Femtosecond laser square-hole drilling of metal with a fourfold rotationally symmetrical polarized laser beam

Femtosecond laser drilling is attractive for micromachining because it enables material removal with minimal thermal damage. However, fabrication of non-circular microholes, especially square holes, remains challenging when conventional focusing optics are used. In this study, we present representative experimental results of square-hole drilling in metals using a fourfold rotationally symmetrical polarized laser beam generated by a polarization-converting element. The beam was designed so that highly absorptive P-polarized light was positioned near the corners of the square drilling pattern, whereas less absorptive S-polarized light was distributed along the edges. This polarization arrangement promotes faster ablation at the corners than along the edges, leading to square-hole formation. Square-hole drilling was carried out using a Ti:sapphire femtosecond laser system operated at a wavelength of 800 nm, a pulse duration of 130 fs, and a repetition rate of 1 kHz. The laser beam was focused by a lens with a focal length of 50 mm, and irradiation was performed in air. Copper foils with a thickness of 0.1 mm were used as samples. Representative through-holes with a lateral size of approximately 35 μm were obtained by static exposure. These results confirm the usefulness of polarization-controlled drilling for forming non-circular microholes in metals.