Strong Chirality Research Articles

Uniaxial strain-induced mechanical and electronic property modulation of silicene.

We perform first-principles calculations of mechanical and electronic properties of silicene under uniaxial strains. Poisson's ratio and the rigidity of silicene show strong chirality dependence under large uniaxial strains. The ultimate strains of silicene with uniaxial strain are smaller than those with biaxial strain. We find that uniaxial strains induce Dirac point deviation from the high-symmetry points in the Brillouin zone and semimetal-metal transitions. Therefore, no bandgap opens under the uniaxial strain. Due to its peculiar structure and variable sp3/sp2 ratio of the chemical bond, the deviation directions of Dirac points from the high-symmetry points in the Brillouin zone and variation of Fermi velocities of silicene exhibit significant difference from those of graphene. Fermi velocities show strong anisotropy with respect to the wave vector directions and change slightly before the semimetal-metal transition. We also find that the work function of silicene increases monotonously with the increasing uniaxial strains.PACS numbers61.46.-w; 62.20.D-; 73.22.Dj

Synthesis and characterization of novel chiral dimers exhibiting highly frustrated liquid crystal phases

Two novel series of optically active dimers comprising cholesterol and biphenyl-4-yl 4-(n-alkoxy)benzoate cores interlinked though either an odd-parity/even-parity spacer have been prepared and characterized. They stabilize an extremely complex, frustrated liquid crystalline state viz., the twist grain boundary (TGB) phase with chiral smectic C structure, denoted as TGBC∗ phase, over a wide (50–110 °C) temperature range. Notably, the dimers with an odd-parity spacer show an additional frustrated liquid crystal phase namely, the blue phase (BP). The presence of such frustrated phases suggests that the synthesized dimers are characterized by high enantiomeric excess and strong molecular chirality. Thus, 12 new optically active, nonsymmetric dimers reported herein constitute new examples of rarely found strongly chiral, optically pure dimers showing frustrated liquid crystal phase over an adequately wide thermal range.

Complete band gaps in one-dimensional photonic crystals with negative refraction arising from strong chirality

In this paper, we propose the idea of using chiral negatively refractive media to realize three-dimensional (3D) complete band gaps in the one-dimensional (1D) photonic crystals with a two-layer unit cell. We explain why at least a three-layer unit cell is necessary for realizing the 3D complete band gaps in 1D achiral photonic crystals, and show how we can reduce the number of layers of the unit cell to two if chirality is introduced. Although the media with strong chirality supply negative refraction only for one of the two eigenwaves, the complete band gaps could be still obtained with an even simpler lattice structure compared to the achiral counterpart.

Cross polarization converter formed by rotated-arm-square chiral metamaterial

Optical activity and cross polarization conversion (CPC) of a rotated-arm-square chiral metamaterial (CMM) is reported in this work. Proposed CMM shows strong chirality owing to the cross coupling between the induced magnetic field and incident electric field of the applied electromagnetic wave. Mechanism of CPC functionality is explained based on the surface current distribution of CMM structure. Microwave experiment is performed to reveal the CPC utility of the proposed CMM. Among various designs on the CPC converters, possibility of near 99% efficiency is revealed in the proposed CMM.

Thermal transport properties of rolled graphene nanoribbons

Using nonequilibrium Green's function method, we investigate the influence of the curvature and edge effects on the thermal transport during the process of rolling graphene nanoribbons (GNRs) into carbon nanotubes (CNTs) in the transverse direction. The curvature effect results in a slight decrease in the thermal conductance of GNRs, which is remarkably different from that in the longitudinal direction. The curvature and edge effects show a strong size and chirality dependence, while the curvature effect is more sensitive to the size. When the size equals to 12.8 nm (49.2 nm) with the zigzag (armchair) edge, the edge effect results in the reduction of thermal conductance of 2.4% (13.0%) as compared to the corresponding CNT, but the curvature effect vanishes.

Electromagnetic energy within single-resonance chiral metamaterial spheres

We derive an exact expression for the time-averaged electromagnetic (EM) energy inside a chiral dispersive sphere irradiated by a plane wave. The dispersion relations correspond to a chiral metamaterial consisting of uncoupled single-resonance helical resonators. Using a field decomposition scheme and a general expression for the EM energy density in bianisotropic media, we calculate the Lorenz-Mie solution for the internal fields in a medium that is simultaneously magnetic and chiral. We also obtain an explicit analytical relation between the internal EM energy and the absorption cross section. This result is applied to demonstrate that strong chirality leads to an off-resonance field enhancement within weakly absorbing spheres.

Chiral metamaterials with giant optical activity and negative refractive index based on complementary conjugate-swastikas structure

A compact chiral metamaterial (CMM) based on the complementary conjugate-swastikas structure was investigated. Both numerical simulations and experiments reveal that the structure exhibits giant optical rotation as well as high transmittances when a plane wave is normally incident on it, indicating strong chirality. The retrieval results reveal that negative refractive indices of the designed CCMM also could be easily realized due to strong chirality. The physical mechanism was illustrated by simulated scattered magnetic field and surface current distributions. Further, the influences of the structural parameters of the complementary CMM to the transmissions spectra, optical activity and negative refraction have been investigated numerically.

Nematic twist cell: Strong chirality induced at the surfaces

A nematic twist cell having a thickness gradient was filled with a mixture containing a configurationally achiral liquid crystal (LC) and chiral dopant. A chiral-based linear electrooptic effect was observed on application of an ac electric field. This “electroclinic effect” varied monotonically with d, changing sign at d=d0 where the chiral dopant exactly compensated the imposed twist. The results indicate that a significant chiral electrooptic effect always exists near the surfaces of a twist cell containing molecules that can be conformationally deracemized. Additionally, this approach can be used to measure the helical twisting power (HTP) of a chiral dopant in a liquid crystal.

Alternating Plasmonic Nanoparticle Heterochains Made by Polymerase Chain Reaction and Their Optical Properties.

Organization of nanoparticles (NPs) of different materials into superstructures of higher complexity represents a key challenge in nanotechnology. Polymerase chain reaction (PCR) was used in this study to fabricate chains consisting of plasmonic NPs of different sizes, thus denoted heterochains. The NPs in such chains are connected by DNA oligomers, alternating in a sequence big-small-big-small-... and spanning lengths in the range of 40-300 nm by varying the number of PCR cycles. They display strong plasmonic chirality at 500-600 nm, the chiral activity revealing nonmonotonous dependence on the length of heterochains. We find the strength of surface-enhanced Raman scattering (SERS) to increase with chain length, while the chiral response initially increased and then decreased with the number of PCR cycles. The relationship between the optical properties of the heterochains and their structure/length is discussed. The length-dependent intense optical response of the plasmonic NP heterochains holds great potential for biosensing applications.

Terahertz chiral metamaterials with giant and dynamically tunable optical activity

We demonstrated giant optical activity using a chiral metamaterial composed of an array of conjugated bilayer metal structures. The chiral metamaterials were further integrated with photoactive inclusions to accomplish a wide tuning range of the optical activity through illumination with near-infrared light. The strong chirality observed in our metamaterials results in a negative refractive index, which can also be well controlled by the near-infrared optical excitation.

Design and studies of novel polyoxysterol-based porphyrin conjugates

New types of steroid-porphyrin conjugates derived from 20-hydroxyecdysone (20E) and 24-epibrassinolide (EBl) were synthesized. An exceptional regioselectivity in the reaction of both steroids with porphyrin boronic acids was found to give side-chain-conjugated boronic esters as sole products. UV–Vis-, fluorescence and NMR spectroscopy yielded similar data for all the studied compounds confirming the solvent driven supramolecular assembly with formation of J-aggregates. CD measurements of water diluted solutions showed a clear difference between 20E and EBl conjugates. The latter showed a strong supramolecular chirality, whereas 20E J-aggregates did not.

Dual- and Multiband Chiral Metamaterial Structures With Strong Optical Activity and Negative Refraction Index

A novel dual-band chiral metamaterial based on the conjugated gammadions structure is proposed. Simulation and experimental results, which are in a very good agreement, indicate that the proposed structure exhibits giant optical activity and a negative refractive index for right and left circularly polarized waves due to its strong chirality in two near frequency bands. The results show that the effective frequency bands of this structure are independent and can be simply adjusted using the presented design curves. Furthermore, our studies prove that generalization for the multiband chiral metamaterials is possible.

GIANT OPTICAL ACTIVITY AND NEGATIVE REFRACTIVE INDEX USING COMPLEMENTARY U-SHAPED STRUCTURE ASSEMBLY

In this paper, a chiral metamaterial (CMM) with complementary U-shaped structure assembly is proposed. The microwave experimental and simulated results of the proposed complementary structure exhibit giant optical activity. The experimental results are in good agreement with the numerical ones. The retrieval results reveal that negative refractive indices for right- handed and left-handed circularly polarized waves could be easily realized due to strong chirality. The mechanism of the chiral behaviors of resonance frequencies will be illustrated by simulated current distributions. Further, the complementary U-shaped structure assembly also exhibits stronger circular dichroism, giant optical activity, and negative index at near-infrared region by simulations.

Helical Metal Nanoparticle Assemblies with Defects: Plasmonic Chirality and Circular Dichroism

Metal nanoparticle structures with a helical arrangement are able to create strong optical chirality and associated circular dichroism (CD). The CD effect in these chiral structures comes from the electromagnetic dipole–dipole interaction between nonchiral nanoparticles. In general, the calculated CD spectra are sensitive to the parameters. The shape of the CD spectrum depends on the helix pitch and radius and, sometimes, on the number of particles in a helix. Simulating several geometries, we were able to identify the sets of parameters for which the CD spectrum is very stable against defects or disorder. These results are important for designing novel nanocomposite materials with strong optical chirality in the visible wavelength region.

Linear Synthesis of Chiral cycloSal‐Pronucleotides

AbstractCycloSal‐nucleosyl‐phosphate triesters are a known class of highly effective nucleotide prodrugs (pronucleotides) of antivirally active nucleoside analogues. Until recently, the synthesis of these compounds always gave diastereoisomeric mixtures. Then, a convergent route for the stereospecific synthesis of cycloSal‐triesters was described to give isomerically pure cycloSal‐prodrugs for the treatment of viral diseases. Here, the development of a stereoselective synthesis of these pronucleotides using various chiral auxiliaries is described. In contrast to pyrrolidine‐ or pyrrolidinone derivatives it was found that a thiazolidine derived from valinol fulfilled all three requirements to act as a suitable chiral moiety, allowing: (i) strong chirality transfer, (ii) the formation of separable diastereoisomeric intermediates, and (iii) a suitable leaving group that allows the introduction of the nucleoside analogue (e.g., d4T) in the final step under mild reaction conditions. The title compounds were obtained with very high diastereoisomeric excesses of more than 95 %.

Complementary chiral metamaterials with giant optical activity and negative refractive index

A complementary bilayer cross-wire chiral metamaterial is proposed and studied experimentally and numerically. It exhibits giant optical activity and a small circular dichroism. The retrieval results reveal that a negative refractive index is realized for right circularly polarized waves due to the strong chirality. Our numerical results show that the mechanism of the chiral behavior at the resonance of lower frequency can be interpreted as the coupling effects between two sets of mutually twisted virtual magnetic dipoles, while the resonance of higher frequency shows complicated nonlocal features.

Helical spin waves, magnetic order, and fluctuations in the langasite compound Ba3NbFe3Si2O14

We have investigated the spin fluctuations in the langasite compound Ba3NbFe3Si2O14 in both the ordered state and as a function of temperature. The low temperature magnetic structure is defined by a spiral phase characterized by magnetic Bragg peaks at q=(0,0,tau ~ 1/7) onset at TN=27 K as previously reported by Marty et al. The nature of the fluctuations and temperature dependence of the order parameter is consistent with a classical second order phase transition for a two dimensional triangular antiferromagnet. We will show that the physical properties and energy scales including the ordering wavevector, Curie-Weiss temperature, and the spin-waves can be explained through the use of only symmetric exchange constants without the need for the Dzyaloshinskii-Moriya interaction. This is accomplished through a set of ``helical" exchange pathways along the c direction imposed by the chiral crystal structure and naturally explains the magnetic diffuse scattering which displays a strong vector chirality up to high temperatures well above the ordering temperature. This illustrates a strong coupling between magnetic and crystalline chirality in this compound.

Chiral nanoparticle assemblies: circular dichroism, plasmonic interactions, and exciton effects

The paper reviews recent progress on chiral nanocrystal assemblies with induced optical chirality and related circular dichroism. Many natural molecules and biomolecules are chiral and exhibit remarkably strong optical chirality (circular dichroism) due to their amazingly uniform atomic composition in a large ensemble. It is challenging to realize artificial nanoscale systems with optical chirality since the atomic structure of artificial nanostructures may not be always controlled or even known. Nevertheless, the artificial optical chirality has been accomplished and it is the main scope of this review. In particular, we discuss assemblies incorporating chiral molecules, metal nanocrystals, and semiconductor quantum dots. Plasmon-induced and plasmon-enhanced circular dichroism effects appear in nanoscale assemblies built with metal nanocrystals, while excitonic and surface-states related phenomena are observed in semiconductor quantum dots conjugated with chiral molecules.

Hydrodynamics of Sperm Cells near Surfaces

Sperm are propelled by an actively beating tail, and display a wide variety of swimming patterns. When confined between two parallel walls, sperm swim either in circles or on curvilinear trajectories close to the walls. We employ mesoscale hydrodynamics simulations in combination with a mechanical sperm model to study the swimming behavior near walls. The simulations show that sperm become captured at the wall due to the hydrodynamic flow fields which are generated by the flagellar beat. The circular trajectories are determined by the chiral asymmetry of the sperm shape. For strong (weak) chirality, sperm swim in tight (wide) circles, with the beating plane of the flagellum oriented perpendicular (parallel) to the wall. For comparison, we also perform simulations based on a local anisotropic friction of the flagellum. In this resistive force approximation, surface adhesion and circular swimming patterns are obtained as well. However, the adhesion mechanism is now due to steric repulsion, and the orientation of the beating plane is different. Our model provides a theoretical framework that explains several distinct swimming behaviors of sperm near and far from a wall. Moreover, the model suggests a mechanism by which sperm navigate in a chemical gradient via a change of their shape.

Theoretical Study of Alkali Atom Doped Outside 4 Å Carbon Nanotubes

Using first-principles calculations, we study the doping of alkali atoms on the outer surface of 4 angstroms single-walled carbon nanotubes. It is found that the exterior doping at the "center" site is more favorable than the interior doping along tube axis. The calculated binding energies show a strong chirality dependence, and the alkali atoms tend to be bound strongest with the (5, 0) tube. The change of band structure upon doping is studied and the charge transfer between tube and alkali atom is shown. Our calculations indicate that the Li storage capacity can be reached to a maximum of LiC2.5, which suggests that the 4 angstroms nanotube could be a plausible candidate for Li-ion battery applications.