  The concept of Feshbach resonances developed for quantum mechanical
scattering is applied in the analysis of classical light scattering off
photonic crystal slabs. It is shown that this concept can be realized almost
perfectly in these systems. As an application guided-mode resonances in the
grating waveguide structure (GWS) are studied in detail. Using simple resonance
dominance approximation the characteristic properties of isolated Feshbach
resonances in light scattering are exhibited. Formation and interaction of
overlapping resonances are investigated. The relevant parameters of the GWS are
identified which control the shape of the reflectivity of interacting
resonances as well as the enhancement of the electromagnetic field. The
differences in the properties of TE and TM resonances is emphasized for both
isolated and interacting resonances.
