Geometry as a Selector of Optical States in Curved Photonic Architectures

Optical response is commonly regarded as an intrinsic property of a photonic structure. Here we show that, in curved photonic systems, geometry plays a dual role: the photonic architecture defines the optical-state landscape, whereas the illumination–observation geometry determines which optical states become experimentally accessible. Self-assembled, millimetre-scale, free-standing curved silica opals coupled to spherical water droplets on Teflon or planar mirrors reveal simultaneous access to multiple Bragg diffraction bands, together with distance-controlled spectral redistribution, spatially organized halos, and pseudo-collimated beams under different illumination–observation geometries. Natural illumination further reveals optical responses relevant to biological and biomimetic photonic structures. Key aspects of this optical functionality are transferred to a graded curved photonic heterostructure formed by partial polystyrene infiltration of a curved silica opal, demonstrating that the underlying geometrical principle extends beyond liquid-confined systems. Curved photonic architectures therefore provide an experimental platform for geometry-controlled optical-state accessibility.