Abstract-Deeply Subwavelength Metasurface Resonators for Terahertz Wavefront Manipulation

Mingkai Liu,  Quanlong Yang,  Ahmmed A. Rifat,   Vidur Raj,   Andrei Komar,   Jiaguang Han,   Mohsen Rahmani,   Haroldo T. Hattori,   Dragomir Neshev,   David A. Powell,   Ilya V. Shadrivov,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201900736?af=R

Metasurfaces offer a highly flexible platform for controlling the propagation and localization of electromagnetic waves. Due to the relatively large size of commonly used resonators, various undesirable effects including spatial dispersion and spurious diffraction occur, thus limiting the metasurface performance. To overcome these problems, one straightforward approach is to utilize deeply subwavelength metaunits. In contrast to conventional approaches that minimize the resonator size by reshaping the metallic patches, the capacitive gaps are reshaped, an approach which is more robust to material loss, minimizing the problem of overdamping. As an example, a novel design based on interdigital capacitors (meander gap) is introduced with extremely subwavelength gaps for use in the terahertz frequency range. The size of the new resonator can be reduced to below λ/30 in a reflective‐type terahertz metasurface, while maintaining the 2 𝜋$\pi$ phase shift required for full wavefront control. Using an advanced electron‐beam lithography technique, a proof‐of‐concept experiment is performed and a 5 mm × 5 mm beam deflector is fabricated, with the capacitive gaps as small as 300 nm (≈λ/1130). The device performance is characterized using angle‐resolved time‐domain spectroscopy. The study provides useful insight for ultracompact metadevices based on deeply subwavelength metaunits working at terahertz frequencies and beyond.

Abstract-Broadband terahertz rotator with an all-dielectric metasurface

Quanlong Yang, Xieyu Chen, Quan Xu, Chunxiu Tian, Yuehong Xu, Longqing Cong, Xueqian Zhang, Yanfeng Li, Caihong Zhang, Xixiang Zhang, Jiaguang Han, and Weili Zhang

Fig. 1. (a) Conceptual description of the metasurface based on two identical dielectric antennas to manipulate the polarization of the terahertz wave. 𝛼$\alpha$ and 𝛽$\beta$ represent the orientations of two dielectric antennas (marked by the orange arrows), and 𝛾$\gamma$ is the effective optical axis orientation from the superposition of two antennas (marked by the navy arrow). (b) Schematic illustration of the two silicon antennas with geometrical parameters 𝑊=45  μm$W=45\mathrm{\mu m}$, 𝐿=180  μm$L=180\mathrm{\mu m}$, 𝐻=200  μm$H=200\mathrm{\mu m}$, and period 𝑃=375  μm$P=375\mathrm{\mu m}$. (c),(d) Schematic diagrams for high-quality polarization generation. Without introducing the phase gradient, both the 𝑥$x$-polarized and 𝑦$y$-polarized light propagates in the normal direction forming dispersive polarization states within the frequency range of interest. The phase gradient enables spatial separation of the two orthogonal polarization components, giving rise to pure linearly polarized components within a broad frequency range.

https://www.osapublishing.org/prj/fulltext.cfm?uri=prj-6-11-1056&id=399205

Polarization manipulation is essential in developing cutting-edge photonic devices ranging from optical communication displays to solar energy harvesting. Most previous works for efficient polarization control cannot avoid utilizing metallic components that inevitably suffer from large ohmic loss and thus low operational efficiency. Replacing metallic components with Mie resonance-based dielectric resonators will largely suppress the ohmic loss toward high-efficiency metamaterial devices. Here, we propose an efficient approach for broadband, high-quality polarization rotation operating in transmission mode with all-dielectric metamaterials in the terahertz regime. By separating the orthogonal polarization components in space, we obtain rotated output waves with a conversion efficiency of 67.5%. The proposed polarization manipulation strategy shows impressive robustness and flexibility in designing metadevices of both linear- and circular-polarization incidences.

© 2018 Chinese Laser Press

Abstract-Interferometric control of dual-band terahertz perfect absorption using a designed metasurface

Ming Kang, Huifang Zhang, Xueqian Zhang, Quanlong Yang, Weili Zhang,  Jiaguang Han

https://journals.aps.org/prapplied/accepted/b3077A50X33E621a909b163840c172b9fbca3365e

Coherent perfect absorber (CPA), a time-reversed counterpart to the laser emission, could cause all energy fed to the system to be absorbed. It can also be as an absorptive interferometer, which could provide applications in controllable optical energy transfer. Here, in order to achieve terahertz CPA, we propose a designed metasurface and experimentally demonstrate that it can serve as a polarization-insensitive CPA at one-frequency channel under normal symmetric excitation, while a transverse electric (TE) CPA at two-frequency channels around oblique ${40}^o$symmetric incidence. Such phenomena in this system can be attributed to Fano resonance consisting of interacting one bright and one dark modes under normal incidence and an additional operative dark mode under oblique symmetric excitation. The experimental results finds good agreement with the fitted coupled mode theory(CMT). Moreover, we show the output amplitude can be effectively tuned from 0 to 1 only by varying the relative phase between the two input waves. The designed CPA could find potential application in effectively controlling absorption for terahertz imaging and terahertz switches.

Abstract-Transmission and plasmonic resonances on quasicrystal metasurfaces

Quanlong Yang, Jianqiang Gu, Yuehong Xu, Yanfeng Li, Xueqian Zhang, Zhen Tian, Chunmei Ouyang, Jiaguang Han, and Weili Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-20-24173&origin=search

The control of light-matter interaction in metasurfaces offers an unexplored potential for the excitation and manipulation of light. Here, we combine experimental terahertz time-domain spectroscopy and near-field scanning terahertz microscopy to demonstrate the role of reciprocal vectors in the transmission and plasmonic resonances of quasicrystal metasurfaces. An investigation of two-dimensional metasurface structures with different rotationally symmetric quasicrystal arrangements demonstrates that the transmission minima resulting from Wood’s anomaly are directly related to the surface plasmon resonances. We also find that the surface plasmon resonances of the quasicrystal metasurface were determined by the reciprocal vectors, which could be well explained by the coupling condition of the resonances, and the characteristic frequencies remain un-shifted under various slit sizes. Our findings demonstrate a new potential in developing novel plasmonic metasurfaces.

© 2017 Optical Society of America

Abstract-Multi-wavelength lenses for terahertz surface wave

Minggui Wei, Quanlong Yang, Quan Xu, Xueqian Zhang, Yanfeng Li, Jianqiang Gu, Jiaguang Han, and Weili Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-21-24872

Metasurface-based surface wave (SW) devices working at multi-wavelength has been continuously arousing enormous curiosity recently, especially in the terahertz community. In this work, we propose a multi-layer metasurface structure composed of metallic slit pairs to build terahertz SW devices. The slit pair has a narrow bandwidth and its response frequency can be altered by its geometric parameter, thereby suppressing the frequency crosstalk and reducing the difficulty of design. By elaborately tailoring the distribution of the slit pairs, a series of achromatic SW lenses (SWLs) working at 0.6, 0.75 and 1 THz are experimentally demonstrated by the near field scanning terahertz microscope (NSTM) system. In addition, a wavelength-division-multiplexer (WDM) is further designed and implemented, which is promising in building multiplexed devices for plasmonic circuits. The structure proposed here cannot only couple the terahertz wave from free space to SWs, but also control its propagation. Moreover, our findings demonstrate the great potential to design multi-wavelength plasmonic metasurface devices, which can be extended to microwave and visible frequencies as well.

© 2017 Optical Society of America

Abstract-Polarization-controlled asymmetric excitation of surface plasmons

Quan Xu, Xueqian Zhang, Quanlong Yang, Chunxiu Tian, Yuehong Xu, Jianbing Zhang, Hongwei Zhao, Yanfeng Li, Chunmei Ouyang, Zhen Tian, Jianqiang Gu, Xixiang Zhang, Jiaguang Han, and Weili Zhang

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-4-9-1044&origin=search

Free-space light can be coupled into propagating surface waves at a metal–dielectric interface, known as surface plasmons (SPs). This process has traditionally faced challenges in preserving the incident polarization information and controlling the directionality of the excited SPs. The recently reported polarization-controlled asymmetric excitation of SPs in metasurfaces has attracted much attention for its promise in developing innovative plasmonic devices. However, the unit elements in these works were purposely designed in certain orthogonal polarizations, i.e., linear or circular polarizations, resulting in limited two-level polarization controllability. Here, we introduce a coupled-mode theory to overcome this limit. We demonstrated theoretically and experimentally that, by utilizing the coupling effect between a pair of split-ring-shaped slit resonators, exotic asymmetric excitation of SPs can be obtained under the 𝑥$x$-, 𝑦$y$-, left-handed circular, and right-handed circular polarization incidences, while the polarization information of the incident light can be preserved in the excited SPs. The versatility of the presented design scheme would offer opportunities for polarization sensing and polarization-controlled plasmonic devices.

© 2017 Optical Society of America

Abstract-All-Dielectric Meta-lens Designed for Photoconductive Terahertz Antennas

 Qing Yu,  Jianqiang Gu, Quanlong Yang,  Ying Zhang,   Yanfeng Li, Zhen Tian, Chunmei Ouyang,   Jiaguang Han, John F. O'Hara, Weili Zhang

Impact Statement:

In this numerical study, we present a metasurface based lens directly integrated to a terahertz PCA transmitter which is rarely reported. Because its all-dielectric nature, the meta-lens not only offers an excellent collimation function, but also has a better transmittance efficiency than the traditional Si hyper-semispheric lens and most metal based terahertz meta-lenses. The meta-lens proposed here have promising applications in next-generation terahertz imaging and spectroscopy techniques.

Abstract:

We present an all-dielectric meta-lens designed to collimate terahertz waves emitted from a terahertz antenna. The meta-lens is not only thinner than a conventional bulk silicon lens, but also promises to eliminate the use of parabolic mirrors in a terahertz time-domain spectroscopy system. A systematic numerical study reveals that the meta-lens exhibits excellent performance in both the emitter and detector modules, converting between the spherical wave of the antennas and the collimated beam. The frequency and alignment dependences of the meta-lens are also investigated to comprehensively map its response characteristics. The all-dielectric meta-lens presented here may pave a way in developing high-performance integrated photoconductive terahertz antenna components.

Abstract-Ultrathin metasurface-based carpet cloak for terahertz wave

Minggui Wei, Quanlong Yang, Xueqian Zhang, Yanfeng Li, Jianqiang Gu, Jiaguang Han, and Weili Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-14-15635

Ultrathin metasurfaces with local phase compensation deliver new schemes to cloaking devices. Here, a large-scale carpet cloak consisting of an ultrathin metasurface is demonstrated numerically and experimentally in the terahertz regime. The proposed carpet cloak is designed based on discontinuous-phase metallic resonators fabricated on a polyimide substrate, offering a wide range of reflection phase variations and an excellent wavefront manipulation along the edges of the bump. The invisibility is verified when the cloak is placed on a reflecting triangular surface (bump). The multi-step discrete phase design method would greatly simplify the design process and is probable to achieve large-dimension cloaks, for applications in radar and antenna systems as a thin, lightweight, and easy-to-fabricate solution for radio and terahertz frequencies.

© 2017 Optical Society of America

Abstract- Broadband and Robust Metalens with Nonlinear Phase Profiles for Efficient Terahertz Wave Control

Quanlong Yang, Jianqiang Gu, Yuehong Xu, Xueqian Zhang, Yanfeng Li, Chunmei Ouyang, Zhen Tian, Jiaguang Han, Weili Zhang,

http://onlinelibrary.wiley.com/doi/10.1002/adom.201601084/abstract

Metasurfaces, 2D artificial electromagnetic media, open up a new frontier of functional device design ranging from radio waves to the visible region. Particularly, metasurface-based lenses are indispensable in various practical terahertz applications. The authors aim at achieving flexible and robust metalenses for efficient terahertz wave control. In general, resolution and efficiency are two inevitable parameters in determining the focusing and imaging abilities of lenses, which however are rarely experimentally demonstrated in the terahertz band. In this Communication, three broadband and robust metalenses with nonlinear phase profiles are proposed, all of which are experimentally investigated by using near-field scanning terahertz microscopy (NSTM) with a spatial resolution of 50 µm. The measurement shows that the metalens can focus a 0.95 THz wave to a spot size of 580 µm and achieve a transmittance efficiency of 45%. In addition, the NSTM system facilitates an experimental investigation of the incidence angle dependence of the terahertz metalens, which proves the robust focusing feature of the proposed device. This demonstration delivers a promising metasurface design for potential applications in imaging and information processing that may be of interest for the entire electromagnetic spectrum.