Direct Observation of Lateral Field Confinement in Symmetry‐Protected THz Bound States in the Continuum

Abstract Electromagnetic field confinement on sub‐wavelength scales is possible at the expense of increasing optical losses. Examples are surface plasmon polaritons and resonant structures, which suffer from absorption losses and radiative outcoupling, respectively. Here, the first experimental evidence of strong electromagnetic field confinement along the lateral (out‐of‐plane) direction in a near lossless metasurface that supports a symmetry‐protected Bound State in the Continuum is shown, which fully suppresses the radiative losses of resonant scatterers. This experimental demonstration, supported by coupled dipole calculations, is measured by a double near‐field probe technique to locally excite and detect the time evolution and confinement of the electric field onto the surface, achieving an amplitude decay length from the metasurface of λ/38 at 0.39 THz.