Abstract-Spatiotemporal Dielectric Metasurfaces for Unidirectional Propagation and Reconfigurable Steering of Terahertz Beams

Longqing Cong,  Ranjan Singh

https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.202001418

Next‐generation devices for low‐latency and seamless communication are envisioned to revolutionize information processing, which would directly impact human lives, technologies, and societies. The ever‐increasing demand for wireless data traffic can be fulfilled by the terahertz band, which has received tremendous attention as the final frontier of the radio spectrum. However, attenuation due to atmospheric humidity and free‐space path loss significantly limits terahertz signal propagation. High‐gain antennas with directional radiation and reconfigurable beam steering are indispensable for loss compensation and terahertz signal processing, which are associated with spatial and temporal dimensions, respectively. Here, experimental demonstration of a spatiotemporal dielectric metasurface for unidirectional propagation and ultrafast spatial beam steering of terahertz waves is shown. The spatial dimension of the metasurface provides a solution to eliminate backscattering of collimated unidirectional propagation of the terahertz wave with steerable directionality. Temporal modulation of the spatial optical properties enables ultrafast reconfigurable beam steering. Silicon‐based spatiotemporal devices amalgamate the rich physics of metasurfaces and technologies that are promising for overcoming the bottlenecks of future terahertz communication, such as high‐speed and secure wireless data transmission, beamforming and ultrafast data processing.

Abstract-All‐Dielectric Active Terahertz Photonics Driven by Bound States in the Continuum

Song Han, Longqing Cong,  Yogesh Kumar Srivastava, Bo Qiang, Mikhail V. Rybin, Abhishek Kumar Ravikumar Jain,  Wen Xiang Lim, Venu Gopal Achanta,   Shriganesh S. Prabhu, Qi Jie Wang, Yuri S. Kivshar, Ranjan Singh,


https://onlinelibrary.wiley.com/doi/10.1002/adma.201901921

The remarkable emergence of all‐dielectric meta‐photonics governed by the physics of high‐index dielectric materials offers a low‐loss platform for efficient manipulation and subwavelength control of electromagnetic waves from microwaves to visible frequencies. Dielectric metasurfaces can focus electromagnetic waves, generate structured beams and vortices, enhance local fields for advanced sensing, and provide novel functionalities for classical and quantum technologies. Recent advances in meta‐photonics are associated with the exploration of exotic electromagnetic modes called the bound states in the continuum (BICs), which offer a simple interference mechanism to achieve large quality factors (Q) through excitation of supercavity modes in dielectric nanostructures and resonant metasurfaces. Here, a BIC‐driven terahertz metasurface with dynamic control of high‐Q silicon supercavities that are reconfigurable at a nanosecond timescale is experimentally demonstrated. It is revealed that such supercavities enable low‐power, optically induced terahertz switching and modulation of sharp resonances for potential applications in lasing, mode multiplexing, and biosensing.

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-Spin Induced Toroidal Dipole in Terahertz Metasurfaces

Longqing Cong, Yogesh K. Srivastava, and Ranjan Singh

https://www.osapublishing.org/abstract.cfm?uri=cleo_qels-2017-JTu5A.37&origin=search

We investigate the dominant role of toroidal dipole in modulating the Fano resonance in planar terahertz metasurfaces by reducing the radiative loss from electric dipole.

© 2017 OSA

Abstract-Active Phase Transition via Loss Engineering in a Terahertz MEMS Metamaterial

Longqing Cong, Prakash Pitchappa, Chengkuo Lee, Ranjan Singh

http://onlinelibrary.wiley.com/doi/10.1002/adma.201700733/abstract

Controlling the phase of local radiation by using exotic metasurfaces has enabled promising applications in a diversified set of electromagnetic wave manipulation such as anomalous wavefront deflection, flat lenses, and holograms. Here, we theoretically and experimentally demonstrate an active phase transition in a micro-electromechanical system-based metadevice where both the phase response and the dispersion of the metamaterial cavity are dynamically tailored. The phase transition is determined by the radiative and the absorptive losses in a metal–insulator–metal cavity that obeys the coupled-mode theory. The complete understanding of the phase diagram in a reconfigurable configuration would open up avenues for designing multifunctional metadevices that can be actively switched between different phases leading to a plethora of applications in polarization control, beam deflectors, and holographic metamaterials.

Abstract-Sharp Toroidal Resonances in Planar Terahertz Metasurfaces

http://onlinelibrary.wiley.com/doi/10.1002/adma.201601611/abstract

A toroidal dipole in metasurfaces provides an alternate approach for the excitation of high-Qresonances. In contrast to conventional multipoles, toroidal dipole interaction strength depends on the time derivative of the surrounding electric field. A characteristic feature of a toroidal dipole is tightly confined loops of oscillating magnetic field that curl around the fictitious arrow of a toroidal dipole vector.