  Electronic Raman scattering in the fully symmetric channel couples to the
charge excitations in the system, including the plasmons. However, the plasmon
response has a spectral weight of $\sim q^2$, where $q$, the momentum
transferred by light, is small. In this work, we show that in inversion
symmetry broken systems where Rashba type spin-orbit coupling affects the
states at the Fermi energy (which is a known low energy effect) as well as the
transition elements to other states (a high energy effect), there is an
additional coupling of the plasmons to the Raman vertex, even at zero momentum
transfer, that results in a spectral weight that is proportional to the
spin-orbit coupling. The high energy effect is due to the breaking of SU(2)
spin invariance in the spin-flip transitions to the intermediate state. We
present a theory for this coupling near the resonant regime of Raman scattering
and show that in giant Rashba systems it can dominate over the conventional
$q^2$ weighted coupling. We also provide experimental support along with a
symmetry based justification for this spin-mediated coupling by identifying a
prominent c-axis plasmon peak in the fully symmetric channel of the resonant
Raman spectrum of the giant Rashba material BiTeI. This new coupling could lead
to novel ways of manipulating coherent charge excitations in inversion-broken
systems. This process is also relevant for spectroscopic studies in ultrafast
spectroscopies, certain driven Floquet systems and topologically non-trivial
phases of matter where strong inversion-breaking spin-orbit coupling plays a
role.
