Abstract-Terahertz Massive MIMO with Holographic Reconfigurable Intelligent Surfaces

                                          

Ziwei Wan, Zhen Gao, Marco Di Renzo, Mohamed-Slim Alouini

https://arxiv.org/abs/2009.10963

We propose a holographic version of a reconfigurable intelligent surface (RIS) and investigate its application to terahertz (THz) massive multiple-input multiple-output systems. Capitalizing on the miniaturization of THz electronic components, RISs can be implemented by densely packing subwavelength unit cells, so as to realize continuous or quasi-continuous apertures and to enable holographic communications. In this paper, in particular, we derive the beam pattern of a holographic RIS. Our analysis reveals that the beam pattern of an ideal holographic RIS can be well approximated by that of an ultra-dense RIS, which has a more practical hardware architecture. In addition, we propose a closedloop channel estimation (CE) scheme to effectively estimate the broadband channels that characterize THz massive MIMO systems aided by holographic RISs. The proposed CE scheme includes a downlink coarse CE stage and an uplink finer-grained CE stage. The uplink pilot signals are judiciously designed for obtaining good CE performance. Moreover, to reduce the pilot overhead, we introduce a compressive sensing-based CE algorithm, which exploits the dual sparsity of THz MIMO channels in both the angular and delay domain. Simulation results demonstrate the superiority of holographic RISs over the nonholographic ones, and the effectiveness of the proposed CE scheme.

Abstract-Realization of a three-dimensional photonic topological insulator

Yihao Yang, Zhen Gao, Haoran Xue, Li Zhang, Mengjia He, Zhaoju Yang, Ranjan Singh, Yidong Chong, Baile Zhang, Hongsheng Chen

Fig. 2: Sample, experimental setup and measured bulk dispersion of the 3D photonic topological insulator.

https://www.nature.com/articles/s41586-018-0829-0

Confining photons in a finite volume is highly desirable in modern photonic devices, such as waveguides, lasers and cavities. Decades ago, this motivated the study and application of photonic crystals, which have a photonic bandgap that forbids light propagation in all directions. Recently, inspired by the discoveries of topological insulators, the confinement of photons with topological protection has been demonstrated in two-dimensional (2D) photonic structures known as photonic topological insulators, with promising applications in topological lasers and robust optical delay lines. However, a fully three-dimensional (3D) topological photonic bandgap has not been achieved. Here we experimentally demonstrate a 3D photonic topological insulator with an extremely wide (more than 25 per cent bandwidth) 3D topological bandgap. The composite material (metallic patterns on printed circuit boards) consists of split-ring resonators (classical electromagnetic artificial atoms) with strong magneto-electric coupling and behaves like a ‘weak’ topological insulator (that is, with an even number of surface Dirac cones), or a stack of 2D quantum spin Hall insulators. Using direct field measurements, we map out both the gapped bulk band structure and the Dirac-like dispersion of the photonic surface states, and demonstrate robust photonic propagation along a non-planar surface. Our work extends the family of 3D topological insulators from fermions to bosons and paves the way for applications in topological photonic cavities, circuits and lasers in 3D geometries.

Letter-Terahertz transparency of optically opaque metallic films

Zhengyong Song1, Zhen Gao1, Youming Zhang1 and Baile Zhang1,2

Hide affiliations

zhysong@ntu.edu.sg blzhang@ntu.edu.sg

1 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University - 21 Nanyang Link, Singapore 637371, Singapore
2 Centre for Disruptive Photonic Technologies, Nanyang Technological University - 21 Nanyang Link, Singapore 637371, Singapore 

http://iopscience.iop.org/0295-5075/106/2/27005

Letter

Here we present an alternative approach to design a freestanding transparent conducting device for wide-angle and polarization-insensitive incidence of electromagnetic waves at terahertz frequencies. It is realized by depositing periodic metallic patches on top and bottom of the subwavelength metallic mesh. Based on the numerical computations, the deposited metallic patches can suppress the reflection and enhance the transmission. The high transmission of the designed system is attributed to the impedance matching to the vacuum. This design of a transparent conducting device can be useful in applications, such as optoelectronic electrodes and micro-electronic displays, where both high electrical conductivity and high optical transmittance are desirable.

Abstract-Terahertz spoof plasmonic coaxial microcavity

Zaihe Yu, Zhuoyuan Wang, Zhen Gao, and Zhengyong Song

http://www.opticsinfobase.org/ao/upcoming_pdf.cfm?id=201245

Abstract: We theoretically demonstrate a subwavelength surface-plasmon-polariton (SPP) microcavity on a planar metallic surface working at terahertz regime with both high quality factor and ultra-small mode volume. The microcavity is based on plasmonic and metamaterial notions, and it consists of an easy-to-manufacture circular aperture and a bell-shaped metallic core. It is shown that such a structure can sustain SPP eigenmodes whose fields are tightly trapped within the microcavity. Using the proposed structure, total Q factor of 1000 (including losses from metals at low temperatures) and subwavelength mode volume of 0.00018(λ/2n)3 can be achieved in the THz range for fundamental surface-plasmonic eigenmodes at room temperature. Moreover, the key figures of merit such as resonance frequency can be flexibly tuned by modifying the geometry of the microcavity, making it attractive for broad applications in filters, light sources, energy storages and on-chip optical communications.