  Symmetry principles and theorems are of crucial importance in optics. Indeed,
from one side they allow to get direct insights into phenomena by eliminating
unphysical interpretations; from the other side, they guide the designer of
photonic components by narrowing down the parameter space of design variables.
In this Letter we illustrate a significant departure from the Babinet spectral
complementarity in a very common and technologically relevant situation: that
of a patterned conducting screen placed on a subwavelength dielectric slab. The
symmetry property predicted by Babinet theorem is correctly recovered for pairs
of geometrically complementary - but less realistic in terms of applications -
free-standing patterned screens. Our analysis merges experimental data with
fully vectorial electromagnetic modeling, and provides also an alternative form
of Babinet theorem that highlights a connection with the concept of
electromagnetic duality.
