Excitation of Quasi‐Bound States in the Continuum With Focused Radially Polarized Beams

Abstract Photonic bound states in the continuum (BICs) are nonradiative resonances embedded in the radiative spectrum of a system, resulting from the complete cancellation of all radiative channels. In absence of material losses, their infinite lifetime and divergent quality factor make BICs ideal for enhancing light–matter interactions. However, in practical scenarios, geometric imperfections and the finite size of the system open radiative channels, transforming these modes into quasi‐BICs (qBICs) with large, but finite, quality factors. Despite this, their nonradiative nature makes efficient far‐field excitation particularly challenging. This work explores an innovative strategy for exciting qBICs in finite periodic arrays of nanostructures using focused radially polarized light beams. These beams possess a strong longitudinal electric field component, enabling efficient excitation of the symmetry‐protected qBIC that emerges at normal incidence from the out‐of‐plane electric dipole induced in the nanostructures of an array. A comparison with excitation using a tilted plane wave demonstrates that a focused radially polarized beam can achieve both higher quality factors and stronger responses. This work lays a foundation for utilizing structured light beams to optimally excite qBICs in arrays of nanostructures, thus helping to unlock the potential of these modes for applications in nanophotonics.