Metasurface-based perfect vortex beams with trigonometric-function topological charge for OAM manipulation.

Topological charge (TC) is generally acknowledged as an important attribute of an optical vortex (OV), which indicates the twisted characterization of the wavefront. In most circumstances, the TC remains constant as an integer or fraction along the azimuthal direction. Herein, by transforming the TCs into the trigonometric functions of the azimuthal angle to tailor the spiral phase distributions, we numerically demonstrate generating perfect vortex beams (PVBs) with sine-function TC based on the all-dielectric geometric metasurfaces, whose unit structure is optimized to an ideal half-wave plate. To seek the intrinsic advancements of the proposed PVBs, their orbital angular momentum (OAM) as well as optical gradient force distributions are calculated for diverse particle manipulation. We believe our proposed scheme is desired to provide an original thought for OAM manipulation, information storage, and optical communication.

Nanoscale Piezoelectric Properties and Phase Separation in Pure and La-Doped BiFeO3 Films Prepared by Sol-Gel Method.

Pure BiFeO3 (BFO) and doped Bi0.9La0.1FeO3 (BLFO) thin films were prepared on Pt/TiO2/SiO2/Si substrates by a modified sol-gel technique using a separate hydrolysis procedure. The effects of final crystallization temperature and La doping on the phase structure, film morphology, and nanoscale piezoelectric properties were investigated. La doping and higher crystallization temperature lead to an increase in the grain size and preferred (102) texture of the films. Simultaneously, a decrease in the average effective piezoelectric coefficient (about 2 times in La-doped films) and an increase in the area of surface non-polar phase (up to 60%) are observed. Phase separation on the films' surface is attributed to either a second phase or to a non-polar perovskite phase at the surface. As compared with undoped BFO, La-doping leads to an increase in the average grain size and self-polarization that is important for future piezoelectric applications. It is shown that piezoelectric activity is directly related to the films' microstructructure, thus emphasizing the role of annealing conditions and La-doping that is frequently used to decrease the leakage current in BFO-based materials.

Inversion Method Characterization of Graphene-Based Coordination Absorbers Incorporating Periodically Patterned Metal Ring Metasurfaces.

In this paper, we propose a tunable coordinated multi-band absorber that combines graphene with metal-dielectric-metal structures for the realization of multiple toward perfect absorption. The parametric inversion method is used to extract the equivalent impedance and explain the phenomena of multiple-peak absorption. With the change of the Fermi level, equivalent impedances were extracted, and the peculiarities of the individual multiple absorption peaks to change were determined. By changing the structure parameters of gold rings, we obtain either multiple narrow-band absorption peaks or a broadband absorption peak, with the bandwidth of 0.8 µm where the absorptance is near 100%. Therefore, our results provide new insights into the development of tunable multi-band absorbers and broadband absorbers that can be applied to terahertz imaging in high-performance coordinate sensors and other promising optoelectronic devices.

Coordinated multi-band angle insensitive selection absorber based on graphene metamaterials.

In this paper, we propose a tunable, multi-band, selective absorber composed of multiple layers. Each layer consisted of SiO2/graphene/SiC, and a layer of silver was used as the ground plane of the entire structure. Simulation results show that we can passively and actively coordinate the resonant frequency of the perfect absorption peak by changing the geometric parameters of the array and the Fermi level of the graphene. The absorber is not sensitive to the angle of incidence and the direction of polarization. We propose a theoretical basis for the formation of multiple absorption peaks. The theoretical calculations are in good agreement with the simulation results. In addition, we simulated the three- and four-layer structures. The results show that in the terahertz (THz) band, composite structures of three and four layers can obtain three and four perfect absorption peaks, respectively. Our results provide new insights into the THz band of harmonizable multi-band absorbers that can be applied to THz imaging to coordinate sensors and other optoelectronic devices.

Independent tunable multi-band absorbers based on molybdenum disulfide metasurfaces.

In this paper, we theoretically and numerically demonstrate a dual-band independently adjustable absorber comprising an array of stacked molybdenum disulfide (MoS2) coaxial nanodisks and a gold reflector that are separated by two dielectric insulating layers. The array plane functionality is explained by the dipole resonances with the MoS2 nanodisks. As a result, strong absorption is achieved at a wide range of incident angles under TE and TM polarizations. The structural parameters of the entire array and the carrier concentration in the MoS2 layers were varied to get the optimized absorption. The absorptance positioning can be adjusted by scaling the diameters of the MoS2 disks. We also proposed the array modification where nanodisks are replaced by a layer with nanoholes. The position of both absorptance peaks can be adjusted individually by changing the carrier concentration in the array. This structure can be useful for the design of chemical sensors, detectors or multi-band absorbers.

Ground-plane-less bidirectional terahertz absorber based on omega resonators.

We present a new ultrathin metamaterial that acts as a frequency-selective absorber of terahertz radiation. The absorber is a square array of pairs of omega-shaped micro-resonators made of high-ohmic-loss metal. The metamaterial provides significant suppression of transmitted and reflected radiation in a bidirectional regime (that is, for both forward and backward propagating radiation). The absorber is efficient in a wide range of angles of incidence. The absence of a ground plane makes the absorber unique in comparison with numerous analogs with a ground plane that operate in a unidirectional regime. The novel metamaterial potentially enables controllable transmission of terahertz radiation in imaging systems. Analytical calculations as well as finite-element electromagnetic modeling are presented for an exemplary case with peak absorption at ∼3 THz.