Strong nonlinear coupling between an ultracold atomic ensemble and a nanomechanical oscillator

A new type of collective nonlinear coupling is presented via the indirect interaction between an ultracold atomic ensemble and a nanomechanical oscillator. More intriguingly, its interaction strength is enhanced largely with a factor of the atomic number, and thus, reaches a strong coupling regime within current experimental parameters. For the large atomic number, this obtained nonlinear coupling describes the interaction between the phonon and a pair of quasiparticle. Physically, this pair of quasiparticle is excited from the ultracold atomic ensemble when a phonon is emitted and vice versa. Based on these collective excitations, the nonlinear optical processes with the χ(2) term are simulated successfully.