Effect of polarization on ripple formation in deep femtosecond laser machined cavities

Polarization has been identified as an important parameter in the generation of micro- and nano-structures during ultrafast laser machining. In this study, deep cavities (500 μm x 500 μm x 100 μm) were machined with a femtosecond laser. The influence of polarization on structure formation was investigated on a stainless steel, a commercially pure titanium, an aluminum alloy, and a silicon sample. Smooth surfaces relatively free of laser induced periodic surface structures (LIPSS or ripples) were achieved using a single set of laser parameters for all examined materials. We show that the influence of polarization on ripple formation in deep cavities is limited to metals with low thermal conductivity and a high electron-phonon coupling constant. A continuously rotating polarization was found to yield optically smooth cavities on both bottom and sidewalls. Obtaining smooth surfaces using ultrafast lasers is relevant in many applications where slower processes are currently used such as in sample preparation for electron microscopy, fabrication of microfluidic channels, 3D serial sectioning, micro-electro-mechanical systems, and micro-optical elements.