Development of a USP four-beam interference setup for submicrometer surface structuring

Abstract

Direct Laser Interference Patterning (DLIP) is a high-precision surface structuring technique, based on the interference of multiple laser beams. Conventional DLIP setups typically use two or three beams and achieve structure periods in the micrometer range (≈1–10 μm), often employing diffractive optical elements combined with lenses or prisms. Structures below one micron are difficult to achieve because the minimum feature size is restricted by the laser wavelength and the achievable incidence angle. This work introduces a novel four-beam interference setup that can generate periodic hole-like structures in the sub-micrometer range. The key innovation is a tailored prism unit that enables precise and symmetric beam deflection toward the sample surface and provides the large incidence angle that is required for these small structures. The system utilizes an ultrashort pulse laser (1 ps, 343 nm) with a gaussian beam profile and was successfully demonstrated on polyimide films. An alignment strategy ensures spatial and temporal coherence across all beams, involving polarization control and power distribution balancing for maximizing the interference contrast of the patterns while minimizing higher-order effects such as zero-order beam or the ±2nd and ±3rd order beams. The interference contrast serves as a key efficiency and quality metric for the ablation process and an evaluation method based on the experimentally achieved modulation was applied to estimate the optimal interference contrast. Laser scanning microscopy and scanning electron microscopy confirmed the formation of the hole-like patterns with a spatial period of approximately 650 nm. These sub-micrometer feature sizes enable applications that require fine structures, such as bacterial adhesion control and cell growth interfaces in the field of bioengineering.

Authors 3

1. Ludwig Pongratz Aachen Fraunhofer ILT

   Fraunhofer Institute for Laser Technology

   Affiliation as printed

   Fraunhofer ILT (Germany)

2. Matthias Trenn Aachen Fraunhofer ILT

   Fraunhofer Institute for Laser Technology

   Affiliation as printed

   Fraunhofer ILT (Germany)

3. Dennis Haasler Aachen Fraunhofer ILT

   Fraunhofer Institute for Laser Technology

   Affiliation as printed

   Fraunhofer ILT (Germany)