  We obtain in TM polarization an analytical expression of the scattering
matrix of one infinitely conducting metallic lamellar grating with
subwavelength slits. The theory is based on the Monomode Modal Method which
consists in considering only one propagative mode in grating slits. Two
expressions are exposed. The first one comes directly from the theory equations
and the second one clearly reveals the Airy-like form of the scattering matrix
terms. The theory is validated on a multi-grating object and the stability of
the numerical results are shown at the same time. This work provides a basic
and very efficient theoretical tool to calculate the diffraction by a stack of
subwavelength metallic gratings.
