  Mechanical motion of photonic devices driven by optical forces provides a
profound means of coupling between optical fields. The current focus of these
optomechanical effects has been on cavity optomechanics systems in which
co-localized optical and mechanical modes interact strongly to enable
wave-mixing between photons and phonons and backaction cooling of mechanical
modes. Alternatively, extended mechanical modes can also induce strong nonlocal
effects on propagating optical fields or multiple localized optical modes at
distances. Here, we demonstrate a novel multi-cavity optomechanical device: a
"photon see-saw", in which torsional optomechanical motion can shuttle photons
between two photonic crystal nanocavities. The resonance frequencies of the two
cavities, one on each side of the see-saw, are modulated anti-symmetrically by
the device's rotation. Pumping photons into one cavity excites optomechanical
self-oscillation which strongly modulates the inter-cavity coupling and
shuttles photons to the other empty cavity during every oscillation cycle in a
well regulated fashion.
