  GaAs disk resonators (typical disk size 5 \mum * 200 nm in our work) are good
candidates for boosting optomechanical coupling thanks to their ability to
confine both optical and mechanical energy in a sub-micron interaction volume.
We present results of optomechanical characterization of GaAs disks by
near-field optical coupling from a tapered silica nano-waveguide. Whispering
gallery modes with optical Q factor up to a few 10^5 are observed. Critical
coupling, optical resonance doublet splitting and mode identification are
discussed. We eventually show an optomechanical phenomenon of optical force
attraction of the silica taper to the disk. This phenomenon shows that
mechanical and optical degrees of freedom naturally couple at the
micro-nanoscale.
