Metasurface device manipulates THz polarization states along propagation path

Published 25 April, 2025

Traditional spatial THz metasurface devices have common limitations: they focus on manipulating polarization states on a single plane, lack consideration for spatial propagation distance factors, and result in polarization states remaining unchanged on every output plane along the propagation path.

So, how can we modify the polarization state of THz waves on different output planes along the propagation path?

To answer this question, it is necessary to consider the factor of spatial propagation distance. Research has found that metamsurfaces constructed based on specific polarization conversion and phase delay units, combined with specially designed phase arrangements, can perform polarization decomposition on incident uniform scalar electromagnetic fields. Additionally, they are capable of modulating the spatiotemporal characteristics of different polarization components to manipulate their phase differences along spatial propagation paths. When they are combined again, different polarization states can be obtained on different output planes.

Previously, the mainstream technology for implementing the above idea in the THz band was spin decoupling, which can output different polarization states at several isolated positions on the propagation path or on different planes within a short distance. However, there have been no reports on the continuous manipulation of THz polarization states on different output planes over relatively long propagation distances, so more technological routes still need to be explored to provide richer candidate solutions for THz polarization manipulation.

To that end, Dr. Li Jitao (currently working at Southwest Petroleum University, China), Prof. Yao Jianquan (Tianjin University, China) and Prof. Zhang Yan (Capital Normal University, China), jointly proposed a metasurface device that can continuously manipulate THz polarization states on different output planes over a relatively long propagation distance.

In particular, the team studied the spatial polarization decomposition and recombination characteristics of THz waves. They employed a simpler technique than spin decoupling to achieve a polarized THz metasurface, which projects a traveling wave function that varies along the propagation distance onto different output planes to perform user-defined polarization changes on the incident wave. The designed metasurface decomposes incident polarized THz waves into two orthogonal circularly polarized THz waves, and applies different phase delays.

Consequently, the two circularly polarized components recombine into linearly polarized THz waves within a relatively long region (greater than 1 cm) along the propagation axis (Fig. 1-2). The phase difference between two circularly polarized components varies with propagation distance, causing the combined linearly polarized THz electric field to continuously rotate, enough to cover the Poincare sphere equator (Fig. 3).

The longitudinal polarization variable THz meta-device provides users with more polarization customization solutions and may benefit some applications. For instance, some electromagnetic response matters (e.g. electro-optical crystals) are sensitive to the intensity and polarization of THz wave; the metasurface can be used to spatially adjust the THz waves to modify the excited intensity and excited mode of the medium, eventually obtaining different output information.

Meanwhile, in the field of THz high-speed communication, the longitudinal variable polarization may make the communication more confidential, because the polarization information intercepted at different locations on the propagation path is different, which brings some difficulties in deciphering information. Furthermore, the terminal receives different polarization states through moving device, which may be used as an information transmission mode. Since the polarization transform is a function of the propagation distance, this feature may also have potential applications in the field of THz radar: the THz radar may identify the movement of the target by emitting THz waves with the longitudinal variable polarization to the target and detecting the polarization change of reflected THz waves. Moreover, the propagation of THz wave is related to the refractive index of the propagation medium, so placing additional media in the propagation path may change the polarization state of the output wave, which can be used as a new sensing scheme to detect the unknown refractive index (Fig. 4).

Fig. 1. (a) THz optical path diagram based on metasurface. By controlling the phase delays of LCP and RCP waves by unit cells, metasurface can transmit the LCP and RCP components of the incident linearly polarized waves to the z-axis in different paths, and each point in a certain region on the z-axis collects the LCP and RCP waves generated by different unit cells respectively. The two kinds of circularly polarized waves are combined again to form linearly polarized waves, and the output linear polarization angle varies with the propagation distance. (b) The phase profile of Eq. (4) makes the output wave form Bessel beam; (c) Functional diagram of a longitudinally polarization variable device, where output linearly polarized THz waves rotate along the propagation path.

Fig. 2. (a) The schematic diagram of sub-units: a complete unit cell is composed of four sub-units, in which two identical purple sub-units control the propagation of LCP component and two identical yellow sub-units control the propagation of RCP component. (b) The required phase profile for RCP and LCP components and the schematic device, corresponding to the SEM images of sample in (c).

Fig. 3. (a) The simulative electric field intensity distribution for the x-polarized and y-polarized components of the output wave in the x-z plane. (b) The simulation results of electric field intensity distribution (|Ex|2+|Ey|2) of output wave in the x-y plane, and the THz spot diameter is about 300-400 μm; (b1-b8) show the simulative polarization states of the THz spots at different positions along the propagation path, where the THz propagation direction vertically points out of the paper. (c) The polarization state distribution corresponding to (b1-b8) on Poincare sphere (left) and their two-dimensional view in the x-y plane (right).

Fig. 4. (a) The schematic concept for the refractive index sensing. Placing additional medium in the propagation path will change the phase difference between LCP and RCP components. The refractive index is determined by detecting the change of polarization state in an output plane. (b) With the change of refractive index, the viewed polarization state in the z=8 mm plane rotates, where the propagation direction vertically points out of the paper. (c-d) Theoretical and simulation results of Prot vs. n.

Contact author:

Jitao Li

¹School of Science, Southwest Petroleum University, Chengdu 610500, China

²School of Precision Instruments and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China

Email: jtlee@tju.edu.cn

Funder: This work was supported by National Key Research and Development Program of China (2021YFB2800703 and 2017YFA0700202).

Conflict of interest: The authors declare that they have no conflicts of interest in this work.

See the article: Jitao Li, Jingyu Liu, Zhen Yue, et al. Polarization variable terahertz metasurface along the propagation path. Fundamental Research 5: 124-131 (2025). https://doi.org/10.1016/j.fmre.2023.03.017

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