Wideband and multi-frequency infrared cloaking of spherical objects by using the graphene-based metasurface

Generalized surface admittance equivalence principle for super-scattering in dielectric particles

A general formulation for controlling the external scattering coefficients of cylindrical harmonics is presented, generalizing previous results for cloaking of a bare dielectric particle. By inserting a suitable surface admittance at the boundary between a dielectric body and the background region, cylindrical harmonic waves can be enhanced by tailoring the admittance value. Two separate limiting cases for super-scattering features are presented and compared against the same bare particle reference case, providing insights on how to enhance the multi-harmonic scattering pattern. Using this formulation, super-scattering systems can be created, which are suitable for future implementation using active or passive thin metasurfaces.

Design of two invisibility cloaks using transmissive and reflective metamaterial-based multilayer frame microstructures

Ultrathin metamaterials provide new possibilities for the realization of cloaking devices because of their ability to control electromagnetic waves. However, applications of metamaterials in cloaking devices have been limited primarily to reflection-type carpet cloaks. Hence, a transmissive free-space cloak was developed using a multilayer frame structure, wherein highly transparent metamaterials were used to guide incident waves into propagating around an object. The cloaking effect was quantitatively verified using near-field and far-field distributions. Metamaterials allow for the cloaking shells of transmissive cloaks to be developed without spatially varying extreme parameters. Moreover, a transmissive invisible cloak with metamaterial-based mirrors was designed. The design principle of this cloak with a frame structure consists of four metamaterial-based mirrors and two metal mirrors. After covered with the designed metamaterials-based mirrors cloak, the outgoing electromagnetic wave is restored greatly as if the wave passes directly through the obstacle without distortion. This cloak used the metamaterials mirrors to adjust the reflected angle, so that the outgoing electromagnetic wave does not change direction, thereby achieving the cloaking effect.

Nonradiating anapole condition derived from Devaney-Wolf theorem and excited in a broken-symmetry dielectric particle

In this work, we first derive the nonradiating anapole condition with a straightforward theoretical demonstration exploiting one of the Devaney-Wolf theorems for nonradiating currents. Based on the equivalent volumetric and surface electromagnetic sources, it is possible to establish a unique compact conditions directly from Maxwell's Equations in order to ensure nonradiating anapole state. In addition, we support our theoretical findings with a numerical investigation on a broken-symmetry dielectric particle, building block of a metamaterial structure, demonstrating through a detailed multiple expansion the nonradiating anapole condition behind these peculiar destructive interactions.

Tunable mantle cloaking utilizing graphene metasurface for terahertz sensing applications

Design of sensors which are able to probe electromagnetic radiation with larger cross section and at the same time with having negligible perturbation in measurement has attracted significant attention. For this purpose, scattering-cancellation sensors or cloaking sensors are introduced. However, tunable cloaking sensors are very challenging. In this regards, here, a metasurface based on graphene strips is proposed to cloak a dielectric cylinder under illumination of TE_z and TM_z polarized incident waves in terahertz range. According to the in plane effective surface impedance tensor for the considered metasurface and the required surface impedance for achieving invisibility under TE and TM polarized impinging waves, the geometrical parameters of the covering structure and characteristics of graphene are obtained. Numerical simulations show radar cross section reduction for both TE and TM polarizations. Furthermore, the introduced metasurface is able to cloak the cylinder for incoming waves with circular polarization. In addition, it is shown that by properly adjusting the chemical potential of graphene, the required surface impedance to have cloaking for the two polarizations in other frequencies can also be achieved, which results in a tunable dual polarized cloaking. The proposed structure provides 2-11 dB reduction in scattering strength relative to the uncloaked configuration for 0.3eV variation of graphene chemical potential.

High-efficiency tunable plasmonically induced transparency-like effect in metasurfaces composed of graphene nano-rings and ribbon arrays and its application

In this paper, a plasmonically induced transparency (PIT)-like phenomenon in a metasurface composed of a periodic graphene ring and ribbon arrays is studied in the terahertz region. We used the Lorentz oscillator model to analyze the metasurface physically and theoretically. This PIT-like effect can be tuned by alternation of the chemical potential and dimension of the nano-graphene ring and ribbon. The resonance frequency of the PIT-like phenomenon is not sensitive to the incident lightwave angle. As an application of the structure, a refractive index sensor is proposed and simulated. Furthermore, we propose a metasurface composed of a double ring and graphene ribbon to realize the PIT-like effect with three dips. Our results express an appropriate approach for the expansion of mid-infrared absorbers and sensors.

Nanoscale, tunable, and highly sensitive biosensor utilizing hyperbolic metamaterials in the near-infrared range

A plethora of research in recent years has been reported on biosensing in the surface plasmon resonant systems. However, very little research has reported a tunable and highly sensitive biosensor in a nanoscale platform. In this regard, we propose a nanoscale hyperbolic metamaterial (HMM)-based prism coupled waveguide sensor (PCWS) in the near-infrared range. The HMM layer makes up one of the constituents of the PCWS-comprised of a periodically arranged assembly of silver nanostrips. The structure is numerically simulated by the finite difference time domain method. It is demonstrated that the sensitivity of the reflected light can be tuned through the refractive index (RI) of the solution. Moreover, the effects of alteration of constituents of PCWS on the sensitivity have been analyzed. Results show that the sensitivity of PCWS can be harnessed by altering the thickness, slant angle of HMM layer, volume fraction (f) of metal in the HMM layer, and the incidence angle of light. For this purpose, the structure is numerically simulated by the finite difference time domain method. In the optimum design of the proposed sensor, the maximum value of sensitivity is achieved as high as S=3450  nm/refractive index unit with θ=10° and ϕ=10° and a metamaterial thickness of 250 nm. Moreover, the structure has a nanoscale footprint of 600  nm×400  nm×200  nm.

Graphene-based metasurface for a tunable broadband terahertz cross-polarization converter over a wide angle of incidence

In this paper, using a graphene-based metasurface, we demonstrate a unique design to develop a highly efficient, broadband, mid-infrared cross-polarization converter. The proposed graphene-based metasurface structure comprises periodical ϕ-shaped graphene on the top surface of a noble-metal-backed dielectric silicon dioxide (SiO₂). The reported structure converts the incident linearly polarized wave into cross-polarized components with a peak polarization conversion ratio of more than 0.9 over a large band. Furthermore, the metasurface structure exhibits the full width at half-maximum bandwidth of 41.98% with respect to its center frequency of 5.98 THz. The physical insights behind electromagnetic polarization conversion are supported by field distributions and retrieved electromagnetic parameters. The structure works as a broadband cross-polarization converter up to 40° incident angle for both TE and TM polarizations. In addition, the structure is found to be as thin as ∼λ/6 with respect to lowermost frequency of the polarization conversion. The period of the unit cell is ∼λ/24 to support the fact that the structure can be treated as a metasurface.