  Increasing the interaction between light and mechanical resonators is an
ongoing endeavor in the field of cavity optomechanics. Optical microcavities
allow for boosting the interaction strength through their strong spatial
confinement of the optical field. In this work, we follow this approach by
realizing a sub-wavelength-long, free-space optomechanical microcavity on-chip
fabricated from an (Al,Ga)As heterostructure. A suspended GaAs photonic crystal
mirror is acting as a highly reflective mechanical resonator, which together
with a distributed Bragg reflector forms an optomechanical microcavity. We
demonstrate precise control over the microcavity resonance by change of the
photonic crystal parameters. The interplay between the microcavity mode and a
guided resonance of the photonic crystal modifies the cavity response and
results in a stronger dynamical backaction on the mechanical resonator compared
to conventional optomechanical dynamics.
