Femtosecond two-photon polymerization (2PP) is widely used in the fabrication of three-dimensional complex architectures with a sub-micrometer resolution. Recent applications are reported in quantum dots, scanning-probe microscopes, microchips, and biomimetic 4D printing. The conventional 2PP procedures utilize the tightly-focused femtosecond laser beam to trigger polymerization inside the photoresist. In recent years, femtosecond 2PP based on adaptive optics has attracted considerable interests due to the enhanced fabrication efficiency with numerous structured lights, such as Bessel beams, optical vortex beams, abruptly autofocusing beams and axilens beams with long focal depths. In these applications, the spatial light modulator (SLM) is introduced to generate arbitrary optical patterns through a computer-generated hologram (CGH) algorithm so that various structures could be rapidly fabricated via one-step exposure. Specifically, Bessel beams are notable for the diffraction-resistance and self-healing behaviors. Besides these properties, high-order Bessel beams also show the potential to synthesize more complex structured lights. Based on the above characteristics, high-order Bessel beams have been widely used in micro-nano manufacturing, biological imaging, optical tweezers, optical communication and other fields.
However, many factors in the 2PP setup including the ultrafast laser and the adaptive optics can introduce undesired aberrations, such as the walk-off effect in the nonlinear crystal, the internal stress and the nonlinear response of the adaptive devices, and the manufacturing defects or assembly errors of the optical element. The high-order Bessel beams are easily distorted in the setup due to their high sensitivity to the detrimental aberrations. Numerous methods can assist in removing aberrations in the optical system, and the SLMs are the critical devices in these adaptive methods. Crucially, these solutions all require relatively "heavy" mathematical tools, such as the Zernike polynomial, the Gerchberg-Saxton or more complex algorithms. Since these advanced skills in mathematics are usually studied by a limited group of professionals, workers in laser processing are usually excluded from further applications of wavefront corrections in laser fabrications.
Fig. 1 Adaptive optics for two-photon polymerization and the correction of high-order Bessel beams.
A research group from China and France has demonstrated the power of aberration correction with the straightforward multi-channel interferometric wave-front sensing technique in the 2PP-based 3D printing. This research is led by Prof. Chen Xie at Tianjin University in collaboration with Prof. Francois Courvoisier from FEMTO-ST institute of CNRS. The research results are published in Chinese Optics Letters, Volume 21, No. 7, 2023 (Erse Jia, Chen Xie, Na Xiao, Francois Courvoisier, Minglie Hu. Two-photon polymerization of femtosecond high-order Bessel beams with aberration correction[J]. Chinese Optics Letters, 2023, 21(7): 071203).
"Any optical field through a system can be expressed as a combination of modes in any orthogonal representation. The optimal focusing is achieved if and only if all the modes present the identical phase. It means the strength of constructive interference between these modes would be the highest." said Erse Jia, the PhD student who performs the experiments. Based on this interpretation, a programmable liquid crystal-based SLM is utilized to decompose the optical field in the 2PP system into M*N orthogonal channels. By interfering the reference channel with all the other test channels successively, the compensational wavefront can be readily achieved. And only a basic knowledge of college physics is necessary for this procedure and no physical wavefront sensor is required.
Fig. 2 Microtubes fabricated with high-order Bessel beams before and after the correction.
Encouraged by the high-quality restored beam restored with this technique, this work performs the two-photon polymerization as a further test. For comparison, the researchers fabricated microtubes with high-order Bessel beams before and after correction, respectively. SEM results showed that the micro-tubes are hideously deformed due to the uncorrected Bessel beams in the non-diffraction region, and the removal of aberrations results in the fabricated microtubes with higher roundness, uniform wall thickness and steeper walls. The researchers attributed the discrepancies in fabricating quality to the structural shrinkage induced by the nonuniform light energy deposition along the distorted Bessel beams.
This work has demonstrated the powerful feasibility of multi-channel interference wave-front sensing in the fabricating quality enhancement with adaptive optics-based two-photon polymerization. Without an additional physical wave-front sensor, more than 4π wave-front fluctuation across the input beam can be well flattened with the integration of the above wave-front sensing scheme. This results in significant improvement of fabrication quality with corrected high-order Bessel beams. Prof. Chen Xie from Tianjin University believes that this wave-front sensing scheme integrated in the setup is universal for laser micro/nano-processing systems despite of the demonstration only with a single family of structured lights (the Bessel beams). Besides, this straightforward solution with no requirements on knowledge of advanced mathematical skills would be highly attractive to promote the adaptive optics in a broader scope of material processing with lasers, esp. where a well-defined wavefront is desired, such as the femtosecond laser-induced periodic surface structures. Furthermore, correcting the fine features of structured lights will be performed with the complex amplitude modulation in the future.
