Bound states in the continuum in all-dielectric metasurfaces

The physics of bound states in the continuum has attracted considerable attention in recent years due to their exceptional ability to enhance light–matter interaction, primarily as a result of their characteristically high Q-factors. This unique property makes bound or quasi-bound states in the continuum highly promising for a wide range of applications, including optical sensing, high-resolution imaging, electromagnetically induced transparency, lasing and strong coupling, nonlinear optical processes, chirality, and the enhancement of spontaneous emission from quantum emitters. Recent advances have expanded the conceptual framework of bound states in the continuum, enabling the exploration of new physical regimes. Notable developments include cloaked quasi-bound states in the continuum and time-domain bound states in the continuum, each offering novel strategies for manipulating light at the nanoscale. In parallel, active control over the Q-factor of quasi-bound states in the continuum is being pursued through various approaches, such as modifying the permittivity contrast between nanoparticles, adjusting the refractive index of the surrounding medium, or tilting meta-atoms. Spectral tuning of bound states in the continuum modes is also being explored using materials with dynamic optical properties, including liquid crystals, phase-change materials, polymers, and magneto-optical media. In this Perspective, we review these emerging mechanisms for the excitation, control, and spectral tuning of quasi-bound states in the continuum in all-dielectric metasurfaces, highlighting current trends and outlining future directions in the field.