  Increasing the sensitivity of chiral spectroscopic techniques such as
circular dichroism (CD) spectroscopy is a current aspiration in the research
field of nanophotonics. Enhancing CD spectroscopy depends upon of two
complementary requirements: the enhancement of the electromagnetic fields
perceived by the molecules under study and the conservation of the helicity of
those fields, guaranteed by duality symmetry. In this work, we introduce a
systematic method to design nanostructured dual periodic photonic systems
capable of enhancing molecular CD spectroscopy resonantly. As an illustration,
we engineer a dual 1D silicon nanoparticle array and show that its collective
optical modes can be efficiently employed to resonantly enhance by two orders
of magnitude the local density of optical chirality and, thus, the CD signal
obtained from a given molecular sample on its vicinity.
