Polarization-dependent formation of side channels during percussion drilling with ultrafast lasers observed by means of high-speed X-ray imaging

• In situ X-ray imaging of side channel formation in metal laser drilling. • Insights into transient laser–material interaction dynamics. • Polarization dependent side channel formation. • Identification of two hole shaping phenomena. The side channels that occur during percussion drilling in stainless steel with ultrafast lasers with linear or circular polarization were observed using high-speed X-ray imaging, capturing the dynamic process in real-time and thus providing primary insights into their formation dynamics. We identified two distinct phenomena directly linked to the formation of side channels: (1) deflection of the borehole tip and (2) melt-induced obstructions that alter the propagation of the laser radiation. These phenomena can occur independently of each other and strongly depend on the state of polarization. With linear polarization, the side channels consistently form in the plane perpendicular to the direction of polarization, while the side channels occur without any preferred orientation when drilling with circular polarization. Additionally, the first side channels form at greater depths when higher pulse energies are applied. This study improves our understanding of the complex interplay between the state of polarization, melt dynamics, and side channel formation. The findings provide valuable information for optimizing laser drilling processes, particularly for high-aspect-ratio hole drilling with high pulse energies, and have promising applications in precision micromachining and industrial laser processing.