Broad dual-band asymmetric transmission of circular polarized waves in near-infrared communication band

Few-layer bifunctional metasurfaces enabling asymmetric and symmetric polarization-plane rotation at the subwavelength scale

We introduce and numerically validate the concept of few-layer bifunctional metasurfaces comprising two arrays of quasiplanar subwavelength resonators and a middle grid (array of rectangular holes) that offer both symmetric and asymmetric transmissions connected, respectively, with symmetric and asymmetric polarization-plane rotation functionalities. The proposed structures are thinner than λ / 7 and free of diffractions. Usually, the structure's symmetry or asymmetry, i.e. unbroken or broken spatial inversion symmetries, are considered for metasurfaces as prerequisites of the capability of symmetric or asymmetric conversion of linearly polarized waves, respectively. Due to the achieved adjustment of the resonances enabling the rotation of the polarization plane simultaneously for both orthogonal polarizations of the incident wave, the symmetric polarization-plane rotation functionality can be obtained within one subwavelength band, whereas the asymmetric polarization-plane rotation functionality associated with the asymmetric transmission is obtained within another subwavelength band. This combination of the functionalities in one subdiffraction structure is possible due to the optimal choice of the grid parameters, since they may strongly affect the coupling between the two resonator arrays. Although normal incidence is required for the targeted bifunctionality, the variations of the incidence angle can also be exploited for the enrichment of the overall functional capability. Variations of the polarization angle give another important degree of freedom. The connection between the polarization-angle dependence of cross-polarized transmission and capability of symmetric and asymmetric polarization-plane rotation functionalities is highlighted. The feasible designs of the bifunctional metasurfaces are discussed.

Switchable bi-functional metasurface for absorption and broadband polarization conversion in terahertz band using vanadium dioxide and photosensitive silicon

We proposed a bi-functional switchable metasurface based on vanadium dioxide (VO2) and photosensitive silicon. The metasurface functions as a transmissive polarization converter in its insulating state with asymmetric transmission characteristics. It attains a remarkable polarization conversion rate (PCR) surpassing 90% and a notable maximum asymmetric transmission (AT) parameter value of 0.73. This performance is observed within the frequency range from 4.31 to 7.86 THz. Dynamic regulation of PCR and AT can be achieved by adjusting the conductivity of photosensitive silicon. To illustrate the underlying factor behind the broadband polarization conversion, the surface current distribution is analyzed at 5.96 THz and 6.08 THz. On the other hand, when VO2is in the metallic state, the metasurface transforms into a bidirectional absorber with near-perfect absorption in both illumination directions. Under forward incidence of terahertz waves, the absorption rates for the transverse electric and transverse magnetic waves are 99.3% at 3.54 THz and 93% at 3.56 THz, respectively. The physical mechanism of near-perfect absorption is explained using impedance matching theory and the electric field distribution. This research expands the applications of transmissive polarization converters within multifunctional metasurfaces, providing new avenues for their practical implementation.

Tunable circular conversion dichroism of single-layer twisted graphene-patterned metasurface

Two-dimensional (2D) chirality-induced asymmetric transmission/reflection has great potential for polarization applications. Usually, asymmetric effects resulting from circular conversion dichroism (CCD) occur in chiral metasurfaces. Here, we propose a single-layer twisted graphene-patterned (with tilted elliptical hole arrays) metasurface and theoretically reveal its tunable CCD in the terahertz (THz) region. The unit cell of the metasurface is achiral. Merely by altering the in-plane orientation of holes for structural 2D chirality, a tunable CCD can be achieved at normal incidence. Interestingly, the reflection phase can be considered an intuitive method to show this metasurface's anisotropy, which complements the conventional CCD measurement in characterizing chiral materials. Furthermore, we can achieve active CCD based on the tunability of graphene. Due to the Fabry-Pérot resonance, a multiband enhancement of CCD spectrum will happen by changing the dielectric layer thickness. The proposed metasurface provides more flexible opportunities for designing active THz devices for polarization manipulation.

Terahertz dual-band asymmetric transmission for a single cross-polarized linear wave

The reported dual-band asymmetric transmission is usually an effect of mutual polarization conversion, where one polarized wave is converted to its cross-polarization in the first band while the other polarized wave is converted to its cross-polarization in the second band. In this work, we experimentally demonstrate a dual-band asymmetric transmission effect only for one-polarized linear wave in the terahertz band. It is measured that the cross-polarization transmission coefficient T_yx reaches two peaks of 0.715 and 0.548 at the frequency of 0.74 THz and 1.22 THz, respectively. While the transmission coefficient T_xy is lower than 0.2 in the wide-band from 0.5 THz to 1.5 THz. Firstly, the multiple interference model is used to discuss the physical mechanism of the dual-band asymmetric transmission. However, the second band of the calculated spectrum is offset due to the strong near field coupling between the two metal layers. The coupled-mode theory is then introduced and the fitting result of the coupled-mode theory is in good agreement with that of the experiment in the two bands. This research would provide new theoretical instructions in designing and analyzing multiband asymmetric transmission in the terahertz, microwave or the optical bands.

Asymmetric transmission for dual-circularly and linearly polarized waves based on a chiral metasurface

We propose a chiral metasurface (CMS) that exhibits asymmetric transmission (AT) of double circularly and linearly polarized waves at the same frequency band. In order to realize the manipulation of electromagnetic (EM) waves in the whole space, the unit cell of CMS consists of three layers of dielectric substrate and four layers of metal patches. The Z-shaped chiral micro-structure and a grating-like micro-structure are proposed and designed to achieve AT. The simulated results show that the x-polarized wave that is incident along one direction can be transmitted into the right-hand circularly polarized (RHCP) wave and the left-hand circularly polarized (LHCP) wave that is incident along the opposite direction can be reflected as the LHCP wave in the frequency band of 4.69GHz-5.84 GHz. The maximum chirality response can be reflected by AT and circular dichroism (CD) and they can reach up to 0.38 and 0.75, respectively. In addition, we also produced the sample of CMS, and the experimental results are in good agreement with the simulated results.

Narrow-band asymmetric transmission based on the dark mode of Fano resonance on symmetric trimeric metasurfaces

Asymmetric transmission (AT) is useful for polarization manipulation. We report narrowband AT that utilizes a triple-layered symmetric trimeric metasurface with near-field coupling of the dark mode of the Fano resonance. The coupling strength of the dark mode was tuned by using a mid-layer to break the dim AT between two slit layers. The peak transmission of linearly polarized waves and percentage bandwidth reached 0.7719 and 1.26% (numerical simulations) and 0.49 and 1.9% (experiments), respectively. Coupled-mode theory and field patterns are utilized to explain the underlying physical mechanisms of the mid-layer assisted field coupling. These results are useful for Fano-resonance-based devices.

Asymmetric transmission of linearly polarized waves based on Mie resonance in all-dielectric terahertz metamaterials

Interest in asymmetric transmission (AT) at terahertz frequencies has increased dramatically in recent years. We present an all-silicon metamaterial to achieve the AT effect for linearly polarized electromagnetic waves in the terahertz regime. The metamaterial is constructed by rectangular silicon pillars and a thick silicon substrate. The magnetic Mie resonance excited by the incident polarized terahertz wave contributes to the AT effect, which is verified by the field distributions. In addition, the rotation angle and dimensions of the silicon pillars are shown to have a great influence on the AT efficiency. The proposed metamaterial with straightforward design has promising applications in polarization control scenarios.

Anisotropic asymmetric transmission of circularly polarized terahertz waves in a three-dimensional spline assembly

Asymmetric transmission (AT) for circularly polarized (CP) electromagnetic (e-m) waves in chiral metamaterial (CMM) is a well-known phenomenon. However, most of the CMMs exhibit AT along only one direction. In this work, AT for CP waves with a magnitude of more than 0.5 along three principal directions of a newly made three-dimensional (3D) spline assembly is reported at terahertz frequencies. Surface current analysis is presented to explain the mechanism of AT for CP waves in the proposed 3D assembly.

Experimental verification of a broadband asymmetric transmission metamaterial in the terahertz region

In this work, a broadband terahertz asymmetric transmission metamaterial is experimentally demonstrated for a linearly polarized wave. The measured transmission coefficient T_yx is larger than 0.6 from 0.55 to 0.82 THz, and reaches a peak value of 0.714 at 0.62 THz, while the transmission coefficient T_xy is lower than 0.2 from 0.4 to 0.9 THz. The calculated asymmetric transmission parameter of the measurement ranges from 0.53 to 0.84 THz for magnitudes over 0.4. The peak value reached 0.65 at the frequency of 0.78 THz. The physical mechanism of the polarization conversion was also analyzed from the distributions of the surface currents and electric fields.

In this work, a broadband terahertz asymmetric transmission metamaterial is experimentally demonstrated for a linearly polarized wave.

Giant spin-selective asymmetric transmission in multipolar-modulated metasurfaces

Spin-selective manipulation of optical waves is widely utilized in various optical techniques and plays a key role in modern nanophotonics. While numerous efficient approaches have been applied in metasurfaces to realize spin-selective manipulation of optical waves, the implementation of giant spin-selective asymmetric transmission remains a challenge. Here, we propose an all-dielectric metasurface to realize giant tri-band spin-selective asymmetric transmission in the near infrared regime. The proposed giant spin-selective asymmetric transmission is attributed to the excitation of overlapping multipolar resonances in the dielectric elliptic cylinders, which can be well manipulated by changing the structure parameters. This research demonstrates the great potential of all-dielectric metasurfaces for spin-selective transmission manipulation, which provide helpful insights and intriguing possibilities for applications in information optics, quantum optics, optical sensing, and imaging.

Controllable linear asymmetric transmission and perfect polarization conversion in a terahertz hybrid metal-graphene metasurface

We propose a three layered metal-graphene-metal metasurface to investigate the controllable linear asymmetric transmission and perfect polarization conversion in THz regime, by using the finite-difference time-domain (FDTD) method. An on-to-off control of asymmetric transmission and perfect polarization conversion is achieved by changing the Fermi energy of graphene from 0.8 eV to 0 eV. We present the electric field distribution and Fabry-Perot-like cavity model to analyze the working mechanisms. By gradually shifting the Fermi energy of graphene, two functions are realized, i.e., controllable linear asymmetric transmission and controllable total transmission with near perfect polarization conversion.

Correlation of circular differential optical absorption with geometric chirality in plasmonic meta-atoms

We report a strong correlation between the calculated broadband circular differential optical absorption (CDOA) and the geometric chirality of plasmonic meta-atoms with two-dimensional chirality. We investigate this correlation using three common gold meta-atom geometries: L-shapes, triangles, and nanorod dimers, over a broad range of geometric parameters. We show that this correlation holds for both contiguous plasmonic meta-atoms and non-contiguous structures which support plasmonic coupling effects. A potential application for this correlation is the rapid optimization of plasmonic nanostructure for maximum broadband CDOA.

Enhanced circular dichroism of tilted zigzag-shaped nanohole arrays

Circular dichroism (CD) of nanostructures is in great demand for applications in biological molecules, photocurrent devices, and photocatalysis. Planar nanostructures can be prepared in a concise manner, and their CD effects have gained much research interest. In this study, tilted zigzag-shaped nanohole (TZSN) arrays are proposed, and the CD effect is studied by the finite element method. A strong resonance occurs in the gap by tuning the charge distributions between adjacent nanoholes. Meanwhile, the CD effect of TZSN arrays is strongly dependent on the structural parameters of TZSN. Results provide a novel method for tuning the CD effects of nanohole arrays on a film.

Propagation and asymmetric behavior of optical pulses through time-dynamic loss-gain-assisted media

We report an asymmetric behavior of optical pulses during their propagation through a time-varying linear optical medium. The refractive index of the medium is considered to be varying with time and complex, such that a sufficient amount of gain and loss is present to realize their effect on pulse propagation. We have exploited the universal formula for optical fields in time-varying media. Numerically simulated results reveal that pulses undergo opposite temporal shifts around their initial center position during their bi-directional propagation through the medium along with corresponding spectral shifts. Moreover, the peak power and accumulated chirp (time derivative of accumulated phase) of the output pulse in both propagation directions are also opposite in nature, irrespective of their initial state. Numerically simulated behavior of the pulses agrees well with the analytical solutions.

Enhanced asymmetric transmissions attributed to the cavity coupling hybrid resonance in a continuous omega-shaped metamaterial

In this paper, the infinite-length metallic bar is folded to a continuous omega-shaped resonator and then arranged as a bi-layer metamaterial, which presents a hybrid resonance and a Fabry-Perot-like cavity mode. The asymmetric transmission (AT) for linearly polarized light is powerfully enhanced at a near-infrared regime by strongly coupling the hybrid resonance to the cavity, with the maximum value of the high-efficiency AT effect reaching 0.8 at around 1364 nm. At this near-infrared band, such a high-efficiency AT effect has never been realized previously by a bi-layer metamaterial. More importantly, we demonstrate that our design is robust to the misalignments, which greatly decreases the difficulties in sample fabrications. Accordingly, the proposed omega-shaped metamaterial provides potential applications in designing polarization filters, polarization switches, and other nano-devices.

Tunable asymmetric transmission through tilted rectangular nanohole arrays in a square lattice

Asymmetric transmission (AT) holds significant applications in controlling polarization and propagation directions of electromagnetic waves. In this paper, tilted rectangular nanohole (TRNH) arrays in a square lattice are proposed to realize an AT effect. Numerical results show two AT modes in the transmission spectrum, and they are ascribed to the localized surface plasmon resonances around the two ends of TRNH and surface plasmon polaritons on the golden film. AT properties of the TRNH strongly depend on structural parameters, such as width, length, thickness, and tilted angle of TRNH. Results provide a novel mechanism for generating AT effect and offer potential plasmonic device applications, such as asymmetric wave splitters and optical isolators.

Experimental verification of asymmetric transmission in continuous omega-shaped metamaterials

A bi-layer continuous omega-shaped metamaterial was proposed and fabricated to measure the asymmetric transmission (AT) effect of a linearly polarized light at near-infrared region.

Optical chirality breaking in a bilayered chiral metamaterial

We propose a planar optical bilayered chiral metamaterial, which consists of periodic metallic arrays of two L-shaped structures and a nanorod twisted on both sides of a dielectric slab, to investigate the optical chirality breaking effect by using finite-difference time-domain (FDTD) method. Even the metamaterial is with chiral symmetry, an optical chirality breaking window in the asymmetric transmission pass band is obtained in chiral metamaterial structure. We analyze the plasmonic mode properties and attribute the mechanism of the optical chirality breaking effect to the plasmonic analogue of EIT. The optical chirality breaking window can be modulated by changing the geometric parameters of the nanorods in the structure.