  Strong acousto-optic interaction in silicon-based waveguides generally
requires releasing of the silicon core to avoid mechanical leakage into the
underlying silica substrate. This complicates fabrication, limits
thermalization, reduces the mechanical robustness and hinders large area
optomechanical devices on a single chip. Here, we overcome this limitation by
employing vertical photonic-phononic engineering. Specifically, the insertion
of a thick silicon nitride layer between the silicon guiding core and the
silica substrate contributes to reduce GHz-frequencies phonon leakage enabling
large values of the Brillouin gain in an unreleased platform. We get values of
the Brillouin gain around 300 (Wm)$^{-1}$ for different configurations, which
could be further increased by operation at cryogenic temperatures. These values
should enable to observe Brillouin-related phenomena in cm-scale waveguides or
in more compact ring resonators. This finding could pave the way towards
large-area unreleased cavity and waveguide optomechanics on silicon chips.
