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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. [ABSTRACT FROM AUTHOR]

Machine learning (ML) methods have gained widespread attention in optimizing nanostructures for target transport properties and discovering unexpected physics. Here, an ML method is developed to discover and experimentally confirm binary CeO2‐MgO aperiodic multilayers (AMLs) for thermal barrier coatings with significantly enhanced reflectance. The effect of varying AML design parameters like total thickness, average period, and randomness in layer thicknesses, on the spectral and total reflectance is demonstrated. Introducing aperiodicity in layer thicknesses is shown to lead to a broadband increase in spectral reflectance due to photon localization. Since the number of possible AML structures increases exponentially with total thickness, a Genetic Algorithm optimizer is developed to efficiently discover AMLs with enhanced reflectance, for total thicknesses of 5–50 µm. Surprisingly, all the optimized structures show an odd number of layers with a CeO2 layer at both ends, deviating from the traditional way of designing binary superlattices with paired layers. The optimized AML and a reference periodic superlattice of 5 µm thickness are fabricated by Pulsed Laser Deposition and characterized by optical reflectance measurements. The fabricated AML, despite considerable fabrication uncertainty, still enhances the reflectance to 48% from 40% of the reference superlattice, validating the effectiveness of our ML‐based optimization process. [ABSTRACT FROM AUTHOR]