  The ever growing need for energy-efficient and fast communications is driving
the development of highly integrated photonic circuits where controlling light
at the nanoscale becomes the most critical aspect of information transfer .
Directional couplers, two interacting optical waveguides placed in close
proximity, are important building blocks in these integrated photonics circuits
and have been employed as optical modulators and switches for high speed
communication, data processing and integrated quantum operations. However,
active control over the coupling between closely packed waveguides is highly
desirable and yet remains a critical barrier towards ultra small footprint
devices. A general approach to achieve active control in waveguide systems is
to exploit optical nonlinearities enabled by a strong control pulse. However
these devices suffer from the nonlinear absorption induced by the intense
control pulse as the signal and its control propagate in the same waveguide.
Here we experimentally demonstrate a unique scheme based on adiabatic
elimination (AE) concept that effectively manipulates the coupling between
densely packed waveguides. We demonstrate active coupling control between two
closely packed waveguides by tuning the mode index of an in-between decoupled
waveguide. This is achieved via a dark mode and thus leaves the signal
unaffected by the induced losses. Such a scheme is a promising candidate for
ultra-dense integrated nano-photonics such as on-chip ultrafast modulators and
tunable filters for optical communication and quantum computing.
