Wire-array metamaterial studies in support of the axion plasma haloscope*

In scenarios where Peccei-Quinn symmetry breaking occurs after inflation, the mass of the axion is expected to be on the high end of the allowed range, with current predictions in excess of 40 μeV (M. Buschmann et al., Nature Communications 13 (2022) 1049.). For a microwave cavity search, such frequencies (>10 GHz) correspond to cavity dimensions whose volume, and thus axion-photon conversion power, are impractically small. A solution has been proposed by which the cavity is supplanted by a wire-array metamaterial whose plasma frequency can be engineered and tuned (M. Lawson et al., 123 (2019) 141802.). As the plasma frequency is a bulk property, a resonator can be made essentially both arbitrarily large and arbitrarily high in frequency. We have carried out a program of transmission measurements of metamaterials comprised of a series of individual planes of wires (50 μm diameter gold-on-tungsten, 5.88 mm pitch) whose spacing and relative position could be easily reconfigured. The basic metamaterial parameters, i.e. plasma frequency, loss term, and overall width of the array were measured and found to be in excellent agreement with theory. Studies of the metamaterial's plasma frequency as a function of its unit cell were likewise in excellent agreement both with theory and simulation, and demonstrate how such a metamaterial-based resonator could be tuned with a dynamic range of ~20%. These results support the feasibility of a wire array metamaterial plasma haloscope, when optimized for each frequency range.