Exploration of Wire Array Metamaterials for the Plasmonic Axion Haloscope*

A plasmonic haloscope has recently been proposed as a feasible approach to extend the search for dark matter axions above 10 GHz (~ 40 μeV), whereby the microwave cavity in a conventional axion haloscope is supplanted by a plasmonic wire array metamaterial. As the plasma frequency is a property of the bulk, a metamaterial resonator of arbitrarily high frequency may be made arbitrarily large, in contrast to a microwave cavity which incurs a steep penalty in volume with increasing frequency. We have investigated the basic properties of wire array metamaterials through S₂₁ measurements in the 10 GHz range. We fit these S₂₁ measurements to the equation for transmission through a medium with a complex permittivity to derive the basic properties of the metamaterial. Excellent agreement with theoretical models is found, including the onset of a well-defined plasma frequency. We project achievable quality factors to be of order 10⁴ in an actual axion search. Furthermore, schemes for tuning the array over a usable dynamic range (~30% in frequency) appear practical from an engineering perspective.