If the grating period is sub-wavelength, then the normally-transparent structure becomes a mirror under resonance conditions. These conditions include the angle, frequency (wavelength), and polarization of the incident light. At resonance, there is also a much higher intensity in the waveguide region.
When the grating period is smaller than the wavelength, the transparent structure turns into a mirror under resonant circumstances. This occurs when considering the incident light's angle, frequency (wavelength), and polarization. Additionally, at resonance, there is notably higher energy within the waveguide area.