Circular Dichroism Effect Research Articles

Chiral metamaterial absorber with high selectivity for terahertz circular polarization waves

In this paper, a chiral metamaterial absorber (CMMA) is proposed for the realization of high selective absorption for right-handed and left-handed circular polarization (RCP and LCP) waves in the terahertz region. The CMMA is composed of a dielectric substrate sandwiched with a bi-layered fourfold twisted via-T-shaped (VTs) structure. The proposed CMMA has strong chiral-selective absorption bands, where absorption peaks for LCP and RCP occur at different resonance frequencies, finally resulting in a significant circular dichroism (CD) effect. The simulation results exhibit that absorption levels are greater than 90% and the maximal CD value is up to 42.2 dB at resonances. The mechanism of the chiral-selective absorption of the CMMA is illustrated and analyzed by surface current and power loss density distributions. Furthermore, the chiral-selective absorption properties can be effectively adjusted by changing the geometric parameters of the unit-cell of the proposed CMMA. Owing to its favorable performance, the proposed CMMA could be found in many potential applications in bolometric imaging, terahertz spectroscopic, detecting, and terahertz communications.

Optically Active Precision Aliphatic Polyesters via Cross‐Metathesis Polymerization

AbstractPrecision synthesis of optically active poly(lactic‐alt‐caprolactic acid) is realized using cross‐metathesis polymerization (CMP) and hydrogenation. Enantiopure monomers ML and MD are generated through two consecutive esterification reactions. CMP of these two monomers in different feed ratios, followed by hydrogenation, afford saturated polymers HPL, HPD, and HPDL comprising L‐lactic and/or D‐lactic acid and ε‐caprolactone‐derived motifs with alternating sequences. CMP kinetic analysis by nuclear magnetic resonance (NMR) is performed by employing a racemic monomer Mrac. Enantiomeric polyesters HPL and HPD display opposite rotation values and mirror‐imaged cotton effects in circular dichroism (CD) spectra, whereas HPDL derived from equimolar ML and MD shows inactive optical rotation and no CD response. Stereocomplex HPD+HPL is obtained by equimolar mixing of HPL and HPD, since this specimen is the only one to exhibit a sharp melting peak on differential scanning calorimetry (DSC) curve at 45.7 °C. Hence, an amorphous, enantiomerically pure polyester becomes crystalline via stereocomplexation with its complementary enantiomer.

Hyperjaponol H, A New Bioactive Filicinic Acid-Based Meroterpenoid from Hypericum japonicum Thunb. ex Murray.

Hyperjaponol H (1), a new filicinic acid-based meroterpenoid, with a 6/6/10 ring system trans-fused by hetero-Diels–Alder cycloaddition between a germacrane sesquiterpenoid and a filicinic acid moiety, was isolated from aerial parts of Hypericum japonicum. The elucidation of its structure and absolute configuration were accomplished by the analyses of extensive spectroscopic data and the comparison of Cotton effects of electron circular dichroism (ECD) with previously reported ones. The bioactivity assay showed that hyperjaponol H exhibited a moderate inhibitory efficacy on lytic Epstein-Barr virus (EBV) DNA replication in B95-8 cells.

Giant circular dichroism of large-area extrinsic chiral metal nanocrecents

In this work, we demonstrate the strong extrinsic chirality of the larger-area metal nanocrescents by experiments and simulations. Our results show that the metal nanocrescent exhibits giant and tunable circular dichroism (CD) effect, which is intensively dependent on the incident angle of light. We attribute the giant extrinsic chirality of the metal nanocrescent to the excitation efficiencies difference of localized surface plasmon resonance (LSPR) modes for two kinds of circularly polarized light at a non-zero incident angle. In experiment, the largest CD of 0.37 is obtained at the wavelength of 826 nm with the incident angle of 60°. Furthermore, the CD spectra can be tuned flexibly by changing the metal nanocrescent diameter. Benefitting from the simple, low-cost and mature fabrication process, the proposed large-area metal nanocrescents are propitious to application.

Circular dichroism and radial Hall effects in topological materials

Under symmetry breaking, a three-dimensional nodal-line semimetal can turn into a topological insulator or Weyl semimetal, accompanied by the generation of momentum-space Berry curvature. We develop a theory that unifies their circular dichroism and highlights the roles of Berry curvature distribution and light incident direction. Nontrivially, these phases exhibit distinct dichroic optical absorption and radial Hall effects, with characteristic scalings with photon energy and electric field. Our findings offer a new diagnosis tool for examining topological phases of matter.

Asymmetric Fission Transmission of Linear-to-Circular Polarization Converter Using Bi-layer Split Ring Structure

A circular polarizer based on bi-layer split ring structure is proposed that could achieve asymmetric fission transmission of linearly polarized wave at the dual band. Firstly, a new approach of "Fission Transmission of Electromagnetic (FTEM) waves" is introduced to understand the polarization transformation behavior for linear-to-circular polarization. The designed structure achieves broadband circularly polarized wave with an asymmetric transmission over resonance frequencies by the principle of FTEM wave. The electronics circuit of proposed structure demonstrates the transformation behavior of EM waves when the electric and magnetic coupling between the upper and lower patterned SRR is reached at the certain strength. The physics of the giant circular dichroism effect and optical activity is illustrated by the surface currents distribution on the structure. The proposed structure achieves a right-handed circularly polarized wave and left hand circularly polarized wave with high transmission at 13.94---15.70 GHz and at 16.0---17.03 GHz, respectively. The axial ratio bandwidth of 11.76 and 6.86% is obtained at the dual band. The simulated and measured results exhibit good correspondence.

Deterministic all-optical switching of synthetic ferrimagnets using single femtosecond laser pulses

We experimentally demonstrate single-pulse all-optical switching in Pt/Co/Gd stacks using linearly polarized laser pulses. This shows that thermal single-pulse switching is not limited to ferrimagnetic alloys, but is also possible in ferrimagnetic multilayers that are highly suitable for future applications due to easy fabrication and (interface) engineering. Moreover, it is shown that the threshold fluence needed for the optical switch strongly depends on the thickness of the Co layer, with a remarkable low threshold fluence of $\ensuremath{\approx}1.2$ mJ/${\mathrm{cm}}^{2}$ for a Co thickness of 0.8 nm. Lastly, helicity-dependent measurements showed no significant effect of the magnetic circular dichroism in these thin magnetic layers.

Synthesis and studies of axial chiral bisbenzocoumarins: Aggregation-induced emission enhancement properties and aggregation-annihilation circular dichroism effects

Axial chiral bisbenzocoumarins were synthesized for the first time by converting naphthanol units in 1,1′-binaphthol (BINOL) molecule to the benzocoumarin rings. The substitute groups on 3,3′-positions of bisbenzocoumarins showed significant influence on their aggregation-induced emission enhancement (AEE) properties. It was also found that BBzC1 with ester groups on 3,3′-positions exhibit an abnormal aggregation-annihilation circular dichroism (AACD) phenomenon, which could be caused by the decrease of the dihedral angle between adjacent benzocoumarin rings in the aggregation state. The single crystal structure of BBzC1 showed that the large dihedral angle in molecule prohibited the strong π-π stacking interactions, which could be main factors for its AEE properties.

Photoconductive detector of circularly polarized radiation based on a MIS structure with a CoPt layer

A photoconductive detector of circularly polarized radiation based on the metal–insulator–semiconductor structure of CoPt/(Al2O3/SiO2/Al2O3)/InGaAs/GaAs is created. The efficiency of detection of circularly polarized radiation is 0.75% at room temperature. The operation of the detector is based on the manifestation of the effect of magnetic circular dichroism in the CoPt layer, that is, the dependence of the CoPt transmission coefficient on the sign of the circular polarization of light and magnetization.

The combination of micro-resonators with spatially resolved ferromagnetic resonance.

We present two new and complementary approaches to realize spatial resolution for ferromagnetic resonance (FMR) on the 100 nm-scale. Both experimental setups utilize lithographically fabricated micro-resonators. They offer a detection sensitivity that is increased by four orders of magnitude compared with resonator-based FMR. In the first setup, the magnetic properties are thermally modulated via the thermal near-field effect generated by the thermal probe of an atomic force microscope. In combination with lock-in detection of the absorbed microwave power in the micro-resonator, a spatial resolution of less than 100 nm is achieved. The second setup is a combination of a micro-resonator with a scanning transmission x-ray microscope (STXM). Here a conventional FMR is excited by the micro-resonator while focused x-rays are used for a time-resolved snap-shot detection of the FMR excitations via the x-ray magnetic circular dichroism effect. This technique allows a lateral resolution of nominally 35 nm given by the STXM. Both experimental setups combine the advantage of low-power FMR excitation in the linear regime with high spatial resolution to study single and coupled nanomagnets. As proof-of-principle experiments, two perpendicular magnetic micro-stripes (5 μm × 1 μm) were grown and their FMR excitations were investigated using both setups.

Circular dichroism effect in a double-layer dolmen array nanostructure.

Chiral plasmonic nanostructures have been studied widely in past years and have various applications in biological sensing, negative refractive indexing, and analytical chemistry. Dolmen is also a commonly used nanostructure in many recent research papers. By placing dolmens in different directions and forming a double-layer dolmen array (DLDA), a new nanostructure with good circular dichroism (CD) effect is designed in this paper. We get up to the maximum of 0.5 of CD effect through numerical simulation. Among three obvious resonance wavelengths of CD effect, magnetic dipole resonance appears in two of them and contributes to the origin of a strong CD effect. In these different coupling modes, the strongest one locating at the largest wavelength is the result of a big magnetic dipolar resonance generated by circulation current in two layers of dolmen together, and the second strongest CD effect is caused by individual magnetic dipolar resonance in the opposite direction located in each layer. At the weakest one, the circulation current disappears and only a regular electric dipolar resonance appears. This simulation result shows that magnetic dipolar resonance of plasmonic nanostructure could create a strong CD effect, and much more effectively in the DLDA we propose. The results can help us in designing novel chiral optical nanostructures and provide applications in the interactions between photons and electrons.

Circular dichroism of a tilted U-shaped nanostructure.

The circular dichroism (CD) effect plays an important role in biological detection, analytical chemistry, and plasmonic sensing. Tilted 3D structures can generate CD signals under normal illumination. However, fabricating tilted 3D structures is complex and expensive. In this study, we fabricate a tilted U-shaped nanostructure (TUSN) on a polystyrene (PS) nanosphere through the glancing angle deposition method. One branch of the U-shaped nanostructure is tilted by raising a sheet of SiO2 on the PS nanosphere. And the CD signal of the TUSN is enhanced because the phase difference increases with increasing thickness of the SiO2 sheet. This work proposes a method for fabricating tilted nanostructures and elucidates the mechanism of the CD effect for future research.

Influence of the geometry of a gammadion stereo-structure chiral metamaterial on optical properties

In this paper, a gammadion stereo-structure (GSS) chiral metamaterial (CMM) is described and numerically analysed at optical frequencies. The influence of the geometry of the proposed CMM on the optical properties was studied systematically. The numerical simulations exhibit that the giant circular dichroism (CD) effect, optical activity and negative refraction can be obtained by properly selecting the geometric parameters, respectively. Given the pronounced optical effects and the negative refraction property of stereo-structure CMM, many applications in ultra-compact polarization components, integrated photonics, plasmonic-enhanced sensing of biochemical substances and CD spectroscopic analysis can be envisioned easily.

Enhanced Circular Dichroism of Gold Bilayered Slit Arrays Embedded with Rectangular Holes.

Gold bilayered slit arrays with rectangular holes embedded into the metal surface are designed to enhance the circular dichroism (CD) effect of gold bilayered slit arrays. The rectangular holes in these arrays block electric currents and generate localized surface plasmons around these holes, thereby strengthening the CD effect. The CD enhancement factor depends strongly on the rotational angle and the structural parameters of the rectangular holes; this factor can be enhanced further by drilling two additional rectangular holes into the metal surfaces of the arrays. These results help facilitate the design of chiral structures to produce a strong CD effect and large electric fields.

Chiral organic photonics: self-assembled micro-resonators for an enhanced circular dichroism effect in the non-linear optical signal

Enantiomeric R and S type organic whispering-gallery-mode (WGM) optical micro-cavities display an enhanced CD-NLO effect due to increased light–matter interaction.

Directed crystallization of isotactic poly(2-vinylpyridine) for preferred lamellar twisting by chiral dopants

As reported in our previous work, banded spherulites of isotactic poly(2-vinylpyridine) (iP2VP) with preferred handedness of twisted lamellae can be formed by the inducement of chiral dopants, i.e., (R)- or (S)-hexahydromandelic acids (HMAs). In the present work, we systematically studied the behaviors of chirality transfer in the crystallization of iP2VP associated with the chiral dopant. As evidenced by the signatures of the split-type Cotton effect of circular dichroism (CD) spectra, the chains of iP2VP exhibit exclusive conformational chirality, i.e., induced circular dichroism (ICD), due to the complexation of chiral dopants (configurational chirality) with the lone-pair electron of the iP2VP. With the same growth axis along the radial direction of the banded spherulites, as indicated by selected area electron diffraction (SAED), the helical sense of the twisted lamellae of the iP2VP with ICD can be driven by the chiral dopants. The handedness of the helical chains remains after crystallization with intrinsic crystalline structure of iP2VP, which was revealed by the corresponding results of CD spectroscopy. As a result, a homochiral evolution from the configurational chirality of chiral dopant to the hierarchical chirality of crystallized iP2VP can be developed through directed crystallization of the iP2VP by chiral HMAs.

Unusual formation and helicity induction in a para-amino-substituted trityl chromophore: a cautionary note

O- and N-trityl derivatives Ph3C∗-X-R∗ of chiral alcohols (X=O) and amines (X=NH) are known to produce strong and stereochemically predictable Cotton effects in the Electronic Circular Dichroism (ECD) spectra due to the preferred M or P helical conformation of the trityl group. With the use of ECD spectra in solution and crystal X-ray diffraction analysis, we demonstrate that in a chiral p-substituted trityl derivative p-R∗N(Me)-C6H4-C∗Ph2H, the stereochemical information is transmitted from a permanent stereogenic center R∗ to the dynamic C∗ stereogenic center of the trityl sensor via the para carbon atom, and not through the central carbon atom.

Optically Active Particles with Tunable Morphology: Prepared by Embedding Graphene Oxide/Fe3O4 in Helical Polyacetylene.

We report a novel and straightforward methodology for constructing hybrid particles with tunable morphology (spherical vs nonspherical) by embedding inorganic components (graphene oxide and/or Fe3O4 nanoparticles) inside chiral helical polyacetylene. Scanning electron microscopic images ascertain the spherical or nonspherical morphology of the particles. The intense circular dichroism effects demonstrate that the hybrid particles (spherical, ellipsoid-like, and cake-like) possess remarkable optical activity. The use of the chiral magnetic hybrid particles in enantioselective crystallization of racemic phenylalanine demonstrates the kind of particles' significant potential applications in chiral technologies and chiral processes. The study not only creates an unprecedented type of chiral hybrid particles, but also provides a versatile strategy for preparing advanced functional hybrid particles with tunable morphology from polymers and even from inorganic and metallic materials.

Circular Differential Scattering of Single Chiral Self-Assembled Gold Nanorod Dimers

Circular dichroism spectroscopy is essential for structural characterization of proteins and chiral nanomaterials. Chiral structures from plasmonic materials have extraordinary strong circular dichroism effects compared to their molecular counterparts. While being extensively investigated, the comprehensive account of circular dichroism effects consistent with other plasmonic phenomena is still missing. Here we investigated the circular differential scattering of a simple chiral plasmonic system, a twisted side-by-side Au nanorod dimer, using single-particle circular dichroism spectroscopy complimented with electromagnetic simulations. This approach enabled us to quantify the effects of structural symmetry breaking, namely, size-mismatch between the constituent Au nanorods and large twist angles on the resulting circular differential scattering spectrum. Our results demonstrate that, if only scattering is considered as measured by dark-field spectroscopy, a homodimer of Au nanorods with similar sizes prod...

Giant circular dichroism induced by silver nanocuboid heterodimers.

Metallic nanocuboid heterodimers are proposed to generate a giant circular dichroism (CD) effect. Two cuboids in the heterodimers have different heights. The dipole and quadrupole charge oscillation modes in the cuboids occur under left- and right-handed circular polarizations. The height difference generates phase difference between charge oscillations in the two cuboids. The two charge oscillations and the phase difference between them are consistent with the Born-Kuhn model for the CD effect. The CD effect of the nanocuboid heterodimers can be tuned by changing the structural parameters of the nanocuboid heterodimers, especially the height difference between two cuboids. The results of this research are not only useful for designing plasmonic structures to generate the CD effect but also for understanding the physical mechanisms of the CD effect.