Electromagnetic field confinement by bound states in the continuum

Confining electromagnetic fields as tightly as possible has driven research in optics and photonics. Conventionally, extreme sub-wavelength confinement is possible at the cost of absorption losses in resonant structures. Here, we experimentally show that it is possible to get an extreme confinement of the electromagnetic field in a resonant system without losses. We exploit symmetry protected bound states in the continuum which fully suppress any radiative losses. Using our unique local excitation and detection terahertz microscope we can measure the confinement of the electric field at the surface of our array, revealing an amplitude decay of λ/38 at 0.39 THz.