Symmetric Metasurfaces Research Articles

Manifestation of super chiral exceptional points in a plasmonic metasurface

The exceptional point (EP), a degenerate point within non-Hermitian parametric space, has attracted considerable attention, especially for its chiral responses. However, achieving ideal circular dichroism (CD) remains challenging due to the existence of parallel components at the EP. Here, we delve into the theoretical condition required to attain a zero-transmission parallel component. This condition, together with the chiral EP condition, leads to a point characterized by near-unity CD, termed the super chiral EP (SCEP). To illustrate our theoretical framework, we introduce a parity-time symmetric metasurface with gain and loss. The observation of SCEP is demonstrated by tuning both the coupling strength and gain–loss ratio. Furthermore, we explore distinctive properties of SCEP, including phase flip and unidirectional invisibility. Leveraging SCEP and the topological phase transition point, we achieve polarization states across the entire Poincaré surface. This work opens avenues for potential applications in polarizing optical elements, holography, logic gates, chiral molecular detection, ultrasensitive sensing, and polarization-sensitive imaging.

Polarization-independent quasi-BIC supported by non-rotationally symmetric dimer metasurfaces.

Asymmetric metasurfaces supporting quasi-bound states in the continuum (-BICs) have recently attracted significant interest in the field of nanophotonics due to their high quality factor and strong light-matter interaction properties. However, asymmetric metasurface structures are susceptible to the polarization state of the incident light, which constrains their potential applications. In this Letter, we present a new, to our knowledge, scheme of polarization-independent quasi-BIC resonance supported by a non-rotationally symmetric nanorod dimer metasurface. By tuning the asymmetry parameter, the designed metasurface exhibits a consistent quasi-BIC response for incident plane waves of arbitrary polarization. The physical mechanism of the quasi-BIC resonance is elucidated by the study of the far-field multipole decomposition and the near-field electromagnetic distribution. We then point out that the realization of the polarization-independent quasi-BIC resonance depends on the transition between magnetic and electric quadrupoles. Furthermore, the designed metasurface is demonstrated to have excellent refractive index sensing performance. This work provides a new idea for the design of polarization-independent and high-performance resonators.

Magnetic quadrupolar resonance in a symmetric three-layered plasmonic metasurface

Dark plasmon mode with even symmetry presents an effective way for manipulating light-matter interactions due to its attractive properties of small radiative loss and strongly intensified near-field, which opens up space for both fundamental explorations and optoelectronic device developments. In this paper, we demonstrate magnetic quadrupolar resonance in a symmetric plasmonic metasurface under oblique light incidence. In a three-layered metasurface made of a cross-shaped gold array on top of a magnesium fluoride film backed with a copper ground plane, a magnetic quadrupolar resonance is excited at 37.3 THz with a quality factor of 34, which is four times of the regular magnetic dipolar resonance excited in the same structure. By increasing the incident angle, asymmetry perturbation on the magnetic quadrupolar mode becomes more pronounced as evidenced from its decreased quality factor. In addition, due to the structure symmetry of the used metal cross, the metasurface exhibits polarization-selective excitation of two separate magnetic quadrupolar modes at 37.3 THz and 42.3 THz for the electric field along four symmetry axes of the structure. Our demonstrated distinctive magnetic quadrupolar resonance offers an alternative mode design in plasmonic metamaterials with an improved quality factor, which could be beneficial for metamaterial applications in sensing, optical filtering and infrared emission manipulation etc.

Anapole mechanism of bound states in the continuum in symmetric dielectric metasurfaces

Anapole mechanism of bound states in the continuum in symmetric dielectric metasurfaces

Simple strategy for the simulation of axially symmetric large-area metasurfaces

Metalenses are composed of nanostructures for focusing light and have been widely explored in many exciting applications. However, their expanding dimensions pose simulation challenges. We propose a method to simulate metalenses in a timely manner using vectorial wave and ray tracing models. We sample the metalens’s radial phase gradient and locally approximate the phase profile by a linear phase response. Each sampling point is modeled as a binary blazed grating, employing the chosen nanostructure, to build a transfer function set. The metalens transmission or reflection is then obtained by applying the corresponding transfer function to the incoming field on the regions surrounding each sampling point. Fourier optics is used to calculate the scattered fields under arbitrary illumination for the vectorial wave method, and a Monte Carlo algorithm is used in the ray tracing formalism. We validated our method against finite-difference time domain simulations at 632 nm, and we were able to simulate metalenses larger than 3000 wavelengths in diameter on a personal computer.

High-Q quasi-bound states in the continuum in C2-symmetric metasurface with enhanced second harmonic generation in two-dimensional materials

Quasi-bound states in the continuum (Quasi-BIC) possess a high quality-factor (Q-factor) that can amplify local electromagnetic fields and nonlinear effects such as second harmonic generation (SHG). In this paper, we propose a silicon metasurface with C2 symmetry. By tuning the structural parameters without breaking the C2 symmetry, three symmetric protected BICs (SP-BICs) are transformed into sharp quasi-BICs dominated by different modes in the nearinfrared spectrum. The quasi-BICs exhibit Q-factors as high as 10370 in C2 symmetric metasurface. Numerical simulations reveal that the toroidal dipole (TD) and magnetic dipole (MD) enhance intercellular coupling and local electromagnetic fields, laying the foundation for generating metasurface with high Q-factor. Furthermore, the longitudinal radiative component of electric quadrupole (EQ) outside the facet promotes enhanced light interaction with the 2D material. Leveraging the robust local electromagnetic fields and radiation characteristics of these quasi-BICs, we have achieved substantial SHG enhancement across multiple wavelengths under continuous wave (CW) operation. The SHG from 2D GaSe flake reaches 14189-fold enhancement. Our results not only offer insights into designing metasurface structures with high Q-factors but also explore the methods of enhancing the interaction between metasurface and 2D materials. The proposed nanostructure has numerous potential applications in nonlinear optics, optoelectronics and signal processing.

Tunable C4-Symmetry-Broken Metasurfaces Based on Phase Transition of Vanadium Dioxide (VO2).

Coupling is a ubiquitous phenomenon observed in various systems, which profoundly alters the original oscillation state of resonant systems and leads to the unique optical properties of metasurfaces. In this study, we introduce a terahertz (THz) tunable coupling metasurface characterized by a four-fold rotation (C4) symmetry-breaking structural array achieved through the incorporation of vanadium dioxide (VO2). This disruption of the C4 symmetry results in dynamically controlled electromagnetic interactions and couplings between excitation modes. The coupling between new resonant modes modifies the peak of electromagnetic-induced transparency (EIT) within the C4 symmetric metasurfaces, simulating the mutual interference process between modes. Additionally, breaking the C4 symmetry enhances the mirror asymmetry, and imparts distinct chiral properties in the far-field during the experimental process. This research demonstrates promising applications in diverse fields, including biological monitoring, light modulation, sensing, and nonlinear enhancement.

Ultra-thin freestanding terahertz frequency selective surface on flexible cyclic olefin copolymer

Frequency selective surfaces (FSSs) are widely employed in spectrometers, selective absorbers, energy harvesting, and sensing devices. However, in the terahertz range, the performance of this ideal component is frequently constrained by the choice of material, which introduces a certain degree of attenuation, thereby diminishing the signal-to-noise ratio. Moreover, these FSS are often bulky and demonstrate a low extinction ratio, which limits their usage in wearables and miniaturised devices. In this work, a multi-band FSS composed of periodic microstructures on an ultrathin cyclic olefin copolymer sheet is proposed, analysed, fabricated, and evaluated using terahertz-time domain spectroscopy. The unit cell is composed of triple, evenly spaced, horizontal gold strips, linked around the middle by a fourth vertically oriented gold strip. By displacing the vertical strip, the asymmetric metasurface shows dual narrowband transmission at 1.04 THz and 1.67 THz. However, only a single narrowband transmission at 1.07 THz can be observed on a symmetric metasurface, with no displacement. The calculated Q factors are 4.52 and 16.63 at 1.04 THz and 1.67 THz, respectively, for the asymmetric metasurface. While for the symmetric metasurface, the calculated Q factor at 1.07 THz is 3.63. The proposed flexible metasurface can be tailored easily as single or dual narrowband frequency selective metasurface for channel filtering and broadband sources in emerging terahertz wireless systems.

Independent control of circularly polarized light with exceptional topological phase coding metasurfaces

Exceptional points, as degenerate points of non-Hermitian parity-time symmetric systems, have many unique physical properties. Due to its flexible control of electromagnetic waves, a metasurface is frequently used in the field of nanophotonics. In this work, we developed a parity-time symmetric metasurface and implemented the 2π topological phase surrounding an exceptional point. Compared with Pancharatnam-Berry phase, the topological phase around an exceptional point can achieve independent regulation of several circular polarization beams. We combined the Pancharatnam-Berry phase with the exceptional topological phase and proposed a composite coding metasurface to achieve reflection decoupling of different circular polarizations. This work provides a design idea for polarimetric coding metasurfaces in the future.

Dynamic and Polarization-Independent Wavefront Control Based on Hybrid Topological Metasurfaces.

Exceptional points (EPs), known as non-Hermitian singularities, have been observed and investigated in parity-time symmetric metasurfaces. However, the chirality and tunability in non-Hermitian metasurfaces still need to be explored. Here, we propose a dynamic topological metasurface with the meta-atom consisting of two orthogonally oriented nanorods, which are placed on the phase change material Ge2Sb2Te5 (GST) and SiO2 dielectric layer, respectively. When GST is converted from the amorphous state (a-GST) to the crystalline state (c-GST), an EP can be dynamically switched from the "ON" state to the "OFF" state in a parameter space. Moreover, based on the topologically protected phase and amplitude modulations of the cross-polarization component, the phase-only hologram and amplitude-only hologram are engineered in the a-GST case and concealed in the c-GST case. Finally, we explore the 2D-chiral symmetry of meta-atoms and further propose two spin-selective meta-deflectors and a hybrid meta-deflector operating with arbitrary polarizations. The GST-based hybrid metasurface offers richer possibilities to realize various wavefront controls.

Generating Angular-Varying Time Delays of THz Pulses via Direct Space-to-Time Mapping of Metasurface Structures.

We experimentally demonstrate the generation of double terahertz (THz) pulses with tailored angular-dependent time delays from a nonlinear metasurface excited by a near-infrared femtosecond pulse. The tailored temporal properties of the generated pulses emerge from a direct mapping of the nonlinear spatial response of the metasurface to the emitted THz temporal profile. We utilize the Pancharatnam-Berry phase to implement symmetric and antisymmetric metasurface configurations and show that the emitted patterns present spatiotemporal "X-shaped" profiles after collimation by a parabolic mirror, with angular-dependent pulse delays corresponding to the intended design. In addition, we show that the addition of polarization multiplexing presents the opportunity to achieve a full range of elliptical THz polarizations. Double pulse generation and spatiotemporal shaping of THz waves in general show potential for THz spectroscopy and molecular dynamics applications, particularly in pump-probe experiments.

Temporal coupled-mode theory for PT-symmetric chiral metasurfaces.

We have developed a temporal coupled-mode theory based on quasi-normal modes to investigate the chiroptical effects in parity-time (PT) symmetric metasurfaces. The PT symmetry enforces a different constraint for the direct scattering matrix and the coupling constants, which is verified by calculating the transmission spectra originating from the chiral quasi-bound states in the continuum. What's more, the scattering matrix can be analytically continued to the complex frequency plane. We find that the zero and pole singularities of the transmission coefficients and scattering matrix play an important role in the optical chirality. The pole singularities carry a quantized topological charge of -1. Our work paves the way for studying the enhanced optical chirality in non-Hermitian metasurfaces.

Achiral nanoparticle trapping and chiral nanoparticle separating with quasi-BIC metasurface.

Dielectric metasurfaces based on quasi-bound states in the continuum (quasi-BICs) are a promising approach for manipulating light-matter interactions. In this study, we numerically demonstrate the potential of silicon elliptical tetramer dielectric metasurfaces for achirality nanoparticle trapping and chiral nanoparticle separation. We first analyze a symmetric tetramer metasurface, which exhibits dual resonances (P1 and P2) with high electromagnetic field intensity enhancement and a high-quality factor (Q-factor). This metasurface can trap achiral nanoparticles with a maximum optical trapping force of 35 pN for 20 nm particles at an input intensity of 100 mW. We then investigate an asymmetric tetramer metasurface, which can identify and separate enantiomers under the excitation of left-handed circularly polarized (LCP) light. Results show that the chiral optical force can push one enantiomer towards regions of the quasi-BIC system while removing the other. In addition, the proposed asymmetric tetramer metasurface can provide multiple Fano resonances (ranging from R1 to R5) and high trap potential wells of up to 33 kBT. Our results demonstrate that the proposed all-dielectric metasurface has high performance in nanoparticle detection, with potential applications in biology, life science, and applied physics.

Controlling the directional excitation of surface plasmon polaritons using tunable non-Hermitian metasurfaces.

In this work, we propose an efficient approach to controlling the directional excitation of surface plasmon polaritons (SPPs) by dynamically modulating the real-part perturbation in a passive parity-time symmetric metasurface. This non-Hermitian system can experience two exceptional points that can induce two unidirectional excitation states of SPPs along opposite directions. Empowered by its superior modulation depth, the energy ratio and energy intensities of two excited SPP states can be effectively manipulated by this non-Hermitian metasurface. To demonstrate these findings, we design and numerically verify non-Hermitian metasurfaces integrated with an Sb2Se3 phase-change material. Our work provides a promising platform for the controllable engineering of SPP excitations, holding significant potential for the development of new plasmonic devices, including on-chip SPP sources, routers and sorters, and integrated optical circuits.

Theoretical analysis of transmission and phase coverage by a symmetric metasurface across dissimilar media

Metasurfaces are widely used for manipulating wavefront control, but so far, they are mainly applied within a single medium. While perfect transmission and entire 2π-phase coverage are possible if metasurfaces are installed in a single medium, there is no theoretical analysis of the transmission efficiency and phase coverage when they are installed to manipulate wavefront across dissimilar media. This study theoretically analyzes the maximum efficiency of the symmetric metasurfaces across dissimilar media and possible phase changes for the metasurface of the maximum efficiency. Our analysis shows that the transmission efficiency can no longer be 100%, and the maximum efficiency can be achieved only at discrete phase changes. If the entire 2π-phase should be covered, the efficiency becomes significantly hampered. We also carried out numerical simulations to validate our theoretical analysis where acoustic wavefronts are manipulated for beam steering and focusing using metasurfaces across two dissimilar media. This study gives insight into the future metasurface research aiming at perfect transmission with nearly entire 2π-phase coverage across dissimilar media.

An ultra-thin high-efficiency plasmonic metalens with symmetric split ring transmitarray metasurfaces

Metasurface lenses (or metalenses) have aroused great attentions and efforts in the community of metamaterials or metasurfaces due to its ultrathin device dimension and superior focusing performances. High-efficiency transmissive metalenses with an ultra-thin device thickness are an important aspect especially in the low frequency by using plasmonic transmitarray antennas. In this paper, an ultra-thin plasmonic metalens with only 0.1λ (λ is working wavelength, the aperture size is 7λ) device thickness is designed by changing the radius of the proposed symmetric complementary split ring resonator antenna metasurfaces. Thanks to its high transmittance and large phase shift of the plasmonic meta-atoms, the designed metalens achieves both high transmissive efficiency of 80% and high focusing efficiency of 50% on the focal plane of F = 4.6λ in the simulations. The designed ultra-thin plasmonic metalens has a moderate large numerical aperture of 0.67 (NA = 0.67). In order to verify its high working efficiency of the proposed plasmonic metalens, a sample is also fabricated and a much higher focusing efficiency of 65% is realized in the measurements. The influence of the open angles of the symmetric split ring transmitarray metasurface on the focusing performances such as working efficiency and NA of the designed metalens is also studied and analyzed finally, which can add new degree of freedoms to optimize its focusing performance. The presented studies can facilitate the development of high-efficiency metalenses in the low frequency and have significant potential applications in high-resolution microwave imaging, high-gain metalens antennas and others.

Plasmonic bound states in the continuum for unpolarized weak spatially coherent light

Plasmonic resonances empowered by bound states in the continuum (BICs) offer unprecedented opportunities to tailor light–matter interaction. However, excitation of high quality-factor ( Q -factor) quasi-BICs is often limited to collimated light at specific polarization and incident directions, rendering challenges for unpolarized focused light. The major hurdle is the lack of robustness against weak spatial coherence and poor polarization of incident light. Here, addressing this limitation, we demonstrate sharp resonances in symmetric plasmonic metasurfaces by exploiting BICs in the parameter space, offering ultraweak angular dispersion effect and polarization-independent performance. Specifically, a high- Q ( ≈ 71 ) resonance with near-perfect absorption ( > 90 % ) is obtained for the input of unpolarized focused light covering wide incident angles (from 0° to 30°). Also, giant electric and magnetic field enhancement simultaneously occurs in quasi-BICs. These results provide a way to achieve efficient near-field enhancement using focused light produced by high numerical aperture objectives.

An all-dielectric metasurface based on Fano resonance with tunable dual-peak insensitive polarization for high-performance refractive index sensing.

A symmetric all-dielectric metasurface based on silicon and GaAs is proposed and numerically studied. In the mid-infrared region, two Fano resonant peaks with a reflectance exceeding 90% are observed. By altering the geometric parameters of the metasurface, the wavelength location and quality factor (Q-factor) of the resonant peaks can be tuned. The highest Q-factors can be 9609.67 and 3476.33, respectively. The proposed metasurface structure for optical refractive index sensing shows high performance and is insensitive to the plane wave's polarization state. In the refractive index range of 1.00 to 1.10, the highest sensitivity and figure of merit (FoM) are 1901.34 nm RIU-1 and 2492.04 RIU-1, respectively. The highest sensitivity is 2248.57 nm RIU-1 and FoM is 977.64 RIU-1 in the refractive index range of 1.30 to 1.40. These research results will help improve and innovate related sensing technologies and devices.

Long-wave infrared to short-wave infrared upconversion through sum-frequency generation in a silicon symmetric metasurface integrated with two-dimensional material

Optical resonances supported by subwavelength nanostructures have significant local-field enhancement effects and are important in the field of nanophotonics such as photoemission and nonlinear optics. In this paper, silicon metasurface with toroidal dipole resonance in the short-wave infrared band is investigated. The formation and properties of the high-quality toroidal dipole resonance are analyzed in detail by the near- and far-field analysis, and the electromagnetic multipole decomposition methods in the metasurface with different structural parameters. In addition, a layer of two-dimensional nonlinear material GaSe is combined with the silicon metasurface, to enhance the long-wave infrared to short-wave infrared upconversion by the nonlinear sum-frequency generation (SFG) process through toroidal dipole resonance. The upconversion efficiency in the GaSe/ silicon metasurface bilayer system is nearly a thousand times more than that in the GaSe/planar silicon bilayer system, indicating the effectiveness of the toroidal dipole resonance generated by the metasurface. This method provides a feasible and viable strategy for upconverting mid-wave infrared or long-wave infrared to near- or short-wave infrared by the nonlinear process induced by the metasurface, which is potentially very attractive for the sensing and imaging of long-wavelength photons that beyond the capability of mature silicon/germinium detectors.

Acoustic impurity shielding induced by a pair of metasurfaces respecting PT symmetry

Impurities usually affect the transmission of acoustic signals, and how to effectively suppress their adverse effects is an important content in the research of acoustic materials. In a recent theoretical proposal, it was pointed out that perfect transmission through impurities can be achieved by adding a gain-loss distribution respecting parity-time ($\mathcal{PT}$) symmetry. In this paper, we demonstrate a similar but not identical way to realize the extraordinary physical property of impurity-shielding, which leads to perfect transmission at the so-called exceptional points. By systematically probing into the system composed of an equivalent medium slab sandwiched by a pair of $\mathcal{PT}$-symmetric metasurfaces, we obtain the two complementary solutions of exceptional points corresponding to perfect transmission. Interestingly, the two solutions of exceptional points coalesce into one when the mass density of the slab approaches zero. At such a critical point, we find that the impurity-shielding effect can be perfectly demonstrated, irrespective of embedded impurities of almost any shape and materials, whether they are Hermitian or non-Hermitian. In addition, the proposed system is frequency independent, indicating that the prototype can work well even in the subwavelength range. Our paper shows that exceptional points can be used to eliminate scattering of impurities in a density-near-zero medium, revealing their potential applications in acoustic sensing, directional imaging, and other related wave disciplines.