在太赫兹频段，近年来具有低损耗以及宽频特性的全介质梯度超表面引起了人们的广泛关注，其中基于该类超表面的超耦合器可以实现自由空间模式(片外模式)和片上模式之间的转换，在成像、传感、片上集成电路、太赫兹通信等领域都得到了广泛应用。本论文研究基于间隙模式全介质超表面的超耦合器，利用亚波长单元内介质柱耦合导致的间隙模式来调控电磁波的相位和振幅，以实现自由空间模式和片上模式的转换。主要创新内容如下：(1) 针对传统全介质超表面设计难以实现超耦合器(90°偏折)的相位调控问题，首次提出了基于间隙模式的相位梯度超表面设计方法。利用介质柱耦合效应(即间隙模式)实现了2?相位覆盖，为耦合现象造成相位调控不准确的问题提供了解决方案，首次采用全介质超表面实现了线偏振空间波到表面波的直接转换，实现了超耦合器的相位调控，单元平均尺寸只有入射波长的1/60，突破了传统全介质超表面设计在大角度偏折情况下的相位调控限制。(2) 针对传统全介质超表面设计难以实现超耦合器的振幅调控问题，首次提出了基于间隙模式的振幅梯度超表面设计方法。在导波传播方向上设计振幅梯度以获得均匀散射效率，成功将导波转为电场振幅均匀的空间波，实现了超耦合器的振幅调控，有效波束宽度可达工作波长的275倍和波导宽度的921倍，和国内外同类超耦合器相比，提高了一个数量级，实现了波束展宽功能。所提出的超耦合器还实现了波束偏折、波束聚焦和涡旋光生成器等功能，充分证明了其波束整形功能，为超耦合器的各类应用打下了理论基础。(3) 针对超耦合器(90°偏折)的低效激发问题，在基于单层结构的间隙模式相位梯度超表面基础上，提出了基于双层增透结构的间隙模式超表面以抑制反射损耗，结合太赫兹表面波传输结构的设计实现了边激发边传输功能以抑制散射损耗。入射区域达到毫米级尺寸时超耦合器效率可达73%，显著优于太赫兹频段其它文献中同类型超耦合器，突破了传统全介质超表面设计在大角度偏折情况下的效率极限。基于上述工作，本文实现了超耦合器的相位、振幅调控及高效激发等目标，指导开展了太赫兹频段新型超耦合器的设计，为这类功能器件的应用提供了坚实的理论基础。

In recent years, all-dielectric gradient meta-surfaces with low loss and wide bandwidth have attracted much attention in the terahertz band. Based on these meta-surfaces, meta-couplers can realize the conversion between free-space modes (off-chip modes) and on-chip modes, which have been widely applied in fields such as imaging, sensing, on-chip integrated circuits, and terahertz communication. This thesis focuses on the meta-coupler based on the all-dielectric gap-mode meta-surfaces and utilizes the gap-mode induced by the coupling between adjacent dielectric pillars in sub-wavelength unit to control the phase and amplitude of electromagnetic waves, for the conversion between free-space modes and on-chip modes. The main research contents and innovative achievements are as follows: (1) Aiming at the problem of the phase modulation required for meta-couplers (90° deflection), which is challenging for the traditional designs of all-dielectric meta-surfaces, for the first time, a design of phase gradient meta-surface based on gap-mode is proposed. The 2? phase coverage is achieved by utilizing the coupling effect of dielectric pillars (i.e. gap-mode) within the unit cell, which provides a solution for the phase control inaccuracy caused by coupling phenomena. This is the first implementation of direct conversion from linearly polarized propagating wave to surface wave using all-dielectric meta-surfaces with an average unit size only 1/60th of the incident wavelength, which demonstrates the gap-mode meta-surface achieves phase control for the meta-coupler, and breaks the phase control limitation of traditional all-dielectric meta-surface designs for large-angle deflection.(2) Aiming at the problem of the amplitude modulation required for meta-couplers, for the first time, a design of amplitude gradient meta-surface based on gap-mode is proposed. By designing an amplitude gradient in the propagation direction of the guided wave to obtain a uniform scattering efficiency, the guided wave is successfully converted to a spatial wave with uniform amplitude, which shows the gap-mode meta-surface realizes amplitude control for the meta-coupler. The effective beamwidth can reach up to 275 times the working wavelength and 921 times the waveguide width, which is an order of magnitude improvement compared to other meta-couplers in the state-of-the-art literature, achieving beam expansion function. The proposed meta-coupler can also realize beam deflection, beam focusing and vortex beam generation function, fully demonstrating its beam shaping capabilities and building a theoretical foundation for various practical applications.(3) Aiming at the problem of the low efficiency for meta-couplers (90° deflection), a gap-mode meta-surface based on a double-layer structure is proposed for transmission enhanced effect and suppressing reflection loss. Combined with a terahertz surface wave waveguide which can support the eigen EM surface modes, the surface waves can be excited and transmitted as the eigen modes simultaneously for suppressing scattering loss. With the incident plane wave having a limited millimeter-scale width, the efficiency of the meta-coupler can reach up to 73%, which is significantly better than the meta-couplers reported in the state-of-the-art literature at the terahertz frequency band, breaking the efficiency limitation of traditional all-dielectric meta-surface designs for large-angle deflection. Based on the above work, this study has successfully achieved the goals of phase and amplitude modulation as well as high efficiency for the meta-couplers, and proposed a new design strategy that is dedicated to the meta-couplers in the terahertz regime, which provides a solid theoretical foundation for the practical applications of such devices.