Polarization-controlled high-efficiency meta-couplers for coupling free-space light into subwavelength on-chip terahertz plasmonic devices

Efficiently converting incident free-space light into surface plasmon polaritons (SPPs) and improving the integration of on-chip plasmonic devices in the terahertz regime remain great challenges. Here, a single ultra-compact device is proposed to efficiently couple propagating waves into on-chip terahertz plasmonic waveguides. We first design a high-efficiency meta-coupler composed of isotropic meta-atoms with an absolute excitation efficiency of 82%. This scheme is further extended using anisotropic meta-atoms to realize another meta-coupler that can efficiently couple orthogonal linearly polarized light into SPPs propagating in orthogonal directions. Next, we use the same anisotropic meta-atoms to efficiently convert free-space light into an SPP beam and then focus the beam to a spot (with a focusing efficiency of approximately 59%), which is then fed into a subwavelength on-chip plasmonic waveguide with an absolute working efficiency of 53%. Finally, a meta-coupler is constructed that can achieve polarization-controlled unidirectional SPP excitation with a focused wavefront in different directions and squeeze the focused SPPs into a dual-waveguide system. Our design scheme that can efficiently couple propagating light into SPPs has potential applications in on-chip devices in the terahertz regime.