Innovative Microwave Resonators for Very High Mass Axion Dark Matter Searches

The most sensitive searches for axion halo dark matter are based on the resonant conversion of axions to photons within a microwave cavity permeated by a strong magnetic field. Current and future experiments such as HAYSTAC and ALPHA seeking to reach recent predictions of the post-inflation axion of masses of 40-180 μeV (~10-45 GHz) are challenged both by the rapidly diminishing volume of conventional microwave cavities with frequency (thus loss of signal power) and by the proliferation of other modes which hybridize with the mode of interest (thus loss of frequency coverage). We will present results on metamaterial-inspired resonators which can simultaneously have the requisite high frequency and large volume, and on photonic band gap structures which can trap the mode of interest (TM₀₁₀) while radiating away most of the interfering modes (TE_lmn). Finally we will present the design and first experimental results from a resonator incorporating both of these features, opening the path to the post-inflation axion.