Yuanhao Lang, Quan Xu, Xieyu Chen, Jie Han, Xiaohan Jiang, Yuehong Xu, Ming Kang, Xueqian Zhang, Andrea Alù, Jiaguang Han and Weili Zhang

Abstract
Since the late 19th century, enormous endeavors have been made in extending the scope and capability of optical interferometers. Recently, plasmonic vortices that strongly confine the orbital angular momentum to surface have attracted considerable attention. However, current research interests in this area have focused on the mechanisms and dynamics of polarization-dependent single plasmonic vortex generation and evolution, while the interference between different plasmonic vortices for practical applications has been unexplored. Here, a method for flexible on-chip spin-to-orbital angular momentum conversion is introduced, resulting in exotic interferograms. Based on this method, a new form of interferometers that is realized by the interference between customized plasmonic vortices is demonstrated. Within wavelength-scale dimension, the proposed plasmonic vortex interferometers exhibit superior performance to directly measure the polarization state, spin and orbital angular momentum of incident beams. The proposed interferometry is straightforward and robust, and can be expected to be applied to different scenarios, fueling fundamental advances and applications alike.

Xieyu Chen, Shoujun Zhang, Kuan Liu, Haiyang Li, Yihan Xu, Jiajia Chen, Yongchang Lu, Qingwei Wang, Xi Feng, Kemeng Wang, Zeru Liu, Tun Cao, and Zhen Tian

Abstract
Reconfigurable terahertz (THz) devices with nonvolatile and multilevel properties are highly desirable in many applications from imaging and high-capacity communication to nondestructive biosensing. Here, we present and experimentally demonstrate a promising platform for dynamic THz devices by exploring the reversible, nonvolatile, and multilevel modulation features of the Ge2Sb2Te5 (GST) material. These properties were fully demonstrated by characterizing the THz response of a centimeter-level area GST layer during the thermally stimulated crystallization and optically stimulated reamorphization process. As a proof of concept, a hybrid plasmonic dimer structure composed of two trapezoidal metallic rings connected by GST islands was designed, fabricated, and characterized. The modulation of this device was experimentally realized by inducing crystallization and reamorphization processes with thermal and optical activation, respectively. Moreover, electrical switching of the designed device was also realized by applying a 5 s duration electrical pulse. With these findings, this study may open an attractive direction for a wide range of design possibilities in terms of reversible, nonvolatile, and multilevel THz modulation devices.

Quan Xu, Xiaoqiang Su, Xueqian Zhang, Lijuan Dong, Lifeng Liu, Yunlong Shi, Qiu Wang, Ming Kang, Andrea Alù, Shuang Zhang, Jiaguang Han, and Weili Zhang

Abstract
Metasurfaces have enabled the realization of several optical functionalities over an ultrathin platform, fostering the exciting field of flat optics. Traditional metasurfaces are achieved by arranging a layout of static meta-atoms to imprint a desired operation on the impinging wavefront, but their functionality cannot be altered. Reconfigurability and programmability of metasurfaces are the next important step to broaden their impact, adding customized on-demand functionality in which each meta-atom can be individually reprogrammed. We demonstrate a mechanical metasurface platform with controllable rotation at the meta-atom level, which can implement continuous Pancharatnam–Berry phase control of circularly polarized microwaves. As the proof-of-concept experiments, we demonstrate metalensing, focused vortex beam generation, and holographic imaging in the same metasurface template, exhibiting versatility and superior performance. Such dynamic control of electromagnetic waves using a single, low-cost metasurface paves an avenue towards practical applications, driving the field of reprogrammable intelligent metasurfaces for a variety of applications.

Dongyang Wang, Biao Yang, Wenlong Gao, Hongwei Jia, Quanlong Yang, Xieyu Chen, Minggui Wei, Changxu Liu, Miguel Navarro-Cía, Jiaguang Han, Weili Zhang & Shuang Zhang

Abstract
Weyl points are discrete locations in the three-dimensional momentum space where two bands cross linearly with each other. They serve as the monopoles of Berry curvature in the momentum space, and their existence requires breaking of either time-reversal or inversion symmetry. Although various non-centrosymmetric Weyl systems have been reported, demonstration of Weyl degeneracies due to breaking of the time-reversal symmetry remains scarce and is limited to electronic systems. Here, we report the experimental observation of photonic Weyl degeneracies in a magnetized semiconductor—InSb, which behaves as a magnetized plasma for electromagnetic waves at the terahertz band. By varying the magnetic field strength, Weyl points and the corresponding photonic Fermi arcs have been demonstrated. Our observation establishes magnetized semiconductors as a reconfigurable terahertz Weyl system, which may prompt research on novel magnetic topological phenomena such as chiral Majorana-type edge states and zero modes in classic systems.

Huifang Zhang, Ming Kang, Xueqian Zhang, Wengao Guo, Changgui Lv, Yanfeng Li, Weili Zhang, and Jiaguang Han

Abstract
Efficient control over the conversion of optical angular momentum from spin to orbital form in a metasurface system is achieved. Under coherent symmetric incidence, it can support nearly 100% conversion and unitary output, while it can support 50% conversion with 25% transmittance under one beam incidence.

Quan Xu, Xueqian Zhang, Yuehong Xu, Chunmei Ouyang, Zhen Tian, Jianqiang Gu, J. Li, S. Zhang, Jiaguang Han, and Weili Zhang

Abstract
The ability of generating arbitrary surface plasmon (SP) profiles in a controllable manner is of particular interest in designing plasmonic imaging, lithography and forcing devices. During the past decades, holography has gained enormous interest and achievements in free‐space three‐dimensional displays. Here, by applying a two‐dimensional version of holography, we experimentally demonstrate a generic method to control the SP profiles. Through controlling the orientation angles of two separated slits under circular polarization incidence, the amplitude and phase of the excited SPs can be freely manipulated, which allows direct generation of the desired SP profiles. A series of controllable SP holography schemes are theoretically and experimentally demonstrated, where the holographic SP profiles with high imaging quality can be dynamically modulated by varying the circular polarization handedness or orientation angle of linear polarization. The universality and simplicity of the proposed design strategies would offer promising opportunities for practical plasmonic applications.

Guosheng Shi, Yaru Dang, Tingting Pan, Xing Liu, Hui Liu, Shaoxian Li, Lijuan Zhang, Hongwei Zhao, Shaoping Li, Jiaguang Han, Renzhong Tai, Yiming Zhu, Jichen Li, Qing Ji, R. A. Mole, Dehong Yu, and Haiping Fang

Abstract
We experimentally observed considerable solubility of tryptophan (Trp) in a CuCl2 aqueous solution, which could reach 2–5 times the solubility of Trp in pure water. Theoretical studies show that the strong cation-π interaction between Cu2+ and the aromatic ring in Trp modifies the electronic distribution of the aromatic ring to enhance significantly the water affinity of Trp. Similar solubility enhancement has also been observed for other divalent transition-metal cations (e.g., Zn2+ and Ni2+), another aromatic amino acid (phenylalanine), and three aromatic peptides (Trp-Phe, Phe-Phe, and Trp-Ala-Phe).

Quan Li, Longqing Cong, Ranjan Singh, Ningning Xu, Wei Cao, Xueqian Zhang, Zhen Tian, Liangliang Du, Jiaguang Han, and Weili Zhang

Abstract
Recent advances in graphene photonics reveal promising applications in the technologically important terahertz spectrum, where graphene-based active terahertz metamaterial modulators have been experimentally demonstrated. However, the sensitivity of the atomically thin graphene monolayer towards sharp Fano resonant terahertz metasurfaces remains unexplored. Here, we demonstrate thin-film sensing of the graphene monolayer with a high quality factor terahertz Fano resonance in metasurfaces consisting of a two-dimensional array of asymmetric resonators. A drastic change in the transmission amplitude of the Fano resonance was observed due to strong interactions between the monolayer graphene and the tightly confined electric fields in the capacitive gaps of the Fano resonator. The deep-subwavelength sensing of the atomically thin monolayer graphene further highlights the extreme sensitivity of the resonant electric field excited at the dark Fano resonance, allowing the detection of an analyte that is λ/1 000 000 thinner than the free space wavelength.

Mitchell Kenney, Shaoxian Li, Xueqian Zhang, Xiaoqiang Su, Teun-Teun Kim, Dongyang Wang, Dongmin Wu, Chunmei Ouyang, Jiaguang Han, Weili Zhang, Hongbo Sun, and Shuang Zhang

Abstract
A dielectric metamaterial approach for achieving spin‐selective transmission of electromagnetic waves is proposed. The design is based on spin‐controlled constructive or destructive interference between propagating phase and Pancharatnam–Berry phase. The dielectric metamaterial, consisting of monolithic silicon herringbone structures, exhibits a broadband operation in the terahertz regime.

Shuo Liu, Tie Jun Cui, Quan Xu, Di Bao, Liangliang Du, Xiang Wan, Wen Xuan Tang, Chunmei Ouyang, Xiao Yang Zhou, Hao Yuan, Hui Feng Ma, Wei Xiang Jiang, Jiaguang Han, Weili Zhang, and Qiang Cheng

Abstract
Metamaterials based on effective media can be used to produce a number of unusual physical properties (for example, negative refraction and invisibility cloaking) because they can be tailored with effective medium parameters that do not occur in nature. Recently, the use of coding metamaterials has been suggested for the control of electromagnetic waves through the design of coding sequences using digital elements '0' and '1,' which possess opposite phase responses. Here we propose the concept of an anisotropic coding metamaterial in which the coding behaviors in different directions are dependent on the polarization status of the electromagnetic waves. We experimentally demonstrate an ultrathin and flexible polarization-controlled anisotropic coding metasurface that functions in the terahertz regime using specially designed coding elements. By encoding the elements with elaborately designed coding sequences (both 1-bit and 2-bit sequences), the x- and y-polarized waves can be anomalously reflected or independently diffused in three dimensions. The simulated far-field scattering patterns and near-field distributions are presented to illustrate the dual-functional performance of the encoded metasurface, and the results are consistent with the measured results. We further demonstrate the ability of the anisotropic coding metasurfaces to generate a beam splitter and realize simultaneous anomalous reflections and polarization conversions, thus providing powerful control of differently polarized electromagnetic waves. The proposed method enables versatile beam behaviors under orthogonal polarizations using a single metasurface and has the potential for use in the development of interesting terahertz devices.

Xueqian Zhang, Quan Xu, Quan Li, Yuehong Xu, Jianqiang Gu, Zhen Tian, Chunmei Ouyang, Yongmin Liu, Shuang Zhang, Xixiang Zhang, Jiaguang Han, and Weili Zhang

Abstract
Control over surface plasmons (SPs) is essential in a variety of cutting-edge applications, such as highly integrated photonic signal processing systems, deep-subwavelength lasing, high-resolution imaging, and ultrasensitive biomedical detection. Recently, asymmetric excitation of SPs has attracted enormous interest. In free space, the analog of electromagnetically induced transparency (EIT) in metamaterials has been widely investigated to uniquely manipulate the electromagnetic waves. In the near field, we show that the dark mode coupling mechanism of the classical EIT effect enables an exotic and straightforward excitation of SPs in a metasurface system. This leads to not only resonant excitation of asymmetric SPs but also controllable exotic SP focusing by the use of the Huygens-Fresnel principle. Our experimental findings manifest the potential of developing plasmonic metadevices with unique functionalities.

Xueqian Zhang, Yuehong Xu, Weisheng Yue, Zhen Tian, Jianqiang Gu, Yanfeng Li, Ranjan Singh, Shuang Zhang, Jiaguang Han, and Weili Zhang

Abstract
Anomalous launch of a surface wave with different handedness phase control is achieved in a terahertz metasurface based on phase discontinuities. The polarity of the phase profile of the surface waves is found to be strongly correlated to the polarization handedness, promising polarization‐controllable wavefront shaping, polarization sensing, and environmental refractive‐index sensing.

Longqing Cong, Ningning Xu, Jiaguang Han, Weili Zhang, and Ranjan Singh

Abstract
It is extremely challenging to control the phase of light at will in free space. Here, Pancharatnam–Berry‐phase‐enabled, tunable phase control of free‐space light is experimentally demonstrated in an ultrathin flexible dispersion‐free metadevice. This metadevice enables the broadband conversion of linearly polarized light into any desired output polarization.

Qiu Wang, Xueqian Zhang, Yuehong Xu, Zhen Tian, Jianqiang Gu, Weisheng Yue, Shuang Zhang, Jiaguang Han, and Weili Zhang

Abstract
A metasurface‐based terahertz flat‐lens array is proposed, comprising C‐shaped split‐ring resonators exhibiting locally engineerable phase discontinuities. Possessing a high numerical aperture, the planar lens array is flexible, robust, and shows excellent focusing characteristics in a broadband terahertz frequency. It could be an important step towards the development of planar terahertz focusing devices for practical applications.

Quan Li, Zhen Tian, Xueqian Zhang, Ranjan Singh, Liangliang Du, Jianqiang Gu, Jiaguang Han, and Weili Zhang

Abstract
Controlling the propagation properties of the terahertz waves in graphene holds great promise in enabling novel technologies for the convergence of electronics and photonics. A diode is a fundamental electronic device that allows the passage of current in just one direction based on the polarity of the applied voltage. With simultaneous optical and electrical excitations, we experimentally demonstrate an active diode for the terahertz waves consisting of a graphene-silicon hybrid film. The diode transmits terahertz waves when biased with a positive voltage while attenuates the wave under a low negative voltage, which can be seen as an analogue of an electronic semiconductor diode. Here, we obtain a large transmission modulation of 83% in the graphene-silicon hybrid film, which exhibits tremendous potential for applications in designing broadband terahertz modulators and switchable terahertz plasmonic and metamaterial devices.

Mark Lawrence, Ningning Xu, Xueqian Zhang, Longqing Cong, Jiaguang Han, Weili Zhang, and Shuang Zhang

Abstract
By utilizing the vector nature of light as well as the inherent anisotropy of artificial meta-atoms, we investigate parity time symmetry breaking in polarization space using a metasurface with anisotropic absorption, whose building blocks consist of two orthogonally orientated meta-atoms with the same resonant frequency but different loss coefficients. By varying their coupling strength, we directly observe a phase transition in the eigenpolarization states of the system, across which the long axis of the eigenpolarization ellipses experience a sudden rotation of 45°. Despite the lack of rotational symmetry of the metasurface, precisely at the phase transition, known as the exceptional point, the eigenmodes coalesce into a single circularly polarized state. The PT symmetric metasurfaces are experimentally implemented at terahertz frequencies.

Lixiang Liu, Xueqian Zhang, Mitchell Kenney, Xiaoqiang Su, Ningning Xu, Chunmei Ouyang, Yunlong Shi, Jiaguang Han, Weili Zhang, and Shuang Zhang

Abstract
By combining the freedom of both the structural design and the orientation of split ring resonator antennas, we demonstrate terahertz metasurfaces that are capable of controlling both the phase and amplitude profiles over a very broad bandwidth. As an example, we show that the phase‐amplitude metasurfaces can be engineered to control the diffraction orders arbitrarily.

Longqing Cong, Ningning Xu, Jianqiang Gu, Ranjan Singh, Jiaguang Han and Weili Zhang

Abstract
Metamaterials offer exciting opportunities that enable precise control of light propagation, its intensity and phase by designing an artificial medium of choice. Inducing birefringence via engineered metamolecules presents a fascinating mechanism to manipulate the phase of electromagnetic waves and facilitates the design of polarimetric devices. In this paper, a high‐efficiency, broadband, tunable and flexible quarter‐wave plate based on a multilayer metamaterial is presented. Excellent achromatic π/2 phase retardance with high transmission is observed upon terahertz propagation through the quarter‐wave plate. The calculated Stokes parameter represents the output polarization state numerically, indicating an excellent broadband conversion of linearly polarized light into circularly polarized light. The metamaterial‐based quarter‐wave plate demonstrated in this work could be an important step forward in the development of functional terahertz polarization conversion devices for practical applications.

Xueqian Zhang, Zhen Tian, Weisheng Yue, Jianqiang Gu, Shuang Zhang, Jiaguang Han, and Weili Zhang

Abstract
A terahertz metasurface is reported to exhibit broadband anomalous deflection with strong phase discontinuities. Weili Zhang, Jiaguang Han, Zhen Tian, and co-workers demonstrate on page 4567 that various frequency components ranging from 0.43 to 1.0 THz with polarization orthogonal to the incidence are deflected into a broad range of angles covering 25° to 84° under normal incidence. Based on this metasurface, a thin Fresnel zone plate was consequently developed that is capable of focusing terahertz radiation within a broad frequency range.

Zhihua Zhu, Xu Yang, Jianqiang Gu, Jun Jiang, Weisheng Yue, Zhen Tian, Masayoshi Tonouchi, Jiaguang Han, and Weili Zhang

Abstract
Plasmon induced transparency (PIT) could be realized in metamaterials via interference between different resonance modes. Within the sharp transparency window, the high dispersion of the medium may lead to remarkable slow light phenomena and an enhanced nonlinear effect. However, the transparency mode is normally localized in a narrow frequency band, which thus restricts many of its applications. Here we present the simulation, implementation, and measurement of a broadband PIT metamaterial functioning in the terahertz regime. By integrating four U-shape resonators around a central bar resonator, a broad transparency window across a frequency range greater than 0.40 THz is obtained, with a central resonance frequency located at 1.01 THz. Such PIT metamaterials are promising candidates for designing slow light devices, highly sensitive sensors, and nonlinear elements operating over a broad frequency range.

Jianqiang Gu, Ranjan Singh, Xiaojun Liu, Xueqian Zhang, Yingfang Ma, Shuang Zhang, Stefan A. Maier, Zhen Tian, Abul K. Azad, Hou-Tong Chen, Antoinette J. Taylor, Jiaguang Han, and Weili Zhang

Abstract
Recently reported metamaterial analogues of electromagnetically induced transparency enable a unique route to endow classical optical structures with aspects of quantum optical systems. This method opens up many fascinating prospects on novel optical components, such as slow light units, highly sensitive sensors and nonlinear devices. In particular, optical control of electromagnetically induced transparency in metamaterials promises essential application opportunities in optical networks and terahertz communications. Here we present active optical control of metamaterial-induced transparency through active tuning of the dark mode. By integrating photoconductive silicon into the metamaterial unit cell, a giant switching of the transparency window occurs under excitation of ultrafast optical pulses, allowing for an optically tunable group delay of the terahertz light. This work opens up the possibility for designing novel chip-scale ultrafast devices that would find utility in optical buffering and terahertz active filtering.

Dachuan Liang, Jianqiang Gu, Jiaguang Han, Yuanmu Yang, Shuang Zhang, and Weili Zhang

Abstract
A large scale homogenous invisibility cloak functioning at terahertz frequencies is reported. The terahertz invisibility device features a large concealed volume, low loss, and broad bandwidth. In particular, it is capable of hiding objects with a dimension nearly an order of magnitude larger than that of its lithographic counterpart, but without involving complex and time‐consuming cleanroom processing.