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Breakthrough in High-Speed Laser Writing Achieves Unprecedented Resolution

In a significant advancement, researchers have harnessed high-speed laser writing techniques to produce lines spaced at just 100 nanometers apart on glass substrates. This novel printing methodology holds promise for enabling super-resolution three-dimensional direct laser writing (DLW) of advanced optical components, including microlenses, photonic crystals, and metamaterials.

DLW is an innovative additive manufacturing process that employs a concentrated laser beam to selectively solidify or polymerize materials with exceptional nanoscale precision. Traditionally, this method utilizes multi-photon polymerization to achieve accurate three-dimensional structures.

“Enhancing the resolution—the minimum distance between adjacent features—poses a challenge, as the intense laser can inadvertently expose surrounding areas during the DLW process,” explained Qiulan Liu, a member of the research team from Zhejiang Lab and Zhejiang University in China. “Nonetheless, by implementing a specialized dual-beam optical configuration and an optimized photoresist, we successfully tackled this issue, achieving super-resolution in DLW.”

In a publication by the Optica Publishing Group, the researchers outline their groundbreaking approach, highlighting that they can achieve a remarkable 100-nanometer lateral resolution at a printing speed of 100 micrometers per second. Remarkably, when the writing speed is increased to 1000 micrometers per second, a lateral resolution of 120 nanometers can still be maintained.

“One of the most exciting applications of our DLW technology is in the fabrication of optical waveguide devices for virtual and augmented reality displays, which require precise, high-resolution patterns,” remarked Liu. “Our rapid and high-precision technique facilitates the swift production of complex optical elements, essential for enhancing the performance of next-generation immersive technologies.”

Innovative Techniques to Minimize Crosslinking

The research involved conducting experiments with both multiphoton DLW and a technique called DLW with peripheral photoinhibition. This latter method employs an inhibition beam to suppress polymerization at the periphery of the laser-exposed regions.

The team developed a new photoresist system that integrates a well-established monomer known as PETA with Bis(2,2,6,6-tetramethyl-4-piperidyl-1-oxyl) sebacate (BTPOS), which functions as a radical quencher. This combination significantly minimizes crosslinking effects, a common issue faced when using DLW for high-resolution patterns.

The optical arrangement included a femtosecond laser emitting at 525 nanometers as the excitation source, complemented by a picosecond laser at 532 nanometers for inhibition purposes. The femtosecond laser activates the picosecond laser through a delay unit that accounts for a 2700 picosecond lag due to the differences in the beams’ optical paths. This inhibition beam is crucial in preventing unintended polymerization, enabling the formation of highly accurate patterns.

“To achieve our high-resolution objectives, we employed a spatial light modulator (SLM) to control both the excitation and inhibition light. We applied Zernike polynomials to the SLM to correct for wavefront aberrations,” Liu stated. “Maintaining overall system stability was also critical, considering factors such as laser focus alignment, power fluctuations, optical system drift, and the memory effects caused by the excitation and inhibition beams.”

Fabricating Minuscule Structures

Through a series of experiments, the team validated the speed and precision of their enhanced DLW technique. They successfully created miniature 3D structures known as woodpiles, with lateral rod separations ranging from 300 nanometers to 225 nanometers. The smallest axial spacing between these wood layers measured 318 nanometers, just shy of the diffraction-limited axial resolution of 320 nanometers, a threshold set by the laser wavelength and the optical system’s focusing capabilities.

Looking forward, the researchers aim to enhance their writing speeds significantly, targeting rates between 10 and 100 millimeters per second while preserving high resolution and fabrication quality. Furthermore, improving the photoresist system remains a priority to make the DLW technique more reliable and practical for broader applications.

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