SPP unidirectional excitation. (a) SPPs launched from a Bragg reflection grating. (b) Schematic of a T-shaped single slit for SPP unidirectional excitation. (c) SPP intensities launched from a T-shaped single slit that are finally scattered from a decoupling grating for two different pump intensities. (d) Schematic of a single slit illuminated by circular polarized light to excite unidirectional SPPs. (e) The experimental results of controllable SPP excitation in (d) under the illumination with different polarization states. Reprinted from references [47–49] with permission.