Reprogrammable Graphene-based Metasurface Mirror with Adaptive Focal Point for THz Imaging

Abstract

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.

Error Analysis of Programmable Metasurfaces for Beam Steering

“…With the introduction of tunable or switchable elements, metasurfaces can adapt to different environments or take multiple functionalities [12], [16], [41], [42]. Tuning has been demonstrated in several forms, including thermal [43], electrical [10], [44], and optical tuning [45]; or the use pin diodes [46], varactors [47], memristors [48], and microelectromechanical systems (MEMS) [49].…”

Toward Intelligent Metasurfaces: The Progress from Globally Tunable Metasurfaces to Software‐Defined Metasurfaces with an Embedded Network of Controllers

“…[128] This concept of tunable/programmable coding metasurfaces has been exemplified with several works in a variety of functionalities, as summarized in Table 3. Most papers make use of diodes for operation in the GHz range [150] ; however, demonstrations at higher frequencies are also appearing, such as in the mmWave band based on liquid crystals, [151] in the THz band based on graphene [152][153][154] and in the NIR based on indium tin oxide (ITO). [155] Note that the number of experimental works showcasing this approach is smaller than for static designs due to a number of manufacturing challenges.…”

“…The approach relying on integrating diodes cannot be scaled well beyond mmWave frequencies, where liquid crystals have been proposed instead. [151] In the THz band, graphene can enable the programmable approach, [152][153][154] but practical local biasing methods for micrometer-sized graphene strips remains a challenge. Even at microwave frequencies, metasurfaces employing PIN diodes need to take into account the parasitics introduced by the package and soldering processes, which change the effective circuit models.…”

A Review on the Development of Tunable Graphene Nanoantennas for Terahertz Optoelectronic and Plasmonic Applications

“…By keeping the relaxation time at τ = 1 ps and chemical potential at μ = 0.2 eV, the radiation patterns at the resonance frequency remain stable, while significant improvement can be seen in radiation efficiency because of graphene as the radiator. Hossein et al proposed a graphene-metal hybrid antenna by employing waveguide feed [103], as depicted in Figure 4a. The authors employed the graphene-metal waveguide structure, which can support TEM mode, thus the transmission line model is adopted for feeding the antenna through a waveguide.…”

“…The authors analyzed several nanoantenna designs with numerical simulations and coupling of metal plasmons to graphene plasmons, as well as studied the effect of different graphene waveguide structure along with metal antennas for the efficient propagation of graphene plasmons [125]. Moreover, in comparison with a dipole antenna, the Yagi-Uda antenna [103] performs better with stronger coupling with graphene plasmons, which allows for more directive propagation. Strong plasmons are activated by dipole antenna in tapered graphene waveguides by the constructive interference of plasmon reflections, generated at the edges of the design structure.…”