Stray Light in 3D Porous Nanostructures of Single‐Crystalline Copper Film

In the design of optical devices and components, geometric structures and optical properties of materials, such as absorption, refraction, reflection, diffraction, scattering, and trapping, have been utilized. Finding the ideal material with certain optical and geometric characteristics is essential for a customized application. Herein, unoxidizable achromatic copper films (ACFs) are fabricated on Al 2 O 3 substrates utilizing an atomic sputtering epitaxy apparatus. ACFs are made up of two regions vertically: a comparatively flat layer region and a 3D porous nanostructured region on top of the flat region. The measured specular reflectance displays low‐pass filter behavior with a sharp cutoff frequency in the infrared spectrum. Furthermore, the measured diffusive reflectance spectra show light‐trapping behavior in the spectral region above the cutoff frequency, where there are no known absorption mechanisms, such as phonons and interband transitions. A focused ion beam scanning electron microscope is utilized to study the thin film's nanostructured region through 3D tomographic analysis in order to comprehend the phenomena that are observed. This work will shed fresh light on the design and optimization of optical filters and light‐trapping employing porous nanostructured metallic thin films.