Active perfect absorber based on planar anisotropic chiral metamaterials

Gradient-assisted metasurface absorber with dual-band chiral switching and quasi-linearly tunable circular dichroism

Chiral metasurfaces with tunable or switchable circular dichroism (CD) responses hold great potential for advanced optical devices. In this work, we theoretically propose and numerically demonstrate a chiral metasurface absorber composed of periodically serrated Ge2Sb2Te5 (GST) resonators. By harnessing strong plasmonic resonance using the gradient geometry, we achieve a strongly enhanced chiral response with a CD value of 0.98 at λ2 = 2359 nm and a CD value of 0.7 at λ1 = 2274 nm. Additionally, by controlling the gradient difference in the serrated GST resonator, we can modify the CD intensity in multiple dimensions and near-perfectly optimize the chiral properties. Furthermore, it is worth noting that the CD value can be strongly varied by adjusting the phase transition characteristics of GST in the range of 0.007 to 0.7 at λ1 and 0.002 to 0.98 at λ2, corresponding to a switch between "on" and "off" states. The findings give new insight into multi-functional chiroptics and hold wide applications.

Tunable metasurface for independent controlling radar stealth properties via geometric and propagation phase modulation

Metasurfaces have been verified as an ideal way to control electromagnetic waves within an optically thin interface. In this paper, a design method of a tunable metasurface integrated with vanadium dioxide (VO₂) is proposed to realize independent control of geometric and propagation phase modulation. The reversible conversion of VO₂ between insulator phase and metal phase can be realized by controlling the ambient temperature, which enables the metasurface to be switched quickly between split-ring and double-ring structures. The phase characteristics of 2-bit coding units and the electromagnetic scattering characteristics of arrays composed of different arrangements are analyzed in detail, which confirms the independence of geometric and propagation phase modulation in the tunable metasurface. The experimental results demonstrate that the fabricated regular array and random array samples have different broadband low reflection frequency bands before and after the phase transition of VO₂, and the 10 dB reflectivity reduction bands can be switched quickly between C/X and Ku bands, which are in good agreement with the numerical simulation. This method realizes the switching function of metasurface modulation mode by controlling the ambient temperature, which provides a flexible and feasible idea for the design and fabrication of stealth metasurfaces.

Terahertz graphene-based multi-functional anisotropic metamaterial and its equivalent circuit model

In this paper, a graphene-based multi-functional anisotropic metamaterial composed of two finite parallel graphene ribbons in each unit cell is designed and proposed in the 0.1-5.5 terahertz (THz) region. Simulations are performed by the finite element method (FEM) in the frequency-domain solver of CST Software. An equivalent circuit modeling (ECM) as a simplified approach has been provided by a MATLAB code to model the performance of the metamaterial. The metastructure is polarization-sensitive because of the geometric non-symmetry. The absorption/reflection spectrum of the metamaterial is dynamically tunable by changing the Fermi energy level of the graphene. The introduced metamaterial can act as a THz switch and inverter at 1.23 and 4.21 THz. It acts as an ON state when the incident electric field is in the x-direction and acts as an OFF state when the incident electric field is in the y-direction. It can also act as a bi-functional mirror: a triple-band mirror for the incident electric field in the x-direction and an ultra-broadband mirror for the incident electric field in the y-direction. The proposed metamaterial has a maximum absorption of 100%, maximum linear dichroism (LD) of 100%, and a maximum switching extinction ratio of 33.01 dB. The metamaterial and its applications could be used as a potential platform in future THz devices and systems.

Chiral Plasmonic Shells: High-Performance Metamaterials for Sensitive Chiral Biomolecule Detection

Low-cost and large-area chiral metamaterials (CMs) are highly desirable for practical applications in chiral biosensors, nanophotonic chiral emitters, and beyond. A promising fabrication method takes advantage of self-assembled colloidal particles, onto which metal patches with defined orientation are created using glancing angle deposition (GLAD). However, using this method to make uniform and well-defined CMs over macroscopic areas is challenging. Here, we fabricate a uniform large-area colloidal particle array by interface-mediated self-assembly and precisely control the structural handedness of chiral plasmonic shells (CPSs) using GLAD. Strong chiroptical signals arise from twisted currents at the main, corner, and edge of CPSs, allowing a balance between strong chiroptical and high transmittance properties. Our shell-like chiral geometry shows excellent sensor performance in detecting chiral molecules due to the formation of uniform superchiral fields. Systematic investigations optimize the interplay between peak and null point resonances in different CPSs and result in a record consistency chiral sensor parameter U, i.e., 3.77 for null points and 0.0867 for peaks, which are about 54 and 1.257 times larger than the highest value (0.068) of previously reported CMs. The geometrical chirality, surface plasmonic resonance, chiral surface lattice resonance, and chiral sensor performance evidence the chiroptical effect and the excellent chiral sensor performance.

Multiple resonant modes coupling enabled strong CD response in a chiral metasurface

The chiral structures with strong circular dichroism (CD) response and narrow linewidth are desirable in chiral sensing, circularly-polarized light detection, and polarization imaging. Here, we theoretically proposed a hybrid chiral metasurface for differential absorption of circularly polarized light. Based on the multiple resonant modes coupling effect in a two-dimensional dielectric slab, it is realizable then to achieve a nearly perfect absorption for right circularly polarized light and simultaneously reflects 90% of left circularly polarized light, suggesting the generation of strong CD of 0.886 within a narrowly spectral linewidth of 4.53 nm. The multipole analysis reveals that the electric dipole, the magnetic dipole, and the electric quadrupole make dominant contributions to chiral absorption and the high CD response in this metsurface. The excitation of guided mode resonance enhances the ability of this metasurface to absorb electric field. Moreover, the optical chirality response can be further manipulated through the geometry features. These findings pave a powerful way to realize the narrowing and strong CD platform for single-band and multiband chirality behaviors.

Engineering a strong and stable ultraviolet chiroptical effect in a large-area chiral plasmonic shell

Ultraviolet chiral metamaterials (UCM) are highly desired for their strong interaction with the intrinsic resonance of molecules and ability in manipulating the polarization state of high energy photons, but rarely reported to date due to their small feature size and complex geometry. Herein, we design and fabricate a kind of novel ultraviolet chiral plasmonic shell (UCPS) by combing the stepwise Al deposition and colloid-sphere assembled techniques. The cancellation effect originated from the disorder lattices of micro-domains in the colloid monolayer has been successfully overcome by optimizing the deposition parameters, and a strong CD signal of larger than 1 deg in the UV region is demonstrated both in simulation and experiment. This strong ultraviolet chiroptical resonances mainly come from the surface chiral lattice resonance mode, the whispering gallery mode and also the interaction between neighbor shells, and can be effectively tuned by changing structural parameters, for example, the sphere diameter, or even slightly increasing the deposition temperature in experiment. To improve the stability, the fabricated UCPSs are protected by N₂ in the deposition chamber and then passivated by UV-ozone immediately after each deposition step. The formed UCPS show an excellent stability when exposing in the atmospheric environment. The computer-aided geometrical model, electromagnetic modes, and the tunable chiroptical resonance modes have been systematically investigated.

Enhanced circular dichroism in Ge2Sb2Te5-loaded metasurface

Circular dichroism (CD) is originally obtained from three-dimensional spiral structures by simultaneously exciting electric and magnetic resonances. To simplify construction, multilayer stacked asymmetric structures and the symmetric structures relying on oblique incidence are proposed for enhancing CD. Herein, we achieved the enhancement of dual-waveband CD by adding a Ge₂Sb₂Te₅ (GST) layer on the top of a Z-shape gold array in a normally incident system. Benefited from the polarization selective excitations of electric and magnetic dipole resonances, the CD in a simple planar structure is immensely enhanced from near zero to 0.73 at 1.58 µm. Furthermore, the CD strengths is dynamically tuned by controlling the phase of GST. With the GST phase transition from amorphous (a-GST) to crystalline state (c-GST), CD magnitudes are switched by about 0.73 and 0.27 at dual wavebands respectively. The enhancement of CD by adding a layer on a simple planar array offers a new method for designing planar metasurfaces with strong chirality.

Three-dimensional artificial chirality towards low-cost and ultra-sensitive enantioselective sensing

Artificial chiral structures have potential applications in the field of enantioselective signal sensing. Advanced nanofabrication methods enable a large diversity in geometric structures and broad selectivity of materials, which can be exploited to manufacture artificial three-dimensional chiral structures. Various chiroptical phenomena exploiting spin and orbital angular momentum at the nanoscale have been continuously exploited as a way to effectively detect enantiomers. This review introduces precisely controlled bottom-up and large-area top-down metamaterial fabrication methods to solve the limitations of high manufacturing cost and low production speed. Particle synthesis, self-assembly, glanced angled vapor deposition, and three-dimensional plasmonic nanostructure printing are introduced. Furthermore, emerging sensitive chiral sensing methods such as cavity-enhanced chirality, photothermal circular dichroism, and helical dichroism of single particles are discussed. The continuous progress of nanofabrication technology presents the strong potential for developing artificial chiral structures for applications in biomedical, pharmaceutical, nanophotonic systems.

Plasmonic topological metasurface by encircling an exceptional point

[Figure: see text].

Equivalent circuit model of graphene chiral multi-band metadevice absorber composed of U-shaped resonator array

In this study, we have designed an equivalent circuit model (ECM) by use of a simple MATLAB code to analyze a single-layered graphene chiral multi-band metadevice absorber which is composed of U-shaped graphene resonator array in terahertz (THz) region. In addition, the proposed metadevice absorber is analyzed numerically by the finite element method (FEM) in CST Software to verify the ECM analysis. The proposed device which is the first tunable graphene-based chiral metadevice absorber can be used in polarization sensitive devices in THz region. It is single-layered, tunable, and it has strong linear dichroism (LD) response of 94% and absorption of 99% for both transverse electric (TE) and transverse magnetic (TM) electromagnetic waves. It has four absorption bands with absorption >50% in 0.5-4.5 THz : three absorption bands for TE mode and one absorption band for TM mode. Proposed ECM has good agreement with the FEM simulation results. ECM analysis provides a simple, fast, and effective way to understand the resonance modes of the metadevice absorber and gives guidance for the analysis and design of the graphene chiral metadevices in the THz region.

Large-area cavity-enhanced 3D chiral metamaterials based on the angle-dependent deposition technique

Large-area and high-performance chiral metamaterials are highly desired for practical applications, such as controlling the polarization state of an electromagnetic wave and enhancing the sensor sensitivity of chiral molecules. In this work, cavity-enhanced chiral metamaterials (CECMs) with a large area (1 cm2) have been fabricated by the convenient angle-dependent material deposition technique. The optimal chiral signal (g factor) resonance in the visible waveband can reach about 0.94 with a figure of merit (FOM) of about 5.2, which is about ten times larger than that of chiral metamaterials (CMs) without a cavity (i.e., a g factor of 0.094 with the FOM of about 1.12). Both the theoretical and experimental results demonstrate that the circular conversion components from the anisotropic geometry of CMs play a crucial role in the final chiroptical effect of CECM, which together with the cavity effect enhance both the chiroptical resonance intensity and FOM. Choosing the appropriate deposition parameters can effectively modify the geometric anisotropy of CM and thus the chiroptical effect of CECM. The geometric nanoscale morphology, electromagnetic properties and sensor performance were investigated carefully in this work. The fabricated CECM working in the visible waveband together with the cavity-enhanced scheme provides a competitive candidate for enhancing the performance and the practical applications of CMs.

Chiral nanohole arrays

Chiral nanohole array (CNA) films are fabricated by a simple and efficient shadow sphere lithography (SSL) method and achieve label-free enantiodiscrimination of biomolecules and drug molecules at the picogram level. The intrinsic mirror symmetry of the structure is broken by three subsequent depositions onto non-close packed nanosphere monolayers with different polar and azimuthal angles. Giant chiro-optical responses with a transmission as high as 45%, a chirality of 21°μm^-1, and a g-factor of 0.17, respectively, are generated, which are among the largest values that have been reported in the literature. Such properties are due to the local rotating current density generated by a surface plasmon polariton as well as a strong local rotating field produced by localized surface plasmon resonance, which leads to the excitation of substantial local superchiral fields. The 70 nm-thick CNAs can achieve label-free enantiodiscrimination of biomolecules and drug molecules at the picogram level as demonstrated experimentally. All these advantages make the CNAs ready for low-cost, high-performance, and ultracompact polarization converters and label-free chiral sensors.