  The fundamental optical effects that are at basis of giant reflection band
and anomalous negative transmission in a self-sustained rectangular dielectric
grating slab in P polarization and for incidence angle not very far from the
Brewster's angle of the equivalent slab, are investigated. Notice, that the
self sustained dielectric grating slab is the simplest system that, due to the
Bragg diffraction, can show both the former optical effects. A systematic study
of its optical response is performed by an analytical exact solution of the
Maxwell equations for a general incidence geometry. At variance of the well
known broad reflection bands in high contrast dielectric grating slab in the
sub-wavelength regime, obtained by the destructive interference between the
travelling fundamental wave and the first diffracted wave (a generalization of
the so called second kind Wood's anomalies), the giant reflection band is a
subtle effect due to the interplay, as well as among the travelling fundamental
wave and the first quasi-guided diffracted one, also among the higher in-plane
wave- vector components of the evanescent/divergent waves. To better describe
this effect we will compare the optical response of the self-sustained high
contrast dielectric grating slab with a system composed by an equivalent
homogeneous slab with a thin rectangular high contrast dielectric grating
engraved in one of the two surfaces, usually taken as a prototype for the
second kind Wood's anomalies generation. Finally, the electromagnetic field
confinement in a patterned planar cavity, where the mirrors are two
self-sustained rectangular dielectric grating slabs, is briefly discussed.
