Tunable wire metamaterials for an axion haloscope

Metamaterials based on regular two-dimensional arrays of thin wires have attracted renewed attention in light of a recently proposed strategy to search for dark-matter axions. The metamaterial-based resonator concept was specifically motivated by recent stringent predictions for the axion mass in one cosmological scenario, which are expected to improve further in the near future. When placed in the external magnetic field, such metamaterials facilitate resonant conversion of axions into plasmons near their plasma frequency. Since the axion mass is not known a priori, a practical way to tune the plasma frequency of metamaterial is required. In this work, we have studied a system of two interpenetrating rectangular wire lattices where their relative position is varied. The plasma frequency as a function of their relative position in two dimensions has been mapped out experimentally and compared with both a semianalytical theory of wire-array metamaterials and numerical simulations. Theory and simulation yield essentially identical results, which in turn are in excellent agreement with experimental data. Over the range of translations studied, the plasma frequency can be tuned over a range of 16%. This dynamic range is well suited for a search amply encompassing the predicted mass range of postinflation axion, enabling discovery or exclusion within a few years. © 2023 American Physical Society.