  We consider Wood anomalies in diffraction spectrum from two-dimensional
dielectric periodic grid embedded in a surrounding media. The grid is of
subwavelength thickness, and diffraction of wave having S-polarization is
investigated in the vicinity of the emergence of first diffraction maximum. We
reduce Maxwell equations to coupled-mode theory with three parameters, which
are integral characteristics of material arrangement in the grid. In
particular, we show that such grids are capable to have full reflectance in
parametrically narrow frequency bandwidth. The effect is accompanied by a
parametric evanescent field enhancement in the region near the grid. In
particular, we consider grids with sinusoidal profile and show that this type
of grids possesses unique diffraction properties due to absence of the guided
mode coupling. For such grids, there is a thin transparency window at a
background of near to zero transmission at slightly nonnormal incidence. We
estimate what are enough grid size and the incident beam flatness to resolve
the singularities.
