0.0 1.0 2.0 3.0 k x s p 7.8 8.0 8.2 8.4 40 50 60 70 β p φ s  (degrees) (a) (b) 7.8 8.0 8.2 8.4 β p FIG. 6. (a) Solutions k$_{x}$ of the equation (30), K ′′ s = 0, associated with different propagation constant β of the wavefields, in the layered structure of Fig. 1. (b) Azimuthal angle φ in the plane k$_{x}$K that corresponds to each solution of the aforementioned self-guiding condition. 28