The polarization conversion of electromagnetic (EM) waves, especially linear-to-circular (LTC) polarization conversion, is of great significance in practical applications. In this study, we propose an ultra-wideband high-efficiency reflective LTC polarization converter based on a metasurface in the terahertz regime. It consists of periodic unit cells, each cell of which is formed by a double split resonant square ring, dielectric layer, and fully reflective gold mirror. In the frequency range of 0.60 - 1.41 THz, the magnitudes of the reflection coefficients reach approximately 0.7, and the phase difference between the two orthogonal electric field components of the reflected wave is close to 90° or -270°. The results indicate that the relative bandwidth reaches 80% and the efficiency is greater than 88%, thus, ultra-wideband high-efficiency LTC polarization conversion has been realized. Finally, the physical mechanism of the polarization conversion is revealed. This converter has potential applications in antenna design, EM measurement, and stealth technology.

As a direct band gap two-dimensional (2D) semiconductor material, black phosphorus (BP) bridges the characteristics of graphene, with a zero or near-zero band gap, and transition metal dichalcogenides, with a wide band gap. In the infrared (IR) regime, 2D BP materials can harvest electromagnetic energy due to losses derived from its surface conductivity. In this paper, we propose an IR absorber design comprising 2D BP metamaterials sandwiched between dielectric layers. The multilayered sandwich-like absorber structure is mounted on a full reflective gold mirror, which forms a Fabry-Perot resonator to strengthen light-matter interactions. Harvested surface plasmons are excited around the 2D BP metamaterial edges, and the incident IR light can be efficiently dissipated by increasing the number of layers of the sandwich-like structure (NLSS). The physical absorption mechanism can be attributed to the destructive interference from the metamaterials, which can be enhanced with increasing NLSS. Here, a phase difference of about 180° is obtained between the directly reflected wave from the first interface and the emergent wave derived from the superposition of the multiple reflections among the resonator, and the amplitude of the emergent wave is steadily reduced to a value close to that of the directly reflected wave with increasing NLSS for incident transverse-magnetic polarized IR illumination.

A broadband tunable metamaterial graphene absorber is investigated in this paper. The unit cell of the proposed metamaterial graphene absorber is composed of four patch resonators. By tuning the chemical potential of graphene and the geometric size of each patch, the simulated total reflectivity is less than -10 dB from 22.02 to 36.61 THz and with the total thickness of 0.76 um (only 0.09λ at the lowest frequency). The analysis of the surface current, magnetic field and power flow distributions has been performed to better understand the absorption mechanism. Moreover, this proposed absorber achieves its bandwidth tunable characteristics through a voltage biasing of the graphene's Fremi level. This proposed metamaterial graphene absorber (MGA) could be used as smart absorbers, photovoltaic devices and tunable sensors.

Modern terahertz (THz) technology offers the advantage of enhanced target detection ability with high spatial and temporal resolutions in the THz band, which makes it a formidable threat to stealth targets. Consequently, THz absorbers have outstanding potential as an electromagnetic countermeasure. In this Letter, we design, fabricate, and characterize a THz absorber based on patterned graphene. We present the transfer, photolithography, and etching processes involved in graphene patterning, as well as the experimental measurements of the fabricated absorber. Our simulations show that with an increase in the Fermi energy, the performance of the designed absorber gradually improves and, finally, decreases slightly. Further, the absorption bandwidth first broadens and then narrows slightly. The effective bandwidth with absorption ≥90% ranges from 1.54 to 2.23 THz, with the relative bandwidth (RBW) reaching about 36.6%. Although the measured RBW (from ∼12% to ∼14% and then to ∼8%) slightly deviates from the simulated one, the position of the resonant frequency is well matched between theory and experiment. Moreover, we illuminate the absorption mechanism using the theory of destructive interference. This Letter can significantly contribute to the design, manufacture, and application of patterned graphene-based THz absorbers.

Two-dimensional (2D) black phosphorus (BP) with direct band gap, bridges the characteristics of graphene with a zero or near-zero band gap and transition metal dichalcogenides with a wide band gap. In the infrared (IR) regime, 2D BP materials can attenuate electromagnetic energy due to losses derived from its surface conductivity. This paper proposes an IR absorber based on 2D BP metamaterials. It consists of multi-layer BP-based nano-ribbon pairs, each formed by two orthogonally stacked nano-ribbons. The multi-layer BP metamaterials and bottom gold mirror together form a Fabry-Perot resonator that could completely inhibit light transmission to create strong absorption through the BP metamaterials. Unlike previously reported BP metamaterial absorbers, this new structure can operate at two frequency bands with absorption > 90% in each owning to the first-order and second-order Fabry-Perot resonant frequencies. It is also polarization independent due to the fourfold rotational structural symmetry. To our best knowledge, this is the first report on using BP metamaterials in an absorber that operates independent of polarization and in dual bands.

Abstract-In the application of two-dimension (2D) finite-difference time-domain (FDTD) to scattering analysis of object embedded in layered media, the incident electromagnetic wave propagation is much more complicated, it can not inject the plane wave source by traditional method. To solve this problem, the Π-shape total-field/scatteringfield (TF-SF) boundary scheme is presented.The side TF-SF boundaries are governed by the modified 1D Maxwell's equations, but the discretization for which to p-wave is more difficult than nwave. Then an auxiliary magnetic variable is used, which can develop the modified 1D-FDTD to p-wave without any approximately. To truncate the modified 1D-FDTD, the convolutional perfectly matched layer (CPML) absorbing boundary condition (ABC) is also given. Examples show the feasibility and applicability of proposed Π-shape TF/SF boundaries scheme.

We present a broadband tunable circular polarization converter composed of a single graphene sheet patterned with butterfly-shaped holes, a dielectric spacer, and a 7-layer graphene ground plane. It can convert a linearly polarized wave into a circularly polarized wave in reflection mode. The polarization converter can be dynamically tuned by varying the Fermi energy of the single graphene sheet. Furthermore, the 7-layer graphene acting as a ground plane can modulate the phase of its reflected wave by controlling the Femi energy, which provides constructive interference condition at the surface of the single graphene sheet in a broad bandwidth and therefore significantly broadens the tunable bandwidth of the proposed polarization converter.

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