The radiation efficiency improvement of graphene plasmonic antennas via synthesized substrates with metamaterial resonators, is introduced in this paper. Graphene, represented as an ultra-thin layer, can support highly-confined surface waves of significantly decreased wavelength compared to the vacuum one. Although, this concept yields more compact devices, its applicability to graphene antennas is limited, due to the degradation of radiation efficiency. Thus, the effect of a substrate material on the surface wave wavelength is thoroughly examined, while radiators of larger dimensions and enhanced efficiency are designed for an epsilon-negative medium. Also, a novel realistic metamaterial substrate is developed for graphene THz antennas, successfully verifying all theoretical estimations. Numerical results are extracted via an accurate finite-difference time-domain algorithm that treats graphene as an efficient surface boundary condition.

In this letter, the finite difference time-domain (FDTD) method and the ray-tracing (RT) technique are systematically revisited and compared as potential tools that can reliably characterize new protocols for emerging nanonetwork applications. To this aim, a set of efficient simulation schemes for the precise prediction of the reception quality in various communication scenarios is presented. In particular, each algorithm involves a similar configuration with a realistic transmitter/receiver model and multiple obstacles sized up to some micrometers. The proposed analysis reveals that, unlike conventional assessments, the RT approach can be successfully employed at nanoscale dimensions, with increasing accuracy at higher frequencies, as a tool for fast received-energy estimations, since its results are in acceptable agreement with the respective FDTD data. These significant deductions are, finally, substantiated by a theoretical formulation equivalent to that of frequency selective surfaces.

An efficient and consistent technique to implement numerically a magnetically biased graphene layer is introduced in this paper. Through the novel scheme and after applying a magnetic bias perpendicular to graphene, its surface conductivity presents anisotropic behavior and this effect is systematically modeled and incorporated in terms of the recursive convolution formulation in the finite-difference time-domain algorithm. The extracted numerical results are comprehensively compared with the corresponding analytical expressions to validate the significant performance of the proposed method over a wide frequency range.

The development of a robust terminating boundary scheme for the transverse magnetic surface plasmon polaritons, supported on graphene, is introduced in this paper and incorporated in the finite-difference time-domain (FDTD) method. First, the 2-D FDTD algorithm is adjusted to efficiently model the graphene sheet as a surface conductivity, and the effect of the scattering rate-the main loss mechanism of graphene-on the surface wave propagation properties, is thoroughly studied. Then, the new scheme is optimally formulated via the prior scattering rate and combined with the FDTD algorithm. Its enhanced performance is successfully validated for several 2-D setups and a 3-D configuration with a surface wave traveling on a graphene microribbon waveguide.

The degradation effects of artificial aging on the “true” pigment color of Byzantine iconography are thoroughly investigated in this work. For this purpose, a multi-material palette is fabricated, consisting of various popular egg-tempera pigments, while the original recipes from the literature are utilized in order to mimic the genuine art of Byzantine painters. Then, artificial aging procedures are appropriately employed to simulate environmental fluctuations in historical buildings, such as churches. A total of four time steps are investigated, including the initial condition, and pigments’ spectra in the ultraviolet/visible (UV/Vis) area are acquired in the diffuse reflectance mode at each individual step. Moreover, a color characterization procedure is realized via the quantification of lightness and saturation by means of the measured UV/Vis spectrum. The main objectives of this work are to determine the color stability, the type of color degradation, and generally the color response through time of the studied pigments. The extracted results indicate that a couple of pigments suffer severe color degradation while the majority present moderate darkening or discoloration.

The development of an accurate convolutional perfectly matched layer for the efficient termination of infinite graphene structures is introduced in this paper. Initially, the popular 2-D material receives the appropriate theoretical analysis, which reveals the necessity of a flexible terminating condition that can enhance the performance of existing absorbing schemes and offer sufficient truncation rates for the graphene's strongly confined surface waves. The consistency and behavior of the novel technique are substantiated via comprehensive comparisons with setups truncated by conventional absorbing boundary conditions. Finally, a graphene microribbon waveguide is addressed to indicate the capabilities of featured algorithm in complex structures.