  We present high-reflectivity mechanical resonators fabricated from AlGaAs
heterostructures for use in free-space optical cavities operating in the
telecom wavelength regime. The mechanical resonators are fabricated in slabs of
GaAs and patterned with a photonic crystal to increase their out-of-plane
reflectivity. Characterization of the mechanical modes reveals residual tensile
stress in the GaAs device layer. This stress results in higher mechanical
frequencies than in unstressed GaAs and can be used for strain engineering of
mechanical dissipation. Simultaneously, we find that the finite waist of the
incident optical beam leads to a dip in the reflectance spectrum. This feature
originates from coupling to a guided resonance of the photonic crystal, an
effect that must be taken into account when designing slabs of finite size. The
single- and sub-$\upmu$m-spaced double-layer slabs demonstrated here can be
directly fabricated on top of a distributed Bragg reflector mirror in the same
material platform. Such a platform opens a route for realizing integrated
multi-element cavity optomechanical devices and optomechanical microcavities on
chip.
