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Stimulated Brillouin scattering from surface acoustic waves in sub-wavelength photonic microwires

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.