  Hybrid opto, electro, and mechanical systems operating at several GHz offer
extraordinary opportunities for the coherent control of opto-electronic
excitations down to the quantum limit. We introduce here a monolithic platform
for GHz semiconductor optomechanics based on electrically excited phonons
guided along the spacer of a planar microcavity (MC) embedding quantum well
(QW) emitters. The MC spacer bound by cleaved lateral facets acts as an
embedded acoustic waveguide (WG) cavity with a high quality factor
($Q\sim10^5$) at frequencies well beyond 6~GHz, along which the acoustic modes
live over tens of $\mu$s. The strong acoustic fields and the enhanced
optomechanical coupling mediated by electronic resonances induce a huge
modulation of the energy (in the meV range) and strength (over 80\%) of the QW
photoluminescence (PL), which, in turn, becomes a sensitive local phonon probe.
Furthermore, we show the coherent coupling of acoustic modes at different
sample depths, thus opening the way for phonon-mediated coherent control and
interconnection of three-dimensional epitaxial nanostructures.
