Giant enhancement of tunable asymmetric transmission for circularly polarized waves in a double-layer graphene chiral metasurface

Dynamically switching the asymmetric transmission and monodirectional absorption of circularly polarized waves using cascade composite resonator-graphene meta-surfaces

We propose a reconfigurable hybrid graphene meta-surface for modulating circularly polarized (CP) waves with dual functionalities of asymmetric transmission (AT) and monodirectional absorption. Such a meta-surface consists of three cascade dielectrics covered with identical metallic resonators and different patterned graphene sheets over the front and the back sides, respectively. The left-handed CP incidences will be transformed into right-handed CP counterparts efficiently with an AT-parameter of 0.8, and the state of transmission will be contrary when the incident CP waves are from the opposite direction. On the other hand, the proposed cascade composite resonator-graphene meta-surface can also function as a super monodirectional absorber to trap the forward incident CP waves at the same frequency range when we simply tune the Fermi energy of graphene. The proposed design should pave the way for building up more advanced meta-devices with multiple functionalities to manipulate CP waves.

Coexistence of circular dichroism and asymmetric transmission in Babinet-complementary metamaterials

Chiral metamaterials with circular dichroism (CD) or asymmetric transmission (AT) draw enormous attention for their attractive applications in polarization transformers, circular polarizers, and biosensing. In this study, a feasible trilayer chiral metamaterials (TCM) is designed and investigated in theory and simulation. The proposed TCM is composed of a nanoslit layer and a Babinet-complementary nanorod layer separated by a nanoslit spacer. Owing to symmetry breaking by the tilted nanoslit in metal film, the TCM shows simultaneous CD and AT effects in the near-infrared region. The simulated electric charge distributions prove that the chirality arises from the excitation of asymmetric electric dipole resonant modes due to the coupling of adjacent unit cells. Moreover, CD and AT can be tuned by the tilted angle of the nanoslit and the thickness of the spacer, the fitting functions of which are consistent with the theoretical formulas based on transmittance matrix analysis. The proposed nanostructure offers a potential strategy for manipulating metamaterials with simultaneous CD and AT effects, allowing a multitude of exciting applications such as ultra-sensitive polarization transformer and biosensor.

Dispersion Theory of Surface Plasmon Polaritons on Bilayer Graphene Metasurfaces

Surface plasmon polaritons (SPPs) on the graphene metasurfaces (GSPs) are crucial to develop a series of novel functional devices that can merge the well-established plasmonics and novel nanomaterials. Dispersion theory on GSPs is an important aspect, which can provide a basic understanding of propagating waves and further guidance for potential applications based on graphene metamaterials. In this paper, the dispersion theory and its modal characteristics of GSPs on double-layer graphene metasurfaces consisting of the same upper and lower graphene micro-ribbon arrays deposited on the dielectric medium are presented. In order to obtain its dispersion expressions of GSP mode on the structure, an analytical approach is provided by directly solving the Maxwell’s equations in each region and then applying periodical conductivity boundary onto the double interfaces. The obtained dispersion expressions show that GSPs split into two newly symmetric and antisymmetric modes compared to that on the single graphene metasurface. Further, the resultant dispersion relation and its propagating properties as a function of some important physical parameters, such as spacer, ribbon width, and substrate, are treated and investigated in the Terahertz band, signifying great potentials in constructing various novel graphene-based plasmonic devices, such as deeply sub-wavelength waveguides, lenses, sensors, emitters, etc.

Equivalent circuit model of graphene chiral multi-band metadevice absorber composed of U-shaped resonator array

In this study, we have designed an equivalent circuit model (ECM) by use of a simple MATLAB code to analyze a single-layered graphene chiral multi-band metadevice absorber which is composed of U-shaped graphene resonator array in terahertz (THz) region. In addition, the proposed metadevice absorber is analyzed numerically by the finite element method (FEM) in CST Software to verify the ECM analysis. The proposed device which is the first tunable graphene-based chiral metadevice absorber can be used in polarization sensitive devices in THz region. It is single-layered, tunable, and it has strong linear dichroism (LD) response of 94% and absorption of 99% for both transverse electric (TE) and transverse magnetic (TM) electromagnetic waves. It has four absorption bands with absorption >50% in 0.5-4.5 THz : three absorption bands for TE mode and one absorption band for TM mode. Proposed ECM has good agreement with the FEM simulation results. ECM analysis provides a simple, fast, and effective way to understand the resonance modes of the metadevice absorber and gives guidance for the analysis and design of the graphene chiral metadevices in the THz region.

Switchable broadband and wide-angular terahertz asymmetric transmission based on a hybrid metal-VO2 metasurface

We propose a switchable broadband and wide-angular terahertz asymmetric transmission based on a spiral metasurface composed of metal and VO₂ hybrid structures. Results show that asymmetric transmission reaching up to 15% can be switched on or off for circularly polarized terahertz waves when the phase of VO₂ transits from the insulting state to the conducting state or reversely. Strikingly, we find that relatively high asymmetric transmission above 10% can be maintained over a broad bandwidth of 2.6-4.0 THz and also over a large incident angular range of 0^°-45^°. We further discover that as the incident angle increases, the dominant chirality of the proposed metasurface with VO₂ in the conducting state can shift from intrinsic to extrinsic chirality. We expect this work will advance the engineering of switchable chiral metasurfaces and promote terahertz applications.

Symmetric accelerating beam generation via all-dielectric metasurfaces

Traditionally, symmetric accelerating beam (SAB) generation requires bulky optical components, which hinder the miniaturization of optical systems. Recently, metasurfaces, which are composed of sub-wavelength features, have provided a captivating boulevard for the realization of ultra-thin and flat optical devices. Therefore, for the first time, we design and simulate all-dielectric metasurfaces based on an optical caustic approach to generate highly efficient SABs by tailoring the phase of an incident wave. The designed metasurface utilizes spatial distribution of optimized Nb₂O₅ nano-rods on SiO₂ substrate to perform the phase modulation. In contrast with conventional accelerating beams, the generated SABs can follow any predefined propagation trajectory with unique features, such as symmetric intensity profile, autofocusing, and thin needle-like structure in their intensity profile. In addition to this, these beams have also shown the ability to avoid obstacles, placed in the direction of propagation of main lobes. We believe that these beams can be useful in applications, including Raman spectroscopy and fluorescent imaging, and multiparticle manipulation.

Tunable asymmetric transmission across stretchable chiral metamaterial

A stretchable chiral metamaterial with L-shaped and T-shaped Au patterns (SCMM-LT) is proposed to generate asymmetric transmission (AT) for circularly polarized waves on the polydimethylsiloxane substrate in the mid-infrared region. The peak value of AT can reach 50.02% at the resonance wavelength of 19.1 µm, owing to the enantiomerically sensitive plasmons. With stretching along the x axis and the y axis. respectively, the band of AT shifts to a longer wavelength, which proves the SCMM-LT can be a candidate as the tunable chiral metamaterial. In the future, the proposed stretchable chiral metamaterial could potentially possess high applicability for wearable electronic devices in a variety of sensor fields.