Dynamically reversible and strong circular dichroism based on Babinet-invertible chiral metasurfaces

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.

Dual-band terahertz chiral metasurface absorber with enhanced circular dichroism based on temperature-tunable InSb for sensing applications

Circular dichroism (CD) in terahertz (THz) regions has been widely used in biomonitoring, analytical chemistry, communication sensing, and other fields. Herein, we present a simple design for a dual-band THz chiral metasurface absorber (CMA) with a stronger CD effect based on temperature-tunable InSb for enhanced sensing applications. The proposed dual-band CMA consisted of a periodic array of the evolved C-shaped InSb adhered to a copper substrate. The designed CMA at 305 K achieved a right-handed circular polarization (RCP)-selective absorbance of 98.86% and 97.43% at 1.65 THz and 1.89 THz, respectively, and left-handed circular polarization (LCP) absorbance of 9.98% and 22.46%, respectively, and exhibited stronger CD values of 0.89 and 0.75. In addition, the CD properties of the designed CMA can be adjusted by changing the geometrical parameters of the unit-cell structure. The simulated electric field and power follow distributions indicate that this dual-band chiral-selective absorption of the designed CMA is due to the different plasma resonance mode excitations for the incident circular polarization (CP) wave. In addition, the CD properties of the designed CMA can be adjusted by changing the geometrical parameters of the unit-cell structure. Furthermore, CD spectra can be dynamically adjusted by varying the outside temperature and refraction index (RI) of the filled analytes. The designed dual-band CMA can function as a high-performance temperature sensor with sensitivities of 4.68 GHz K-1 and 5.52 GHz K-1 and also as an RI sensor with sensitivities of 1080 GHz RIU-1 and 860 GHz RIU-1, respectively. Our proposed tunable dual-band CMA with its exquisite performance has the potential to be widely applied in diverse areas such as detection, sensing, and other related optoelectronic fields.

Enhancement and sensing applications of ultra-narrow band circular dichroism of the chiral nanopore films based on Bragg reflector

Narrow-band circular dichroism (CD) has attracted considerable attention in the high-sensitivity detection of chiral molecules and chiral catalysis. However, achieving dynamic adjustment of narrow-band CD signals is challenging. In this study, we introduce a disruption layer (DL) and molybdenum disulfide (MoS2) into an L-shaped chiral nanohole array based on a distributed Bragg reflector (DBR), forming L-shaped chiral nanoholes (LCNAs/DL-DBR/MoS2), and investigate the mechanism of CD signal generation. Simulation results show that LCNAs/DL-DBR/MoS2 generate three narrow-band CD signals in the visible region. Analysis of the near-field electric field maps reveals that the three CD peaks of LCNAs/DL-DBR/MoS2 are caused by three Tamm resonances in the DBR layer. The producing and adjusting mechanisms of the CD signals are achieved by changing the structural parameters and the number of MoS2 layers. Dynamic adjustment of the CD signals of LCNAs/DL-DBR/MoS2 can be achieved by changing the environmental temperature. Furthermore, by altering the refractive index of the environment and the DBR layer, it is demonstrated that LCNAs/DL-DBR/MoS2 has a high-quality factor. Our theoretical simulations aid in the design of UNB chiral devices, opening up new avenues for environmental monitoring and the detection of chiral molecules with exceptional sensitivity.

Broadband and strength-switchable circular dichroism in a Ge2Sb2Te5-based metasurface

Circular dichroism (CD) is highly required in the applications of biological detection and analytical chemistry. In this paper, we achieved a giant, broadband, and strength-switchable CD effect in a quadruple z-shaped G e 2 S b 2 T e 5 (GST) metasurface. At the amorphous state of GST (a-GST), the giant CD reaches 0.92 and the width of the absorption >0.80 is about 100 nm. The giant and broadband CD originates from polarization selective excitations of Mie resonances and the coupling between subunit resonators. With the transition from a-GST to crystalline GST, CD could be dynamically switched from 0.92 to 0.05. The GST-based metasurfaces with giant and wide-range switching CD will promote the development of active chiral devices.

Terahertz broadband tunable chiral metamirror based on VO2-metal hybrid structure

Aiming at the problems of narrow working bandwidth, low efficiency, and complex structure of existing terahertz chiral absorption, we propose a chiral metamirror composed of C-shaped metal split ring and L-shaped vanadium dioxide (VO₂). This chiral metamirror is composed of three layers of structure, a gold substrate at the bottom, the first polyethylene cyclic olefin copolymer (Topas) dielectric layer and VO₂-metal hybrid structure as the top. Our theoretical results led us to show that this chiral metamirror has a circular dichroism (CD) value greater than 0.9 at 5.70 to 8.55 THz and has a maximum value of 0.942 at f = 7.18 THz. In addition, by adjusting the conductivity of VO₂, the CD value can be continuously adjustable from 0 to 0.942, which means that the proposed chiral metamirror supports the free switching of the CD response between the on and off states, and the CD modulation depth exceeds 0.99 in the range of 3 to 10 THz. Moreover, we discuss the influence of structural parameters and the change of incident angle on the performance of the metamirror. Finally, we believe that the proposed chiral metamirror has important reference value in the terahertz range for constructing chiral light detectors, CD metamirrors, switchable chiral absorbers and spin-related systems. This work will provide a new idea for improving the terahertz chiral metamirror operating bandwidth and promote the development of terahertz broadband tunable chiral optical devices.

Adjustable strong circular dichroism based on a tricircular arc metasurface

Circular dichroism has promising applications in biology, molecular chemistry, and other fields. The key to obtaining strong circular dichroism is to introduce symmetry breaking into the structure, which leads to a great difference in the response to different circularly polarized waves. Here, we propose a metasurface structure based on three circular arcs, which produces strong circular dichroism. The metasurface structure combines the split ring with the three circular arcs and increases the structural asymmetry by changing the relative torsional angle. The causes of the strong circular dichroism are analyzed in this paper, and the influence of metasurface parameters on it is discussed. According to the simulation data, the response of the proposed metasurface to different circularly polarized waves varies greatly, with absorption of up to 0.99 at 5.095 THz for a left-handed circularly polarized wave and a maximum circular dichroism of over 0.93. In addition, the incorporation of the phase change material vanadium dioxide on the structure allows flexible modulation of circular dichroism and modulation depths of up to 98.6%. The change of angle within a certain range has little effect on the structural performance. We believe that this flexible and angle robust chiral metasurface structure is suitable for complex reality, and large modulation depth is more practical.

Actively tunable linear and circular dichroic metamirrors based on single-layer graphene

Aiming at the problems of low efficiency, single function and complex structure of the existing dichroic metamirrors, the actively tunable linear and circular dichroic metamirrors based on single-layer graphene are proposed in this study. The designed metamirrors are mainly composed of the ion-gel, patterned graphene, polyimide, polysilicon and gold substrates. The anisotropy of the achiral structures can be used to realize circular dichroism (0.8) and linear dichroism (0.9) in two directions at the same time without functional switching. Additionally, the incidence angle of electromagnetic waves, rather than the structural chirality, is used to create the exceptionally strong dichroism. The proposed metamirrors not only increase the integration, but also reduce the angular dispersion and complexity of the structure. What's more, by changing the Fermi level of graphene, the CD function of the metamirrors can be tuned in the range of 0 - 0.8, and the LD function can be tuned in the range of 0.22 - 0.9. The designed metamirrors can achieve dual functions under a wide range of incident angles, and can be widely used in various fields such as terahertz imaging, biological detection, optical sensing, and spectrometry.

Dual-band tunable and strong circular dichroism in a metal-graphene hybrid zigzag metasurface

Most active chiral metasurfaces operate in a single band and have an unidirectionally tunable circular dichroism (CD). Here, we propose a zigzag metasurface composed of a Z-shaped metallic strip and a L-shaped graphene strip to realize the dual-band tunable and strong CD. Two strong CD values of -0.88 and 0.88 are found at f₁= 0.86 THz and f₂= 1.23 THz, respectively. The strengths and resonant frequencies of these two CD signals can be tuned by varying graphene's Fermi energy (E_F). Strikingly, the CD value at 0.86 THz undergoes a continuous adjustment in a large range from 0.79 to -0.88 when E_F increases from 0.32 eV to 1.00 eV, implying that the proposed metasurface supports the switching of CD signal between on-, off- and reverse-states. Based on the strong CD signals, the capability of the metasurface as a biosensor to detect Avian Influenza viruses is demonstrated. This work will advance the development of broadband tunable chiral-optical devices.

Mid-infrared reconfigurable all-dielectric metasurface based on Ge2Sb2Se4Te1 phase-change material

In this paper, a reconfigurable all-dielectric metasurface based on Ge₂Sb₂Se₄Te₁ (GSST) phase-change material is proposed. By changing GSST from amorphous state to crystalline state, the metasurface can achieve high circular dichroism (CD) and wideband polarization conversion for circularly polarized waves in the mid-infrared (MIR) band. The maximum CD value reaches 0.95 at 74 THz, and circular polarization conversion efficiency is more than 90% in the wideband range of 41 THz - 48 THz. In addition, based on Pancharatnam-Berry phase, three kinds of wavefront manipulation of light have been realized: abnormal refraction, orbital angular momentum vortex beam and orbital angular momentum vortex beam splitting. This work has potential applications in the future MIR optical integrated system.

Polarization-sensitive tunable extraordinary terahertz transmission based on a hybrid metal-vanadium dioxide metasurface

Thermally tunable extraordinary terahertz transmission in a hybrid metal-vanadium dioxide (VO₂) metasurface is numerically demonstrated. The metasurface consists of a metal sheet perforated by square loops, while the loops are connected with strips of VO₂. The frequency and amplitude of the transmission resonance are modulated by controlling the conductivity of VO₂. For a y-polarized incident field, the resonance transmission peak redshifts from 0.88 to 0.81 THz upon insulator-to-metallic phase transition of VO₂. For an x-polarized incident field, the transmission resonance at 0.81 THz is observed in the insulator phase. However, in the metallic phase of VO₂, the electromagnetic field is effectively reflected in the 0.5-1.1 THz range with a transmission level lower than 0.14. The proposed metasurface can be utilized as a terahertz modulator, reconfigurable filter, or switch.

Ultra-narrow-band circular dichroism by surface lattice resonances in an asymmetric dimer-on-mirror metasurface

Narrow-linewidth circular dichroism (CD) spectroscopy is a promising candidate to push the limits of molecular handedness detection toward a monolayer or even to a single molecule level. Here, we designed a hybrid metasurface consisting of a periodic array of symmetry-breaking dielectric dimers on a gold substrate, which can generate strong CD of 0.44 with an extremely-narrow linewidth of 0.40 nm in the near-infrared. We found that two surface lattice resonance modes can be excited in the designed metasurface, which can be superimposed in the crossing spectral region, enabling a remarkable differential absorption with a high Q-factor for circular polarizations. The multipole decomposition of the resonance modes shows that the magnetic dipole component contributes most to the CD. Our simulation results also show that the CD response of the chiral structure can be engineered by modulating the structural parameters to reach the optimal CD performance. Ultra-narrow-linewidth CD response offered by the proposed metasurface with dissymmetry provides new possibilities towards design of the high-sensitive polarization detecting, chiral sensing and efficient chiral light emitting devices.

Dual-band terahertz all-silicon metasurface with giant chirality for frequency-undifferentiated near-field imaging

Chiral metasurfaces are widely used in imaging and biosensing due to their powerful light field control capabilities. Most of the work is devoted to achieving the goals of chirality enhancement and tunability, but lacks consideration of design complexity, loss, cost, and multi-band operation. In order to alleviate this situation, we propose a pair of dual-frequency giant chiral structures based on all-silicon, which can achieve excellent and opposite spin-selective transmission around 1.09 THz and 1.65 THz. The giant chirality derives from the in-plane electric and magnetic dipole moments excited in different degrees. Theoretically, the maximum circular dichroism at the two frequencies are both as high as 0.34, and the coverage bandwidths of the two giant chirality are 85.5 GHz and 41.4 GHz, respectively. The experimental results are in good agreement with the simulation results. Based on the dual-band giant chiral patterns, the terahertz near-field imaging of different Chinese character images is demonstrated at two frequencies. The frequency-undifferentiated characteristics, good intensity contrast and three-dimensional imaging information are shown by the results. This work provides new ideas for the design of terahertz devices with simple structure and multi-functions, which are expected to be applied in the field of terahertz imaging or multi-channel communication.

Design and simulation of a GST-based metasurface with strong and switchable circular dichroism

Circular dichroism (CD) is required in the applications of biological detection, analytical chemistry, etc. Here, we numerically demonstrated large-range switchable CD by controlling the phase change of Ge₂Sb₂Te₅ (GST) in a zigzag array. At the amorphous state of GST (a-GST), the strong and dual-waveband CD effects are realized via the selective excitations of electric, magnetic, and toroidal resonances. With the transition from a-GST to crystalline state GST, CD strengths are tailored dynamically in large ranges. In detail, the CD magnitudes change by about 0.93 and the modulation depths exceed 94% at dual wavebands. The strong CD effects and large-range switch capability in the GST-based metasurfaces will boost the development of active chiroptical devices.

Switchable Chiral Metasurface for Terahertz Anomalous Reflection Based on Phase Change Material

A switchable chiral metasurface based on a phase change material Ge2Sb2Te5, which can switch between a right-handed circularly polarized mirror and a left-handed circularly polarized mirror, is theoretically discussed. When the conductivity of Ge2Sb2Te5 σ is 0 S/m, the metasurface will reflect incident right-handed circularly polarized light and absorb incident left-handed circularly polarized light at 0.76 THz. As σ is set to 3 × 105 S/m, the response of the metasurface to circularly polarized light will be reversed. That is, it reflects the incident left-handed circularly polarized light and absorbs the incident right-handed circularly polarized light at 0.66 THz. The circular dichroism is from 76% to −64%. Then, we also study the performance of the mirror structure of the initial metasurface. By simulating the reflected spectra with different conductivities and the surface current distribution, the reason for the switchable function is clear. Moreover, the switchable chiral metasurface can be applied in spin-selective beam deflectors, which is proven by simulation. This work provides a new strategy for the development of tunable chiral devices.

Switchable polarization manipulation at the telecom wavelength based on L-shaped hybrid Au-VO2 nanoholes

We propose to achieve switchable polarization manipulation at the telecom wavelength at nanoscale based on L-shaped plasmonic nanoholes in an Au-VO₂ film. The L-shaped nanohole acts as a quarter-wave plate or a half-wave plate owing to the phase differences between different plasmon resonant modes, which is controlled by the insulator or metallic phases of VO₂. In addition, by changing the structure and removing the bottom Au layer, a switchable full-/quarter-wave plate can be achieved when VO₂ transits from the insulating state to the metallic state. Furthermore, we vary the geometrical parameters of the L-shaped hole to tune its resonant spectra and achieve a switchable full-wave plate/polarizer. The multifunctional switchable polarization manipulation abilities together with large bandwidths enable the proposed structures promising applications in nanophotonics and integrated optics.

A dual band spin-selective transmission metasurface and its wavefront manipulation

Chiral metasurfaces which can achieve different optical responses for left-handed and right-handed circularly polarized (CP) light have been proposed. Most of the research studies on chiral metasurfaces focus on improving circular dichroism (CD) and realizing dynamic manipulation of the chiro-optical response. However, there have only been a few reports on the multi-band chiro-optical response. Here, we propose an all-silicon chiral meta-atom which can realize spin-selective transmission in a dual band. In addition, a terahertz metasurface with spin-selective transmission through phase arrangement is designed by using chiral meta-atoms satisfying a gradient geometric phase. Under left-hand circularly polarized (LCP) incidence, the metasurface generates a focused right-hand circularly polarized (RCP) beam which is focused at a distance of 4.8 mm from the exit surface of the metasurface. Our work broadens the concept of metasurface design and may attract more researchers' attention on the applications of chiral metasurfaces.

Strong extrinsic chirality in biaxial hyperbolic material α-MoO3 with in-plane anisotropy

Chirality has always been a hot research topic because it possesses potential applications in polarization optics, chemical and biosensing. In the previous works, intrinsic chirality has been extensively explored, but its development is limited due to the complexity in fabrication of chiral metamaterials. Therefore, there is an urgent need to simplify fabrication and design of compact devices with chiral response. Extrinsic chirality has shown great potential because it can be realized in nonchiral anisotropic planar structures with low-cost fabrication techniques. In this paper, the extrinsic chirality of biaxial hyperbolic material $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ with in-plane anisotropy has been investigated. By analyzing the effect of thickness of $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film, the angle of incidence, azimuth angle, and wavelength of incidence on the circular dichroism (CD), the maximum CD can reach 0.77. This strong extrinsic chirality of the $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film results from the mutual orientation of the $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film and the incident light. In addition, $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film can still maintain strong extrinsic chirality when the azimuthal angle ranges from approximately 20°-57° and the angle of incidence is from 55°-80°.