Polarization-dependent, Beam Reflection-Regulated Hole Shaping Dynamics Visualized by In-Situ Imaging of Ultrafast Laser Drilling Process

Flexible hole-shape control during ultrafast laser micro-drilling requires in-depth understanding of the processing dynamics, which remains a challenge. We use an in-situ imaging method to visualize the dynamic evolution of hole shape during the ultrafast laser drilling of metals. The direct visualization of the hole shaping dynamics shows that the laser beams are transmitted towards the hole bottom with polarization-dependent reflections on the hole wall. There exists a depth threshold from which the reflected laser beams start to dominate the hole shape under all conditions, and the depth threshold is jointly determined by the pulse energy and polarization of the laser, the trepanning radius, and the material. Due to the polarization-dependent beam reflections, the hole shape evolves uniformly under circularly polarized laser, whereas in the case of linearly polarized laser, two distinct shape evolution patterns are observed on the sections parallel and perpendicular to the polarization direction. These results provide fresh understanding on hole shaping dynamics, and the proposed in-situ imaging method is a powerful platform for the in-depth research of laser drilling process.