Exploring the Directional Thermal Emissivity of Angled Microstructures Produced via Femtosecond Laser Surface Processing

In recent years, interest in radiative cooling technologies has spurred a flurry of research in spectral control of thermal emissivity. However, angular control of spectral emissivity remains a less studied topic. Creating surfaces that exhibit both broadband and polarization independent directional emissivity is extremely challenging, typically requiring complex microstructures that are difficult and costly to fabricate. One technology that may be able to meet this challenge is Self-Organized Laser Functionalization (SPLF). SOLF is an advanced manufacturing technique that can create self-organized quasiperiodic micro- and nano-structures on the surface of a material. Recently, broadband directional emissivity was experimentally demonstrated from self-organized angled microstructures produced via oblique angled SOLF applied to stainless steel. While the emissivity from these structures was shown to be broadband and highly directional, little was understood about the underlying physical mechanisms and geometric dependencies producing the polarization independent directional response. In this work, full-wave simulations are carried out using COMSOL Multiphysics® to model the electromagnetic response of the laser produced angled microstructures. Experimental measurements of the directional emissivity are replicated with reasonable accuracy. Simulated electric field distributions indicate that the directional emissivity is due to light trapping in angled cavities beneath the structure. Geometric changes to the model are used to show how the angle of directional emissivity can be tuned by changing the width of the opening to the angled cavities. Experimentally feasible routes for tuning the directional emissivity and the possibility of expanding to other materials are explored using the model. This work enables a deeper understanding of the optical characteristics of angled FLSP structures that will guide their future use in advanced radiative heat transfer applications.