  We present here precise measurement of the resonance frequency, lifetime and
shape of confined acoustic modes in the tens of GHz regime in GaAs/AlAs
superlattice planar and micropillar cavities at low temperature ($\sim
20\,\textrm{K}$). The subharmonic resonant pump-probe technique, where the
repetition rate of the pump laser is tuned to a subharmonic of the cavity
resonance to maximize the amplitude of the acoustic resonance, in combination
with a Sagnac interferometer technique for high sensitivity ($\sim 10
\,\textrm{fm}$) to the surface displacement, has been used. The cavity
fundamental mode at $\sim 20\,\textrm{GHz}$ and the higher order cavity
harmonics up to $\sim 180\,\textrm{GHz}$ have been clearly resolved. Mechanical
Q-values up to $2.7 \times 10^4$ have been measured in a planar superlattice,
and direct spatial mapping of confined acoustic modes in a superlattice cavity
micropillar has been demonstrated. The Q-frequency product obtained is $ \sim 5
\times 10^{14}$ demonstrating the suitability of these superlattice cavities
for optomechanical applications.
