We propose a terahertz (THz) vortex emitter that utilizes a high-resistance silicon resonator to generate vortex beams with various topological charges. Addressing the challenge of double circular polarization superposition resulting from the high refractive index contrast, we regulate the transverse spin state through a newly designed second-order grating partially etched on the waveguide’s top side. The reflected wave can be received directly by a linearly polarized antenna, simplifying the process. Benefiting from the tuning feature, a joint detection method involving positive and negative topological charges identifies and detects rotational Doppler effects amid robust micro-Doppler interference signals. This emitter can be used for the rotational velocity measurement of an on-axis spinning object, achieving an impressive maximum speed error rate of ∼2 % . This approach holds promise for the future development of THz vortex beam applications in radar target detection and countermeasure systems, given its low cost and potential for mass production.

基于Kekulé晶格，验证了物质拓扑相与晶格原子间耦合作用之间的关系，研究了非厄米效应对拓扑绝缘体的影响。设计了两种格点增益损耗分布方式，分别说明了不同增益损耗对体态能谱、边缘态能谱的影响。随着增益损耗值的增大，体态能谱和边缘态能谱将经历能带间隙减小，能带在临界值处关闭形成狄拉克点，随后狄拉克点劈裂形成一对奇异点的过程。区别于传统对Kekulé晶格的研究，在保持系统胞内耦合作用相同的基础上，将胞间耦合作用分化为水平方向和垂直方向的两个量，分别进行调控，验证了拓扑边缘态能谱中能带间隙的有无不仅与几何边界相关，也受系统胞间耦合相互作用的调控。

为了提高太赫兹超透镜成像的纵向容忍度，设计了一种纯几何相位全介质长焦深超构表面透镜器件。采用纯几何相位自旋解耦的设计方法，并结合时域有限差分（FDTD）方法，对设计的超构表面透镜进行了数值仿真。研究结果表明，所设计的太赫兹超构表面透镜具有偏振可控和长焦深的特性，在沿着传播方向上焦深达到了8 mm，而普通超构表面透镜只有4.5 mm焦深。设计并数值仿真验证了两个太赫兹长焦深聚焦复用的超构表面透镜，并使得横向复用的两个长焦深焦点具有相互正交的偏振态。相关研究有望应用于层析成像和信息加密等方面。

The investigation of converged twisted beams with a helical phase structure has a remarkable impact on both fundamental physics and practical applications. Geometric metasurfaces consisting of individually orientated metal/dielectric meta-atoms provide an ultracompact platform for generating converged vortices. However, it is still challenging to simultaneously focus left-handed and right-handed circularly polarized incident beams with pure geometric phase modulation, which hinders the independent operation on topological charges between these two helical components. Here we propose and experimentally demonstrate an approach to design terahertz geometric metasurfaces that can generate helicity-independent converged vortices with homogeneous polarization states by the superposition of two orthogonal helical vortices with identical topological charges. Furthermore, the multiplexing of polarization-rotatable multiple vortices in multiple dimensions, i.e., in both longitudinal and transverse directions, and a vortex with an extended focal depth is confirmed by embedding polarization modulation into the geometric metasurfaces. The demonstrated approach provides a new way to simultaneously manipulate orthogonal helical components and expand the design dimension, enabling new applications of geometric metasurface devices in polarization optics, twisted-beam related image and edge detection, high capacity optical communication, and quantum information processing, to name a few.

Low-loss dielectric terahertz (THz) chips are efficient platforms for diverse THz applications. One of the key elements in the chip is the coupler. Most of the available THz couplers are in-plane and couple the THz wave from the metal waveguide to the dielectric waveguide. However, out-of-plane couplers are more suitable for wafer-scale testing and tolerant of alignment variation. In this work, we propose an out-of-plane THz coupler for coupling the antenna to the dielectric waveguide. The device is constructed using a grating and a compact spot-size converter. As the conventional optical spot-size converters that apply directly to THz chips are too large, we have designed a compact spot-size converter based on a tapered waveguide with a lens. The total device is 2.9 cm long and can couple a 7 mm diameter THz beam to a 500 µm wide waveguide. The device can scan the THz beam, radiate the input rectangular waveguide mode to free space, and drive the rotation angle of the fan beam through the scanning frequency. We fabricated the device using a single lithography step on a silicon wafer. The out-of-plane coupling efficiency was found to be ∼5 dB at 194 GHz. The fan-beam steering range was found to be around 40° in the frequency range of 170–220 GHz. The proposed out-of-plane coupling technique may provide an effective way for THz wafer-scale testing with a higher degree of freedom for on-chip integration. Also, the proposed technique being non-mechanical, beam steering using it, may therefore find applications in THz radar, communication, and sensing.

The control of spin electromagnetic (EM) waves is of great significance in optical communications. Although geometric metasurfaces have shown unprecedented capability to manipulate the wavefronts of spin EM waves, it is still challenging to independently manipulate each spin state and intensity distribution, which inevitably degrades metasurface-based devices for further applications. Here we propose and experimentally demonstrate an approach to designing spin-decoupled metalenses based on pure geometric phase, i.e., geometric metasurfaces with predesigned phase modulation possessing functionalities of both convex lenses and concave lenses. Under the illumination of left-/right-handed circularly polarized (LCP or RCP) terahertz (THz) waves, these metalenses can generate transversely/longitudinally distributed RCP/LCP multiple focal points. Since the helicity-dependent multiple focal points are locked to the polarization state of incident THz waves, the relative intensity between two orthogonal components can be controlled with different weights of LCP and RCP THz waves, leading to the intensity-tunable functionality. This robust approach for simultaneously manipulating orthogonal spin states and energy distributions of spin EM waves will open a new avenue for designing multifunctional devices and integrated communication systems.

提出了一种基于单层超表面的太赫兹复用器件。该器件基于PB(back propagation)相位理论、广义斯涅耳定律以及马吕斯定律，实现了空间分复用成像。计算结果表明，所设计的器件能将一束线偏振入射光离轴出射，并产生多通道的复用图像。该器件的研究对拓展THz波段的高分辨成像、多通道信息传输等功能具有重要意义。

设计出基于几何相位的介质超表面实现线偏振光聚焦功能。该设计打破了传统设计中几何相位（Pancharatnam–Berry phase）的手性限制，不再是局限于左旋圆偏振光（LCP）和右旋圆偏振光（RCP）入射。通过有限时域差分仿真来证明这种线偏振聚焦相位调控的特性。该类功能器件可能用于设计新颖的THz元器件、高分辨成像、功能探测等方面。

Asymmetric transmission, defined as the difference between the forward and backward transmission, enables a plethora of applications for on-chip integration and telecommunications. However, the traditional method for asymmetric transmission is to control the propagation direction of the waves, hindering further applications. Metasurfaces, a kind of two-dimensional metamaterials, have shown an unprecedented ability to manipulate the propagation direction, phase, and polarization of electromagnetic waves. Here we propose and experimentally demonstrate a metasurface-based directional device consisting of a geometric metasurface with spatially rotated microrods and metallic gratings, which can simultaneously control the phase, polarization, and propagation direction of waves, resulting in asymmetric focusing in the terahertz region. These dual-layered metasurfaces for asymmetric focusing can work in a wide bandwidth ranging from 0.6 to 1.1 THz. The flexible and robust approach for designing broadband asymmetric focusing may open a new avenue for compact devices with potential applications in encryption, information processing, and communication.