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In addition to selecting appropriate processing methods, other important design factors must be considered, such as strategies for achieving high efficiency, methods for converting data points from 3D models, and materials for microoptical elements. The following section will discuss these three aspects.
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Generally, FsLDW uses a point-by-point processing method with a very small voxel, which is time-consuming when processing 3D structures with millions of pixels89,90. The single-point scanning mode of femtosecond lasers can no longer satisfy the requirements of efficient processing. Therefore, developing various methods to improve processing efficiency is essential.
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The energy of the laser focus controls the size of the processing voxels. On the one hand, smaller voxels can be obtained to fabricate high-precision 3D structures by reducing the energy of the femtosecond laser. However, this requires extended processing time. On the other hand, high exposure energy leads to an increase in voxel size, which significantly reduces the processing time. However, this results in high surface roughness. Therefore, by taking advantage of small and large voxels, different voxel sizes can be combined for processing to meet complex requirements. For example, large voxels are used to fabricate a microoptical element base quickly, and small voxels are used to obtain a 3D structure with a smooth surface, as shown in Fig. 2a91. Two-photon grayscale lithography is a voxel modulation method that quickly and accurately modulates the irradiation dose through a single scanning plane92,93. This technique provides the high precision required for high-speed printing and manufacturing complex micropatterned structures.
Fig. 2 a Processing method of high-speed voxel-modulation laser scanning (HVLS) [obtained from ref. 91]. b System of multiple-spot parallel processing (MSPP) with diffractive optical elements (DOEs) [obtained from ref. 99]. c Processing system of maskless optical projection nanolithography (MLOP-NL) based on digital mirror device (DMD) [obtained from ref. 102]. d Hybrid 3D printing method and system [obtained from ref. 105].
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In addition to increasing the voxel size, the processing efficiency can be improved by increasing the number of femtosecond laser spots94-98. For example, Yang et al. established a multifocus TPP system based on a spatial light modulator (SLM)99. In their system, the laser beam completely irradiated the SLM with only phase modulation after passing through the beam expander. When computationally generated holograms (CGH) were loaded onto the SLM, the gray levels on the computational holograms modulated the 0–2π phase shift of the corresponding laser beam. Consequently, the femtosecond laser beam reflected from the SLM is modulated into multiple beams with a specific distribution, and multiple laser beams were subsequently focused on the platform to fabricate the structures. Maibohm et al. used diffractive optical elements (DOEs) to produce nine parallelized multibeamlets. The experimental setup is shown in Fig. 2b96. SLMs (such as microlens arrays (MLA) or DOEs) are introduced into TPP manufacturing systems through which the femtosecond laser beam can be split into tens or even hundreds of light points100,101. These methods enable parallel processing of the femtosecond laser to improve the processing efficiency.
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Maskless optical projection lithography is a crucial technology in micro- and nanoscale graphics. Combining it with a femtosecond laser changes the point exposure mode to the planar exposure mode, significantly improving the processing efficiency. Liu et al. proposed and demonstrated an MLOP-NL system (Fig. 2c) for efficient cross-scale patterns102. The essential component of this system is a digital mirror device (DMD). The DMD modulates the laser beam by controlling the micromirror array, which produces a target exposure pattern for each layer. The MLOP-NL technology not only dramatically improves efficiency but also provides a powerful tool for fabricating multiscale integrated microsystems. Notably, the diffraction of the incident laser on the DMD and gap between the micromirrors results in significant energy loss, which requires a high laser power.
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The collaborative use of multiple 3D printing technologies can improve processing efficiency and achieve cross-scale processing103,104. For example, Tan et al. combined the advantage of TPP and DMD micro stereolithography (DMDMSL) (TPP-DMDMSL) for hybrid 3D printing (Fig. 2d)105,106. The TPP system can produce voxels with a diameter of 400 nm; however, its processing efficiency is low. Conversely, the processing speed of the DMDMSL system is fast, but the minimum resolution is only ~1 μm. The two systems were used for different parts of the 3D structures with different precision requirements to balance the processing time and resolution. To fabricate the aspheric lens, the TPP method was used for the high-precision processing of the surface, whereas the internal steps of the lens without high precision were processed using DMDMSL. Because the two systems are independent, they mark the substrate with circular marks to enable the two systems to process on the same substrate. High-precision positioning was performed by identifying circular marks in the CCD. Overall, TPP-DMDMSL combines the high resolution and superior design flexibility of TPP with the rapid processing capabilities of DMDMSL for efficient, high-precision printing. Table 1 provides a comparison of the processing methods used in this study. Notably, the obtained spatial resolution varies depending on the processed material and the projection lens magnification of the femtosecond laser processing system.
Technology Light source Energy intensity Spatial resolution Characteristic Basic methods TPP Fs laser Low 10 nm-μm145 Nonlinear effect Single point scanning, low efficiency FLA Fs laser High 250 nm-μm70 Non-thermal ablation FLM Fs laser Medium ~1 μm85 Processed inside the hard material Highly effective strategies HVLS Fs laser Low 100 nm-400 nm91 Voxel-modulation, bigger voxels enhance efficiency MSPP Fs laser Medium 90 nm-μm263 Multibeam processing, parallelized processing enhance efficiency MLOP-NL Fs laser High 32 nm-μm102 Planar exposure mode besed on DMD High processing efficiency, can achieve cross-scale processing TPP-DMDMSL Fs laser and LED Low 500 nm-μm105 Multiple 3D printing technology synergy Table 1. A comparison of various FsDLW technologies.
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Before processing, the 3D model must be translated into individual data-point coordinates, each of which is the location of a voxel107,108. During TPP, the femtosecond laser performs point-by-point processing according to the data points, and the processing area is photopolymerized to form a 3D structure. Because of its ablative properties, the data points used for processing in FLA are from structures that need to be removed. For the FLM, the area processed according to the data points was the laser-modified structural area. This section introduces a method for transforming a 3D model into data points. Notably, given the various efficiency improvement methods in section of “Strategies for achieving high efficiency, the methods of transforming the 3D model into data points need to be adjusted accordingly.
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In general, we convert the entire structure of the printed model into data points. During the conversion, the 3D model must be converted into a layer-by-layer 2D plane structure, where each layer is a single 2D data point (i.e., X and Y coordinate points). We also need to get the $ \text{Z} $ coordinate of each 2D plane structure, which is called slicing. The slicing process generally uses the equal-height slicing method109,110, which refers to an equal distance between each layer of a 3D model. However, for 3D curved microstructures (such as optical lenses and spherical microstructures), the thickness of the slice is a key factor that determines the surface quality. An image of the lens surface obtained by equal-height slicing with a layer spacing $ \text{h} $ of 100 nm is shown in Fig. 3b. Apparent solenoid traces are observed owing to the lateral offset of the voxel position, particularly at the top of the lens. This lateral offset has a significant effect on the roughness of the lens surface. Therefore, equal-height slicing is not an effective method for dealing with 3D microstructures of lenses. An equal-arc slicing method has been proposed to compensate for the large offset of voxels in the equal-height slicing method111-113, which implies that the layer height $ {\Delta }{h = \Delta L}\text{}\text{×}\text{}\text{sinθ} $ is variable. $ {\Delta }\text{h} $ depends on the arc length ${\Delta }\text{L} $ and sloping angle ${θ} $. A microlens with a smooth surface is obtained by selecting an appropriate arc for the isoarc slicing.
Fig. 3 a 3D model transformation data point method. b All 3D models are converted into data points. c 3D models are converted into data points partially.
Then, we transformed each 2D slice into a series of points containing $ \text{X} $ and $ \text{Y} $ coordinates114,115. The 2D slices are generally transformed into parallel point coordinates that are suitable for most microstructures. However, because each layer of the microlens exhibits a circular structure, the lines formed by the coordinates of parallel points not only distort the outline of the circle but also create some protrusions on the edges of the lens surface. Therefore, to obtain optical elements with smooth edges, annular point coordinates are generally chosen to ensure circular symmetry and radial consistency of the lens.
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To obtain a reduced number of data points, the 3D model can be partially (i.e., the contour) converted into data points116,117 (Fig. 3c). During TPP, the voxel size can be controlled by changing the laser power or scanning speed to create a shell with sufficient mechanical strength114. The unpolymerized portion of the outer layer of the contour was then removed by developing. Finally, the inner part of the contour is irradiated with UV light to obtain a completely polymerized structure. The fabrication of the shell, development, and post-processing with UV light significantly reduced the processing time. However, the contour scanning method requires a processed material with high mechanical strength. Otherwise, the development causes the collapse of the polymerization contour, resulting in processing failure. For FLA processing, detaching the part outside the target structure is also possible by processing only the contour data points.
Two types of data point conversion methods are available for single-point scanning processing, which are the most basic data point conversion methods. To improve the efficiency of the different processing methods mentioned in section of “Strategies for achieving high efficiency”, we need to improve the data point conversion methods correspondingly. For example, we need to modify the distance between voxels for HVLS. In the case of the MSPP, the SLM needs to be controlled to generate multiple focuses. For the MLOP-NL, the data points must be modified to the plane projection data of the DMD. In the hybrid 3D printing method, models and data points must be split.
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Materials suitable for optical element processing can also be categorized according to the three optical element processing methods in FsLDWs. The first is photopolymerization materials applicable to TPP, which mainly include polymeric materials and organic-inorganic hybrid materials118. Polymeric materials include resins119-123, polydimethylsiloxane (PDMS)124, and proteins125,126. Organic–inorganic hybrid materials include silicon nanocomposite127-129, sol–gel glass130,131, and polyhedral oligomeric silsesquioxane (POSS) glass materials, which can form glass after heat treatment132,133. Second, hard and brittle materials with excellent stability and optoelectronic properties are suitable for FLA, including crystalline (sapphire and silicon) and noncrystalline (glass) materials. The materials used for FLM include glass, crystalline materials, graphene, and hydrogels, which partially overlap with the materials used for FLA. A partial overlap occurs between photopolymerized and ablative materials, such as certain photoresists and other polymeric materials that can be processed by both photopolymerization and laser ablation. However, the ablation process usually results in high surface roughness, which cannot meet the high-precision requirements of optical elements. By contrast, the structures obtained by photopolymerization exhibit smoother surfaces with smaller voxels134,135. Therefore, overlapping materials are primarily processed using photopolymerization. The classification and corresponding auxiliary processing methods for the processed materials are shown in Fig. 4.
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Various photopolymerization materials are suitable for TPP. Several resin materials can be self-configured, the advantage of which is that the ratio of each part of the material can be adjusted according to the device requirements to obtain the best printing results. In addition, mature commercialized photoresists and other resin materials can be purchased. The commonly used photoresists are the SU-8 series125 (manufacturer Microlithography Chemical) and IP series120. SU-8 is a near-UV photoresist based on an epoxy resin (from the rubber industry), which exhibits excellent imaging properties. Owing to its low absorption in the wavelength range of 365–400 nm, each layer of SU-8 can obtain uniform exposure. Therefore, it can process thick-film structures with vertical sidewalls and high aspect ratios. IP series photoresists have a wide variety of IP series photoresists, mainly IP-S and IP-n162. The optical, mechanical, electronic, and biological properties of these photoresists were engineered to make them suitable for optical, optoelectronic, and biomedical applications.
Commercial bovine serum albumin (BSA, aqueous solution) was used to process the optical microdevices. When methylene blue is used as a photosensitizer, the absorbed laser energy is transferred to ground-state oxygen, forming reactive oxygen species such as singlet oxygen. Singlet oxygen can easily catalyze the inter- or intramolecular covalent crosslinking of oxidizable protein residues (such as Tyr, Trp, and His), forming a 3D protein microhydrogel125,126. After processing, proteins at higher concentrations (400–500 mg/mL) can achieve a more robust mechanical structure. The natural structure of a protein undergoes reversible conformational isomerization at different pH values. Microoptical elements processed with proteins have flexibility and extensibility, as well as good biocompatibility and biodegradability.
PDMS exhibits 100% light transmittance and is an ideal hydrophobic silica gel material for processing optical elements124,136. PDMS is nontoxic, odorless, physiologically inert, chemically stable, electrically insulating, weather-resistant, soft, and can be processed into soft lenses. Moreover, PDMS is highly sensitive to organic solvents, and most organic solvents diffuse into PDMS materials, leading to material expansion. Solvent-tunable PDMS microlenses are fabricated by exploiting the solvent responsiveness.
Some of the aforementioned soft materials have poor robustness and are unsuitable for use in extreme environments (such as strong acids, strong bases, and high temperatures). Hard materials such as quartz glass are the materials of choice for several high-performance elements in optics because of their high optical transparency and high thermal, chemical, and mechanical stability137. Fused silica microstructures are particularly attractive for optical and biomedical applications. Fused silica glass can be fabricated by firing some organic-inorganic hybrid materials. Three major types of organic–inorganic hybrid materials, including silica nanocomposites, sol–gel glass, and POSS glass materials, can be processed into glass by TPP and heat treatment. The first type is a mixture prepared by adding silica nanoparticle fillers to photopolymer precursors127,138,139. Photopolymerization precursors generally consist of two main components: small-molecule polymeric monomers and large-molecule crosslinkers. On the one hand, polymeric monomer solution allows the dispersion of large amounts of silica nanoparticles, while crosslinker can provide more chemical bonds for photopolymerization. By contrast, the polymeric monomers and crosslinkers exhibit different refractive indices. By adjusting the ratio in the solution, the overall refractive index can be made very close to that of silica. This reduces the scattering of light spots during processing, thereby improving the processing resolution. After curing by TPP and thermal degreasing and sintering (>1100 ℃), silica nanocomposites can be transformed into fused silica glass, achieving a resolution of several hundred nanometers and a surface roughness of several nanometers140. Glass ceramics are formed from silica nanocomposites at precisely controlled temperatures (~1200 ℃) during sintering, which exhibit superior optical properties compared to ordinary glass materials, such as better robustness and lower scattering141,142.
Unlike silicon nanocomposites, sol–gel glass materials do not require such high heat treatment temperature130,143,144. The sol–gel method uses inorganic salts containing silica as precursors, which are then gradually gelated through hydrolytic condensation. Finally, sol–gel glass materials (also known as liquid glass) were obtained after post-treatment and drying. Because the inorganic silica filler is introduced at the molecular scale by mixing the precursors, it allows the incorporation of silicon groups to control the homogeneity of the material better; thus, the particle cluster problem does not arise130. In addition, these precursors contain a photopolymerizable fraction (acrylate or epoxy resin). The sol–gel material was photocured using TPP to form a 3D structure. Then, it was degreased and sintered at 600 ℃. 3D structures with glass as the main body can be formed after cooling. Owing to the low heat-treatment temperature, sol–gel materials can be used to fabricate structures on the surface of devices that are not resistant to high temperatures. With the continuous development of sol–gel glass materials, further reducing the processing temperature and broadening the application scenarios is possible145.
Another material that can be converted into fused silica at low temperatures (650 °C) is POSS glass145,146. The POSS glass consists of three components: 89 wt% acrylate-functionalized POSS monomer, 9 wt% trifunctional acrylic monomer, and 2 wt% photoinitiator of the α-aminoketone family. POSS is an organic–inorganic hybrid polymer consisting of a caged silicon–oxygen framework147,148. When sintered into a solid, POSS exhibits a better high-temperature resistance and robustness than most pure organic polymers. Long-armed, branched trifunctional acrylic provides important resilience against cracking, which is the key to printing structures with sufficiently dense silica-oxygen nanoclusters at a low temperature (650 °C). Furthermore, the viscosity of the resin can be regulated by varying the concentration of the trifunctional acrylates. The photoinitiator exhibited a high quantum yield for generating free radicals when excited at the 780 nm wavelength of the TPP system, thereby facilitating efficient polymerization of the materials. Overall, POSS glass materials exhibit excellent optical properties, mechanical elasticity, ease of processing, and high spatial resolution (down to 10 nm), which are suitable for the micro- and nano-3D printing of optical elements.
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Hard and brittle materials are also known as difficult-to-process materials, and their complex, brittle, and stable characteristics render fine processing much more complicated than that of ordinary materials75,149. Moreover, hard and brittle materials contain many material systems, and materials composed of different atoms exhibit significant differences in their physical and chemical properties. Other micro- and nanoprocessing techniques are challenging to apply to hard, brittle materials. Owing to the instantaneous high-intensity energy of the femtosecond laser, high-precision machining of hard and brittle materials can be achieved using FLA combined with several auxiliary methods in section of "Processing principle of FLA". Graphene oxide150-152, optical glass137,153,154, and crystals (such as sapphire 71,155 and silicon146,156-158) are the most widely used laser-ablative materials. These materials have excellent physical and chemical stabilities, including high hardness, high-temperature resistance, and corrosion resistance, and are widely used in extreme fields, such as the military and aerospace industries. They also have critical applications in microoptics and microelectronics, owing to their broad spectral transmittance, nonlinear optics, and other optoelectronic properties. Their combination of functionality and stability make them ideal materials for preparing micro- and nanooptical devices. For instance, except for diamonds, sapphire is the hardest natural mineral on Earth. Sapphire glass has excellent thermal properties, infrared transmission properties, and good chemical stability. Therefore, it is often used to fabricate optical elements and infrared-transparent optical windows and is widely used in infrared and far-infrared military equipment. Graphene is a novel material composed of carbon atoms arranged in a tightly packed single layer with a 2D honeycomb lattice structure that exhibits exceptional optical, electrical, and mechanical properties. Graphene is one of the most robust materials and is ductile and bendable. It holds significant potential for applications in micro- and nanoprocessing, materials science, energy, and biomedicine.
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Some laser-ablative materials such as glass, crystals, and graphene oxide82,150 can also be used as laser-modified materials. Glass and crystalline materials exhibit good light transmission, and FLM can change their refractive indices, absorption coefficients, nonlinear optical sensitivities, and crystal structures159. Graphene oxide turns into reduced graphene oxide under laser irradiation, and its optical properties significantly change during this process160. The optical reduction also results in much higher refractive index modulation ($ \text{Δn} $ of ~0.8) than conventional optical materials161. 2D layered materials generally refer to graphene and transition metal dichalcogenides (TMDCs) MX2, where M is a transition metal atom (such as Mo and W), and X is a chalcogen atom (S, Se, or Te)162-165. Nanoparticles can be generated in single-layer TMDC crystals using femtosecond lasers to obtain amplitude or phase modulation by strong scattering of incident light166. 2D layered materials with sufficient phase or amplitude modulation can be processed into ultrathin lenses that can reach the thickness of a single atom167.
In addition to the aforementioned materials, which can be directly fabricated into optical elements after modification, some materials, such as hydrogels, require post-treatment after modification. Femtosecond lasers can alter the polymer network of polyacrylate–polyacrylamide hydrogels in pure water, thereby reducing scaffold density and improving the ability to form hydrogen bonds168. Hydrogels can trap various materials through different interactions, including hydrogen bonds, charge effects, or dense scaffolds169,170. Therefore, various materials, such as metals, 2D materials, molecular crystals, semiconductors, biomaterials, and fluorescent substances, can be assembled onto hydrogels to form 3D structures. Moreover, to achieve processing resolutions below the diffraction limit (20–35 nm), pretuning the properties of the patterned gel before material deposition is necessary.
Imaging/nonimaging microoptical elements and stereoscopic systems based on femtosecond laser direct writing
- Light: Advanced Manufacturing 4, Article number: (2023)
- Received: 08 June 2023
- Revised: 26 October 2023
- Accepted: 30 October 2023 Published online: 31 October 2023
doi: https://doi.org/10.37188/lam.2023.037
Abstract: The development of modern information technology has led to significant demand for microoptical elements with complex surface profiles and nanoscale surface roughness. Therefore, various micro- and nanoprocessing techniques are used to fabricate microoptical elements and systems. Femtosecond laser direct writing (FsLDW) uses ultrafast pulses and the ultraintense instantaneous energy of a femtosecond laser for micro-nano fabrication. FsLDW exhibits various excellent properties, including nonlinear multiphoton absorption, high-precision processing beyond the diffraction limit, and the universality of processable materials, demonstrating its unique advantages and potential applications in three-dimensional (3D) micro-nano manufacturing. FsLDW has demonstrated its value in the fabrication of various microoptical systems. This study details three typical principles of FsLDW, several design considerations to improve processing performance, processable materials, imaging/nonimaging microoptical elements, and their stereoscopic systems. Finally, a summary and perspective on the future research directions for FsLDW-enabled microoptical elements and stereoscopic systems are provided.
Research Summary
Femtosecond lasers: Fabrication of imaging/nonimaging microoptical elements
Femtosecond laser direct writing (FsLDW) exhibits various excellent properties, including high-precision processing beyond the diffraction limit, and the universality of processable materials, demonstrating its unique advantages in 3D micro-nano manufacturing. Liu Hua's team from Northeast Normal University reported the development of FsLDW-enabled microoptical elements and stereoscopic systems, including three typical principles of FsLDW, several design considerations to improve processing performance, processable materials, imaging/nonimaging microoptical elements, and their stereoscopic systems. Finally, the team provided a summary and perspective on the future research directions for FsLDW-enabled microoptical elements and stereoscopic systems.
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