Bound states in the continuum (BICs) emerge throughout physics as leaky/resonant modes that remain, however, highly localized. They have attracted much attention in optics and photonics, and especially in metasurfaces, that is, planar arrays of sub‐wavelength meta‐atoms. One of their most outstanding features is the arbitrarily large Q‐factors they induce upon approaching the BIC condition, which is exploited here to achieve a narrow transparency band. It is first shown how to shift a canonical BIC in an all‐dielectric metasurface, consisting of high‐refractive disks exhibiting in‐ and out‐of‐plane magnetic dipole (MD) resonances, by tuning the periodicity of the array. By means of the coupled electric/magnetic dipole formulation, it is shown analytically that when the quasi‐BIC overlaps with the broad (in‐plane MD) resonance, a full transparency band emerges with diverging Q‐factor upon approaching the BIC condition in parameter space. Finally, the experimental measurements in the microwave regime with a large array of high‐refractive‐index disks confirm the theoretical predictions. The results reveal a simple mechanism to engineer an ultra‐narrow BIC‐induced transparency band that can be exploited throughout the electromagnetic spectrum with obvious applications in filtering and sensing.

Bound states in the continuum (BICs) represent a new paradigm in photonics due to the full suppression of radiation losses. However, this suppression has also hampered the direct observation of them. By using a double terahertz (THz) near-field technique that allows the local excitation and detection of the THz amplitude, we are able to map for the first time the electromagnetic field amplitude and phase of BICs over extended areas, unveiling the field-symmetry protection that suppresses the far-field radiation. This investigation, done for metasurfaces of dimer scatterers, reveals the in-plane extension and formation of BICs with antisymmetric phases, in agreement with coupled-dipole calculations. By displacing the scatterers, we show experimentally that a mirror symmetry is not a necessary condition for a BIC formation. Only π-rotation symmetry is required, making BICs exceptionally robust to structural changes. This work makes the local field of BICs experimentally accessible, which is crucial for the engineering of cavities with infinite lifetimes.

The directionality and polarization of light show peculiar properties when the scattering by a dielectric sphere can be described exclusively by electric and magnetic dipolar modes. Particularly, when these modes oscillate in phase with equal amplitude, at the so-called first Kerker condition, the zero optical backscattering condition emerges for nondissipating spheres. However, the role of absorption and optical gain in the first Kerker condition remains unexplored. In this work, we demonstrate that either absorption or optical gain precludes the first Kerker condition and, hence, the absence of backscattered radiation light, regardless of the particle’s size, incident wavelength, and incoming polarization. Finally, we derive the necessary prerequisites of the second Kerker condition of the zero forward light scattering, finding that optical gain is a compulsory requirement.

Bound states in the continuum (BICs) have been thoroughly investigated due to their formally divergent Q‐factor, especially those emerging in all‐dielectric, nanostructured metasurfaces from symmetry protection at the Γ point (in‐plane wavevector k_|| = 0). Less attention has been paid to accidental BICs that may appear at any other in the band structure of supported modes, being in turn difficult to predict. Here, a coupled electric/magnetic dipole model is used to determine analytical conditions for the emergence of accidental BICs, valid for any planar array of meta‐atoms that can be described by dipolar resonances, which is the case of many nanostructures in the optical domain. This is explored for all‐dielectric nanospheres through explicit analytical conditions that allow in turn to predict accidental BIC positions in the parameter space (ω, k_||). Finally, such conditions are exploited to determine not only single, but also double (for both linear polarizations) accidental BICs occurring at the same position in the dispersion relation ω − k_|| for realistic semiconductor nanodisk meta‐atoms. This might pave the way to a variety of BIC‐enhanced light–matter interaction phenomena at the nanoscale such as lasing or nonlinear conversion, that benefit from emerging at wavevectors away from the Γ point (off‐normal incidence) overlapping for both linear polarizations.