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The piezopotential in floating, homogeneous, quasi-1D piezo-semiconductive nanostructures under axial stress is an anti-symmetric (i.e., odd) function of force. Here, after introducing piezo-nano-devices with floating electrodes for maximum piezo-potential, we show that breaking the anti-symmetric nature of the piezopotential-force relation, for instance by using conical nanowires, can lead to better nanogenerators, piezotronic and piezophototronic devices.

The problem of accelerating the calculation of the periodic Green's function is addressed here for both 3-D and 2-D free-space configurations. In the 3-D case, periodicity is considered both along one axis and along two, generally skew, axes. A comprehensive review of the existing methods is first presented and some extensions are developed. The possibility of treating the case of complex phase shifts between unit cells, necessary for the study of complex modes in periodic structures, is also investigated. Comparisons among the various acceleration methods are performed, thus providing fundamental information on their actual efficiency in typical problems.

Analytical expressions are presented for the intraband conductivity tensor of graphene that includes spatial dispersion for arbitrarily wave-vector values and the presence of a nonzero Fermi energy. The conductivity tensor elements are derived from the semiclassical Boltzmann transport equation under both the relaxation-time approximation and the Bhatnagar-Gross-Krook model (which allows for an extra degree of freedom to enforce number conservation). The derived expressions are based on linear electron dispersion near the Dirac points, and extend previous results that assumed small wave-vector values; these are shown to be inadequate for the very slow waves expected on graphene nanoribbons. The new expressions are also compared to results obtained by numerical integration over the first Brillouin zone using the exact (tight-binding) electron dispersion relation. Very good agreement is found between the new analytical expressions and the exact numerical results. Furthermore, a comparison with the longitudinal random-phase conductivity is also made. It is shown analytically that these new expressions lead to the correct value of the quantum capacitance of a graphene sheet and that ignoring spatial dispersion leads to serious errors in the propagation properties of fundamental modes on graphene nanoribbons. DOI: 10.1103/PhysRevB.87.11542

| This review paper summarizes various aspects of directive beaming and explains these aspects in terms of leaky waves. Directive beaming occurs in antenna design where a narrow beam is obtainable by using fairly simple planar structures excited by a single source. These structures include Fabry-Pérot cavity structures as well as metamaterial structures made from artificial low-permittivity media. Directive beaming also occurs in the optical area where it has been observed that highly directive beams can be produced from small apertures in a metal film when an appropriate periodic patterning is placed on the film. One aspect that these phenomena all have in common is that they are due to the excitation of one or more weakly attenuated leaky waves, the radiation from which forms the directive beam. This is established in each case by examining the role of the leaky waves in determining the near-field on the aperture of the structure and the far-field radiation pattern of the structure.

Tunable THz antennas based on a single unpatterned graphene sheet placed inside a grounded dielectric multilayer are studied with the aim of characterizing their performance in terms of pattern reconfigurability, directivity, and radiation efficiency. The considered structures belong to the class of Fabry-Perot cavity (FPC) antennas, whose radiation mechanism relies on the excitation of cylindrical leaky waves with an ordinary (i.e., non-plasmonic) sinusoidal transverse modal profile. This allows for achieving radiation efficiencies considerably higher than those of alternative graphene-based radiators based on the excitation of surface-plasmon polaritons (SPPs) either in bound or leaky propagation regimes. A customized efficient circuit model has been employed in order to obtain all the radiation characteristics of such graphene FPC antennas, which have been also fully validated by means of a CAD tool. The role of the graphene quality is explicitly taken into account in this comprehensive investigation, proving that it plays a remarkable role in establishing the antenna performance. In particular, it is expected that the standard quality of graphene allows for designing low-efficiency reconfigurable THz antennas based on SPPs and, conversely, high-efficiency FPC antennas with slightly reduced reconfigurability.

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