  The interaction between light and acoustic phonons is strongly modified in
sub-wavelength confinement, and has led to the demonstration and control of
Brillouin scattering in photonic structures such as nano-scale optical
waveguides and cavities. Besides the small optical mode volume, two physical
mechanisms come into play simultaneously: a volume effect caused by the strain
induced refractive index perturbation (known as photo-elasticity), and a
surface effect caused by the shift of the optical boundaries due to mechanical
vibrations. As a result proper material and structure engineering allows one to
control each contribution individually. In this paper, we experimentally
demonstrate the perfect cancellation of Brillouin scattering by engineering a
silica nanowire with exactly opposing photo-elastic and moving-boundary
effects. This demonstration provides clear experimental evidence that the
interplay between the two mechanisms is a promising tool to precisely control
the photon-phonon interaction, enhancing or suppressing it.
