Tunable Super-Mode Dielectric Resonators for Axion Haloscopes

Abstract

We present frequency tuning mechanisms for dielectric resonators, which undergo "super-mode" interactions as they tune. The tunable schemes are based on dielectric materials strategically placed inside traditional cylindrical resonant cavities, necessarily operating in Transverse Magnetic modes for use in axion haloscopes. The first technique is based on multiple dielectric disks with radii smaller than that of the cavity. The second scheme relies on hollow dielectric cylinders similar to a Bragg resonator, but of different location and dimension. In particular we engineer a significant increase in form factor for the TM030 mode utilising a variation of a Distributed Bragg Reflector Resonator. Additionally, we demonstrate application of traditional Distributed Bragg Reflectors in TM modes, which may be applied to a haloscope. This is the first demonstration of Bragg resonators applied to TM modes, as well as the first application of super-modes to tune Bragg resonators, or haloscope resonators. Theory and experimental results are presented showing an increase in Q-factor and tunability due to the super-mode effect. The TM030 ring resonator mode offers between 1 and 2-orders-of-magnitude improvement in axion sensitivity over current conventional cavity systems and will be employed in the forthcoming ORGAN experiment.