An antiresonant hollow core effective single-mode terahertz fiber is proposed and successfully fabricated with a photosensitive resin (SomosEvoLVe 128) by a 3D printer. The single mode characteristics of the fiber are found to be related to the area of the semielliptical tubes in the cladding. By optimizing the cladding structure parameters, a Higher Order Mode Extinction Ratio (HOMER) of above 6 can be achieved. A 30 cm long sample with optimized structural parameters is obtained by cascading two 15 cm long fiber samples fabricated by a 3D printer. The transmission loss and the electric field distribution are measured by a terahertz time domain spectroscopy (TDS) system. The electric field distribution measured at the output end of the fiber sample has the Gaussian profile, which accords with single-mode field distribution. Experimental results show that the average loss is 0.048 cm ^-1 within the frequency range from 0.2 to 1 THz, and the minimum loss is obtained as low as 0.009 cm ^-1 at the frequency of 0.82 THz.

具有螺旋波前的电磁波是携带轨道角动量的涡旋波束, 涡旋波束存在的相位奇点, 使其在微粒操控和通讯等领域有特殊的应用. 本文提出了一种基于反射型超表面的太赫兹宽带涡旋波束产生器, 该器件由超表面-电介质-金属三层结构构成, 顶层为两个正交I形金属结构单元组成的超表面, 中间层是聚酰亚胺介质, 最底层为金属. 通过对超表面单元结构参数的优化设计, 可以实现在不同旋转角度下反射波振幅高达90%以上, 同时反射波相位随旋转角线性变化的目的. 进一步利用这些单元结构按照Pancharatnam-Berry相位原理进行超表面布阵, 在0.8—1.4 THz频率范围内, 可以将圆偏振太赫兹波束转换为具有轨道角动量的涡旋波束, 这一器件的工作带宽相对较宽, 结构简单, 转换效率高, 在太赫兹涡旋波束产生方面具有潜在的应用价值.

A novel high-birefringence and low-loss hollow-core (HC) antiresonant (AR) THz waveguide is proposed and experimental demonstrated. The waveguide is a circular tube embedded with two pairs of symmetrical parallel slabs. With the support of a circular tube, one pair of slabs near the waveguide center is used to form a rectangular-like core to achieve high birefringence, and another pair of slabs is placed in the cladding to further reduce the waveguide loss. Simulation results indicate that the birefringence reaches the magnitude order of 10 ⁻³ within the transmission window from 0.38 to 0.50 THz. The highest birefringence reaches 3.42 × 10 ⁻³ at 0.38 THz with a loss of 0.09 cm ⁻¹ . We manufacture the HC-AR THz waveguide with photosensitive resin by using three-dimensional printing technology. And the waveguide properties are investigated by THz time-domain spectroscopy system. Experimental results demonstrate that the birefringence variation of the HC-AR THz waveguide is within the range of 1.57 × 10 ⁻³ to 4.77 × 10 ⁻³ in the 0.39 to 0.51 THz transmission window. The highest birefringence is up to 4.77 × 10 ⁻³ at 0.39 THz and the corresponding loss is 0.12 cm ⁻¹ . To the best of authors’ knowledge, the proposed HC-AR THz waveguide achieves the highest birefringence and relatively low loss in current HC-AR THz waveguides.

Single-function metasurfaces can only perform one task and thus cannot satisfy the demands of many advanced applications. Employing multifunctional and multiplexing metasurfaces considerably increases the integration density of functional devices. Therefore, orbital angular momentum (OAM) multiplexing metasurfaces have been designed to increase transmission capacity. In this study, a bilayer Pancharatnam–Berry phase unit cell operating at two frequencies in the terahertz (THz) band is proposed. The unit cell has a sub-wavelength thickness that can be used as a functional element for constructing ultrathin and compact metasurfaces for wavefront manipulation. Cross-polarization conversion ratio of 86.3% and 88.5% are obtained at the two frequencies, respectively. We design three multifunction metasurfaces to verify the method. First, two vortex metalenses are designed to focus vortex beams in the sub-wavelength scale at various frequencies. Furthermore, multichannel OAMs with various modes and arbitrary shape of OAM beam arrays can be realized using segmented metasurfaces. Finally, multichannel OAM multiplexing with the frequency selection and polarization dependence were simultaneously realized on a metasurface. Such metasurfaces can be used to realize large-capacity and high-spectrum-efficiency OAM communication. This paper contributes to the disciplines of photonic integration, multiplexing, and multifunctional metasurfaces.

The terahertz (THz) vortex beam generators are designed and theoretically investigated based on single-layer ultra-thin transmission metasurfaces. Noncontinuous phase changes of metasurfaces are obtained by utilizing Pancharatnam–Berry phase elements, which possess different rotation angles and are arranged on two concentric rings centered on the origin. The circularly polarized incident THz beam could be turned into a cross-polarization transmission wave, and the orbital angular momentum (OAM) varies in value by lℏ. The l values change from ± 1 to ± 5, and the maximal cross-polarization conversion efficiency that could be achieved is 23%, which nearly reaches the theoretical limit of a single-layer structure. The frequency range of the designed vortex generator is from 1.2 THz to 1.9 THz, and the generated THz vortex beam could keep a high fidelity in the operating bandwidth. The propagation behavior of the emerged THz vortex beam is analyzed in detail. Our work offers a novel way of designing ultra-thin and single-layer vortex beam generators, which have low process complexity, high conversion efficiency and broad bandwidth.