  Brillouin scattering in optical fibres is a fundamental interaction between
light and sound with important implications ranging from optical sensors to
slow and fast light. In usual optical fibres, light both excites and feels
shear and longitudinal bulk elastic waves, giving rise to forward guided
acoustic wave Brillouin scattering and backward stimulated Brillouin
scattering. In a subwavelength-diameter optical fibre, the situation changes
dramatically, as we here report with the first experimental observation of
stimulated Brillouin scattering from surface acoustic waves. These
Rayleigh-type hypersound waves travel the wire surface at a specific velocity
of 3400 m.s$^{\mathrm{-1}}$ and backscatter the light with a Doppler shift of
about 6 GHz. As these acoustic resonances are highly sensitive to surface
defects or features, surface acoustic wave Brillouin scattering opens new
opportunities for various sensing applications, but also in other domains such
as microwave photonics and nonlinear plasmonics.
