Experimental observation of a symmetry-protected bound state in the continuum in a chain of dielectric disks

The existence of bound states in the continuum (BIC) manifests a general wave phenomenon first predicted in quantum mechanics by John von Neumann and Eugene Wigner [J. von Neumann and E. Wigner, Phys. Z. 30, 465 (1929)]. Today it is being actively explored in photonics, radiophysics, acoustics, and hydrodynamics. We report an experimental observation of an electromagnetic bound state in the radiation continuum in a one-dimensional array of dielectric particles. By measurement of the transmission spectra of the ceramic disk chain at GHz frequencies, we demonstrate how a resonant state in the vicinity of the center of the Brillouin zone turns into a symmetry-protected BIC with increase in the number of the disks. We estimate a number of the disks when the radiation losses become negligible in comparison to material absorption and, therefore, the chain could be considered practically as infinite. The presented analysis is supplemented by measurements of the near fields of the symmetry-protected BIC. All measurements are in a good agreement with the results of the numerical simulation and analytical model based on a tight-binding approximation. The obtained results provide useful guidelines for practical implementations of structures with bound states in the continuum that opens up horizons for the development of optical and radio-frequency metadevices.