Directional broadband emissivity with fin-shaped microstructures produced via femtosecond laser surface processing

Surfaces with high directional electromagnetic absorption or emission in the infrared (IR) region of the electromagnetic spectrum have numerous potential applications, however many of the relevant surfaces suffer from extremely narrow bandwidth and/or polarization dependence. Here we demonstrate broadband directional emissivity in the mid-infrared range of 7.5 to 14 µm, that is not dependent on polarization. This was achieved with angled micro-scale structures that are overlaid with nano-scale features on stainless steel 304 produced using an emerging fabrication technique known as femtosecond laser surface processing (FLSP). FLSP is an advanced surface functionalization technique that produces hierarchical micro- and nano-scale quasi-periodic surface features in a single laser processing step. Here we report a surface with peak emission for an angle of 55° using FLSP to create fin-shaped micro- and nano-scale surface features that are tilted at a 55° angle. Cross sectioning of the fin-shaped structures using focused ion beam milling was performed to understand the morphology and subsurface microstructure. Cross-sectional images and energy dispersive X-ray spectroscopy analysis show the structure consists of a thin redeposited oxide layer and the bulk of the fin structure is consistent with the original stainless-steel alloy. The emission results are verified by full-wave electromagnetic simulations which consider all the diffraction-orders performed utilizing the finite element method software, COMSOL Multiphysics, that predicts with reasonable accuracy the resulting directional emissivity of the laser processed surface.