During the past decade, metasurfaces have provided a versatile platform for controlling the amplitude, phase, and polarization of electromagnetic waves, thus showing great potential for developing compact photonic devices. However, compact and broadband terahertz (THz) components are still scarce for high‐speed communication and spectroscopy analysis. Here, a free‐standing metasurface is designed to realize broadband and efficient wavefront manipulation in the THz regime. The proposed metasurface consists of a single patterned metallic layer on an ultra‐thin flexible substrate. The full‐span phase control of circularly cross‐polarized waves can be achieved based on the Pancharatnam‐Berry phase mechanism only by spatially varying the element orientation. Besides, it can operate in a broadband range of 0.35–1.21 THz (relative bandwidth of 111%), with the efficiency approaching the theoretical limit of the ultra‐thin single‐layer metasurface. It is demonstrated that the free‐standing metasurface can be used for numerous applications, including but not limited to THz beam steering, focusing, and vortex generation. The broadband performances allow the pulsed spectral imaging system to record the evolution process of broadband vortex beam in real‐time. The proposed metasurface may trigger many exciting opportunities in THz wireless communication, medicine, and spectroscopy applications.

Dynamic manipulation of electromagnetic (EM) waves with multiple degrees of freedom plays an essential role in enhancing information processing. Currently, an enormous challenge is to realize directional terahertz (THz) holography. Recently, it was demonstrated that Janus metasurfaces could produce distinct responses to EM waves from two opposite incident directions, making multiplexed dynamic manipulation of THz waves possible. Herein, we show that thermally activated THz Janus metasurfaces integrating with phase change materials on the meta-atoms can produce asymmetric transmission with the designed phase delays. Such reconfigurable Janus metasurfaces can achieve asymmetric focusing of THz wave and directional THz holography with free-space image projections, and particularly the information can be manipulated via temperature and incident THz wave direction. This work not only offers a common strategy for realizing the reconfigurability of Janus metasurfaces, but also shows possible applications in THz optical information encryption, data storage, and smart windows.

Asymmetric manipulation of electromagnetic waves exhibits tremendous capabilities in the applications of encryption, ultrafast information processing, and communication. However, compact and broadband terahertz (THz) components for asymmetric applications are still scarce. Here, we demonstrate broadband, ultrathin, and efficient metasurface-based asymmetric transmission and wavefront manipulation in the THz region. The metasurface consists of tri-layer metallic structures separated by two flexible polyimide dielectrics. This ultrathin structure exhibits superior asymmetric response which can act as an efficient device with a high linear polarization conversion ratio over 0.9 ranging from 0.5 to 1.1 THz. We also develop THz asymmetric applications based on the flexible metasurface for beam deflector and orbital angular momentum generation. The THz time-domain spectroscopy system and spectral imaging system are used to characterize the broadband asymmetric properties. The experimental results confirm that the beam deflection and vortex waves can be realized over broadband under one linearly polarized incidence while blocked under its orthogonal incidence. This work may provide a promising route to achieve broadband asymmetric applications in information encryption, THz wireless communication, and spectroscopy applications.

In this study, we describe the design, fabrication, and characterization of a cross-shaped structure reflective broadband THz polarization converter. The operating bandwidth of the proposed polarization converter was in the range of 0.8–1.6 THz. The polarization conversion rate (PCR) exceeded 85% in the frequency range of 1.07–1.35 THz and was as high as 91% at 1.35 THz. The cross PCR value exceeded 85% in the frequency range of 1.05–1.35 THz and reached a maximum value of 91%. The performance characteristics (bandwidth and PCR) of the proposed polarization converter were compared with those of similar previously reported devices, and the results indicated that the proposed polarization converter exhibits better performance. The proposed THz polarization converter is suitable for a wide range of applications in communication and polarization manipulation devices.