Metasurfaces, the two-dimensional counterpart of metamaterials, have caught great attention thanks to their powerful capabilities on manipulation of electromagnetic waves. Recent times have seen the emergence of a variety of metasurfaces exhibiting not only countless functionalities, but also a reconfigurable response. Additionally, digital or coding metasurfaces have revolutionized the field by describing the device as a matrix of discrete building block states, thus drawing clear parallelisms with information theory and opening new ways to model, compose, and (re)program advanced metasurfaces. This paper joins the reconfigurable and digital approaches, and presents a metasurface that leverages the tunability of graphene to perform beam steering at terahertz frequencies. A comprehensive design methodology is presented encompassing technological, unit cell design, digital metamaterial synthesis, and programmability aspects. By setting up and dynamically adjusting a phase gradient along the metasurface plane, the resulting device achieves beam steering at all practical directions. The proposed design is studied through analytical models and validated numerically, showing beam widths and steering errors well below 10 o and 5% in most cases. Finally, design guidelines are extracted through a scalability analysis involving the metasurface size and number of unit cell states.

Recent emergence of metasurfaces has enabled the development of ultra-thin flat optical components through different wavefront shaping techniques at various wavelengths. However, due to the non-adaptive nature of conventional metasurfaces, the focal point of the resulting optics needs to be fixed at the design stage, thus severely limiting its reconfigurability and applicability. In this paper, we aim to overcome such constraint by presenting a flat reflective component that can be reprogrammed to focus terahertz waves at a desired point in the near-field region. To this end, we first propose a graphene-based unit cell with phase reconfigurability, and then employ the coding metasurface approach to draw the phase profile required to set the focus on the target point. Our results show that the proposed component can operate close to the diffraction limit with high focusing range and low focusing error. We also demonstrate that, through appropriate automation, the reprogrammability of the metamirror could be leveraged to develop compact terahertz scanning and imaging systems, as well as novel reconfigurable components for terahertz wireless communications.

Digital metasurfaces have opened unprecedented ways to accomplish novel electromagnetic devices thanks to their simple manipulation of electromagnetic waves. However, the metasurfaces leveraging phase-only or amplitude-only modulation restricted the full-functionality control of the devices. Herein, a digital graphene-based metasurfaces engineering wavefront amplitude and phase are proposed for the first time to tackle this challenge in the terahertz (THz) band. The concept and its significance are verified using reprogrammable multi-focal meta-lens based on a 2/2-bit digital unit cell with independent control of 2-bit states of amplitude and phase individually. Moreover, we introduce a novel method to directly transmit digital information over multiple channels via the reprogrammable digital metasurface. Since these metasurfaces are composed of digital building blocks, the digital information can be directly modulated to the metasurface by selecting specific digital sequences and sent them to predetermined receivers distributed in the focal points. Following that, a multi-channel THz high-speed communication system and its application to build three-dimensional wireless agile interconnection are demonstrated. The presented method provides a new architecture for wireless communications without using complicated components of conventional systems. This work motivates versatile meta-devices in many applications envisioned for the THz frequencies, which will play a vital role in modern communications.

Graphene plasmonic antennas possess two significant features that render them appealing for short-range wireless communications, notably, inherent tunability and miniaturization due to the unique frequency dispersion of graphene and its support for surface plasmon waves in the terahertz band. In this letter, dipole-like antennas using few-layer graphene are proposed to achieve a better trade-off between miniaturization and radiation efficiency than current monolayer graphene antennas. The characteristics of few-layer graphene antennas are evaluated and then compared with those of antennas based on monolayer graphene and graphene stacks, which could also provide such improvements. To this end, first, the propagation properties of one-dimensional and two-dimensional plasmonic waveguides based on the aforementioned graphene structures are obtained by transfer matrix theory and finite-element simulation, respectively. Second, the antennas are investigated as three-dimensional structures using a full-wave solver. Results show that the highest radiation efficiency among the compared designs is achieved with the few-layer graphene, while the highest miniaturization is obtained with the even mode of the graphene stack antenna.

This paper proposes an ultra-compact Near-field Focusing (NFF) setup at 60 GHz. The proposed configuration is included a planar substrate integrated waveguide (SIW) slot array as a feeder for a three-layer transmissive coded metasurface lens. A comprehensive design methodology is presented, encompassing unit cell design, coded metasurface lens synthesis, and planar slot array design. The transmissive metasurface lens performance is studied and validated by numerical and analytical approaches. Then, the planar slot array design considerations are elaborated, investigating the amplitude and phase of the resulting waves to ensure that quasi-plane waves are produced in the near-field region. Finally, the slot array is employed to illuminate the designed metasurface where the distance between the feeder and metasurface lens is 2 mm (0.4λ), which shows the integrability and being packed of the proposed setup, contrary to the conventional metasurface-based NFF structures. There are fair agreements between analytical, numerical, and measurement methods to verify the presented approach. It turns out that the proposed device can focus waves close to the diffraction limit, which results in a high-resolution efficiency.

Engineered electromagnetic surfaces enable enhancing the strength of a signal in the desired direction(s) using anomalous reflection/transmission. However, to boost the gain of the conventional surfaces, the common solution is to increase its aperture size where this method limits the gain performance for limited space in scenarios for large-distance communication. This paper proposes a gain-controlled beam-steering polarization-engineered transmissive surface to tackle this challenge. To implement such a functionality, a transmissive unit cell properly integrated with transistors and phase shifters is introduced with the ability of simultaneous manipulation of phase, amplitude (reduction and amplification), and polarization. Then, a supercell including the array of unit cells with desired linear phase gradient is designed and analyzed using Floquet approach to tilt the transmitted beam. Finally, to verify the proposed idea, a surface including 12×12 unit cells is designed, fabricated and verified. It is demonstrated that a maximum gain of 25 dB compared to air aperture and 11 dB compared to the passive one are achieved by assuming a constant size at 5 GHz. Based on this gain improvement, the active surface’s aperture efficiency is 12.58 times greater than that of the passive surface. Moreover, the surface gain can be reconfigured by the amplification level of the transistors, providing efficient dynamic way for changing the gain instead of the static resizing of aperture.