  Reconfigurable photonic circuits have applications ranging from
next-generation computer architectures to quantum networks, coherent radar and
optical metamaterials. However, complete reconfigurability is only currently
practical on millimetre-scale device footprints. Here, we overcome this barrier
by developing an on-chip high quality microcavity with resonances that can be
electrically tuned across a full free spectral range (FSR). FSR tuning allows
resonance with any source or emitter, or between any number of networked
microcavities. We achieve it by integrating nanoelectronic actuation with
strong optomechanical interactions that create a highly strain-dependent
effective refractive index. This allows low voltages and sub-nanowatt power
consumption. We demonstrate a basic reconfigurable photonic network, bringing
the microcavity into resonance with an arbitrary mode of a microtoroidal
optical cavity across a telecommunications fibre link. Our results have
applications beyond photonic circuits, including widely tuneable integrated
lasers, reconfigurable optical filters for telecommunications and astronomy,
and on-chip sensor networks.
