Femtosecond Laser Two-Photon High-Efficiency Processing Based on Spatial Light Modulator(Invited)
Significance Femtosecond laser two-photon polymerization(TPP)technology enables the direct three-dimensional(3D)printing of micro-nano structures with submicron precision.This technology has broad application prospects in many fields.Conventional femtosecond laser TPP uses a point-by-point scanning strategy to shape 3D structures.Specifically,it requires the focused spot to traverse the 3D spatial coordinates of the structure to be processed,thus rendering the efficient fabrication of 3D devices challenging.Consequently,the application of this technology is limited in various fields.Optical modulation technology can be applied to modulate incident light into the target light field,which significantly improves the processing efficiency and reduces the processing time.Thus,it can serve as a basis for high-throughput large-area manufacturing.A spatial light modulator(SLM)is a diffractive optical device that modulates the wavefront of a light wave.It can modulate a light beam into a predesigned spatial light field.SLMs can be categorized into phase and amplitude types depending on the modulation physical quantities.The most commonly used devices are liquid-crystal spatial light modulators(LC-SLMs)and digital micromirror devices(DMDs).In recent years,researchers have used SLMs to modulate various forms of light fields.Based on the modulated light fields,they have efficiently processed and fabricated micro-nano structures with specific shapes,thereby improving the efficiency and processing accuracy of femtosecond laser TPP.Herein,the main research achievements of SLM-assisted TPP technology in the past decade are summarized,including holographic processing based on iterative algorithms,holographic processing based on structured light fields,interferometric holographic processing,and spatiotemporal focusing.The advantages and disadvantages of various methods are analyzed,and their applications in various fields are discussed.Finally,existing issues associated with the processing methods are discussed,and future endeavors are proposed.Progress Light field modulation can improve processing efficiency and accuracy.Achieving high-quality light field modulation is a priority for many researchers.Using various hologram iterative algorithms,such as the Gerchberg-Saxton(GS)and weighted Gerchberg-Saxton(GSW)algorithms,researchers have successfully achieved the rapid machining of repetitive structures by modulating the incident laser into a multifocal shape(Fig.1).Furthermore,researchers have realized the flexible processing of multiple foci by dynamically loading holograms combined with the movement of the carrier table(Fig.2).Additionally,the efficiency of TPP has been improved by modulating the light field into a patterned or 3D light field and then enhancing the uniformity of the light field,thus enabling complex micro-nano structures to be processed in a single exposure(Fig.3).Another commonly used modulated light field is the structured light field,which enables the effective modulation of the incident laser light via the simulation of existing optical devices.Researchers have simulated optical devices such as cone lenses,Fresnel lenses,and helical phase plates to modulate beams into long-focus beams(Fig.4)and higher-order Bessel beams(Fig.5),toroidal Fresnel beams(Fig.6),Airy beams(Fig.7),and Mathieu beams(Fig.8).Various micro-nano structures,including micropillars,hollow microtubes,and micro-cages with high aspect ratios,have been fabricated.Meanwhile,some researchers have adopted optical interference to generate more complex light fields.Specifically,multibeam laser interference was simulated by loading computational holograms on an SLM to form periodic or custom-shaped light field distributions,and functional structures such as helical structures or chiral microstructures were prepared in a single exposure(Figs.9 and 10).Finally,femtosecond laser TPP has yielded impressive results in micro-nano structure processing.However,further improvements are required to efficiently prepare cross-scale(nano-micron-centimeter)structures.Furthermore,conventional femtosecond lasers result in elliptical asymmetric focal cross-sections owing to the inequality between the beam focal radius and Rayleigh length.This renders it challenging to achieve a spherical focal point with isotropic resolution in 3D space.Hence,researchers have combined SLMs with spatiotemporal focusing to efficiently process cross-scale micro-nano structures with high precision(Fig.11).Conclusions and Prospects In this paper,domestic and international studies pertaining to SLMs in femtosecond laser TPP are reviewed.In particular,holographic processing based on iterative algorithms,holographic processing based on structured light fields,interferometric holographic processing,and spatiotemporal focusing high-efficiency processing are discussed.The implementation methods and development process of femtosecond laser TPP for efficient processing are discussed,and the advantages and disadvantages of each method are summarized.Although femtosecond laser TPP based on SLMs has achieved significant breakthroughs,some of its aspects can be further improved:1)The range of materials suitable for femtosecond laser TPP can be further broadened to accommodate other fabrication processes to further expand the functionality of micro-nano structures.2)To satisfy the requirements of practical applications,the efficiency of SLM-assisted TPP must be further improved.3)The algorithms for the iterative generation of high-quality 3D light fields should be further investigated.4)The generation and modulation of novel structured light fields should be further developed.
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