Chiral Absorber Research Articles

Tunable and strong circular dichroism metamaterial absorber based on gold-VO2 hybrid structure

The existing terahertz chiral absorber faces a series of issues, including low efficiency, un-tunable absorption, and complex structure. We present a chiral metamaterial absorber integrated with gold and phase-change material vanadium dioxide (VO2) to address the issues above. Our designed metamaterial absorber’s maximum circular dichroism (CD) value, as an ideal chirality-selective meta-absorber, reaches 0.95 when VO2 behaves as a metallic state. However, when VO2 is in the insulator state, the CD value is merely 0.003. Notably, the intrinsic mechanisms of the CD effect are revealed by analyzing the difference in electromagnetic response under left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) illumination. The designed chiral metamaterial absorber supports the free switching of the CD signal between ‘on’ and ‘off’ states by continuously adjusting the conductivity of VO2 with a CD value ranging from 0 to 0.95. Moreover, the absorption performance of chiral metamaterials is affected by geometric parameters and the angle of incidence. To evaluate the capability of chiral structure in biosensing, three typical avian influenza viruses are sensed by detecting changes in their resonant frequencies and CD values. Finally, the presented chiral-selective absorber with phase-change materials has an important reference value in the terahertz range for constructing chiral light detectors, chiral thermal switching, biosensors, and tunable chiral photonics.

High quality factor and unity circular dichroism enabled by dielectric-Weyl semimetal-dielectric stack

A mid-infrared (mid-IR) chiral absorber based on multilayer films of silicon (Si)/Weyl semimetal (WSM)/Si is proposed and numerically studied. The proposal exhibits high quality factor circular dichroism (CD) with different absorptance properties for right-hand circular polarization (RCP) and left-hand circular polarization (LCP) light. The absorption characteristics have been explored with the generalized 4 × 4 transfer matrix algorithm, and the effects of structural parameters on the circularly polarized (CP) absorptance are systematically investigated. The circular polarization efficiency (η) and CD are defined as crucial parameters for quantifying the chirality response. The results prove that the chiral absorber exhibits extraordinary efficiencies of 1 and 0 for LCP and RCP light at a wavelength of 11.5 μm, respectively. Thus a maximum CD in absorption of unity with a resonant quality factor (Q-factor) of 295 can be obtained. The performance of dual-band chiral responses can be engineered via the thickness of each layer. Moreover, it is identified that the design results in angle-independent performance, which is essential trait for an efficient chiral absorber.

Chiral photothermoelectric effect driven by high-Q quasi-bound states in the continuum

Bound states in continuum (BIC) have been proposed as a means to efficiently improve light-matter interactions in metasurfaces. While breaking the mirror symmetry of structure and developing BIC into a reachable and observable quasi-BIC, it is usually accompanied by the chiral phenomenon with high quality (Q) factor. Here, we report a spin-sensitive photodetector in the infrared (NIR) region comprising a silicon metasurface with chiral quasi-BIC, a silver layer, and a thermoelectric layer. A chiral quasi-BIC supported by a silicon metasurface can be realized under normal incidence. Based on finite element method simulations, a silicon metasurface with a silver layer shows a high Q-factor of 958.6 with a giant absorption circular dichroism of 0.83. Subsequently, we study the thermal performance of the chiral absorbers by using the heat transfer module of COMSOL. Combined with the thermoelectric material bismuth telluride, we calculate the differential photothermoelectric effects of the system under circular-polarized light irradiation. When the incident flux is 100 W cm−2, the output voltage under right-circular-polarization (left-circular-polarization) light reaches 0.59 mV (0.08 mV), which can be used for polarization detection. Therefore, our designed structure incorporating thermoelectricity broadens the applications of chiral BIC in sensors and detectors.

Chiral metasurface absorber with near-infrared excitation-induced dual circular dichroism

Microscopic components in metasurfaces allow precise control of electromagnetic waves, enabling chiral metasurface absorbers to adapt to different frequencies and wavelengths of electromagnetic waves, providing higher controllability. In this paper, we present a near-infrared chiral absorber utilizing a Metal-Insulator-Metal (MIM) structure, achieving selective absorption in the dual wavelength ranges of 1500–1750 nm and 2250–2750 nm. The absorber's structural layout adopts a pseudo-π configuration, featuring a unique design that establishes distinct resonance cavities between structural periods. This results in disparate responses to left-handed circularly polarized (LCP) light and right-handed circularly polarized (RCP) light, thereby attaining dual-band selectivity and adjustability. To illustrate the physical mechanism, we explore the electric and magnetic field distributions within this structure, offering deeper insights into the fundamental mechanism of selective absorption. This analysis illustrates that the charge distribution in different wavelength bands can vary with the polarization direction, forming orthogonal electric dipoles. Under the incidence of LCP and RCP, we achieved circular dichroism (CD) of 0.75 and 0.7, respectively. By adjusting parameters within individual resonance cavities, we achieve independent control over each resonance wavelength. This near-infrared chiral absorber provides unparalleled design flexibility, with implications for advancements in applications such as molecular detection and optical switching.

Single-Particle Photoluminescence Measures a Heterogeneous Distribution of Differential Circular Absorbance of Gold Nanoparticle Aggregates near Constricted Thioflavin T Molecules.

The chirality of biomacromolecules is critical for their function, but the optical signal of this chirality is small in the visible range. Plasmonic nanoparticles are antennas that can couple to this chiral signal. Here, we examine the molecular-scale mechanism behind the induced circular dichroism of gold nanorods (AuNRs) in solution with insulin fibrils and the fibril-intercalating dye thioflavin T (ThT) with polarization-resolved single-molecule fluorescence and single-particle photoluminescence (PL) imaging. We compared the PL upon excitation by left- and right-handed circularly polarized light to calculate the differential absorbance of AuNRs near insulin fibrils with and without ThT. Overall, our results indicate that AuNRs do not act as chiral absorbers near constricted ThT molecules. Instead, we hypothesize that fibrils promote AuNR aggregation, and this templating is mediated by subtle changes in the solution conditions; under the right conditions, only a few chiral aggregates with significantly higher circular dichroism signal contribute to a large net circular dichroism.

Weyl semimetal/dielectric/Weyl semimetal stack for highly circularly polarized thermal radiation.

Highly circularly polarized (CP) infrared thermal radiation is greatly in demand because of its significant potential in mid-infrared (mid-IR) applications. To exploit the magnitude and quality factor of circular dichroism (CD) simultaneously, a lithography-free platform consisting of a Weyl semimetal (WSM)/dielectric (Ge)/WSM stack sitting on a metallic substrate (Mo) is proposed. A chiral response and varying CD values from -1 to 0.957 have been demonstrated. The numerical results from a generalized 4 × 4 transfer matrix algorithm verify that the chiral structure manifests a remarkably high quality factor (Q-factor) of 605. The effect of the thickness of each layer in the stack on the CD value is investigated. Moreover, it is identified that the design results in an angle-independent performance. A dual-channel chiral absorber operating in the 18-21 μm wavelength range has also been achieved. Our simple yet powerful paradigm could offer a new way of manipulating the Q-factor and resonance wavelength of a chiral absorber while maintaining near-unity CD, offering a new approach for the advancement of more efficient and tunable chiral optical devices. The approach is generally applicable to other planar configurations with different WSMs and can be extended to other wavelengths.

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.

Chiral metasurface absorbers with tunable circular dichroism in the mid-infrared via phase transition of vanadium dioxide

Chiral metasurface absorbers are vital for many applications such as polarization imaging, CD spectroscopy, and thermal emitters. Herein, we present a chiral metasurface integrated with η-shaped vanadium dioxide (VO2) to achieve dynamic tuning of circular dichroism (CD) in the mid-infrared region. The proposed chiral absorber can achieve near-perfect absorption due to the excitation of strong magnetic dipole (MD) resonance under the illumination of the right-handed circularly polarized (RCP) light, while the absorption is comparably low in the case of the left-handed circularly polarized (LCP) light as the electric dipole (ED) mode associated with the LCP light is weak, thus broadband CD effect with maximum CD of 0.63 can be realized due to the remarkable chiral-selective absorption performances. In addition, the CD value and the absorption peak of the chiral metasurface can be dynamically tuned by changing the volume fraction of the metallic state of VO2, and good CD effect can be maintained even if the structural parameters are significantly altered.

Chiral Absorber-Based Frequency Selective Rasorber With Identical Filtering Characteristics for Distinct Polarizations

In this article, we propose a new paradigm to design frequency-selective rasorber (FSR) with two-sided efficient absorptions and broadband transmission in a chiral structure. The proposed FSR is implemented via combining two chiral absorber layers with a bandpass frequency-selective surface (FSS). An absorptive split-ring resonator (ASRR) inserted with a resistive film is designed to provide efficient spin-sensitive absorption. Then, four ASRRs are arranged to form a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{4}$ </tex-math></inline-formula> symmetric split-ring absorber (SRA) with identical absorption for distinct polarized incident waves. The physical mechanisms of spin-selective absorption and polarization-insensitive absorption are discussed theoretically. The broadband bandpass FSS is introduced to provide a transparent window and ground plane for the absorbers. A prototype is fabricated and measured to verify the feasibility of the method. The simulation and experimental results manifest over 90% absorptive bands of 1.5–3.6 and 15.2–21.6 GHz and a transmission band of 8–12 GHz with insertion loss less than 2 dB. Compared with previous FSRs, the proposed FSR has a broadband transmission, efficient absorptions with high-frequency selectivity, and easy geometric structure.

Interface modulation of chiral PPy/Fe3O4 planar microhelixes to achieve electric/magnetic-coupling and wide-band microwave absorption

Adjusting the heterogeneous dielectric/magnetic interfaces have been affirmed to be an impactful strategy for obtaining superior electromagnetic wave absorbing (EMWA) materials. Herein, PPy planar microhelixes anchored with Fe3O4 nanoparticles were constructed via a simple chemical oxypolymerization coupled with a solvothermal method. Kinetic factors (i.e., H2O volume and Fe3+ concentration) can be easily used to adjust PPy/Fe3O4 dielectric/magnetic interfaces by controlling the size and content of Fe3O4 nanoparticles in an exquisitely designed continuously adjusted process. The interface modulation not only forms superb impedance matching and significant electric/magnetic-coupling effects, but also tunes the conduction loss/polarization loss balance, beneficial to the excellent absorption performance. Interestingly, the prepared PPy/Fe3O4 planar microhelixes deliver the best EMWA performance compared with the other PPy-based absorbing materials reported so far, with the lowest reflection loss of −45.48 dB and the widest effective bandwidth of 10.24 GHz. The strategies for adjusting magnetic/electric properties and synthesizing chiral absorbers are very instructive for the development of high-performance EMW materials and devices.

Direct Detection of Circularly Polarized Light Using Chiral Copper Chloride-Carbon Nanotube Heterostructures.

The emergent properties of chiral organic-inorganic hybrid materials offer opportunities in spin-dependent optoelectronic devices. One of the most promising applications where spin, charge, and light are strongly coupled is circularly polarized light (CPL) detection. However, the performance of state-of-the-art CPL detectors using chiral hybrid metal halide semiconductors is still limited by the low anisotropy factor, poor conductivity, and limited photoresponsivity. Here, we synthesize 0D chiral copper chloride hybrids, templated by chiral methylbenzylammonium (R/S-MBA), i.e., (R-/S-MBA)2CuCl4, that display circular dichroism for the ligand-to-metal charge transfer transition with an absorption anisotropy factor (gCD) among the largest reported for chiral metal halide semiconductor hybrids. To circumvent the poor conductivity of the unpercolated inorganic framework of this chiral absorber, we develop a direct CPL detector that utilizes a heterojunction between the chiral (MBA)2CuCl4 absorber layer and a semiconducting single-walled carbon nanotube (s-SWCNT) transport channel. Our chiral heterostructure shows high photoresponsivity of 452 A/W, a competitive anisotropy factor (gres) of up to 0.21, a current response in microamperes, and low working voltage down to 0.01 V. Our results clearly demonstrate a useful strategy toward high-performance chiral optoelectronic devices, where a nanoscale heterostructure enables direct CPL detection even for highly insulating chiral materials.

Polarization-dependent and tunable absorption of terahertz waves based on anisotropic metasurfaces

Metamaterial absorbers can achieve high-efficiency electromagnetic absorption in a specific band, which have been used in biochemical sensing, photoelectric detection, imaging and other fields. Tunable metamaterial absorbers provide more possibilities for the development of multifunctional electromagnetic absorption devices. Here we propose a tunable and polarization-dependent terahertz metamaterial absorber which can work for both linearly and circularly polarized waves. By introducing single layer graphene and vanadium dioxide (VO2), switching between the two working states and wide-range tuning of the absorption efficiency are realized. When VO2 is in insulating state, the absorber shows different tunable absorption performance for the x- or y-polarized terahertz waves, in which the maximum absorption rate is close to 100%. When VO2 is in metallic state, the metasurface behaves as a chiral absorber, and the maximum absorption difference between the two circular polarizations is about 0.45, while the tuning efficiency reaches 86.3%. Under the two working conditions, the absorber can maintain efficient absorption with a large incident angle. In addition, as an application exploration of the absorber, we demonstrated its application in tunable and polarization multiplexed near-field image display.

New spin-resolved thermal radiation laws for nonreciprocal bianisotropic media

A chiral absorber of light can emit spin-polarized (circularly polarized) thermal radiation based on Kirchhoff’s law which equates spin-resolved emissivity with spin-resolved absorptivity for reciprocal media at thermal equilibrium. No such law is known for nonreciprocal media. In this work, we discover three spin-resolved Kirchhoff’s laws of thermal radiation applicable for both reciprocal and nonreciprocal planar media. In particular, these laws are applicable to multi-layered or composite slabs of generic bianisotropic material classes which include (uniaxial or biaxial) birefringent crystals, (gyrotropic) Weyl semimetals, magnetized semiconductors, plasmas, ferromagnets and ferrites, (magnetoelectric) topological insulators, metamaterials and multiferroic media. We also propose an experiment to verify these laws using a single system of doped indium antimonide (InSb) thin film in an external magnetic field. Furthermore, we reveal a surprising result that the planar slabs of all these material classes can emit partially circularly polarized thermal light without requiring any surface patterning, and identify planar configurations which can experience nontrivial thermal optomechanical forces and torques upon thermal emission into the external environment at lower temperature (nonequilibrium). Our work also provides a new fundamental insight of detailed balance of angular momentum (in addition to energy) of equilibrium thermal radiation, and paves the way for practical functionalities based on thermal radiation using nonreciprocal bianisotropic materials.

Broadband infrared circular dichroism in chiral metasurface absorbers

Chirality is ubiquitous in nature and it is essential in many fields, but natural materials possess weak and narrow-band chiroptical effects. Here, chiral plasmonic metasurface absorbers are designed and demonstrated to achieve large broadband infrared circular dichroism (CD). The broadband chiral absorber is made of multiple double-rectangle resonators with different sizes, showing strong absorption of left-handed or right-handed circularly polarized (LCP or RCP) light above 0.7 and large CD in absorption more than 0.5 covering the wavelength range from 1.35 µm to 1.85 µm. High broadband polarization-dependent local temperature increase is also obtained. The switchable infrared reflective chiral images are further presented by changing the wavelength and polarization of incident light. The broadband chiral metasurface absorbers promise future applications in many areas such as polarization detection, thermophotovoltaics, and chiral imaging.

Circular dichroism of honeycomb-shaped elliptical hole absorber

The circular dichroism of chiral structure has been widely used in analytical chemistry, industrial pharmacy, biological monitoring, etc. However, the light-matter interaction between natural chiral structures is extremely weak. Plasmonic nanostructures can significantly enhance light-matter interaction. During the fabrication of the visible-to-near-infrared chiral plasmonic metamaterial absorbers, there exists usually a trade-off between the absorption and the sample area, that is, the circular dichroism signal of the large-area structure is small. Besides, the preparation of chiral absorbers working in the visible and near-infrared region usually requires expensive etching or lithography equipment, such as reactive ion etching or electron beam lithography. Therefore, preparing cost-effective chiral absorbers with large circular dichroism is attractive for practical applications. In order to improve the circular dichroism of large-scale chiral absorbers, a honeycomb-shaped elliptical hole absorber is proposed in this paper, and its absorption, circular dichroism, and optical &lt;i&gt;g&lt;/i&gt;-factor are studied. By reasonable design, the numerical calculation results show that the circular dichroism can reach about 0.8 under the excitation of chiral polarized light, and the corresponding optical &lt;i&gt;g&lt;/i&gt;-factor can reach about 1.7 at 920 nm. Compared with the reported absorber, our chiral absorber has a maximum &lt;i&gt;g&lt;/i&gt;-factor value. The giant circular dichroism originates from the symmetry breaking of the structure by tilting ellipse structures, and the tilt angle has a significant influence on circular dichroism. To further explain the absorption difference, the electric profile, surface current distribution, and absorption loss of the chiral absorption at resonant wavelength are analyzed. Finally, we point out that the structure can be prepared by existing technologies, such as nanosphere photolithography: first, a layer of polystyrene (PS) balls is formed by self-organization, which can control the period of the structure; then the size of the PS balls is reduced to a suitable size and spacing by the reactive ion etching; finally, a metallic layer is deposited by oblique angle evaporation. This work provides useful guidance for fabricating the large-scale chiral plasmonic absorbers.

Chiral Absorbers Based on Polarization Conversion and Excitation of Magnetic Polaritons

Chiral Absorbers Based on Polarization Conversion and Excitation of Magnetic Polaritons

Reconfigurable Chiral Metasurface Absorbers Based on Liquid Crystals

We propose a liquid-crystal-based reconfigurable chiral metasurface absorber and numerically investigate its chiro-optical properties. The chiral metasurface absorber consists of a metal–insulator–metal structure with the substrate, which can strongly absorb a circularly polarized wave of one spin state and reflects that of the opposite spin, resulting a strong circular dichroism (CD). A birefringent liquid crystal (LC) is exploited to serve as the insulator layer in the metal–insulator–metal structure. We could then vary the circular state of the incident light by controlling the alignment of the LC molecules, hence inversing the CD. The simulation results show that the CD will change the sign as the LC molecules are realigned from 0° to 90°. The absorption efficiency for the specific circularly polarized wave is larger than 80% and the CD is nearly 70%. The simple and compact design of our proposed chiral metasurface absorber is especially favorable for integration, and such a reconfigurable chiral absorber could find many potential applications in biological detection/sensing, polarimetric imaging, and optical communications.

A new absorbing boundary condition for FDTD

In this work, we present a new absorbing boundary condition for the finite-difference time-domain (FDTD) method. This boundary condition is based on the use of chiral absorbers, which are well suited for this application. Further, we present several numerical examples to illustrate the efficacy of this approach.

A new absorbing boundary condition for FDTD

In this work, we present a new absorbing boundary condition for the finite-difference time-domain (FDTD) method. This boundary condition is based on the use of chiral absorbers, which are well suited for this application. Further, we present several numerical examples to illustrate the efficacy of this approach. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 25: 86–90, 2000.

The absorption of electromagnetic waves at microwave frequencies by synthetic chiral and racemic materials

Physical principles are discussed which may improve the absorption of electromagnetic waves, especially in the microwave regime, by synthetic chiral and racemic materials. These principles are used to design a chiral composite for enhanced electromagnetic wave absorption at about 14 GHz. The chiral absorber consists of single-turn copper helices, each wound on a ferrite core and randomly dispersed in a dielectric absorber made of tiny spheres of graphite-coated expanded polystyrene. Its constitutive parameters, and those of some related dielectric, chiral, non-chiral (bihelix) and racemic absorbing composites, are studied in the 10 to 18 GHz band. The materials are analyzed as third-order D¨ layers to compare their ability to reduce normal incidence specular reflection by a flat metal plate. The presence of half-wave resonant helices moderately increases the absorption bandwidth of the dielectric host. A chiral composite and its racemic counterpart have essentially identical properties as absorbers of specular reflection.