Building with Light: Photoactive Colloids as the Next Generation Meta-Atoms.

At the micro- and nanoscale, active colloids generate mechanical forces that promote aggregation and crystallization. Unlike passive colloids, which assemble at or near thermodynamic equilibrium, active colloids operate inherently far from equilibrium, allowing them to transcend several fundamental constraints of equilibrium systems including detailed balance and reciprocity. Notably, due to the untethered nature and high spatiotemporal precision, light can be employed to modulate the interactions of photoactive colloids with remarkable accuracy and versatility through multiple-mode control, making them promising candidates for next-generation artificial meta-atoms for constructing complex architectures and realizing innovative functionalities. As the material properties of colloidal systems are fundamentally governed by the strength of colloidal interactions, lattice arrangement, and symmetry, the photoactive colloids promised novel active materials with dynamic photonic band gaps, scattering spectra, anisotropy, and effective refractive indices, thereby providing a platform for constructing next-generation optical materials.