  Probing the polarization of gravitational waves (GWs) would provide an
evidence of graviton, indicating the quantization of gravity. Motivated by the
next generation of gravitational wave detectors, we make an attempt to study
the possible helicity coupling of structured lights to GWs. With the analogue
between of gravitational fields and the generic electromagnetic media, we
present a 4-vector optical Dirac equation based on the Maxwell theory under the
paraxial approximation. It is found that twisted lights propagating in a
gravitational field can be viewed as a non-Hermitian system with the
$\mathcal{PT}$ symmetry. We further demonstrate that the coupling effect
between angular momentums of the GWs and twisted lights may make photons
undergo both dipole and quadrupole transitions between different
orbital-angular-momentum(OAM) eigenstates and leads to some measurable optical
features, including the central intensity brightening and macroscopic rotation
of the intensity pattern for twisted lights. The former is spin-independent
while the later is spin dependent phenomena, both of which can be viewed
alternatively as the spin-orbital-Hall effect of structured lights in the GWs
and can serve as an indicator of the particle nature of GWs.
