is the orders of magnitude of 104 with the 11 × 11 ‘on’ DMD micromirrors, which means the minimum intensity unit that one micromirror controls is in the level of 102. Also, the uniformity nearly remains unchanged while the ${I_{Thresh}}$=102 in our iteration process. If the lowest focal intensity obviously deviates from the final average intensity, this focus should be abandoned to ensure the overall uniformity and system efficiency.
To further verify the effectiveness of the optimization, the uniformity of the fabricated dot array is also evaluated by the following Eq. (2). The dots with the maximal and minimal size can be searched via the dot-array SEM image recognition by MATLAB, then the diameter of which will be specifically measured by SEM.
Where ${U^{\prime}}$ is the uniformity of the dot array; D is the diameter of the fabricated dot.4. Result
4.1 Flexible writing with complex foci pattern
As illustrated in Fig. 5, subdistricts of varied distributions are generated based on the DMD pattern generation method in section 3.1, in which the designed binary image is successively replaced by different patterns of a circle, Chinese characters ‘之’ and ‘江’. With the loaded DMD patterns shown in Fig. 5 (a)-(c), the corresponding multi-foci pattern are generated as present in Fig. 5 (d)-(f). These foci arrays are further applied to parallel laser writing on silicon wafer. Figure 5 (g)-(i) show that patterns of fabricated structures are well-defined by designed foci array, which demonstrates the ability of individually switching of foci in our parallel TPL system.
Fig. 5. Parallel writing with laser foci of various distributions. (a)-(c) DMD subdistricts with designed patterns: circle, Chinese characters ‘之’ and ‘江’. (d)-(f) Corresponding foci arrays generated with DMD patterns of (a)-(c) respectively. (g)-(i) Fabricated structures with foci arrays of (d)-(f) respectively.
4.2 Large parallel TPL based on high-uniformity thousand-fold laser beams
To demonstrate the large parallelization capability and high uniformity of our TPL system, the uniformity optimization of 40 × 40 foci array is implemented as shown in Fig. 6. By loading the designed DMD pattern with 40 × 40 subdistricts shown in Fig. 6(a), the non-optimized 40 × 40 foci array is generated. It can be seen from the two zoomed-in views Fig. (e) and (f) that the foci array exhibits large deviation in intensity. According to the calculation method in section 3.3, intensity of each focus and uniformity of 74.39% are further deduced as illustrated in Fig. 6(g), in which each focus corresponds a blue dot. Referring to the iteration method in Fig. 4(c)-(d), DMD pattern is refreshed successively and the intensity uniformity also change correspondingly, tendency of which is displayed in Fig. 6(i). After 40 iterations, the intensity uniformity can be improved to 97.65% with fresh DMD pattern shown in Fig. 6(b), subplot of which is the enlarged view of four DMD subdistricts circled within red square. Figure 6(d) and Fig. 6(h) respectively illustrate the detected optimized 40 × 40 foci and its intensity distribution after 40 iterations, which shows high uniformity in intensity.
Fig. 6. Uniformity optimization of multi-foci intensity. (a) Non-optimized DMD pattern including 40 × 40 subdistricts. (b) Optimized DMD pattern after 40 iterations. Subplot illustrates the enlarged view of four DMD subdistricts in red square. (c) The non-optimized 40 × 40 foci by loading DMD pattern of Fig. 6(a). (d) The optimized 40 × 40 foci with DMD pattern of Fig. 6(b). (e)-(f) Enlarged view of two foci marked respectively with red and yellow circle in Fig. 6(c). (g) The original intensity distribution of each focus and uniformity of overall foci. (h) The optimized intensity distribution and uniformity after 40 iterations. (i) The tendency curve of intensity uniformity through 40 iterations.
The performance of the non-optimized and optimized 40 × 40 foci array in Fig. 6(c) and Fig. 6(d) is then experimentally verified by our TPL system, as shown in Fig. 7(a) and Fig. 7(b). The polymerized structure in Fig. 7(a) shows an inferior uniformity owing to the severely poor uniformity of writing foci without optimization. And the tousled part is probably due to the overexposure of foci. In contrast, Fig. 7(b) shows the diameters of the optimized 40 × 40 dots are all below 200 nm (distributed from 150 nm to 190 nm) with a significantly improved uniformity of ∼88%. The uniformity of the fabricated dots in Fig. 7(b) is little lower than that of foci, which is due to the polymerization characteristic and proximity effect.
Fig. 7. (a) Polymerized structure writing with non-optimized 40 × 40 foci in Fig. 6(c). (b) 40 × 40 TPP dot lattice fabricated with optimized 40 × 40 foci in Fig. 6(d). (c)-(d) Enlarged view of four dots circled within red and yellow square respectively in Fig. 7(b). The diameters of the enlarged dots in (c) and (d) are around 180 nm and 150 nm respectively. Scale bars are 200 nm for (c)-(d).
5. Conclusion
We have introduced a TPL system based on DMD and MLA. Specifically, the fs laser is partitioned into thousands of beams by dividing the DMD into distinctive subdistricts and each beam is matched with a microlens of MLA to generate thousands of foci. By independently switching each DMD subdistrict, complex foci patterns can be realized. In addition, via individually adjusting the quantity of ‘on’ micromirrors in each DMD subdistrict, intensity of every focus can be precisely controlled with a tuning step of 0.83%. We also provide an effective iteration method to optimize the uniformity of the multi-foci array, which only needs to be performed once after system construction. An intensity uniformity of ∼97.65% is achieved in 40 × 40 foci array after 40 iterations, and the sizes of all polymerized dots via this method are all below 200 nm with 88% uniformity. The results proved that our system can realize large parallelized, highly uniform TPL with thousands of individually controlled laser foci, which will be an effective solution for printing complex micro- and nanostructures with greatly improved throughput, uniformity and controllability. For non-dot fabrication like processing large-scale continuous structure, a scanning strategy of thousands of foci will be further developed in our future work.
Funding
National Key Research and Development Program of China (2021YFF0502700); National Natural Science Foundation of China (62205304, 52105565 6220032333, 62105298); China Postdoctoral Science Foundation (2022TQ0311); Natural Science Foundation of Zhejiang Province (LQ22F050015, LQ22F050017); Major Program of Natural Science Foundation of Zhejiang province (LD21F050002); Key Project of Zhejiang Laboratory (2020MC0AE01).
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
References
1. K. Sugioka and Y. Cheng, “Femtosecond laser three-dimensional micro-and nanofabrication,” Appl. Phys. Rev. 1(4), 041303 (2014). [CrossRef]
2. L. M. Fang, Z. Qiao, J. Zhang, and G. D. Kumar, “Study on Micromachining of Femtosecond Laser Biomedical Polymer Materials,” Key Eng. Mater. 852, 109–118 (2020). [CrossRef]
3. R. Huang and W. H. Knox, “Quantitative photochemical scaling model for femtosecond laser micromachining of ophthalmic hydrogel polymers: effect of repetition rate and laser power in the four photon absorption limit,” Opt. Mater. Express 9(3), 1049–1061 (2019). [CrossRef]
4. G. H. Balbus, M. L. P. Echlin, C. M. Grigorian, T. J. Rupert, T. M. Pollock, and D. S. Gianola, “Femtosecond Laser Rejuvenation of Nanocrystalline Metals,” Acta Mater. 156, 183–195 (2018). [CrossRef]
5. S. I. Kudryashov, A. O. Levchenko, P. A. Danilov, N. A. Smirnov, A. A. Rudenko, N. N. Melnik, N. I. Busleev, and A. A. Lonon, “Direct femtosecond-laser writing of optical-range nanoscale metagratings/metacouplers on diamond surfaces,” Appl. Phys. Lett. 115(7), 073102 (2019). [CrossRef]
6. W. Li, J. Zheng, Y. Zhang, F. Yuan, and P. Lyu, “Temperature and depth evaluation of the in vitro effects of femtosecond laser on oral soft tissue, with or without air-cooling,” Lasers Med Sci 34(4), 649–658 (2019). [CrossRef]
7. L. Wang, Q. D. Chen, X. W. Cao, R. Buividas, X. Wang, S. Juodkazis, and H. B. Sun, “Plasmonic nano-printing: large-area nanoscale energy deposition for efficient surface texturing,” Lasers Med Sci 6(12), e17112 (2017). [CrossRef]
8. Tommaso Baldacchini, Three-Dimensional Microfabrication Using Two-Photon Polymerization2nd Edition (William Andrew,2019).
9. K. M. T. Ahmmed, C. Grambow, and A. M. Kietzig, “Fabrication of micro/nano structures on metals by femtosecond laser micromachining,” Micromachines 5(4), 1219–1253 (2014). [CrossRef]
10. J. K. Gansel, M. Thiel, M. S. Rill, M. Decker, K. Bade, V. Saile, G. V. Freymann, S. Linden, and M. Wegener, “Gold Helix Photonic Metamaterial as Broadband Circular Polarizer,” Science 325(5947), 1513–1515 (2009). [CrossRef]
11. S. Hengsbach and A. D. Lantada, “Rapid prototyping of multi-scale biomedical microdevices by combining additive manufacturing technologies,” Biomed. Microdevices 16(4), 617–627 (2014). [CrossRef]
12. M. Nawrot, Ł. Zinkiewicz, B. Włodarczyk, and P. Wasylczyk, “Transmission phase gratings fabricated with direct laser writing as color filters in the visible,” Opt. Express 21(26), 31919–31924 (2013). [CrossRef]
13. D. B. Phillips, M. J. Padgett, S. Hanna, Y. L. D. Ho, D. M. Carberry, M. J. Miles, and S. H. Simpson, “Shape-induced force fields in optical trapping,” Nat. Photonics 8(5), 400–405 (2014). [CrossRef]
14. D. Wu, S. Z. Wu, L. G. Niu, Q. D. Chen, R. Wang, J. F. Song, H. H. Fang, and H. B. Sun, “High numerical aperture microlens arrays of close packing,” Appl. Phys. Lett. 97(3), 031109 (2010). [CrossRef]
15. B. Spagnolo, V. Brunetti, G. Leménager, E. D. Luca, L. Sileo, T. Pellegrino, P. P. Pompa, M. D. Vittorio, and F. Pisanello, “Three-dimensional cage-like microscaffolds for cell invasion studies,” Sci. Rep. 5(1), 10531–10 (2015). [CrossRef]
16. U. T. Sanli, T. Messer, M. Weigand, L. Lötgering, G. Schütz, M. Wegener, C. Kern, and K. Keskinbora, “High-Resolution Kinoform X-Ray Optics Printed via 405 nm 3D Laser Lithography,” Adv. Mater. Technol. 7(9), 2101695 (2022). [CrossRef]
17. S. W. Hell and J. Wichmann, “Breaking the Diffraction Resolution Limit by Stimulated-Emission-Depletion Fluorescence Microscopy,” Opt. Lett. 19(11), 780–782 (1994). [CrossRef]
18. H. W. Yoo, S. Ito, and G. Schitter, “High speed laser scanning microscopy by iterative learning control of a galvanometer scanner,” Control. Eng. Pract. 50, 12–21 (2016). [CrossRef]
19. R. D. Loor, “Polygon scanner system for ultra short pulsed laser micro-machining applications,” Phys. Procedia 41, 544–551 (2013). [CrossRef]
20. J. Schille, L. Schneider, A. Streek, S. Kloetzer, and U. Loeschner, “High-throughput machining using a high-average power ultrashort pulse laser and high-speed polygon scanner,” Opt. Eng. 55(9), 096109 (2016). [CrossRef]
21. J. Kato, N. Takeyasu, Y. Adachi, H. B. Sun, and S. Kawata, “Multiple-spot parallel processing for laser micronanofabrication,” Appl. Phys. Lett. 86(4), 044102 (2005). [CrossRef]
22. F. Formanek, N. Takeyasu, T. Tanaka, K. Chiyoda, A. Ishikawa, and S. Kawata, “Three-dimensional fabrication of metallic nanostructures over large areas by two-photon polymerization,” Opt. Express 14(2), 800–809 (2006). [CrossRef]
23. E. Stankevicius, M. Malinauskas, M. Gedvilas, B. Voisiat, and G. Raciukaitis, “Fabrication of periodic micro-structures by multi-photon polymerization using the femtosecond laser and four-beam interference,” Mater. Sci. 17(3), 244–248 (2011). [CrossRef]
24. X. Z. Dong, Z. S. Zhao, and X. M. Duan, “Micronanofabrication of assembled three-dimensional microstructures by designable multiple beams multiphoton processing,” Appl. Phys. Lett. 91(12), 124103 (2007). [CrossRef]
25. V. Hahn, P. Kiefer, T. Frenzel, J. Qu, E. Blasco, C. B. Kowollik, and M. Wegener, “Rapid Assembly of Small Materials Building Blocks (Voxels) into Large Functional 3D Metamaterials,” Adv. Funct. Mater. 30(26), 1907795 (2020). [CrossRef]
26. S. D. Gittard, A. Nguyen, K. Obata, A. Koroleva, R. J. Narayan, and B. N. Chichkov, “Fabrication of microscale medical devices by two-photon polymerization with multiple foci via a spatial light modulator,” Biomed. Opt. Express 2(11), 3167–3178 (2011). [CrossRef]
27. L. Yang, A. El-Tamer, U. Hinze, J. Li, Y. Hu, W. Huang, J. Chu, and B. N. Chichkov, “Parallel direct laser writing of micro-optical and photonic structures using spatial light modulator,” Opt. Lasers Eng. 70, 26–32 (2015). [CrossRef]
28. X. Li, Y. Cao, N. Tian, L. Fu, and M. Gu, “Multifocal optical nanoscopy for big data recording at 30 TB capacity and gigabits/second data rate,” Optica 2(6), 567–570 (2015). [CrossRef]
29. B. Xu, W. Q. Du, J. W. Li, Y. L. Hu, L. Yang, C. C. Zhang, G. Q. Li, Z. X. Lao, J. C. Ni, J. R. Chu, D. Wu, S. L. Liu, and K. Sugioka, “High efficiency integration of three-dimensional functional microdevices inside a microfluidic chip by using femtosecond laser multifoci parallel microfabrication,” Sci. Rep. 6(1), 19989–9 (2016). [CrossRef]
30. Y. Hu, W. Feng, C. Xue, Z. Lao, S. Ji, Z. Cai, W. Zhu, J. Li, D. Wu, and J. Chu, “Self-assembled micropillars fabricated by holographic femtosecond multi-foci beams forin situ trapping of microparticles,” Opt. Lett. 45(17), 4698–4701 (2020). [CrossRef]
31. Q. Geng, D. Wang, P. Chen, and S. C. Chen, “Ultrafast multi-focus 3-D nano-fabrication based on two-photon polymerization,” Nat. Commun. 10(1), 2179 (2019). [CrossRef]
32. D. Chen, S. Gu, and S. C. Chen, “Study of Optical Modulation based on Binary Masks with Finite Pixels,” Opt. Lasers Eng. 142, 106604 (2021). [CrossRef]
