The superstructure surface is composed of sub-wavelength quasi-two-dimensional microstructure in accordance with a specific arrangement, which can flexibly regulate the wave front of electromagnetic waves to obtain high-quality sub-wavelength and even focus spots that break the diffraction limit, and effectively realize the functions of traditional three-dimensional "volume" functional devices with flat devices, effectively improving the integrability of functional devices. It provides a new solution for the miniaturization and integration of the future system. In recent years, under the guidance of Academician Zhuang Songlin, Professor Zang Xiaofei and Zhu Yiming of the Institute of Terahertz Technology Innovation at the University of Shanghai for Science and Technology have developed a series of planar lenses based on the superstructure surface (such as polar-controlled superstructure surface lenses, telephoto deep superstructure surface lenses, unidirectional superstructure surface lenses, etc.) and applied to terahertz high-resolution imaging. Relevant research is carried out in the Key Research and Development Program of the Ministry of Science and Technology (2017YFF0106300, 2017YFA0701005, 2018YFF01013003), the Key Project of the National Natural Science Foundation (3218302024), the Talent Program of the Ministry of Science and Technology and the Outstanding Youth Science Fund Project (3119302003). With the support of 3218302015, 3220302008), a total of 7 SCI Region I papers were published, and the detailed progress is as follows：
(1) polarization-controlled terahertz meta-surface lens and high-resolution imaging
Figure 1. Polarization-controlled metasurface lens: (a,b) bifocal focusing and polarization rotation functions; (c,d) terahertz polarization dependent high-resolution scanning imaging.
Figure 1(a) shows the numerical simulation of the incident THZ wave of X-polarization to the designed metasurface lens. It can be clearly found that only the focused spots polarized along the Y-axis are present in the outgoing field. Similarly, it can be seen from the experiment in FIG. 1(b) that a beam of terahertz wave incident along the X-polarization is incident on the metasurface lens designed by us, and the polarization state of the focused spot is along the Y-axis, which also proves that the metasurface lens designed by us has both focusing and polarization regulation functions. As shown in Figure 1(c, d), the image of the polarization-dependent sample to be measured is "E" in the former focus area and "" in the latter focus area, realizing the polarization-dependent high-resolution imaging detection function.
(2) Terahertz lens focal depth control
A pure geometric phase spin-solution coupled terahertz superstructure lens is proposed, which realizes the simultaneous focusing of left - and right-circularly polarized terahertz waves. By adjusting the focal length of the spin-solution coupled superstructure lens to the left and right circular polarization terahertz waves, a pair of orthogonal circular polarization terahertz focusing spots are formed. Furthermore, the focal depth of the polarization-independent terahertz superstructure surface lens is fused to form the regulation of the ultra-long focal depth (focal depth up to 23λ) of polarization-independent terahertz waves (as shown in FIG. 2(a), 2(b)), and the scanning imaging of longitudinal high resolution and high tolerance is realized (as shown in FIG. 2(c),2(d),2(e)).
Figure 2. Polarization-independent terahertz telephoto deep metasurface lens and longitudinal high tolerance imaging.
(3) Terahertz superstructure surface lens: unidirectional focusing control
At present, almost all the focusing spots formed by the research of hyperlens on the superstructure surface (for example, single focusing spot, multiple focusing spot, chromatic aberration focusing spot and broadband focusing spot) are reversible. That is, the electromagnetic wave incident from any side of the lens on the superstructure surface will form a focused spot. Such meta-surface lenses will not have any "secret" properties in terms of imaging and detection. Based on this, we integrated the three functions of traditional focusing lens, 1/2 wave plate and grating into the double-layer superstructure surface, and designed an ultra-thin asymmetric superstructure lens with a thickness of 25.2μm to realize the THZ asymmetric focusing function (FIG. 3).
FIG. 3. Field distribution of a lens with a unidirectional focusing superstructure surface: (a1),(b1) terahertz wave of 0.6THz incident from the back and front, respectively; (a2) and (b2) are the field distributions on the focusing plane of the respective transmission plane.
(4) Terahertz superstructural surface lens: ultra-focused terahertz spot that breaks the diffraction limit
Figure 4. Chiral surface and terahertz hyperfocus spot
We extended the principle of metasurface wavefront regulation to the regulation of terahertz hyperfocus spots that break the diffraction limit in terahertz near-field. By designing a chiral dependent metasurface structure and combining the coherence effect, the focused isolator is emitted into free space to form a single frequency polarization controllable terahertz hyperfocus spot (FWHM ~ 0.38λ) that breaks the diffraction limit (as shown in FIG. 4). The results are expected to be applied to terahertz superresolution imaging.
(5) Review progress of terahertz superstructure surface:
At the invitation of Light: Advanced Manufacturing, the Institute published a review paper entitled "Metasurfaces for manipulating terahertz waves" (terahertz wave regulation based on ultrastructural surfaces). In this paper, the relevant principles, functional design and applications of the wave front control of the superstructure surface in terahertz are reviewed. With resonant, geometric phase and tunable surface as the carrier and related functions of wavefront regulation as the main line, the research work of focusing regulation, holographic generation, polarization regulation, special beam generation and tunable function of terahertz waves on the superstructure surface is summarized. The work by American physicist organization network coverage (https://phys.org/news/2021-03-metasurfaces-terahertz.html)
The above related research results:
[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).
