Scalable, Highly Uniform, and Robust Colloidal Mie Resonators for All‐Dielectric Soft Meta‐Optics

All‐dielectric nanoparticles (NPs) with high (>3.0) or moderate (1.7–3.0) refractive indices have become fundamental to meta‐optics, as they enable low‐loss, low‐heating, and quenching‐free magnetodielectric Mie resonances, which are difficult to achieve by use of plasmonic counterparts. However, a scalable and versatile synthetic route for such magnetodielectric NPs retaining high uniformity, roundness, and robustness has remained elusive. Thus, soft self‐assembly still represents an underutilized method in the optical engineer's toolset, which in turn limits the accessible range of meta‐optics. Herein, a gram‐scale and versatile synthesis of dielectric colloidal Mie resonators is presented, in which selenous acid precursors are converted into highly uniform, crystalline colloids by a low demand reaction. These crystalline selenium (c‐Se) colloids enable strong electric and magnetic resonances due to their moderate refractive index (2.8–3.2 at optical frequencies), while simultaneously satisfying the requirements of high uniformity, roundness, and robustness. Even with these exotic properties, c‐Se colloids are successfully self‐assembled into various all‐dielectric meta‐optics systems including (i) metafluids exhibiting directional scattering, (ii) metamolecules with nanogap‐dielectric resonances, and (iii) metacrystals with magnetodielectric bandgaps. The design space of all‐dielectric meta‐optics will be greatly expanded by utilizing soft self‐assembly of c‐Se colloids. Highly uniform, robust, and scalable high‐index selenium colloids are successfully synthesized under mild conditions. With explicit demonstration of selenium colloids self‐assembled into various platforms including metafluids, metamolecules, and metacrystals, selenium colloids are proven to serve as general building block for “soft” meta‐optics.