  Radiation pressure, electrostriction, and photothermal forces have been
investigated to evidence backaction, non-linearities and quantum phenomena in
cavity optomechanics. We show here through a detailed study of the relative
intensity of the cavity mechanical modes observed when exciting with pulsed
lasers close to the GaAs optical gap that optoelectronic forces involving real
carrier excitation and deformation potential interaction are the strongest
mechanism of light-to-sound transduction in semiconductor GaAs/AlAs distributed
Bragg reflector optomechanical resonators. We demonstrate that the ultrafast
spatial redistribution of the photoexcited carriers in microcavities with
massive GaAs spacers leads to an enhanced coupling to the fundamental 20 GHz
vertically polarized mechanical breathing mode. The carrier diffusion along the
growth axis of the device can be enhanced by increasing the laser power, or
limited by embedding GaAs quantum wells in the cavity spacer, a strategy used
here to prove and engineer the optoelectronic forces in phonon generation with
real carriers. The wavelength dependence of the observed phenomena provide
further proof of the role of optoelectronic forces. The optical forces
associated to the different intervening mechanisms and their relevance for
dynamical backaction in optomechanics are evaluated using finite-element
methods. The results presented open the path to the study of hitherto seldom
investigated dynamical backaction in optomechanical solid-state resonators in
the presence of optoelectronic forces.
