  Controlling the polarization state of electromagnetic radiation enables the
investigation of fundamental symmetry properties of matter through chiroptical
processes. Many strategies have been developed to reveal structural or
dynamical information about chiral molecules, from the microwave to the extreme
ultraviolet range. Most schemes employ circularly or elliptically polarized
radiation, and more sophisticated configurations involve, for instance, light
pulses with time-varying polarization states. In all these schemes, the
polarization state of light is always considered as constant over one optical
cycle. In this study, we zoom into the optical cycle in order to resolve and
control a subcyle attosecond chiroptical process. We engineer an electric field
whose instantaneous chirality can be controlled within the optical cycle, by
combining two phase-locked orthogonally polarized fundamental and second
harmonic fields. While the composite field has zero net ellipticity, it shows
an instantaneous optical chirality which can be controlled via the two-color
delay. We theoretically and experimentally investigate the photoionization of
chiral molecules with this controlled chiral field. We find that electrons are
preferentially ejected forward or backward relative to the laser propagation
direction depending on the molecular handedness, similarly to the
well-established photoelectron circular dichroism process. However, since the
instantaneous chirality switches sign from one half cycle to the next,
electrons ionized from two consecutive half cycles of the laser show opposite
forward/backward asymmetries. This chiral signal provides a unique insight into
the influence of instantaneous chirality in the dynamical photoionization
process. Our results demonstrate the important role of sub-cycle polarization
shaping of electric fields, as a new route to study and manipulate chiroptical
processes.
