When the optical parameters of a planar waveguide exhibit a transversal periodic modulation, it's possible to observe one or more sharp peaks in the reflection and transmission spectra. This rapid variation in the intensity of the fields propagating outside the device is called Guided Mode Resonance (GMR). GMR occurs when an evanescent diffraction order is parametrically close to a guided wave. The coupling between the evanescent wave and the guided wave leads to a redistribution of the energy between the propagative orders. The efficiency of the process is maximum when the grating presents only the zero diffractive order in transmission and reflection (ZOG: Zero Order Grating). In this case, the coupling occurs by the way of the +1 and -1 evanescent orders. The GMR phenomenon enable, for example, the construction of narrow bandpass filters.
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Scheme of a guided mode resonance filter working in normal incidence. The coupling occurs by the way of the evanescent diffraction orders +1 and -1. When the resonance condition is verified, an important par of the energy is reflected.