  Very weak left/right asymmetry in reflection and transmission is offered by a
layer of a topological insulator on top of a layer of an anisotropic dielectric
material, but it can be enhanced very significantly by using a periodic
multilayer of both types of materials. This is an attractive prospect for
realizing one-way terahertz devices, because both types of materials can be
grown using standard physical-vapor-deposition techniques.
