High-efficiency meta-couplers enabled by multilayer gap-mode metasurfaces with enhanced transmission

All-dielectric metasurfaces control light through tailored planar arrangements of high and low refractive index materials. However, achieving efficient large-angle wavefront bending, especially converting freely propagating waves (PWs) into surface waves (SWs), remains challenging. Here, we propose a gap mode metasurface with a double-layer antireflection structure for PW-SW conversion. This design employs compact meta-unit elements (one-eighth of the wavelength) and improves transmittance by approximately 8% compared to single-layer structures. By further integrating a surface wave transmission structure, scattering losses are minimized, satisfying the requirements for millimeter-scale incident beams. The conversion efficiency of the proposed multilayer gap-mode meta-coupler reaches 73%, the highest value reported for all-dielectric terahertz (THz) metasurface converters. This performance surpasses the efficiency limit of conventional all-dielectric metasurface designs for large-angle deflection, which typically remains below 60% in previously reported studies on PW–SW conversion. Our meta-couplers achieve high-efficiency conversion between free-space modes (off-chip) and on-chip modes (e.g., SWs), enabling applications in imaging, biosensing, integrated photonic circuits, and THz communication systems.