Cooperative coupling of ultracold atoms and surface plasmons

Cooperative coupling between optical emitters and light fields is one of the outstanding goals in quantum technology. It is both fundamentally interesting for the extraordinary radiation properties of the participating emitters and has many potential applications in photonics. Although this goal has been achieved using high-finesse optical cavities, attention has turned to broadband, easy to build cavity-free approaches. Here we demonstrate cooperative coupling of ultracold atoms with surface plasmons propagating on a plane gold surface. While the atoms are moving towards the surface they are excited by an external laser pulse. The interaction between the excited atom fluorescence and surface plasmons is probed by detecting the photons emitted into the substrate when the plasmon excitations decay. A maximum Purcell factor of eta(P) = 4.9 is reached at an optimum distance of z = 250 nm from the surface. The coupling leads to the observation of a Fano-like resonance in the spectrum.