Phase-Controllable Spoof Surface Plasmon Coupling from Bull's Eye Aperture to Planar Silicon Waveguide in the Terahertz Band

We demonstrate spoof surface plasmon coupling of a terahertz wave propagating in free space into a planar silicon waveguide through a bull's eye structure with a subwavelength aperture. Spoof surface plasmon polaritons induced by the bull's eye structure on the backside of the substrate propagate to the frontside through the aperture and couple into the waveguide. Electromagnetic field simulations revealed that the spoof surface plasmon polaritons propagating to the frontside show directivity along the incident polarization direction, and that the phase can be controlled by placing a Y-branched silicon waveguide beside the aperture. A prototype device was fabricated by bonding a copper-plated substrate with a bull's eye aperture and a waveguide fabricated by silicon micromachining. A monochromatic wave of 0.42–0.49 THz from a backward terahertz-wave parametric oscillator was injected onto the bull's eye structure, and the intensity of the emitted wave from the end of the waveguide was measured. Directional coupling into the waveguide was confirmed from the intensity change depending on the incident polarization direction when using a straight waveguide. In addition, the phase difference between the two ends of the Y-branched waveguide was confirmed from the intensity change showing constructive or destructive interference depending on the polarization direction. These results indicate that it is possible to couple an incident wave into a planar waveguide perpendicular to it with controlling the phase via spoof surface plasmon coupling, suggesting its applicability to new experimental and practical systems in the terahertz band such as Beyond 5G/6G communications.