(a) Schematic of a magnetic antenna for unidirectional SPP excitation. (b) Leakage radiation microscopy shows the propagation of SPPs excited by the magnetic antenna. (c) Unidirectional SPP excitation with asymmetric optical slot nanoantennas. Inset shows the scanning electron microscope (SEM) image of the slot nanoantenna. (d) SPP launched from both sides of a symmetric slot nanoantenna for comparison. Inset shows the SEM image. Reprinted and modified from references [59, 60] with permission.