  Geometrical chirality is a universal phenomenon that is encountered on many
different length scales ranging from geometrical shapes of various 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 would efficiently
couple to only a specific (right or left) molecular enantiomer, and not couple
to the opposite one. Here, we demonstrate a single-handedness chiral optical
cavity supporting only an eigenmode of a given handedness without the presence
of modes of other helicity. Resonant excitation of the cavity with light of
appropriate handedness enables formation of a helical standing wave with a
uniform chirality density, while the 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 coupling light-matter strength. The proposed system
expands the set of tools available for investigations of chiral matter and
opens the door to studies of chiral electromagnetic vacuum.
