Electrostatic tuning of mechanical and microwave resonances in 3D superconducting radio frequency cavities

Abstract

We implement a non-contact, external method of simultaneously fine-tuning a mechanical resonator and a superconducting radio frequency (SRF) cavity using a capacitor formed between a silicon nitride membrane and a copper electrode at cryogenic temperatures. The silicon nitride membrane forms a variable boundary condition for the SRF cavity thereby creating the optomechanical cavity. By controlling the DC voltage applied between an external electrode and the silicon nitride membrane we are capable of tuning the resonance frequency internal to the SRF cavity up to 25 kHz for a cavity with loaded quality factor of 2.5 million, corresponding to six cavity linewidths. At the same time we observe the electrostatic frequency shift of the membrane. This approach has the unique benefit of avoiding any dielectric insertion or added gaps due to a moving end-wall thereby limiting the loss of the cavity. Furthermore, this design avoids applied pressure typically used with piezoelectric devices in accelerator cavities. This work seeks to have strong impact in tuning high-Q cavities due to its ability to maintain low losses.