Foams‐Induced Hierarchy for Multiscale Photonics on Flat Substrates: Lasing and Mie‐Bragg Diffraction as Case Studies.

Abstract Densely packed arrays of monodisperse dielectric spheres exhibit strong optical diffraction down to the single‐layer limit, making them highly attractive for the implementation of order‐ and disorder‐based functionalities in photonic devices. Colloidal techniques are among the most convenient methods for creating these densely packed arrays, performing well on flat substrates. However, forming complex geometries with colloidal techniques usually requires patterned substrates and postprocessing routes. Here, a facile colloidal route capable of producing a complex configuration with two levels of functionality is described. Specifically, by foaming and casting colloidal suspensions of monodisperse spherical particles, both ordered and disordered arrays of densely packed spheres on unpatterned flat substrates are achieved. These arrays outline a complex cellular configuration consisting of a ridge‐trough network structure that supports random lasing action at the macroscale, while enabling Bragg diffraction (order) or Mie scattering (disorder) across the visible and near‐infrared spectral ranges at the microscale. Notably, the optical properties of the disordered foams can be effectively modeled using Mie calculations for individual spheres, serving as a valuable proof of concept for developing particulate portable detectors based on Mie resonances and a hierarchical configuration built upon a simple self‐assembly process carried out on flat substrates.