Nano-scale photonic ripples, dots, grids, and heterostructures on polymeric substrates produced by UV laser processing

Periodic nano-structures can be produced on poly(ethylene terephthalate) (PET) foils using a $\mathrm{KrF}^{*}$ excimer-laser with a wavelength of 248 nm and a pulse duration of about 20 ns. In this contribution, we show that nano-ripples, or so-called laser-induced periodic surface structures (LIPSS), as well as nano-dots can be fabricated in a reproducible way. Especially the latter ones have not been systematically investigated up to now. The periodicity of the ripples can be varied by the angle of incidence of the laser beam. For pattern formation, various irradiation parameters (pulse number, pulse energy, and polarization) were varied systematically, allowing the fabrication of interesting grids and hybrid patterns of ripples superimposed with dots. All nano-structures are characterized by scanning electron microscopy and relevant morphological parameters, such as periodicity, are assessed. In the case of the epoxy-photoresist $\mathrm{SU}-8, \mathrm{KrF}^{*}$ laser processing can be employed along standard ultraviolet (UV) lithography to create hierarchical structures, that is, a nano-ripple pattern superimposed onto $\mu \mathrm{m}$-scale lines.