Metasurface device manipulates THz polarization states along propagation path

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).

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Contact the 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

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