200 nm K p 2 π -2 π k$_{x}$ p 4 -4 59º FIG. 5. Contour plot of the intensity | h$_{x}$ | 2 in the xy plane corresponding to a localized diffraction- free beam of normalized propagation constant βp = 8 . 503 and transverse frequency γp = 2 . 008 leading to ( k$_{‖}$p ) max = 8 . 737. Intensity distributions along the coordinate axes are shown at the top and left sides. Inset: Isofrequency curve where shaded region corresponds to the excited spatial bandwidth. 27