  By applying the properties of Fabry-Perot resonance and Rayleigh anomaly, we
have shown that a photonic crystal slab can scatter the light from an incident
plane wave into a diffracted light with a very large reflection or transmission
coefficient. The enhanced field is either a propagating diffraction order (with
a grazing angle of diffraction) or a weakly evanescent order, so it can be
particularly useful for applications requiring an enhanced propagating field
(or an enhanced field with a low attenuation). An efficient effective medium
technique is developed for the design of the resonant photonic crystal slabs.
Numerical simulations have shown that photonic crystal slabs with low index
contrast, such as the ones found in the cell wall of diatoms, can enhance the
intensity of the incident light by four orders of magnitude.
