Distinguishing trivial and topological quadrupolar insulators by Wannier-Stark ladders

I study theoretically quadrupolar topological insulators under applied static electric field rotated along the crystal axis. I demonstrate, that the energy spectrum of this structure is a Wannier-Stark ladder that is quantized and directly distinguishes between the topological phase, possessing localized corner states, and the trivial phase, lacking the corner states. These results may find applications in the characterization of rapidly emerging higher-order topological phases of light and matter.