Formation of Deep-Subwavelength Structures on Organic Materials by Femtosecond Laser Ablation

Femtosecond (fs-) laser induced subwavelength and deep-subwavelength periodic structures have attracted attention due to their sub-diffraction feature size and rich physics of laser-matter interaction. However, the formation mechanisms of fs-laser induced deep-subwavelength structures on organic materials have not been reported nor systematically studied. Herein, based on the degree of laser induced free-carrier excitation, the formation of deep-subwavelength structures by fs-laser ablation was systematically studied on popular materials: negative tone resists SU8 and conductive polymer poly-3, 4-ethylenedioxythiophene: poly-(styrenesulfonate) (PEDOT: PSS). A weak propensity to form deep-subwavelength structures was found on poly-methylmethacrylate and polyvinyl alcohol. The photo-excited electrons forming the surface plasmonic wave contributed to energy localization and absorption, which led to the imprinting of deep-subwavelength structures on polymers. It has been found that materials with π-bonds and benzene rings were most susceptible to deep-subwavelength structure formation. Based on a laser-induced surface plasmonic model, the period of regular patterns was estimated and was in a good accordance with experimental results. The proposed formation mechanism of the deep-subwavelength structures on organic materials extends an application field of fs-laser micro/nanomachining of polymers.