Metamaterials have optical properties that are unprecedented in nature. They have opened new horizons in light manipulation, with the ability to bend, focus, completely reflect, transmit, or absorb an incident wave front. Optically active metamaterials in particular could be used for applications ranging from 3D information storage to photovoltaic cells. Silicon (Si) particles are some of the most promising building blocks for optically active metamaterials, with high scattering efficiency coupled to low light absorption for visible frequencies. However, to date ideal Si building blocks cannot be produced by bulk synthesis techniques. The key is to find a synthetic route to produce Si building blocks between 75-200 nm in diameter of uniform size and shape, that are crystalline, have few impurities, and little to no porosity. This Review provides a theoretical background on Si optical properties for metamaterials, an overview of current synthetic methods and gives direction towards the most promising routes to ideal Si particles for metamaterials.

Due to their high porosity and chemical versatility, metal–organic frameworks (MOFs) exhibit physical properties appealing for photonic‐based applications. While several MOF photonic structures have been reported, examples of applications thereof are mainly limited to chemical sensing. Herein, the range of application of photonic MOFs is extended to local thermal and photothermal sensing by integrating them into a new architecture: MOF photonic balls. Micrometric‐sized photonic balls are made of monodispersed MOFs colloids that are self‐assembled via spray‐drying, a low‐cost, green, and high‐throughput method. The versatility of the process allows tuning the morphology and the composition of photonic balls made of several MOFs and composites with tailored optical properties. X‐ray nanotomography and environmental hyperspectral microscopy enable analysis of single objects and their evolution in controlled atmosphere and temperature. Notably, in presence of vapors, the MOF photonic balls act as local, label‐free temperature probes. Importantly, compared to other thermal probes, the temperature detection range of these materials can be adjusted “on‐demand.” As proof of concept, the photonic balls are used to determine local temperature profiles around a concentrated laser beam. More broadly, this work is expected to stimulate new research on the physical properties of photonic MOFs providing new possibilities for device fabrication.