Efficient micromachining of nickel-titanium by laser ablation in liquids

Ultrashort pulsed laser processing in liquid environments is a highly precise tool for micromachining and, due to minimal heating of the sample, well-suited for heat-sensitive materials. Consequently, this technique is particularly relevant for the fabrication of filigree NiTi stent structures, where heat input directly affects the shape memory properties of the material. Typically, additional post-processing steps such as electropolishing are required, as conventional laser processes do not meet the surface quality requirements for these medical devices. LAL has proven capable of producing clean cuts on 100 μm thick NiTi foils without melt splashes or debris adhesion, demonstrating its potential to reduce post-processing effort for medical devices.
However, laser ablation in liquids is a highly complex physical process, where nonlinear optical effects, plasma generation, and cavitation bubbles prevent upscaling the process by simply increasing pulse energy or repetition rate. Therefore, pulse bursts allow a substantial increase in energy input without exceeding critical intensities for self-focusing or causing interactions between the laser beam and bubbles. Experiments with MHz and GHz burst modes reveal that burst frequency and sub-burst number strongly influence the kerf flank topography. By systematically varying these parameters, the kerf flank topography was controlled to produce either clean cuts or kerf flanks with periodic sawtooth-like microstructure.