Fabrication of highly uniform laser-induced periodic structures on polycarbonate via UV femtosecond pulses

Abstract

In this work, we focus on the fabrication of highly uniform laser-induced periodic surface structures (LIPSS) on bulk polycarbonate (PC) using 258 nm femtosecond laser pulses. A systematic approach was pursued to investigate the influence of various laser parameters such as fluence, effective number of pulses, energy dose and polarisation on the features of the generated LIPSS. Experimental results showed that linearly polarized beams produce LIPSS with period comparable to the laser wavelength. Moreover, it was observed that the orientation of LIPSS is either parallel or perpendicular to the laser polarization, depending on the excitation level. These features are similar to the LIPSS formed on dielectrics, despite the considerably higher absorption and larger extinction coefficient (~10-2) of polycarbonate at 258 nm. The orientation and features of the patterns at different excitation levels are explained using Sipe's theory, while the excitation and induced carrier densities were quantified through the application of a theoretical physical model that describes ultrafast dynamics in polymers. Furthermore, observations revealed that low excitation levels with linearly polarized enhance the uniformity of LIPSS, whereas irradiating PC targets with circularly polarized beams leads to the formation of microscale topographies decorated with protruding nanosphered-like structures, with their size varying based on the laser conditions. A detailed analysis of the impact of the induced topographies on their wetting and optical properties demonstrated a steady increase in hydrophilicity over time, in contrast to a modest increase in optical absorbance. The results regarding the morphological features and properties of the induced topographies on PC are anticipated to support future efforts in creating patterned surfaces on polymeric materials for potential applications.