Tuning of giant 2D-chiroptical response using achiral metasurface integrated with graphene

Polarization and incident angle independent multifunctional tunable terahertz metasurface based on graphene

Motivated by the imperative demand for design integration and miniaturization within the terahertz (THz) spectrum, this study presents an innovative solution to the challenges associated with singular functionality, limited application scope, and intricate structures prevalent in conventional metasurfaces. The proposed multifunctional tunable metasurface leverages a hybridized grapheme-metal structure, addressing critical limitations in existing designs. Comprising three distinct layers, namely a graphene-gold resonance layer, a Topas dielectric layer, and a bottom gold film reflective layer, this terahertz metasurface exhibits multifunctionality that is both polarization and incident-angle independent. The metasurface demonstrates a broadband circular dichroism (CD) function when subjected to incident circularly polarized waves. In contrast, under linear incidence, the proposed design achieves functionalities encompassing linear dichroism (LD) and polarization conversion. Remarkably, graphene's chemical potential and the incident light's state can be manipulated to tune each functional aspect's intensity finely. The proposed tunable multifaceted metasurface showcases significant referential importance within the terahertz spectrum, mainly contributing to advancing CD metamirrors, chiral photodetectors, polarization digital imaging systems, and intelligent switches.

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.

Giant 2D-chiroptical response in an achiral metasurface integrated with black phosphorus

In this work, we proposed a black phosphorus (BP) achiral metasurface and theoretically study the chiroptical response arising from extrinsic 2D-chirality in the mid-infrared regime. The achiral metasurface is composed of a monolayer BP sheet sandwiched by a silver ring array and dielectric spacer stacking on a silver substrate. The giant circular conversion dichroism (CCD) of the achiral metasurface is allowed at oblique incidence for the cooperative interaction of BP anisotropic surface plasmon modes and localized surface plasmons in metal rings, and the integrated BP can dynamically modulate the chiroptical response by controlling the doping concentration of BP. Furthermore, we found that a multiband phenomenon for CCD response occurs when tuning the thickness of the spacer. The proposed hybrid achiral metasurface provides more flexible opportunities to realize active polarization modulator, biosensor and chiral detection.

Graphic-processable deep neural network for the efficient prediction of 2D diffractive chiral metamaterials

We propose a novel, to the best of our knowledge, graphic-processable deep neural network (DNN) to automatically predict and elucidate the optical chirality of two-dimensional (2D) diffractive chiral metamaterials. Four classes of 2D chiral metamaterials are studied here, with material components changing among Au, Ag, Al, and Cu. The graphic-processable DNN algorithm can not only handle arbitrary 2D images representing any metamaterials that may even go beyond human intuition, but also capture the influence of other parameters such as thickness and material composition, which are rarely explored in the field of metamaterials, laying the groundwork for future research into more complicated nanostructures and nonlinear optical devices. Notably, the rigorous coupled wave analysis (RCWA) algorithm is first deployed to calculate circular dichroism (CD) in the higher-order diffraction beams and simultaneously promote the training of DNN. For the first time we creatively encode the material component and thickness of the metamaterials into the color images serving as input of the graphic-processable DNN, in addition to arbitrary graphical parameters. Especially, the smallest intensity is found in the third-order diffraction beams of E-like metamaterials, whose CD response turns out to be the largest. A comprehensive study is conducted to capture the influence of shape, unit period, thickness, and material component of arrays on chiroptical response. As expected, a satisfied precision and an accelerated computing speed that is 4 orders of magnitude quicker than RCWA are both achieved using DNN. This work belongs to one of the first attempts to thoroughly examine the generalization ability of the graphic-processable DNN for the study of arbitrary-shaped nanostructures and hypersensitive nanodevices.

Magnetically active terahertz wavefront control and superchiral field in a magneto-optical Pancharatnam-Berry metasurface

Nowadays, the manipulation of the chiral light field is highly desired to characterize chiral substances more effectively, since the chiral responses of most molecules are generally weak. Terahertz (THz) waves are related to the vibration-rotational energy levels of chiral molecules, so it is significant to actively control and enhance the chirality of THz field. Here, we propose a metal/magneto-optical (MO) hybrid Pancharatnam-Berry (PB) phase structure, which can serve as tunable broadband half-wave plate and control the conversion of THz chiral states with the highest efficiency of over 80%. Based on this active PB element, MO PB metasurfaces are proposed to manipulate THz chiral states as different behaviors: beam deflector and scanning, Bessel beam, and vortex beam. Due to the magnetic-tunablibity, these proposed MO PB metasurfaces can be turned from an "OFF" to "ON" state by changing the external magnetic field. We further investigate the near-field optical chirality and the chirality enhancement factors in far field of the chiral Bessel beam and vortex beam, achieving the superchiral field with the highest chiral enhancement factor of 40 for 0th Bessel beam. These active, high efficiency and broadband chiral PB metasurfaces have promising applications for manipulation the THz chiral light and chiroptical spectroscopic techniques.

Amplification of optical activity in a fiber loop resonator

In this paper, we have theoretically studied an effective amplification of optical activity by a fiber loop resonator. We propose a scheme in which an optically active element is placed in the loop segment of the resonator. Assuming that the coupling in the resonator is polarization-independent, we have shown that initially small polarization plane rotation, which arises due to the optically active element, can be significantly amplified by tuning the resonator's closed-path phase. We have also studied the influence of losses on the amplification of optical activity. We have shown that the maximal amplification takes place under the condition of critical coupling, at which the attenuation parameter is equal to the resonator's effective reflection coefficient. We have also studied effective dichroism in such a system and shown the relevance of a critical coupling regime to that effect.

Planar Achiral Metasurfaces-Induced Anomalous Chiroptical Effect of Optical Spin Isolation

Planar chiral structures respond differently for oppositely handed incident light, and thus can produce extraordinary chiroptical effects such as circular conversion dichroism (CCD) and asymmetric transmission (AT). Such chiroptical effects are powerful tools to realize the fundamental principle of optical spin isolation, which leads to a plethora of applications such as optical conversion diodes, chiral imaging, and sensing. Here, we demonstrate the chiroptical effects of simultaneous CCD and AT through meticulously designed single-layered achiral nanofins. Our metamolecule consists of four achiral hydrogenated amorphous silicon (a-Si:H) nanofins that are carefully oriented and optimized to exhibit considerable CCD and AT. The device demonstrates a circular conversion dichroism of 55% and an asymmetric transmission of 58% at a wavelength of 633 nm. Right-hand circularly polarized light (RHCP) is completely absorbed, while left-hand circularly polarized light (LHCP) is transmitted with a polarization conversion, making it a perfect circular polarization wave isolator with negligible backscattering (due to low reflectance). This unique design and its underlying working mechanism are described comprehensively with three different techniques. These methods validate the proposed design and its methodology. For practical applications such as imaging, the proposed design realizes the Pancharatnam-Berry (PB) phase, achieving a 0-2π phase coverage for transmitted circular polarization. For the proof of concept, a metahologram is designed and demonstrated by employing the achieved full-phase control. The measured response of the fabricated metadevice not only validates the CCD and AT but also exhibits a simulated polarization conversion efficiency of up to 71% and measured efficiency up to 52%, comparable to state-of-the-art metahologram demonstrations.

Dual-band transmissive circular polarization generator with high angular stability

Metasurfaces (MSs) offer us an efficient way to control electromagnetic wave polarization due to its capability of flexible wave manipulation and compact configurations. However, the design of dual-band polarization conversion MS with high angular stability is still a challenge, especially in transmission geometry. Here, we propose a dual-band linear-to-circular (LTC) polarization conversion MS with high angular stability by using an array of multi-resonance meta-atoms. The meta-atom consists of two outer double split-ring layers and a central bar layer with circle-slot and can realize circular polarization at two bands with high efficiency and angular stability. The MS can transform the x-polarized wave into right-hand circular polarization (RHCP) at lower band and left-hand circular polarization (LHCP) at higher band and an opposite role for the y-polarized wave. The results show that the MS operates with insertion loss less than 0.5 dB and 0.3 dB and axial ratio below 3 dB in the frequency range of 9.05-9.65 GHz and 12.55-13.1 GHz, respectively. Moreover, our MS is insensitive to the oblique incident waves and can operate at high performance with the incident angle less than 55°. The proposed MS provides a new avenue to design meta-devices with dual frequency property and also high angular stability.

Giant optical activity in plasmonic chiral structure via double-layer graphene moiré stacking in mid-infrared region

The plasmonic metamaterials and metasurfaces play a critical role in manipulating lights in the mid-infrared spectral region. Here, we first propose a novel plasmonic chiral structure with the giant optical activity in the mid-infrared spectral region. The chiral structure consists of the moiré patterns, which are formed by stacking double-layer graphene nanoribbons with a relative in-plane rotation angle. It is demonstrated that the graphene-based plasmonic structure with moiré patterns exhibits the strong circular dichroism. The giant chiroptical response can be precisely controlled by changing the rotation angle and Fermi level of graphene. Furthermore, a dielectric interlayer is inserted between two layers of graphene to obtain the stronger circular dichroism. Impressively, the strongest circular dichroism can reach 5.94 deg at 13.6 µm when the thickness of dielectric interlayer is 20 nm. The proposed structure with graphene-based moiré patterns can be superior to conventional graphene chiral metamaterials due to some advantage of rotation-dependent chirality, flexible tunability and cost-effective fabrication. It will advance many essential mid-infrared applications, such as chiral sensors, thermal imaging and chiroptical detectors.

Plasmonic metamaterials for chiral sensing applications

Plasmonic metamaterials are artificially designed materials which exhibit optical properties that cannot be found in nature. They have unique and special abilities related to electromagnetic wave control, including strong field enhancement in the vicinity of the surfaces. Over the years, scientists have succeeded in dramatically improving the detection limit of molecular chirality utilizing a variety of plasmonic metamaterial platforms. In this mini-review, we will discuss the principles of most recent issues in chiral sensing applications of plasmonic metamaterials, including suggested formulas for signal enhancement of chiroptical plasmonic sensors, and studies on various platforms that employ different sensing mechanisms.

Chalcogenide-gold dual-layers coupled to gold nanoparticles for reconfigurable perfect absorption

Recently, tunable high absorptance from various nanophotonic structures has been demonstrated. However, most of these structures require nano-lithography, which is expensive and slow. Lithography-free tuneable absorbers are rarely explored for tuneable visible and near-infrared photonics. Herein, we demonstrate a gold (Au)/chalcogenide dual-layer that is resonantly coupled to Au nanoparticles (NPs). The structure exhibits angle and polarisation-independent high absorptance. At resonance, waveguide cavity-like modes are excited between the film and NPs whilst gap plasmon modes are excited between the NPs. Coalescence of the waveguide cavity-like modes, the gap plasmon modes, and the highly absorbing chalcogenide semiconductor not only leads to perfect absorptance but also a reconfigurable response via reversible structural phase transitions in the chalcogenide film. In the amorphous state, the design provides nearly perfect absorptance for both p- and s-polarisation states at an incident angle of 20°. However, after switching to the crystalline state, the peak absorptance spectrally broadens and redshifts from 980 to 1520 nm. This experimental observation was theoretically validated by the finite element method. Thermal-electric modeling was performed to show that the structural transition from crystalline to amorphous states is possible in just 5 ns, thus allowing high-speed reconfigurable perfect absorbers.

Reconfigurable parity-time symmetry transition in phase change metamaterials

Although the conceptually remarkable, experimental illustration of parity-time (PT) symmetric quantum systems stays unexplored, their photonics analogues are explored by taking advantage of the similarity between the Helmholtz and Schrödinger equations. However, the static nature of the constitutive parameters of photonics structure inherently limits an achievement of dynamic PT-symmetry transition. We propose mid-infrared metamaterials (MMs) consisting of two orthogonally orientated metallic meta-atoms with one resonator integrated by a chalcogenide glass, GeTe. The PT-symmetry breaking can be actively transited to PT-symmetry by varying the structural state of GeTe between amorphous and crystalline while fixing the MM geometry. An electric-thermal model is constructed to illustrate that reversible switching between PT-symmetry and PT-symmetry breaking can be obtained in ∼270 ns. Our theoretical work lays the basis for designing high-speed reconfigurable PT-symmetry photonics systems that provide a promising approach for dynamically engineering non-Hermitian quantum symmetry.

Tunable chiroptical response of graphene achiral metamaterials in mid-infrared regime

We numerically investigate a tunable and extrinsic chiroptical response of a graphene achiral metamaterial in mid-infrared regime. The achiral metamaterial is composed of cascaded metallic split ring apertures and complementary graphene rings patterned on a dielectric layer. The strong extrinsic chiroptical responses of the metamaterial are allowed at oblique incidence and the integrated graphene can dynamically modulate extrinsic chirality by changing its Fermi level. The spectra of the chiroptical responses will show a blue shift with increasing the Fermi level of the patterned graphene. The maximal values of circular dichroism in the reflection and transmission modes can reach 80% and 50%, respectively. The maximal values of polarization rotation angle in the reflection and transmission modes can reach 80° and 60°, respectively. This graphene-based metamaterial design paves a way for a myriad of important terahertz (THz) and mid-infrared applications, such as optical modulators, absorbers and polarizers.

Chiral CdSe nanoplatelets as an ultrasensitive probe for lead ion sensing

As opposed to traditional photoluminescence and ultra-violet based optical sensing, we present here a sensing system based on resolved optically active polarization with promising applications. It is based on the ultrathin CdSe nanoplatelets (NPLs) when modified with either l or d-cysteine molecules (l/d-cys) as bio-to-nano ligands. The chiral ligand transfers its chiroptical activity to the achiral nanoplatelets with an anisotropy factor of ∼10-4, which unlocks the chiral excitonic transitions and allows lead ion detection with a limit of detection (LOD) as low as 4.9 nM. Simulations and modelling based on time-dependent density functional theory (TD-DFT) reveal the chiral mechanism of l/d-cys capped CdSe NPLs. The presented CD-based sensing system illustrates an alternative possibility of using chiral CdSe NPLs as competitive chiral sensors for heavy metal ion detection.

3D-printed chiral metasurface as a dichroic dual-band polarization converter

We propose a novel design of a 3D chiral metasurface behaving as a spatial polarization converter with asymmetric transmission. The metasurface is made of a lattice of metallic one-and-a-half-pitch helical particles. The proposed metasurface exhibits a dual-band asymmetric transmission accompanied by the effect of complete polarization conversion. Regarding circularly polarized waves, the metasurface demonstrates a strong circular dichroism. A prototype of the metasurface is manufactured for the quasi-optic experiment by using a 3D printing technique utilizing a cobalt-chromium alloy, which exhibits good performances against thermal fatigue and corrosion at high temperatures.

Chiral plasmonic nanocrescents: large-area fabrication and optical properties

Large-area arrays of substrate-supported plasmonic gold crescents are fabricated by using the new colloidal lithography technique, which is based on an in situ-deposited silica resistance layer. The method provides the means to control the particles' asymmetry just by changing the mutual deposition angle of gold and silica. Asymmetric crescent structures exhibit a pronounced circular dichroism in near-infrared region, with the chiral asymmetry factor reaching 0.2. According to the simulation, the optical chirality enhancement reaches between one and two orders of magnitude and is localized near the crescents' tips.

Manipulation of visible-light polarization with dendritic cell-cluster metasurfaces

Cross-polarization conversion plays an important role in visible light manipulation. Metasurface with asymmetric structure can be used to achieve polarization conversion of linearly polarized light. Based on this, we design a quasi-periodic dendritic metasurface model composed of asymmetric dendritic cells. The simulation indicates that the asymmetric dendritic structure can vertically rotate the polarization direction of the linear polarization wave in visible light. Silver dendritic cell-cluster metasurface samples were prepared by the bottom-up electrochemical deposition. It experimentally proved that they could realize the cross - polarization conversion in visible light. Cross-polarized propagating light is deflected into anomalous refraction channels. Dendritic cell-cluster metasurface with asymmetric quasi-periodic structure conveys significance in cross-polarization conversion research and features extensive practical application prospect and development potential.

Graphene-metal hybrid metamaterials for strong and tunable circular dichroism generation

A strong and dynamically controlled circular dichroism (CD) effect has aroused great attention due to its desirable applications in modern chemistry and life sciences. In this Letter, we propose a graphene-metal hybrid chiral metamaterial to generate mid-infrared CD with an intensity of more than 10%, which can be actively controlled over a wide wavelength range. In addition to the strong tunability, the CD signal intensity of our nanostructure is drastically larger than that of the purely graphene-based chiroptical nanostructures. Our design offers a new strategy for developing tunable chiral metadevices, which could be used in various applications, such as biochemical detection and information processing.

Reconfigurable, graphene-coated, chalcogenide nanowires with a sub-10-nm enantioselective sorting capability

Chiral surface plasmon polaritons (SPPs) produced by plasmonic nanowires can be used to enhance molecular spectroscopy for biosensing applications. Nevertheless, the switchable stereoselectivity and detection of various analytes are limited by a lack of switchable, chiral SPPs. Using both finite-element method simulations and analytic calculations, we present a graphene-coated chalcogenide (GCC) nanowire that produces mid-infrared, chiral SPPs. The chiral SPPs can be reversibly switched between "on" (transparent) and "off" (opaque) by non-volatile structural state transitions in the dielectric constants of the chalcogenide glass Ge2Sb2Te5. Furthermore, by controlling the Fermi energy of the graphene-coating layer, the nanowire can output either non-chiral or chiral SPPs. A thermal-electric model was built to illustrate the possibility of ultrafast on/off switching of the SPPs at the terminus of the nanowire. Finally, we show that a selective, lateral sorting of sub-10-nm enantiomers can be achieved via the GCC nanowire. Chiral nanoparticles with opposite handedness experience transverse forces that differ in both their sign and magnitude. Our design may pave the way for plasmonic nanowire networks and tunable nanophotonic devices, which require the ultrafast switching of SPPs, and provide a possible approach for a compact, enantiopure synthesis.

High-efficiency tunable circular asymmetric transmission using dielectric metasurface integrated with graphene sheet

High-efficiency tunable asymmetric transmission (AT) based on simple-constructed metasurface is highly desired for next generation optical polarization devices. Here, we numerically investigate high-efficiency and frequency-tunable circular AT effect in mid-infrared region by combining simple-shape silicon array with a graphene sheet. The asymmetric parameter of the dielectric nanostrip structure reaches 0.92 at 12.68 THz and the width of tunable spectra (AT > 0.7) is 1100 nm, which represent a major advance compared with previously reported AT. The AT behavior originates from extrinsic chirality induced by oblique illumination, and the high AT efficiency results from the constructive and deconstructive interferences of selectively excited electric and magnetic resonances. In addition, the working waveband of AT is shifted by dynamically modulating graphene's Fermi energy, which offers a new degree of freedom to achieve multifunctions without refabricating structures. The proposed array system possessing the merits of high efficiency, simple inclusions and frequency-tunability has significant potentials for practical applications in polarization devices such as polarization sensor, polarizer, etc.

Meta-Optical Chirality and Emergent Eigen-polarization Modes via Plasmon Interactions

The response of an individual meta-atom is often generalized to explain the collective response of a metasurface in a manner that neglects the interactions between meta-atoms. Here, we study a metasurface composed of tilted achiral meta-atoms with no spatial variation of the unit cell that derives appreciable optical chirality solely from the asymmetric interactions between meta-atoms. The interactions between meta-atoms are considered to stem from the Lorentz force arising from the Larmor radiation of adjacent plasmonic resonators because their inclusion in a simple model accurately predicts the bonding/anti- bonding modes that are measured experimentally. We also experimentally observe the emergence of multiple polarization eigenmodes, among other polarization-dependent responses, which cannot be modeled with the conventional formalism of transmission matrices. Our results are vital to the precise characterization and design of metasurfaces.