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.
If the grating period is below the wavelength, the structure that normally allows light to pass through transforms into a mirror under resonant conditions. These conditions encompass the angle, frequency (wavelength), and polarization of the incoming light. At resonance, there is also an increased intensity in the waveguide region.