Optical bound states in the continuum in metallic gratings with compound lattices

Bound states in the continuum (BICs) are completely localized modes with no radiation but existing within the continuous spectrum of radiating waves. In photonic crystals, accidental BICs arise from total complete interference and cancellation of two or more resonances in radiation channels. However, for a perfect conducting grating with subwavelength grooves, achieving an accidental BIC is challenging due to lacking of modes. Here, we realize and manipulate accidental BICs in perfect conducting gratings with subwavelength grooves. The gratings feature compound lattice structures, and BICs emerge as a result of the coupling of fundamental modes supported by different grooves in the superlattice. The evolution of BICs is revealed by investigating the surface plasmon polaritons (SPPs) of uniform gratings. Furthermore, we demonstrate that BICs inherit the properties of SPPs in uniform gratings and it is confirmed by comparing the filed distributions of BIC and SPPs. Our work presents a promising method for realizing and tuning of accidental BICs in metallic gratings at low frequencies, which would contribute to the application of plasmonic BICs.