Pulse-driven self-reconfigurable meta-antennas.

Wireless communications and sensing have notably advanced thanks to the recent developments in both software and hardware. Although various modulation schemes have been proposed to efficiently use the limited frequency resources by exploiting several degrees of freedom, antenna performance is essentially governed by frequency only. Here, we present an antenna design concept based on metasurfaces to manipulate antenna performances in response to the time width of electromagnetic pulses. We numerically and experimentally show that by using a proper set of spatially arranged metasurfaces loaded with lumped circuits, ordinary omnidirectional antennas can be reconfigured by the incident pulse width to exhibit directional characteristics varying over hundreds of milliseconds or billions of cycles, far beyond conventional performance. We demonstrate that the proposed concept can be applied for sensing, selective reception under simultaneous incidence and mutual communications as the first step to expand existing frequency resources based on pulse width.

Maximizing the forward scattering of dielectric nanoantennas through surface impedance coatings.

In this Letter, we discuss a novel, to the best of our knowledge, approach for designing passive nanoantennas with maximum forward and almost-zero backward scattering. The proposed approach is based on the use of high-index dielectric spheres supporting dipolar magnetic resonances, which are coated by ultra-thin surface impedance coatings. It is shown that, by properly engineering the radius of the coat and its surface reactance, it is possible to introduce an additional electric dipolar resonance and to make this overlap with the magnetic one sustained by the high-index dielectric sphere. A realistic design that is based on graphene and works in the low-THz range is also proposed and verified with full-wave simulations. Compared to earlier techniques based on the combination of multipoles or on the use of ellipsoidal particles, the proposed one is quite robust toward realistic ohmic losses and preserves the isotropic behavior of the nanoantenna.

Light propagation through metamaterial temporal slabs: reflection, refraction, and special cases.

Time-varying metamaterials are artificial materials whose electromagnetic properties change over time. Similar to a spatial medium discontinuity, a sudden change in time of the metamaterial refractive index induces the generation of reflected and refracted light waves. The relationship between the incident and emerging fields at one temporal interface has been subject of investigation in earlier studies. Here, we extend the study to a temporal slab, i.e., a uniform homogeneous medium that is present in the whole space for a limited time. The scattering coefficients have been derived as a function of the refractive indices and application time, demonstrating that the response of the temporal slab can be controlled through the application time, which acts similarly to the electrical thickness of conventional spatial slabs. The results reported in this Letter pave the way to creating novel devices based on temporal discontinuities, such as temporal matching networks, Bragg grating, and dielectric mirrors, which exhibit zero space occupancy by exploiting the time dimension, instead of the spatial dimension.

Narrowband transparent absorbers based on ellipsoidal nanoparticles.

In this paper, we propose the design of an optical device that is able to selectively absorb impinging light in a desired frequency range while being almost completely transparent outside this range. The proposed absorber is a variant of the optical Salisbury screen we recently proposed [Opt. Lett.41, 3383 (2016)OPLEDP0146-959210.1364/OL.41.003383] but, differently from this earlier version, is transparent for any electromagnetic wave whose frequency is outside the absorption spectrum. Such an absorber also exhibits excellent performance in terms of angular bandwidth and may find application in all scenarios where narrowband absorption is required, such as for light filters or digital sensors. Full-wave simulations confirming the effectiveness of the proposed absorber as well as its robustness toward geometrical defects are provided.

Exploiting the surface dispersion of nanoparticles to design optical-resistive sheets and Salisbury absorbers.

In this Letter, we propose a method to implement resistive sheets exhibiting a desired value of the intrinsic surface resistance at optical frequencies. Considering the sheet made by arrays of plasmonic nanoparticles, the idea is to tailor the surface dispersion occurring when the dimensions of the nanoparticles are smaller than the mean free path of electrons in the constituent material. An analytical model of the surface resistance is proposed and its effectiveness assessed through full-wave simulations. Finally, the applicability of the proposed resistive sheets to implement optical Salisbury screens is discussed and validated through full-wave simulations.

Optical cloaking of cylindrical objects by using covers made of core-shell nanoparticles.

In this Letter, we propose an engineered design of optical cloaks based on the scattering cancellation technique and intended to reduce the observability of cylindrical objects. The cover, consisting of a periodic arrangement of core-shell nanospheres, is designed in such a way to exhibit near-zero values of the real part of the homogenized effective permittivity at optical frequencies. Full-wave numerical simulations, considering the measured data of the dielectric function of the plasmonic material composing the shell, show that the cloak is able to reduce by about 6 dB the scattering cross section of a finite-length cylinder at around 740 THz with a -3 dB fractional bandwidth of about 7%. We show also that this result is not significantly affected by the perturbation of the periodic alignment of the core-shell nanospheres, due to possible fabrication issues or to an amorphous arrangement.

Cloaking apertureless near-field scanning optical microscopy tips.

We numerically demonstrate that properly designed plasmonic covers can be used to enhance the performance of near-field scanning optical microscopy (NSOM) systems based on the employment of apertureless metallic tip probes. The covering material, exhibiting a near-zero value of the real permittivity at the working frequency, is designed in such a way to dramatically reduce the undesired scattering due to the strongly plasmonic behavior of the tip. Though the light scattering by the tip end is necessary for the correct operation of NSOMs, the additional scattering due to the whole probe affects the signal-to-noise ratio and thus the resolution of the acquired image. By covering the whole probe but not the very tip, we show that unwanted scattering can be effectively reduced. A realistic setup, working at mid-IR frequencies and employing silicon carbide covers, has been designed and simulated to confirm the effectiveness of the proposed approach.

Optimization and tunability of deep subwavelength resonators for metamaterial applications: complete enhanced transmission through a subwavelength aperture.

In the present work, we studied particle candidates for metamaterial applications, especially in terms of their electrical size and resonance strength. The analyzed particles can be easily produced via planar fabrication techniques. The electrical size of multi-split ring resonators, spiral resonators, and multi-spiral resonators are reported as a function of the particle side length and substrate permittivity. The study is continued by demonstrating the scalability of the particles to higher frequencies and the proposition of the optimized particle for antenna, absorber, and superlens applications: a multi-spiral resonator with lambda/30 electrical size operating at 0.810 GHz. We explain a method for tuning the resonance frequency of the multi-split structures. Finally, we demonstrate that by inserting deep subwavelength resonators into periodically arranged subwavelength apertures, complete transmission enhancement can be obtained at the magnetic resonance frequency.

Split-ring-resonator-coupled enhanced transmission through a single subwavelength aperture.

We report the enhanced transmission of electromagnetic waves through a single subwavelength aperture by using a split-ring resonator (SRR) at microwave frequencies. By placing a single SRR at the near field of the aperture, strongly localized electromagnetic fields are effectively coupled to the aperture with a radius that is 20 times smaller than the resonance wavelength (r/lambda=0.05). We obtained 740-fold transmission enhancement by exciting the electric resonance of SRR. A different coupling mechanism, through the magnetic resonance of SRR, is also verified to lead to enhanced transmission.