We use polarization-resolved Raman spectroscopy to demonstrate that for a three-dimensional giant Rashba system the bulk plasmon collective mode can directly couple to the Raman response even in the long-wavelength $\mathbf{q}\to \mathit{0}$ limit. Although conventional theory predicts the plasmon spectral weight to be suppressed as the square of its quasimomentum 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 plasmons in Raman scattering.

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• Plasmons
• Rashba coupling
• Spin-orbit coupling
• Doped semiconductors
• Inelastic light scattering
• Raman spectroscopy