Collective nature of high-Q resonances in finite-size photonic metastructures

We study high quality-factor (high Q) resonances supported by periodic arrays of Mie resonators from the perspectives of both Bloch wave theory and multiple scattering theory. We reveal that, unlike a common belief, the bound states in the continuum (BICs) derived by the Bloch-wave theory do not directly determine the resonance with the highest Q value in large but finite arrays. Higher Q factors appear to be associated with collective resonances formed by nominally guided modes below the light line associated with strong effect of both electric and magnetic multipoles. Our findings offer valuable insights into accessing the modes with higher Q resonances via bonding modes within finite metastructures. Our results underpin the pivotal significance of magnetic and electric multipoles in the design of resonant metadevices and nonlocal flat-band optics. Moreover, our demonstrations reveal that coupled arrays of high-Q microcavities do not inherently result in a stronger light-matter interaction when compared to coupled low-Q nanoresonators. This result emphasizes the critical importance of the study of multiple light-scattering effects in cavity-based systems.