  We study the propagation of electromagnetic (EM) waves in a slab-type
waveguide which walls consist of three-dimensional topological insulator (3D
TI). The results show that a cutoff frequency with topological stability limits
the spectrum that propagates along the waveguide and are in agreement with
experimental observations. Our approach also provides a way to measure the
penetration length of surface metallic states in 3D TI.
