Radiation control by defects in dark-state resonant photonic lattices

In this Letter, we present and explain novel radiation properties enabled by defects in resonant photonic lattices (PLs). Incorporating a defect breaks the lattice symmetry and generates radiation through the stimulation of leaky waveguide modes near the non-radiant bound (or dark) state spectral location. Analyzing a simple one-dimensional (1D) subwavelength membrane structure, we show that the defects produce local resonant modes that correspond to asymmetric guided-mode resonances (aGMRs) in spectra and near-field profiles. Without a defect, a symmetric lattice in the dark state is neutral, generating only background scattering. Incorporating a defect into the PL induces high reflection or transmission by robust local resonance radiation depending on the background radiation state at the bound state in the continuum (BIC) wavelengths. With the example of a lattice under normal incidence, we demonstrate defect-induced high reflection as well as high transmission. The methods and results reported here have significant potential to enable new modalities of radiation control in metamaterials and metasurfaces based on defects.