Ultrafast 3D nanofabrication via digital holography

Three-dimensional (3D) printing has revolutionized fabrication processes, and two-photon lithography (TPL) is a leading technique for nanoscale 3D printing. However, the inherent limitations of conventional TPL, namely its slow writing speed and high cost due to serial scanning, restrict its large-scale applications. Existing attempts to increase fabrication rate through multi-focus strategies, such as multi-beam interference, micro-lens arrays, and diffractive optical elements (DOEs), are often limited to periodic structures due to a lack of individual focus control. While programmable beam shapers offer some control, they are typically limited by low pattern rates or insufficient laser power. Projection-based TPL approaches using a depth-resolved fs light sheet, although capable of parallel printing, are restricted to layer-by-layer fabrication, hindering the creation of complex overhanging structures. This study addresses these challenges by presenting a novel multi-focus TPL platform leveraging the high peak power of a fs regenerative laser amplifier, overcoming the power limitations of typical fs laser oscillators.

The literature extensively covers the applications of TPL in various fields, including photonics, robotics, machine design, biomimetic materials, and metamaterials. However, the slow speed and high cost associated with conventional serial scanning TPL have been widely acknowledged as major limitations. Several approaches have been proposed to improve the fabrication rate, focusing on multi-focus strategies. Multi-beam interference, micro-lens arrays, and DOEs have been explored, but these methods generally lack the individual control of laser foci necessary for creating complex, non-periodic structures. Prior work using programmable beam shapers has demonstrated limited success, often constrained by low pattern rates or the inability to generate a sufficient number of controllable foci due to laser power restrictions. The use of fs regenerative amplifiers, despite their high peak power, presents unique challenges related to polymerization kinetics at low repetition rates, requiring careful optimization of optical configurations, photoresists, and printing parameters.

This research developed a multi-focus TPL platform based on digital holography, employing a Ti:sapphire fs regenerative laser amplifier (800 nm, 1 kHz, 100 fs pulse width, 4 W average power) to generate up to 2000 individually programmable laser foci. A digital micromirror device (DMD) displays synthesized holograms (using the weighted Gerchberg-Saxton algorithm) to control the amplitude, phase, and location of each focus. A custom-designed photoresist, incorporating a bis-donor photoinitiator with a large two-photon absorption (2PA) cross-section (∼800 GM), is used to optimize polymerization kinetics under the high-peak-power laser. The photoresist's composition includes a monomer mixture of PETA and BPADA, along with an inhibitor (4-hydroxyanisole). The optical setup includes a blazed grating and 4-f systems to compensate for angular dispersion introduced by the DMD. A high NA oil immersion objective is used for dip-in fabrication. The system enables random-access scanning and 3D fabrication, achieving a volumetric printing speed of up to 54.0 mm³/h and a resolution of 90 nm (lateral) and 141 nm (axial). The single-pulse exposure strategy minimizes diffusion effects observed at the low repetition rate of the laser amplifier, optimizing both resolution and speed. The system also includes a hexapod six-axis positioner to extend the build volume. Various characterization techniques, including scanning electron microscopy (SEM), Raman spectroscopy, and mechanical testing (using a rotating rheometer), are employed to evaluate the fabricated structures. The mechanical properties of metamaterials fabricated by the system were characterized through compression, stress-strain, and cyclic loading-unloading tests.

The researchers successfully demonstrated ultrafast 3D nanoprinting using their digital holography-based TPL platform. The system achieved a remarkable printing speed of up to 2,000,000 voxels/sec and a resolution of 90 nm (lateral) and 141 nm (axial). This significantly surpasses the capabilities of conventional TPL systems and other parallel printing methods. The custom-designed photoresist played a crucial role in enabling efficient polymerization under the high-peak-power laser. The single-pulse exposure strategy proved effective in minimizing diffusion effects and achieving high-resolution structures. Fabrication of large-scale metastructures (up to 1.08 × 1.08 × 1 mm³), including octahedral trusses, validated the system's capability for high-throughput manufacturing of complex 3D architectures. These metastructures exhibited good mechanical properties, including resilience and compressibility, as demonstrated through mechanical testing. The ability to create complex grayscale structures (accuracy > 99%), such as arrays of numbers and alphabets, along with large-scale patterns (e.g., a 1 × 4 cm² "CUHK" pattern) further showcased the system's versatility and precision. The fabrication of functional micromachines, such as magnetic micro-gears, using a magnetic photoresist, proved the system's potential for creating functional devices. These micro-gears displayed remote controllability and intricate movements in an aqueous environment, illustrating the system's applications in micro-robotics.

The findings of this study demonstrate a significant advancement in 3D nanofabrication technology. The reported multi-focus TPL platform offers a powerful combination of high resolution, ultrafast printing speed, and flexible grayscale control, overcoming many limitations of current TPL techniques. The combination of digital holography, a high-power fs regenerative amplifier, and a custom-designed photoresist allows for efficient parallel printing of intricate 3D structures. The single-pulse exposure strategy is a particularly important innovation, enabling the system to break the trade-off between resolution and speed often encountered in other approaches. The successful fabrication and characterization of large-scale metamaterials highlight the potential of this technology for manufacturing complex functional components with unprecedented throughput. The creation of functional micromachines like magnetic micro-gears further expands the application scope to micro-robotics and other areas requiring high-precision, complex microstructures.

This research successfully developed a high-throughput, high-resolution, and cost-effective 3D nanofabrication platform based on digital holography and a fs regenerative laser amplifier. The system's superior performance, in terms of speed, resolution, and flexibility, opens exciting avenues for various applications in nanotechnology, photonics, micro-robotics, and beyond. Future research could focus on further increasing the number of controllable foci, exploring novel photoresist materials with enhanced properties, and expanding the system's applications in diverse fields.

While the study demonstrates significant improvements in TPL, some limitations exist. The current system's build volume is limited by the DMD's field of view, although this can be addressed by employing a low-magnification objective lens. The use of a magnetic photoresist, while demonstrating functionality, introduced a higher writing threshold compared to the standard photoresist, although the high-power laser easily compensated for this. Future research could explore methods to further improve the mechanical properties of the printed structures and address the challenges associated with high-density printing. The reported resolution and speed are highly dependent on the optimized photoresist and laser parameters; optimization for different materials may be necessary for broader applications.