  In the past decade, there has been a surge in research at the boundary
between photonics and phononics. Most efforts centered on coupling light to
motion in a high-quality optical cavity, typically geared towards observing the
quantum state of a mechanical oscillator. It was recently predicted that the
strength of the light-sound interaction would increase drastically in nanoscale
silicon photonic wires. Here we demonstrate, for the first time, such a giant
overlap between near-infrared light and gigahertz sound co-localized in a
small-core silicon wire. The wire is supported by a tiny pillar to block the
path for external phonon leakage, trapping $\mathbf{10} \; \textbf{GHz}$
phonons in an area below $\mathbf{0.1 \;
\boldsymbol\mu}\textbf{m}^{\mathbf{2}}$. Since our geometry can be coiled up to
form a ring cavity, it paves the way for complete fusion between the worlds of
cavity optomechanics and Brillouin scattering. The result bodes well for the
realization of low-footprint optically-pumped lasers/sasers and delay lines on
a densely integrated silicon chip.
