Synthetic Axion Response with Space-Time Crystals

Here, we show that space-time modulations provide an exciting route to realize complex nonreciprocal couplings, and, in particular, the elusive axion response. We develop an analytical formalism to homogenize anisotropic space-time crystals in the long wavelength limit. It is found that space-time crystals with suitable glide-rotation symmetry can have a giant axion-type response, several orders of magnitude larger than in natural materials. The nonreciprocal axion response may have interesting applications in optics, for example, in electromagnetic isolation, and in addition may enable exciting forms of light-wave interactions.