  Fine control of the chiral light-matter interaction at the nanoscale, by
exploiting designed metamaterial architecture, represents a cutting-edge craft
in the field of biosensing, quantum and classic nanophotonics. Recently,
artificially engineered 3D nanohelices have demonstrated programmable wide
chiroptical properties by tuning materials and architecture, but fundamental
diffractive aspects that are to the origin of chiral resonances still remain
elusive. Here, we proposed a novel concept of three-dimensional chiral
MetaCrystal, where the chiroptical properties are finely tuned by in-plane and
out-of-plane diffractive coupling. Different chiral dipolar modes can be
excited along the helix arms, generating far field optical resonances and
radiation pattern with in-plane side lobes and suggesting that a combination of
efficient dipole excitation and diffractive coupling matching controls the
collective oscillations among the neighbor helices in the chiral MetaCrystal.
This concept enables the tailorability of chiral properties in a broad spectral
range for a plethora of forefront applications, since the proposed compact
chiral MetaCrystal can be suitable for integration with quantum emitters and
can open perspectives in novel schemes of enantiomeric detection.
