  Geometrical chirality is a universal property encountered on very different
length scales ranging from geometrical shapes of living organisms to protein
and DNA molecules. Interaction of chiral matter with chiral light - that is,
electromagnetic field possessing a certain handedness - underlies our ability
to discriminate enantiomers of chiral molecules. In this context, it is often
desired to have an optical cavity that efficiently couples to only a specific
(right or left) molecular enantiomer, and does not couple to the opposite one.
Here, we demonstrate a single-handedness chiral optical cavity supporting only
an eigenmode of a given handedness and lacking modes having the opposite one.
Resonant excitation of the cavity with light of appropriate handedness enables
formation of a helical standing wave with a uniform chirality density, while
the light of opposite handedness does not cause any resonant effects.
Furthermore, only chiral emitters of the matching handedness efficiently
interact with such a chiral eigenmode, enabling the handedness-selective
strength of light-matter coupling. The proposed system expands the set of tools
available for investigations of chiral matter and opens the door to studies of
chiral electromagnetic vacuum.
