  Designing broadband enhanced chirality is of strong interest to the emerging
fields of chiral chemistry and sensing, or to control the spin orbital momentum
of photons in recently introduced nanophotonic chiral quantum and classical
optical applications. However, chiral light-matter interactions have an
extremely weak nature, are difficult to be controlled and enhanced, and cannot
be made tunable or broadband. In addition, planar ultrathin nanophotonic
structures to achieve strong, broadband, and tunable chirality at the
technologically important visible to ultraviolet spectrum still remain elusive.
Here, we tackle these important problems by experimentally demonstrating and
theoretically verifying spectrally tunable, extremely large, and broadband
chiroptical response by nanohelical metamaterials. The reported new designs of
all-dielectric and dielectric-metallic (hybrid) plasmonic metamaterials permit
the largest and broadest ever measured chiral Kuhn dissymmetry factor achieved
by a large-scale nanophotonic structure. In addition, the strong circular
dichroism of the presented bottom-up fabricated optical metamaterials can be
tuned by varying their dimensions and proportions between their dielectric and
plasmonic helical subsections. The currently demonstrated ultrathin optical
metamaterials are expected to provide a substantial boost to the developing
field of chiroptics leading to significantly enhanced and broadband chiral
light-matter interactions at the nanoscale.
