An example of guided-mode resonance is a grating coupler, which is a region on top of or below a waveguide where there is a grating. Off-resonance light incident on the grating behaves almost the same as it would if it was incident in an area where there is no grating. Waveguides are usually made of dielectric and are transparent. For specific combinations of incident angles and light frequency, there is resonance, allowing the grating to couple light into a guided mode of the waveguide.
Typically, the grating coupler has only a few periods, so light can be coupled into the waveguide, but not back out. In such a case, light will be guided in the waveguide until it reaches the waveguide edge, or an additional coupling element, which will couple the light out. The larger the diffraction efficiency of the grating, the larger percent of light that would be coupled in. If the grating is used as a coupling-out element, the larger the diffraction efficiency, the fewer periods would be needed to couple the light out.
Read more about this topic: Guided-mode Resonance