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.

We present an experimental investigation of vibrational strong coupling of C=O bonds in poly(methyl methacrylate) to surface lattice resonances (SLRs) on arrays of gold particles in infrared and Raman spectra. SLRs are generated from the enhanced radiative coupling of localized resonances in single particles by diffraction in the array. Compared to previous studies in Fabry–Perot cavities, particle arrays provide a fully open system that easily couples with external radiation while having large field confinement close to the array. We control the coupling by tuning the period of the array, as evidenced by the splitting of the C=O vibration resonance in the lower and upper vibro-polaritons of the IR extinction spectra. Despite clear evidence of vibrational strong coupling in IR transmission spectra, both Raman spectroscopy and micro-Raman mapping do not show any polariton signatures. Our results suggest that the search for vibrational strong coupling in Raman spectra may need alternative cavity designs or a different experimental approach.

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 their direct observation. 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 of BICs over extended areas, unveiling the field-symmetry protection that suppresses far-field radiation. This investigation, done for metasurfaces of dimer rod resonators, reveals the in-plane extension and formation of BICs with anti-symmetric phases, in agreement with coupled-dipole calculations. By displacing the rods, we demonstrate that mirror symmetry is not a necessary condition for 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.