  A III-V compound semiconductor nanowire is an attractive material for a novel
hybrid quantum interface that interconnects photons, electrons, and phonons
through a wavelength-tunable quantum structure embedded in its free-standing
structure. In such a nanomechanical element, however, a challenge is how to
detect and manipulate a small number of phonons via its tiny mechanical motion.
A solution would be to couple an optical cavity to a nanowire by introducing
the ``cavity optomechanics'' framework, but the typical size difference between
them becomes a barrier to achieving this. Here, we demonstrate near-field
coupling of a silica microsphere cavity and an epitaxially grown InP/InAs
free-standing nanowire. The evanescent optomechanical coupling enables not only
fine probing of the mechanical motion by balanced homodyne interferometry but
also tuning of the resonance frequency, linewidth, Duffing nonlinearity, and
vibration axis in the nanowire. Combining this cavity optomechanics with
epitaxial nanowire engineering opens the way to novel quantum metrology and
information processing.
