Eccentric 1-D magnetic core–shell photonic crystal balls: ingenious fabrication and distinctive optical properties

This paper demonstrates eccentric one-dimensional (1-D) core-shell photonic crystal balls (PCBs) with self-displaying structural colours and responsiveness to both the direction and strength of the external magnetic field (H), and a controllable and versatile light intensity-guided morphology-controllable polymerization method. In this protocol, the irradiation of unidirectional UV light on photopolymerizable magnetically responsive photonic crystal (MRPC) droplet templates in a vessel with a reflective bottom substrate enables the light intensity within the droplets to decrease from the outside to the inside in a noncentrosymmetric manner, generating PCBs with an MRPC liquid core eccentrically encapsulated in the 1-D PC hydrogel shell under H by controlling the light irradiation time. Besides the magnetic rotation, the resulting PCBs are capable of collectively self-displaying structural colours without input energy due to the inner-eccentric 1-D PC hydrogel shells. Furthermore, they display dynamically tunable bi-photonic band-gaps under H benefiting from the coexistence of the PC hydrogel shell and the MRPC liquid core. When dispersed in viscous media suppressing the magnetic rotation, they may also exhibit orthogonal bi-photonic band-gaps, enabling a wide-angle display characteristic of 1-D photonic crystals with different colour patterns. These unique modulations in optical properties promise important applications of the as-developed PCBs in bionic chromatic materials, energy-saving displays, switchable signage and anti-counterfeiting coatings.