  We reveal all the linear optical responses, reflection, transmission, and,
diffraction, of typical one-dimensional metallic photonic crystal slabs (MPhCS)
with the periodicity of a half micrometer. Maxwell equations for the structure
of deep grooves are solved numerically with good precision by using the
formalism of scattering matrix, without assuming perfect conductivity. We
verify characteristic optical properties such as nearly perfect transmission
and reflection. Moreover, we present large reflective diffraction and show
that, in the energy range where diffraction channels are open, the photonic
states in the MPhCS originate from surface plasmon polaritons.
