Unveiling the Symmetry Protection of Bound States in the Continuum with Terahertz Near-Field Imaging.

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.