X-ray tomography of polarization effects on deep laser-machined microgrooves

Ultrafast laser machining has been researched extensively over the last few decades to create features such as holes in a variety of materials. The effects of laser parameters including power and polarization on the dynamics of hole formation and resulting hole geometry have been studied. Grooves formation, especially deep ones, on the other hand, has not attracted as much attention, even though grooves are essential to most laser cutting operations. One aspect limiting the study of deep machined features such as grooves is the difficulty in imaging not only the geometry but also the associated collateral damage produced in the material during machining. Here, we employed x-ray tomography for three-dimensional imaging of deep ultrafast laser-machined grooves in various metals. The 3D images of the deep grooves were quantitatively analyzed, revealing the significant effect of laser polarization on groove morphology. Under rotating polarization (also called “scrambled polarization” or “polarization trepanning”), the deep grooves are smooth and uniform, while under linear polarization, extensive branching is observed along the groove, and becomes more pronounced with increasing laser energy and groove entrance length. A mechanistic picture based on laser light reflection off the groove walls is proposed to qualitatively explain the polarization-dependent groove branching observed experimentally. These findings provide new insights into high-precision deep groove laser machining, highlighting the effectiveness of x-ray tomography as a powerful tool for in-depth three-dimensional studies of laser machining processes.