High-Repetition-Rate Projection Multiphoton Lithography for Large-Area Sub-Micron 3D Printing

Technical Summary: High-Repetition-Rate Projection Multiphoton Lithography for Large-Area Sub-Micron 3D Printing

Problem Statement
High-resolution additive manufacturing, specifically Multiphoton Polymerization (MPL) via Direct Laser Writing (DLW), is the premier technique for fabricating sub-micron 3D structures, including micro-optics and metamaterials. However, standard point-by-point MPL is fundamentally limited by low volumetric throughput, making it unsuitable for large-area or industrial-scale manufacturing. While projection lithography using Digital Micromirror Devices (DMDs) offers a pathway to parallelize fabrication, scaling these systems to the sub-micron regime presents significant optical and thermal challenges. Specifically, maintaining axial confinement without scanning is difficult due to elongated focal volumes in standard holographic beams. Furthermore, increasing laser repetition rates to boost throughput often induces thermal instability, leading to "hot spots," uncontrolled bulk polymerization, and damage to DMD micro-mirrors. Additionally, the Gaussian intensity profiles typical of femtosecond lasers result in non-uniform energy distribution, preventing the fabrication of large arrays with consistent dimensional accuracy.

Methodology
The authors present a high-throughput projection 3D printing platform that integrates spatiotemporal beam shaping with a high-repetition-rate laser source. The system architecture consists of three primary zones:

1. Source and Pulse Compression: The setup utilizes a compact, high-power femtosecond fiber laser (Aeropulse FS10) operating at 1030 nm with a tunable repetition rate up to 20 MHz. For this study, the laser operates at 500 kHz. A custom pulse compressor (MIKS1S) introduces spatial chirp to stretch the pulse in space, which is subsequently compressed in time. Following Second Harmonic Generation (SHG) to 515 nm, the pulse duration is reduced from an initial 180 fs to < 60 fs. This temporal compression is critical for maximizing two-photon excitation efficiency while keeping the average power distributed across millions of pulses, thereby avoiding DMD damage.
2. Beam Shaping and Projection: To ensure uniform intensity across the projection field, a $\pi$-shaper converts the Gaussian beam into a top-hat profile, which is expanded to 15 mm to slightly overfill the DMD active area. The DMD (DLP6500) acts as a programmable diffractive optical element. The system is aligned such that the DMD's micromirror tilt axes are perpendicular to the plane of incidence, satisfying the blazed grating condition to maximize diffraction efficiency into the fundamental order.
3. Simultaneous Spatiotemporal Focusing (SSTF): The system exploits the angular dispersion introduced by the DMD. A 4f optical relay system recombines the spectrally dispersed components at the focal plane using a high-numerical-aperture objective (NA 1.4). This results in isochronous superposition of spectral components only at the focus, where the pulse reaches its minimum duration (< 60 fs) and peak intensity. Outside the focal volume, the pulse remains temporally stretched, suppressing out-of-focus polymerization and ensuring axial confinement.
4. Material Optimization: The authors investigated the impact of photoinitiators on thermal stability at 500 kHz. They compared 4,4'-bis(diethylamino)benzophenone (BIS) against Irgacure 369 within a SZ2080 hybrid resin. BIS was found to cause thermal blooming and solvent vaporization due to residual linear absorption at 515 nm. In contrast, Irgacure 369, with negligible absorption in the green spectrum, enabled a thermally stable process driven exclusively by non-linear two-photon absorption.

Key Results

• Resolution and Throughput: The system achieved a stable volumetric throughput of 2.3 $\times$ 10$^6$ voxels/s while maintaining a lateral resolution of < 400 nm (minimum feature size of 352 nm).
• Micropillar Arrays: High-aspect-ratio micropillar arrays were fabricated with uniform cross-sections and high verticality, confirming the system's ability to suppress axial elongation typical of standard projection beams.
• Large-Area Fabrication: Using a stitching process with high-precision linear stages, the authors fabricated seamless woodpile lattices and uniform micropillar arrays over macroscopic areas (e.g., a 1.2 cm $\times$ 1.2 cm area patterned in 2 minutes).
• Gray-Scale Lithography: The platform successfully fabricated analog micro-optical elements, specifically hemispherical microlenses (10–33 $\mu$m diameter). By modulating exposure duration, the authors controlled the curvature and sag height, achieving optical-grade smoothness free from the "hatching artifacts" common in point-scanning systems.
• Thermal Stability: The use of Irgacure 369 and the 500 kHz repetition rate successfully eliminated thermal crosstalk and proximity effects, allowing for the fabrication of dense structures without uncontrolled bulk polymerization.

Significance and Claims
The paper claims to bridge the gap between standard point-scanning Two-Photon Polymerization (TPP) and complex, high-end projection systems. By utilizing a compact, cost-efficient fiber laser instead of expensive, bulky regenerative amplifiers, the system offers a scalable pathway for mass production. The integration of spatiotemporal pulse shaping allows for the simultaneous printing of millions of voxels while maintaining sub-micron resolution and thermal stability. The authors assert that this workflow establishes a robust foundation for the scalable manufacturing of functional micro-structures, including micro-optical components and metasurfaces, without the need for high-power amplified laser sources. The work validates that high-speed volumetric manufacturing is achievable by optimizing the spatiotemporal pulse profile and photoinitiator chemistry.