Microwave frequency comb generation in a niobium-based superconducting electromechanical device

Superconducting electromechanical devices realize optomechanical coupling between microwave photons and mechanical phonons. To date, most optomechanical phenomena like back-action cooling and amplification can be sufficiently explained based on the linear approximation of the coupling. Here, we study the regime where this linear approximation no longer holds due to the presence of intense microwave and mechanical excitations. This nonlinear photon-phonon interaction results in an optomechanical instability which leads to the generation of a frequency comb with a frequency spacing precisely locked to the mechanical resonant frequency (8 MHz). We explore the dynamics of the frequency combs in both the frequency- and time-domains, and investigate the behavior their threshold pump powers for different pump frequencies as well as for different cavity decay rates. Finally, we numerically model our system considering the optomechanical coupling and confirm that the results match well with our experimental data. Since the information of mechanical systems is contained in GHz pulse trains, the use of electromechanical combs could enable fast electrical readout of mechanical frequencies useful to nanomechanical sensing.