K p 0 π - π 2 π -2 π k$_{x}$ p 0 4 2 -4 -2 8.28 8.28 7.92 8.73 8.73 7.92 8.28 K p 0 π - π 2 π -2 π k$_{x}$ p 0 4 2 -4 -2 7.92 7.92 7.92 8.28 8.28 (a) (b) FIG. 3. Isofrequency curves at different propagation constants β$_{c}$ < β < β$_{max}$ are shown in (a) for the first sheet and in (b) for the second sheet of the dispersion curve. Contour lines are labelled following the normalization βp 25