Dynamic control of anapole states with phase-change alloys.

High-index dielectric nanoparticles supporting a distinct series of Mie resonances have enabled a new class of optical antennas with unprecedented functionalities. The great wealth of multipolar responses and their delicate interplay have not only spurred practical developments but also brought new insight into fundamental physics such as the recent observation of nonradiating anapole states in the optical regime. However, how to make such a colorful resonance palette actively tunable and even switchable among different elemental multipoles is still elusive. Here, for the first time, we demonstrate both theoretically and experimentally that a structured phase-change alloy Ge$_2$Sb$_2$Te$_5$ (GST) can support a diverse set of multipolar Mie resonances with active tunability and switchability. By harnessing the dramatic optical contrast ({Delta}n u003e 2) and the intermediate phases of the GST material, we realize continuous switching between a scattering bright state (electric dipole mode) and a dark state (anapole mode) in a broadband range ({Delta}{lambda} u003e 600 nm). Dynamic control of higher-order anapoles and resulting multimodal switching effects are also systematically investigated, which naturally make the structured GST serve as a multispectral optical switch with high extinction contrasts (u003e 6 dB) and multi-level control capabilities. With all these findings, our study provides an entirely new design principle for realizing active nanophotonic devices.