WASHINGTON — For the first time, researchers have used high-speed laser writing to create lines spaced just 100 nm apart on a glass substrate. The optimized printing approach could enable super-resolution 3D direct laser writing (DLW) of microlenses, photonics crystals, micro-optical devices, metamaterials and more.
DLW is an additive manufacturing technique that uses a focused laser beam to selectively solidify, or polymerize, a material with nanoscale precision. DLW typically uses multi-photon polymerization to polymerize materials in a precise, 3D manner.
"Increasing the resolution — the minimum distance between two adjacent features — is difficult because the intense laser light can cause unwanted exposure in nearby areas during DLW," said Qiulan Liu, a member of the research team from Zhejiang Lab and Zhejiang University in China. "However, by using a unique dual-beam optical setup and a special photoresist, we were able to overcome this challenge and achieve super-resolution DLW."
In the Optica Publishing Group journal Optics Letters, the researchers describe their new approach and show that the record-breaking 100-nm lateral resolution can be achieved at 100 µm/s printing speeds. When an even faster writing speed of 1000 μm/s is used, a 120-nm lateral resolution could still be achieved.
"One exciting application of our DLW technique is printing optical waveguide devices for virtual reality or augmented reality displays with precise, high-resolution structuring," said Liu. "This fast and high-precision approach allows rapid fabrication of complex optical elements, which are crucial for the performance of next-generation immersive technologies."
Reducing crosslinking
In the new work, the researchers performed experiments using both multiphoton DLW and DLW with peripheral photoinhibition, which uses an inhibition beam to suppress polymerization at the edges of the laser-exposed region.
They developed a photoresist system that consisted of a commonly used monomer known as PETA combined with Bis(2,2,6,6-tetramethyl-4-piperidyl-1-oxyl) sebacate (BTPOS). BTPOS acted as a radical quencher that helped reduce the crosslinking that can occur when using DLW to print high-resolution line patterns.
The optical setup included a 525-nm femtosecond laser as the excitation light source and a 532-nm picosecond laser used for inhibition. The femtosecond laser triggers the picosecond laser through a picosecond delay unit, which introduces a 2700 ps delay due to the difference in the optical paths of the two beams. The inhibition beam prevents unwanted polymerization and ensures that the desired pattern is formed with high resolution and precision.
"To achieve high resolution, we also used a spatial light modulator (SLM) to modulate the excitation and inhibition light, and applied Zernike polynomials onto the SLM to correct wavefront aberrations," said Liu. "We also had to make sure the entire system was very stable, taking into account laser focus alignment, laser power fluctuation, the optical system drift, and memory effect, which stems from the excitation and inhibition beams."
Printing tiny structures
The researchers performed several experiments demonstrating the speed and resolution of their optimized DLW approach. They also fabricated tiny 3D woodpiles with lateral rod spacing from 300 nm to 225 nm. The smallest axial period between the wood layers was 318 nm, which reaches the diffraction-limited axial resolution of 320 nm. This diffraction limit is determined by the laser wavelength and the optical system's ability to focus the laser beam.
The researchers are now working to further improve the writing speed, with a goal of reaching speeds of 10 and 100 mm/s while maintaining high writing quality and resolution. They also want to improve the photoresist system to make the DLW technique more stable and practical.
Paper: X. Liu, Q. Liu, M. Luo, L. Xu, C. Kuang, X. Liu, "Super-resolution direct laser writing via multiphoton and peripheral photoinhibition photolithography," Opt. Lett., 50, 1675-1678 (2025).
DOI: https://doi.org/10.1364/OL.552034
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