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We present here precise measurement of the resonance frequency, lifetime andshape of confined acoustic modes in the tens of GHz regime in GaAs/AlAssuperlattice planar and micropillar cavities at low temperature ($\sim20\,\textrm{K}$). The subharmonic resonant pump-probe technique, where therepetition rate of the pump laser is tuned to a subharmonic of the cavityresonance to maximize the amplitude of the acoustic resonance, in combinationwith a Sagnac interferometer technique for high sensitivity ($\sim 10\,\textrm{fm}$) to the surface displacement, has been used. The cavityfundamental mode at $\sim 20\,\textrm{GHz}$ and the higher order cavityharmonics up to $\sim 180\,\textrm{GHz}$ have been clearly resolved. MechanicalQ-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 cavitymicropillar has been demonstrated. The Q-frequency product obtained is $ \sim 5\times 10^{14}$ demonstrating the suitability of these superlattice cavitiesfor optomechanical applications.

High spectral resolution of GaAs/AlAs phononic cavities by subharmonic resonant pump-probe excitation