Progress in High-resolution Imaging Made by Terahertz Technology Innovation Research Institute

Metasurfaces, which are two-dimensional counterpart of metamaterials, and consist of micro-nano structures with predesigned in-plane orientations, can flexibly control the wavefront of electromagnetic waves. The ultrathin nature of metasurfaces, their ease of fabrication, and subwavelength resolution for manipulating of EM waves make metasurfaces ideal candidates for device miniaturization (ultra-compact THz devices) and system integration. In recent years, under the guidance of Prof. Songlin Zhuang, Prof. Xiaofei Zang and Yiming Zhu (from terahertz technology innovation research institute, USST) have proposed a variety of THz metalens, i.e., polarization-controllable multi-foci metalens, metalens with extended focal depth, and metalens with asymmetric focal point, and these THz metalens have been applied into THz high-resolution imaging. They have published a series of papers under the support of the National Key Research and Development Program of China and National Natural Science Foundation of China.
(1) Polarization-controllable multi-foci metalens and high-resolution imaging
For a linearly polarized incident THz beam with polarization along x-axis, one focal spot is clearly observed with the polarization axis rotating by 90 degree (see Fig. 1(a)), and another x-polarized focal point is shown in Fig. 1(b), demonstrating a polarization-controllable multi-foci metalens. When a Chinese character of “Sun” consisting of two symmetrically placed capital letters “E” is embedded the region of the left focal point, the left “E” can be observed, while the right “∃” is revealed when the imaging sample is located at the region of the right focal point (see Fig. 1(c)).

(2) Metalens with extended focal depth and high-tolerance imaging
They have proposed an approach to realize a polarization-insensitive metalens with an extended focal depth using anisotropic dielectric micropillars (geometric metasurfaces). Under the illumination of LCP and RCP THz waves, two identical focal points can be observed at the same position, leading to the polarization-insensitive metalens. For the incidence of arbitrarily polarized THz waves, this metalens shows a focal depth of ≈23λ along the propagation direction. Longitudinal high-tolerance imaging is experimentally demonstrated based on such a THz metalens.

(3) Metalens for asymmetric focusing
As is well known, lenses can be applied to imaging. Recently, various types of metalenses such as dual-polarity plasmonic metalense, achromatic metalenses, metalens array, and polarization-dependent metalenses have been developed for imaging. However, all of the previous metalenses are symmetric in the forward and backward directions, meaning that we can grab information/imaging from the “enemy” side and cannot avoid our own information being detected. In this work, we have designed a untrathin (with thickness of 30.3 μm) metalens with asymmetric functionality by putting together a lens, a halfwave plate, and a linear polarizer into a dual layer metasurface. The asymmetric focal points are obtained based on such a metalens.

(4) THz metalens with superfocusing.
They have proposed a polarization-dependent terahertz (THz) plasmonic lens consisting of chiral spiral arrays and a concentric groove, aiming to obtain application oriented THz super-focusing. Since the chiral spiral array is polarization-dependent, it can be applied to control the intensity of the focal point, resulting in the intensity-tunable functionality. An intensity-tunable THz focal spot beyond the diffraction limit (FWHM≈0.38λ) was realized by controlling the polarization of the incident THz waves. Such plasmonic lens may enable the capability of super-resolution imaging.

(5) Review article: Metasurfaces for manipulating terahertz waves.
The traditional THz devices should be thick enough to realize the desired wave-manipulating functions, hindering the development of THz integrated systems and applications. As a review article, Prof. Songlin Zhuang from Terahertz Technology Innovation Research Institute, and co-workers have summarized recent advancements of metasurfaces for the manipulation of THz waves. These ultra-compact devices with unusual functionalities render metasurface devices very attractive for applications such as imaging, encryption, information modulation and THz communications.

Relative research achievements:

[1]Xiaofei Zang, Bingshuang Yao, Lin Chen,Jingya Xie, Xuguang Guo, Alexei V. Balakin, Alexander P. Shkurinov, Songlin Zhuang, “Metasurfaces for manipulating terahertz waves,” Light: Advanced Manufacturing 2:10 (2021).
[2]Xiaofei Zang, Weiwei Xu, Min Gu, Bingshuang Yao, Lin Chen, Yan Peng, Jingya Xie, Alexey V. Balakin, Alexander P. Shkurinov, Yiming Zhu, Songlin Zhuang, “Polarization-insensitive metalens with extended focal depth and longitudinal high-tolerance imaging,”Advanced Optical Materials 8(2), 1901342(2020).
[3] Bingshuang Yao, Xiaofei Zang, Zhen Li, Lin Chen, Jingya Xie, Yiming Zhu, Songlin Zhuang, “Dual-layered metasurfaces for asymmetric focusing,” Photonics Research 8(5), 830(2020).
[4] Xiaofei Zang, Bingshuang Yao, Zhen Li, Yang Zhu, Jingya Xie, Lin Chen, Alexey. V. Balakin, Alexander. P. Shkurinov, Yiming Zhu, SongLin Zhuang, “Geometrric phase for multidimensional manipulation of photonics spin Hall effect and helicity-dependent imaging,” Nanophotonics 9(6), 1501-1508(2020).
[5] XiaoFei Zang, Hongzhen Ding, Yuttana Intaravanne, Lin Chen, Yan Peng, Jingya Xie, Qinghong Ke, Alexey V. Balakin, Alexander P. Shkurinov, Xianzhong Chen, Yiming Zhu, Songlin Zhuang, “A multi-foci metalens with polarization-rotated focal points,” Laser & Photonics Reviews 13, 1900182 (2019).
[6] Xiaofei Zang, Yiming Zhu, Chenxi Mao, Weiwei Xu, Hongzhen Ding, Jingya Xie, Qingqing Cheng, Lin Chen, Yan Peng, Qing Hu, Min Gu, Songlin Zhuang, “Manipulating terahertz plasmonic vortex based on geometric and dynamic phase,” Advanced Optical Materials 7, 1801328(2019).
[7] Xiaofei Zang, Fengliang Dong, Fuyong Yue, Chunmei Zhang, Lihua Xu, Zhiwei Song, Ming Chen, Pai‐Yen Chen, Gerald S. Buller, Yiming Zhu, Songlin Zhuang, Weiguo Chu, Shuang Zhang, Xianzhong Chen,“Polarization encoded image embedded in a dielectric metasurface,” Advanced Materials.30, 1707499 (2018).
[8] Xiaofei Zang, Chenxi Mao, Xuguang Guo, Guanjun You, He Yang, Lin Chen, Yiming Zhu, and Songlin Zhuang, “Polarization-controlled terahertz super-focusing”, Appl. Phys. Lett. 113, 071102(2018).