K p k$_{x}$ p 4 -4 200 nm 48 º K p k$_{x}$ p 4 -4 200 nm K p k$_{x}$ p 4 -4 200 nm (a) (b) (c) FIG. 7. Transverse intensity | h$_{x}$ | $^{2}$( x, y ) for diffraction-free beams of βp = 7 . 899 and spatial fre- quency (a) γp = 2 . 552, (b) γp = 3 . 148, and (c) γp = 3 . 734. Insets: Isofrequency curve at βp = 7 . 899. The blue shade indicates the spectral window in k$_{x}$ under excitation. Light blue regions refer to gap-induced frustrated excitation. 29