Catching Axions with Density Waves

Axions saturate the local dark matter halo of the galaxy and have rest energy in the sub-millimeter range. In this frequency range, topological density wave materials, such as, topological insulators in the spin density wave phase (SDW) and Weyl semi-metals in the charge density wave phase (CDW), are usually transparent. Turning on a magnetic field, however, causes light polarized parallel to the field to couple linearly to modulations of the SDW (CDW) amplitude (phase). The resulting axionic polariton transforms under discrete space-time symmetries in the same way as the axion. Rotating the light polarization perpendicular to the magnetic field restores transparency in the absence of axions but gives rise to a measurable excitation amplitude at the observed axion density.