  We demonstrate Resonant Tunneling Diodes, embedded in double metal cavities,
strongly coupled to the cavity field, while maintaining their electronic
properties. We measure the polariton dispersion and find a relative vacuum Rabi
splitting of 16%, which explicitly qualifies for the strong-coupling regime.
Additionally we show that electronic transport has a significant influence on
the polaritons by modulating the coupling strength. The merge between
electronic transport and polaritonic physics in our devices opens up a new
aspect of cavity quantum electro-dynamics and integrated photonics.
