Dynamically reconfigurable photonic circuits are expected to have a rich variety of applications, enabling high-bandwidth optical interconnects and memories in next generation computer architectures, chip-based quantum networks, and on-chip coherent radar and microwave communication systems. Widely tuneable high quality microcavities are a key component for such circuits. Their passive response allows controllable optical phase shifts, memories and add-drop filters which together provide the reconfigurability of the circuit; their strong optical confinement enhances light-matter interactions and thereby enables components such as lasers, sensors, optical frequency combs, and quantum processors.
However, wide tuneability 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 provided by a double-disk microcavity 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 fiber link. Our results have applications beyond photonic circuits, including widely tuneable integrated lasers, cavity quantum electrodynamics, reconfigurable optical filters for telecommunications and astronomy, and on-chip sensor networks.