Topological cutoff frequency in a slab waveguide: Penetration length in topological insulator walls

Electromagnetic waves propagating in a slab waveguide, whose walls consist of three-dimensional topological insulator, experience a topological cutoff frequency, ωθ. Once ωθ depends upon the penetration length of the metallic surface states into the bulk, l, our findings suggest TI-made waveguides as useful apparatus to estimate the microscopic quantity l by measuring the optical response carried out by the macroscopic signature of ωθ.