  Brillouin scattering has been widely exploited for advanced photonics
functionalities such as microwave photonics, signal processing, sensing,
lasing, and more recently in micro- and nano-photonic waveguides. So far, all
the works have focused on the opto-acoustic interaction driven from the core
region of micro- and nano-waveguides. Here we observe, for the first time, an
efficient Brillouin scattering generated by an evanescent field nearby a
sub-wavelength waveguide embedded in a pressurised gas cell, with a maximum
gain coefficient of $18.90 \pm 0.17$ m$^{-1}$W$^{-1}$. This gain is 11 times
larger than the highest Brillouin gain obtained in a hollow-core fibre and 79
times larger than in a standard single-mode fibre. The realisation of strong
free-space Brillouin scattering from a waveguide benefits from the flexibility
of confined light while providing a direct access to the opto-acoustic
interaction, as required in free-space optoacoustics such as Brillouin
spectroscopy and microscopy. Therefore, our work creates an important bridge
between Brillouin scattering in waveguides, Brillouin spectroscopy and
microscopy, and opens new avenues in light-sound interactions, optomechanics,
sensing, lasing and imaging.
