  Optical activity is a fundamental phenomenon originating from the chiral
nature of crystals and molecules. While intrinsic chiroptical responses of
ordinary chiral materials to circularly polarized light are relatively weak,
they can be enhanced by specially tailored nanostructures. Here, nanorod
metamaterials, comprising a dense array of vertically aligned gold nanorods, is
shown to provide significant enhancement of the circular dichroism response of
an embedded material. A nanorod composite, acting as an artificial uniaxial
crystal, is filled with chiral mercury sulfide nanocrystals embedded in a
transparent polymer. The nanorod based metamaterial, being inherently achiral,
enables optical activity enhancement or suppression. Unique properties of
inherently achiral structures to tailor optical activities pave a way for
flexible characterization of optical activity of molecules and
nanocrystal-based compounds.
