Studies of a Wire Array Metamaterials for a Tunable Plasmonic Haloscope*

The microwave cavity experiment based on the resonant conversion of axions to photons in a magnetic field proposed by Sikivie in 1983 has provided the strongest limits to date on the QCD axion. Current searches are extendable up to the range of 10 GHz (\textasciitilde 40 \textmu eV), but concepts for practical resonators for higher frequencies that possess both large volume and good form factor are lacking. A concept has recently been published however, based on a metamaterial comprised of a 2D array of thin wires, whereby in a magnetic field the axion couples directly to its plasmonic modes [1]. With wire arrays similar to those used in high energy physics detectors (tens of microns diameter, few millimeter spacing), the plasma frequency can be adjusted to be \textgreater 10 GHz and importantly, the volume can be arbitrarily large. We have carried out experimental studies of 2D wire array metamaterials built up of individual wire planes extending earlier measurements [2,3], and finding excellent agreement with theoretical models [2,4]. Furthermore, we have explored new configurations to find a practical mechanism for tuning the array over a wide dynamic range in frequency, with promising results.