  Circular dichroism is the differential rate of absorption of right- and
left-handed circularly polarized light by chiral particles. Optical vortices
which convey orbital angular momentum (OAM) possess a chirality associated with
the clockwise or anti-clockwise twisting of their wavefront. Here it is
highlighted that both oriented and randomly oriented chiral particles absorb
photons from twisted beams at different rates depending on whether the vortex
twists to the right or the left through a dipole coupling scheme. This is in
contrast to previous studies that investigated dipole couplings with vortex
modes in the paraxial approximation and showed no such chiral sensitivity to
the vortex handedness: only in oriented media where electric quadrupole
coupling contributes to optical activity effects due to absorption does such a
mechanism exist for paraxial vortices. The distinct difference in the scheme
highlighted in this work is that longitudinal fields are taken account of and
these allow for OAM transfer even during dipole interactions. Due to the vortex
dichroism persisting in randomly oriented collections of chiral particles, the
mechanism has a distinct advantage in its potential applicability in chemical
and biochemical applications where the systems under study are invariably in
the liquid phase. Additionally, the result is put into context in terms of the
quantifiable optical chirality, highlighting that optical OAM can in fact
increase the optical chirality density of an electromagnetic field.
