Chiral Metasurface Research Articles

Independent Amplitude Control of Arbitrary Orthogonal States of Polarization via Dielectric Metasurfaces.

Exquisite polarization control using optical metasurfaces has attracted considerable attention thanks to their ability to manipulate multichannel independent wavefronts with subwavelength resolution. Here we present a new class of metasurface polarization optics, which enables imposition of two arbitrary and independent amplitude profiles on any pair of orthogonal states of polarization. The implementation method involves a polarization-dependent interference mechanism achieved by constructing a metasurface composed of an array of nanoscale birefringent waveplates. Based on this principle, we experimentally demonstrate chiral grayscale metasurface and chiral shadow rendering of structured light. These results illustrate a general approach interlinking amplitude profiles and orthogonal states of polarization and expands the scope of metasurface polarization shaping optics.

Controllable chiral emissions from free-electron driven plasmonic metasurfaces

Arbitrary control of the light emission in nanoscale is of great importance to high-performance, compact and integrated on-chip systems. Free-electron impact interactions with metasurfaces provide an excellent platform for the realization of a point source while the generation and engineering of chiral polarized emission is still unresolved. In this work, we propose a method to flexibly generate and control the chiral emission to the desired direction at specified wavelength(s) with plasmonic chiral metasurfaces. Emission chirality dependences on the impacting positions and wavelength are demonstrated. The same chirality is achieved within a broad bandwidth from 750 nm to 1100 nm at a fixed impacting position. Additionally, we also illustrate a specified wavelength at which the chirality is insensitive to the impacting positions. Simulated far field intensity distributions confirm the approach to implement different chiral emissions. To solve the divergent distributions at chirality 0, we also propose an anisotropic metasurface to confine the left circular polarized and right circular polarized lights in a tight solid angle. The competition between the chiral emission enhancement and ohmic loss on the nanoparticles number is also investigated. Our work promises effective manipulation of the chiral emissions and may find applications in quantum communication and biomolecule detections.

Terahertz polarization and chirality sensing for amino acid solution based on chiral metasurface sensor

The detection and identification of chiral materials, especially chiral biochemical samples, have a wide range of applications. Terahertz (THz) sensing has obvious advantages in online real-time non-contact and label-free biochemical detection, but it is difficult to detect the chiral solution samples. Herein, we propose a new THz sensing method based on THz reflective time-domain polarization spectroscopy (RTDPS) system and a chiral metasurface sensor. The chiral enantiomers of three kinds of amino acids aqueous solution were measured by this sensing method, which improves sensitivity and detection accuracy for amino acid aqueous solutions compared with the traditional THz resonance sensing. The sensing accuracies of the different amino acid samples are all in the order of 10−5 ∼ 10−4 g/mL, and the minimum value reaches 1 × 10-5 g/mL. Moreover, the D- and L- enantiomers with the same concentration can be distinguished by the significant difference in THz polarization parameters at the characteristic frequency. This method is not only suitable for amino acid aqueous solutions, but also other chiral biochemical solutions. Therefore, THz polarization spectroscopy and chiral metasurface sensor have great potential for high-sensitive quantitative detection and chirality identification in the analysis of biochemical materials.

An efficient chiral polarization rotator with asymmetric transmission for large incidence angles

A linear polarization converting chiral metasurface with asymmetric transmission is designed, analyzed, and characterized experimentally for operation in the Ku band of the microwave frequency regime. The proposed structure consists of two metasurface layers of asymmetric square-shaped unit cells with quarter moon polygon split-ring resonators. By breaking certain symmetries of the chiral structure, the proposed structure achieves efficient asymmetric transmission with the asymmetric transmission parameter of more than 80% and a polarization conversion ratio of more than 90% for 13.87–16.04 GHz. Moreover, the proposed structure gives a stable response to the variations in the incidence angle up to 45° both for transverse-electric and transverse-magnetic polarizations. The high efficiency for both asymmetric transmission and polarization conversion, wide bandwidth, and angular stability qualify the proposed design for numerous microwave applications.

Coherent Chiral‐Selective Absorption and Wavefront Manipulation in Single‐Layer Metasurfaces

AbstractRecently, chiral metasurfaces have drawn enormous attentions due to their flexibility in realizing nearly arbitrary chiral effect with designable strength. One intriguing direction in this field is chiral‐selective absorption. However, previous studies mostly require multilayer design and meanwhile often work in a passive way. Here, for the first time to the authors’ knowledge, a chiral‐selective metasurface absorber is experimentally demonstrated with a single‐layer free‐standing metasurface using coherent control method, where the strength of the chiral‐selective behavior can be actively tuned with a 100% modulation depth by simply changing the phase difference between the two coherent inputs. Specifically, under coherent symmetric illumination, near unity absorption is achieved for one circularly polarized incidence, while high‐efficiency cross‐polarization conversion is obtained for the other circularly polarized incidence. Such chiral‐selective absorption property is quite suitable for achieving coherent chiral‐selective wavefront control for the unabsorbed circularly polarized incidence. Three coherent chiral metagratings are designed and experimentally characterized by further employing Pancharatnam–Berry phase method and multiplexed method, where very good performances are achieved. The proposed approach on coherent control can enrich the functionalities and also improve the performances of the conventional single‐layer metasurfaces, which can find extending applications in various practical cases.

Experimental investigation of Kramers–Kronig relations in chiral metasurfaces with reduced rotational symmetry

We examine the Kramers–Kronig relations between the circular dichroism and the optical rotation dispersion in the k-space obtained from a chiral metasurface with a reduced rotational symmetry. We operate a leakage-radiation microscopy system to probe the near-field plasmonic modes and measure the polarization effects of the grating. By using our system we were able to capture and analyze the plasmonic modes at both air/gold and gold/glass interfaces. The k-space mapping of the chirality parameters allows us to fully analyze the optical activity of the metasurface. We experimentally find that the reduction in rotational symmetry affects the locality condition which unavoidably leads to the deviation from the Kramers–Kronig relation.

Chiral Bilayer All-Dielectric Metasurfaces

Three-dimensional chiral plasmonic metasurfaces were demonstrated to offer enormous potential for ultrathin circular polarizers and applications in chiral sensing. However, the large absorption losses in the metallic systems generally limit their applicability for high-efficiency devices. In this work, we experimentally and numerically demonstrate three-dimensional chiral dielectric metasurfaces exhibiting multipolar resonances and examine their chiro-optical properties. In particular, we demonstrate that record high circular dichroism of 0.7 and optical activity of 2.67 × 105 degree/mm can be achieved based on the excitation of electric and magnetic dipolar resonances inside the chiral structures. These large values are facilitated by a small amount of dissipative loss present in the dielectric nanoresonator material and the formation of a chiral supermode in a 4-fold symmetric metasurface unit cell. Our results highlight the mechanisms for maximizing the chiral response of photonic nanostructures and offer important opportunities for high-efficiency, ultrathin polarizing elements, which can be used in miniaturized devices, for example, integrated circuits.

A Bilayered, Broadband, Angularly Robust Chiral Metasurface for Asymmetric Transmission

Asymmetric transmission (AT) through metasurfaces yields a direction-sensitive behavior for electromagnetic (EM) wave propagation. To date, majority of the reported works operate at normal incidences alone and are, therefore, highly prone to variations under oblique incidences. In contrast, the chiral metasurface presented in this letter is robust up to 60 $^{\circ }$ incident angle while yielding a transmission magnitude above 80% for an operational-range of 8.1–9.3 GHz in a thin bilayered configuration. Furthermore, since a miniaturized unit-cell is the key to angular robustness of the metasurface, a lumped-model development methodology to determine its resonant frequency is presented. Lastly, it is shown that the reported metasurface can geometrically be scaled to the lower ends of microwave spectrum, thus increasing its utility for mainstream applications. The results have been analytically and experimentally validated and yield state-of-the-art features in comparison to previous works in microwave regime.

Tunable circular dichroism in a graphene extrinsically chiral L-shaped metasurface

In this manuscript, we propose a graphene metasurface composed of an L-shaped pattern to achieve a tunable circular dichroism under oblique incidence in the mid-infrared region. For our graphene extrinsically 3D-chiral structure, there are two graphene localized surface plasmons resonant modes at the wavelength of 11.05 and 16.69 μm. A broad bandwidth circular dichroism between the two modes is generated, so are two bands of negative circular dichroism (circular dichroism is minus). The circular dichroism can be enhanced by increasing the incident angle. Maximal circular dichroism can reach 24.64% when the incident angle is 60°. The mechanism of circular dichroism generation is attributed to the strong interaction between electric and magnetic dipoles. Besides, the circular dichroism can be tuned by changing the Fermi energy and is affected by the intrinsic relaxation time. It will have a wide application in photonic devices, which can promote the development of analytical chemistry and numerous other areas.

Wide-Angle Circular Polarization Converter Based on a Metasurface of Z-Shaped Unit Cells

Since it’s designable, metasurface was widely used in various fields, especially the design of polarization converters. However, most of the polarization converters currently designed can only work under normal incidence or small angle incidence, which hugely limits the application of the device. In this paper, a chiral metasurface based on the z-shaped unit cell which can manipulate circular polarization wave is proposed. The simulation result shows that this converter can maintain the polarization state of circular polarization after reflection from 8.18 to 13.988GHz with a polarization conversion ratio more than 90%. Moreover, the structure is insensitive to incidence angle，which can keep a stable performance both for left-handed circularly polarized wave and right-handed circularly polarized waves as the incident angle increase to 75°. The proposed metasurface with simple structure, angular stability can be used in communication and polarization manipulating devices.

Asymmetric transmission in bilayer chiral metasurfaces for both linearly and circularly polarized waves

In this paper, we numerically and theoretically propose an architecture with bilayer chiral metasurfaces which can achieve asymmetric transmission (AT) for both linearly and circularly polarized waves in the near-infrared region. The research results show that the structure can obtain a maximum AT ∼ 0.65 and polarization conversion ratio over 80% for linearly polarized waves. In addition, dual-band AT can even be achieved for circularly polarized waves. The electric field distribution and current vector are presented to explain the physical mechanisms of the AT effects. Our research work may have some potential applications in designing optical diode devices, such as asymmetric wave splitters, polarizers, rotators, etc.

Dynamic control of reflective chiral terahertz metasurface with a new application developing in full grayscale near field imaging

The chiral metasurface possesses wide applications in terahertz (THz) band. It is still an important issue to control the chiral properties of metasuface by a simple and effective method. We design a reflective chiral THz metasurface that reflection medium in bottom is based on monolayer graphene on low resistance Si substrate. This structure allows a more reasonable approach to tune Fermi level (EF) of graphene compared with previous report. THz reflection change in graphene as varying EF can control in a large dynamic range the chiral properties of metasurface, inducing the state switch between ‘On’ and ‘Off’. The dynamic control is also effective for various further applications such as vortex focusing and near field imaging. Significantly, we report for the first time the variation rule of reflection amplitude for reflecting suppressed circularly polarized light, with the change in rotation angle of top metal ring of metasurface. Based on this, we develop an innovation application, called full grayscale near field imaging (FGNFI) that can show full grayscale details compared with traditional near field imaging that can only show two grayscale states.

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.

Chiral plasmonic metasurface absorbers in the mid-infrared wavelength range.

Chiral metamaterials in the mid-infrared wavelength range have tremendous potential for studying thermal emission manipulation and molecular vibration sensing. Here, we present one type of chiral plasmonic metasurface absorber with high circular dichroism (CD) in absorption of more than 0.56 across the mid-infrared wavelength range of 5-5.5 µm. The demonstrated chiral metasurface absorbers exhibit a maximum chiral absorption of 0.87 and a maximum CD in absorption of around 0.60. By adjusting the geometric parameters of the unit cell structure of the metasurface, the chiral absorption peak can be shifted to different wavelengths. Due to the strong chiroptical response, the thermal analysis of the designed chiral metasurface absorber further shows the large temperature difference between the left-handed and right-handed circularly polarized light. The demonstrated results can be utilized in various applications such as molecular detection, mid-infrared filter, thermal emission, and chiral imaging.

Tunable and Reconfigurable Dual-Band Chiral Metamirror

Chiral metasurfaces have recently undergone a dramatic development, due to their important applications in optics, chemistry and biology. However, the tunability and reconfigurability of the chiral metasurfaces remain challenges. Then, we proposed an S-shaped metamirror consisting of a MIM (Au-LiNbO3−Au) structure with a dual-band chiral absorption in the near-infrared range. The proposed structure has two resonant wavelengths with a giant chiro-optical effect. The giant chiro-optical effect originates from the different plasmonic resonances induced by the incident circularly polarized light (CPL) with opposite spin states. By tuning the voltage and thus tuning the refractive index of the lithium niobate (LiNbO3), chiral resonant wavelengths can be dynamically adjusted in a large range as desired. Furthermore, by tuning the refractive index of LiNbO3, we can move the short-wavelength resonance peak to the position of another long-wavelength resonance peak. Therefore, the chiral metamirror can be dynamically reconfigured at the position of the long-wavelength resonance peak. The work offers a further step in developing tunable and reconfigurable chirality for possible applications including reconfigurable chiral hologram, chiral-selective absorber and other chiral components in near-infrared region.

Vertical Routing of Spinning-Dipole Radiation from a Chiral Metasurface

We propose a perfect photonic router based on a specially designed chiral bi-metasurface membrane for spin-polarized point light sources. Due to the mirror symmetry breaking in the chiral metamembrane, the radiation power flux of the clockwise and counterclockwise spinning dipoles to the opposite sides of the slab becomes different. We show that spinning dipoles in the specially designed chiral D$_4$-symmetrical bi-metasurface membrane can emit light either upwards or downwards depending on their rotation direction. We attribute this phenomenon to the Fano-resonance effect which is a result of the guided modes coupling with the far field. We show the advantage of D$_4$-symmetrical structures for the achievement of 100\% routing efficiency. This phenomenon can find applications in spintronics for spin-selective inter-chip coupling or as a measurement tool of spin polarization in memory cells.

Metasurfaces with Maximum Chirality Empowered by Bound States in the Continuum.

We demonstrate that rotationally symmetric chiral metasurfaces can support sharp resonances with the maximum optical chirality determined by precise shaping of bound states in the continuum (BICs). Being uncoupled from one circular polarization of light and resonantly coupled to its counterpart, a metasurface hosting the chiral BIC resonance exhibits a narrow peak in the circular dichroism spectrum with the quality factor limited by weak dissipation losses. We propose a realization of such chiral BIC metasurfaces based on pairs of dielectric bars and validate the concept of maximum chirality by numerical simulations.

Single-layer dielectric metasurface with giant chiroptical effects combining geometric and propagation phase

Planar chiral optics play an important role in modern optical detections, for example, imaging systems. Here, we extend the scheme to design planar chiral devices by combining both the geometric phase and propagation phase. Both the propagation and geometric phase can be used to modulate wavefront for the single-layer chiral metasurface. The supercells, containing interference effect among adjacent building blocks inside, are constructed to achieve a strong chiral effect. The circular dichroism in transmission of metasurface with periodic arrangement of supercells can reach as high as 0.937 at mid-infrared range. Meanwhile, the supercells on metasurface exhibit the ability of geometric and propagation phase modulation, which is employed to realize chiral metalenses supporting spin-selective focusing effects. We believe that the proposed scheme could be readily extended for designing compact and miniaturized chiral devices and promote their practical applications.

Chiral Plasmons with Twisted Atomic Bilayers.

van der Waals heterostructures of atomically thin layers with rotational misalignments, such as twisted bilayer graphene, feature interesting structural moiré superlattices. Because of the quantum coupling between the twisted atomic layers, light-matter interaction is inherently chiral; as such, they provide a promising platform for chiral plasmons in the extreme nanoscale. However, while the interlayer quantum coupling can be significant, its influence on chiral plasmons still remains elusive. Here we present the general solutions from full Maxwell equations of chiral plasmons in twisted atomic bilayers, with the consideration of interlayer quantum coupling. We find twisted atomic bilayers have a direct correspondence to the chiral metasurface, which simultaneously possesses chiral and magnetic surface conductivities, besides the common electric surface conductivity. In other words, the interlayer quantum coupling in twisted van der Waals heterostructures may facilitate the construction of various (e.g., bi-anisotropic) atomically-thin metasurfaces. Moreover, the chiral surface conductivity, determined by the interlayer quantum coupling, determines the existence of chiral plasmons and leads to a unique phase relationship (i.e., ±π/2 phase difference) between their transverse-electric (TE) and transverse-magnetic (TM) wave components. Importantly, such a unique phase relationship for chiral plasmons can be exploited to construct the missing longitudinal spin of plasmons, besides the common transverse spin of plasmons.

3D micro/nanoshaping of metal strip arrays by direct imprinting for chiral metasurfaces

An original hybrid method for three-dimensional (3D) shaping of micro- and nanostructures by direct imprinting of lithographically patterned metal 2D-elements on a polymer substrate is proposed. The technological features of the method are demonstrated by transforming Au micro/nanostrip arrays into 3D half-turn helices. The metasurface formed by an array of 3D half-turn Au microhelices in the surface layer of a polymer substrate rotates the polarization plane of transmitted 3 THz radiation through an angle in excess of 50°. Both experimentally and numerically, it is established that the metasurface permits to control the state of polarization and the intensity of transmitted linearly polarized THz radiation using half-wavelength and guided-mode resonances as well as diffraction effects. The proposed approach enables the fabrication of double-layer arrays of half-helices, designed for infrared metasurfaces, from 80 nm wide Au nanostrips. These studies not only pave the way towards the fabrication of large-area chiral metamaterials but, also, open a new avenue in the development of 3D micro/nanostructures for other applications.