Silicon-air nanofilm based on the ⟨110⟩ projection of a diamond lattice to enhance radiative cooling

Photonic crystals built with 3D diamond structures have potentially large bandgaps, which can benefit a wide range of applications, such as daytime radiative cooling, where emissivity needs to be targeted at certain wavelengths in the infrared. However, building simple and low-cost 3D diamond-lattice photonic crystals remains a challenge. In this work, we develop a new processing technique to fabricate a 3D diamond-lattice photonic crystal based on a $⟨$110 $⟩$ surface-terminated structure to ease the creation of a deep lattice structure through reactive ion etching through the surface. This fabrication results in a significant amount of broadband selectivity, characterized by relatively high reflection in the visible wavelength range, as well as low reflection and high emissivity in the mid- to long-wavelength infrared range. Simulations performed in the Stanford Stratified Structure Solver indicate a reasonable agreement with experimental measurements, while suggesting approaches for further improvement.