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The helicity of 3D topological insulator surface states has drawn significantattention in spintronics owing to spin-momentum locking where the carriers'spin is oriented perpendicular to their momentum. This property can provide anefficient method to convert charge currents into spin currents, and vice-versa,through the Rashba-Edelstein effect. However, experimental signatures of thesesurface states to the spin-charge conversion are extremely difficult todisentangle from bulk state contributions. Here, we combine spin- andangle-resolved photo-emission spectroscopy, and time-resolved THz emissionspectroscopy to categorically demonstrate that spin-charge conversion arisesmainly from the surface state in Bi$_{1-x}$Sb$_x$ ultrathin films, down to fewnanometers where confinement effects emerge. We correlate this large conversionefficiency, typically at the level of the bulk spin Hall effect from heavymetals, to the complex Fermi surface obtained from theoretical calculations ofthe inverse Rashba-Edelstein response. %We demonstrate this for film thicknessdown to a few nanometers, Both surface state robustness and sizeable conversionefficiency in epitaxial Bi$_{1-x}$Sb$_x$ thin films bring new perspectives forultra-low power magnetic random-access memories and broadband THz generation.

Spin-momentum locking and ultrafast spin-charge conversion in ultrathin epitaxial Bi$_{1-x}$Sb$_x$ topological insulator