Fig. 1. Bilayer meta-atom design and its optical properties under ideal conditions. (a) Schematic of the designed meta-atom structure. (b) Two different configurations of the meta-atom. (c) Simulated cross-polarization amplitudes for configurations 1 and 2 within the frequency range from 0.4 to 1.0 THz. (d) Simulated cross-polarization phase distributions for configurations 1 and 2 within the frequency range from 0.66 to 0.8 THz. (e), (f) Simulated cross-polarization amplitudes for different scales of the dark and bright modes within the frequency range from 0.4 to 1.0 THz, respectively.

Fig. 2. Far-field measurement system and results of the unit cells. (a) Schematic diagram of the THz-TDS system and microscopy images of the fabricated metasurface. (b) Experimental cross-polarization amplitudes for configurations 1 and 2 within the frequency range from 0.2 to 1.2 THz. (c) Experimental cross-polarization phase distributions for configurations 1 and 2 within the frequency range from 0.6 to 0.78 THz.

Fig. 3. Excitation properties of the bilayer metacoupler. (a) Three-dimensional diagram of the overall metasurface design. The incident wave is a transverse electric mode, a TM mode of the orthogonal polarization is obtained through bright–dark mode coupling, and a TM SP mode is then excited. (b) Dispersion relations of the coated metal surface and the free-space wave. (c) Simulated distribution of the real part of the Ez-field distribution at 0.73 THz in the xz-plane. (d) Comparison of simulated SP excitation intensities obtained with positive and negative x-axis probes. (e) Simulated distribution of the real part of the Ez-field at 0.73 THz in the xy-plane.

Fig. 4. Near-field measurement system and results of the bilayer metacoupler and slit coupler. (a) Schematic of the scanning near-field THz microscopy system and microscopy images of the fabricated metacoupler and slit coupler. (b) Experimental distribution of the real part of the Ez-field from the metacoupler at 0.73 THz in the xy-plane. (c) Comparison of SP excitation intensities of the metacoupler measured with positive and negative x-axis probes. (d) Comparison of measured SP excitation intensities of the metacoupler and slit coupler within the frequency range from 0.55 to 0.95 THz.

Fig. 5. Efficiencies of the bilayer metacoupler and another two SP couplers. (a) Schematic of the designed bilayer metacoupler. (b) Simulated SP excitation efficiency of the metacoupler with ideal parameters within the frequency range from 0.55 to 0.95 THz. (c) Schematic of the designed reflectarray coupler. (d) Comparison of simulated SP excitation efficiencies of the bilayer metacoupler and reflectarray coupler (both with ideal parameters) as a function of supercell number. (e) Schematic of the designed slit coupler. (f) Comparison of simulated SP excitation efficiencies of the metacoupler and slit coupler (both with non-ideal parameters) within the frequency range from 0.55 to 0.9 THz.

Fig. 6. Relative permittivities of dielectric materials of PI.

Fig. 7. Cross-polarized electric field distributions of the top and bottom layers at the working frequency of 0.73 THz.

Fig. 8. Transmission spectra of bilayer meta-atoms under non-ideal conditions. (a) Simulated transmission amplitudes for configurations 1 and 2 within the frequency range from 0.2 to 1.2 THz. (b) Simulated transmission phases for configurations 1 and 2 within the frequency range from 0.6 to 0.78 THz.

Fig. 9. Simulated SP excitation intensities obtained with different vertical and lateral coupling. (a) Schematic of the designed supercell structure. (b) Vertical change between two resonators. (c) x-axis change of the CWS. (d) x-axis change of the SRS. (e) y-axis change of the CWS. (f) y-axis change of the SRS.

Fig. 10. Excitation properties of the bilayer metacoupler under non-ideal conditions. (a) Dispersion relation of the coated metal surface and the free-space wave. (b), (d) Simulated distributions of the real part of the Ez-field at 0.73 THz in the xz- and xy-planes, respectively. (c) Comparison of simulated SP excitation intensities obtained with positive and negative x-axis probes.