  We use polarization resolved Raman spectroscopy to demonstrate that for a 3D
giant Rashba system the bulk plasmon collective mode can directly couple to the
Raman response even in the long wavelength $\mathbf q \rightarrow 0$ limit.
Although conventional theory predicts the plasmon spectral weight to be
suppressed as the square of its quasi-momentum and thus negligibly weak in the
Raman spectra, we observe a sharp in-gap plasmon mode in the Raman spectrum of
BiTeI below the Rashba continuum. This coupling, in a polar system with
spin-orbit coupling, occurs without assistance from phonons when the incoming
photon excitation is resonant with Rashba-split intermediate states. We discuss
the distinctive features of BiTeI's giant Rashba system band structure that
enable the direct observation of plasmon in Raman scattering.
