  Chirality is ubiquitous from microscopic to macroscopic phenomena in physics
and biology, such as fermionic interactions and DNA duplication. In photonics,
chirality has traditionally represented differentiated optical responses for
right and left circular polarizations. This definition of optical chirality in
the polarization domain includes handedness-dependent phase velocities or
optical absorption inside chiral media, which enable polarimetry for measuring
the material concentration and circular dichroism spectroscopy for sensing
biological or chemical enantiomers. Recently, the emerging field of
non-Hermitian photonics, which explores exotic phenomena in gain or loss media,
has provided a new viewpoint on chirality in photonics that is not restricted
to the traditional polarization domain but is extended to other physical
quantities such as the orbital angular momentum, propagation direction, and
system parameter space. Here, we introduce recent milestones in chiral
light-matter interactions in non-Hermitian photonics and show an enhanced
degree of design freedom in photonic devices for spin and orbital angular
momenta, directionality, and asymmetric modal conversion.
