Broadband and switchable terahertz polarization converter based on graphene metasurfaces

Abstract

In this work, we propose broadband and switchable terahertz (THz) polarization converters based on either graphene patch metasurface (GPMS) or its complementary structure (graphene hole metasurface, GHMS). The patch and hole are simply cross-shaped, composed of two orthogonal arms, along which plasmonic resonances mediated by Fabry-Perot cavity play a key role in polarization conversion (PC). An incidence of linear polarization will be converted to its cross-polarization (LTL) or circular polarization (LTC), as the reflected wave in the direction of two arms owning the same amplitude and π phase difference (LTL), or ±π/2 phase difference (LTC). Such requirements can be met by optimizing the width and length of two arms, thickness of dielectric layer, and Fermi level EF of graphene. By using GPMS, LTL PC of polarization conversion ratio (PCR) over 90% is achieved in the frequency range of 2.92 THz to 6.26 THz, and by using GHMS, LTC PC of ellipticity χ ≤ −0.9 at the frequencies from 4.45 THz to 6.47 THz. By varying the Fermi level, the operating frequency can be actively tuned, and the functionality can be switched without structural modulation; for instance, GPMS supports LTL PC as EF = 0.6 eV and LTC PC of χ ≥ 0.9 as EF = 1.0 eV, in the frequency range of 2.69 THz to 4.19 THz. Moreover, GHMS can be optimized to sustain LTL PC and LTC PC of |χ| ≥ 0.9, in the frequency range of 4.96 THz to 6.52 THz, which indicates that the handedness of circular polarization can be further specified. The proposed polarization converters of broad bandwidth, active tunability, and switchable functionality will essentially make a significant progress in THz technology and device applications, and can be widely utilized in THz communications, sensing and spectroscopy.

The method of lines extension for the analysis of multilayered graphene-loaded structures in cylindrical coordinates

“…New nano-sized electronic and electromagnetic components and devices such as antennas, flexible electronic devices, touch screens and high-speed transistors are examples of the states of the art in the graphene world. The unique properties of graphene are due to its gapless electronic band structure 7 – 10 .…”

A Review: The Functional Materials-Assisted Terahertz Metamaterial Absorbers and Polarization Converters

“…To achieve multi-functional metamaterial structures, most of the previously reported graphene-based metamaterial structures are used to achieve the LTC and LTL functions simultaneously [105][106][107][108][109][110][111][112][113][114]. These metamaterial structures can realize multi-frequency or broad-band LTL and LTC polarization conversion in a single metamaterial device.…”

“…For single-layer discrete graphene patterns, many classical structures, including graphene patches [118], cross double-ellipse graphene patch [107], cross-shaped graphene patch [109], graphene ribbons [110,115], graphene disk with a hollow rectangle [108], graphene split-ring resonator [117], and so on, are used to achieve the polarization conversion function. In this case, Z. Liu et al demonstrated that a layer of ion-gel lies on the top of graphene layer to serve as a transparent spacer layer, forming high capacitive properties between the top metal gate contacts and the underlying layer of the graphene microstructures [117].…”

“…Switching of the two functions is caused by the obvious dispersion of the orthogonal eigenmodes and the overlap of graphene and dielectric resonant modes. In addition, some continuous graphene layers, including continuous graphene microstructures (π-shaped hollows [113], cross-shaped graphene patch [109], reflective etched L-shaped structures on graphene [105], and so on) and nonstructured graphene films [106,114], are used to achieve the polarization conversion functions. These structures can effectively reduce the difficulty of manufacturing and electric tuning to a certain extent.…”

Broadband Graphene-Based Electro-Optic Chiral Polarization Conversion for Terahertz Pulse Shaping

“…Despite important advances in the exploration of the active control of polarization, electrical control of THZ chiral polarization remains largely elusive and suffers from a limited dynamic range of polarization conversion and limited response frequency bandwidth. − Because of the highly tunable conductivity in the THz region, two-dimensional (2D) materials (e.g., graphene and black phosphorus) show great potential in THz modulation. − Combining graphene with metamaterials/metasurface, electrical modulation of the polarization state of THz beams has been widely demonstrated. − However, the working frequency is limited at a certain narrowband because of the resonance nature of metamaterials.…”