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.

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.

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 $\ge 90\%$ ranges from 1.54 to 2.23 THz, with the relative bandwidth (RBW) reaching about 36.6%. Although the measured RBW (from $\sim 12\%$ to $\sim 14\%$ and then to $\sim 8\%$) 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.

We propose a broadband reflective linear-to-circular (LTC) polarization converter in a mid-infrared regime based on monolayer black phosphorus (phosphorene) metamaterial. The proposed converter consists of periodic unit cells, each cell of which is formed by multiple phosphorene layers, a dielectric layer, and a fully reflective gold mirror. In the frequency range of 14.20 and 23.10 THz, the magnitudes of the reflection coefficients are approximately equal and the phase difference value between the two orthogonal electric field components of the reflected wave is close to π/2 or −3π/2, which indicates the reflective wave can be deemed the circular polarization with the incidence of linearly polarized wave. The simulation results show that the relative bandwidth of LTC polarization conversion reaches 58% and 47.8%, respectively, when the pucker ridge alignment of the phosphorene is perpendicular to the y-direction and x-direction. Finally, the physical mechanism is revealed via the field decomposition and current distribution. This polarization converter has great potential for future applications in electronic measurement, photonic design, and in some other optoelectronic systems.

We propose a tunable terahertz (THz) absorber design that utilizes a bulk Dirac semimetal (BDS)-based metasurface. The absorber consists of a BDS film with a periodic ring slot, dielectric layer, and fully reflective gold mirror. Simulations indicate that localized surface plasmon resonance is excited within certain regions close to the ring slot, leading to dipolar plasmon resonance of periodic arrays along the polarization direction, thereby producing resonant absorption. The BDS conductivity scales linearly with the Fermi level that can be tuned by bias application, enhancing the electromagnetic-field concentration and shrinking the effective current path, thereby blue-shifting the operating frequency within an ultrawideband. The thickness of the proposed design is significantly less than the resonant wavelength; therefore, the design yields an ultrathin metasurface absorber. This paper may lead to the investigations of the tunable metasurface device based on the BDS.