Broadband Directional Thermal Radiation with Asymmetric Femtosecond Laser‐Processed Surfaces

Photonic engineering of thermal radiation has resulted in the development of a plethora of new structures and optical designs that tailor the spectral and angular characteristics of thermal radiation at will. However, directional emissivity insensitive to the polarization of thermal radiation remains an open challenge. Moreover, the lack of cost‐effective scalable fabrication techniques severely limits any realistic applications of directional thermal emission. Here, we tackle these issues by presenting broadband asymmetric unpolarized directional thermal radiation from “shark‐skin” like angled microstructures created on a stainless‐steel surface through oblique angled self‐organized laser functionalization (SOLF). Extensive experimental measurements corroborated by rigorous full‐wave simulations demonstrate that the emissivity from these surfaces exhibits strong directionality and asymmetry operating for both polarizations. The presented robust directional control of incoherent thermal radiation can find applications to asymmetrically enhance radiative cooling efficiency without resorting to complicated approaches requiring external bias to break reciprocity and radiation symmetry.