The unidirectional waves arise through interference in the 'near field'. This electromagnetic interference is similar to that seen when two or more waves meet on the surface of a pond. The 'near field' refers to the proximity of the waveguide to the nanostructure illuminated with the polarised light.

Unfortunately in conventional explanation electromagnetic waves could propagate without using any medium, but by their mutual creation between electric field and magnetic field. The problem is that both electric field and magnetic field are rise and fall at the same time, so how could they create each other, maybe this physical idea could help to understand it.
http://www.vacuum...21〈=en