Plasmonic rod dimers as elementary planar chiral meta-atoms

We theoretically calculate the electromagnetic response of metallic rod dimers for the arbitrary planar arrangement of rods in the dimer. It is shown that dimers without an in-plane symmetry axis exhibit elliptical dichroism and act as "atoms" in planar chiral metamaterials. Because of a very simple geometry of the rod dimer, such planar metamaterials are much easier to fabricate than conventional split-ring or gammadion-type structures and lend themselves to a simple analytical treatment based on a coupled dipole model. Dependencies of the metamaterial's directional asymmetry on the dimer's geometry are established analytically and confirmed in numerical simulations.     

Magnetism of ferriprotoporphyrin IX monomers and dimers

The SQUID and the ⁵⁷Fe Mössbauer spectroscopy studies of the magnetic properties of monomeric and dimeric forms of iron porphyrin were performed between 2 and 305 K. The effective magnetic relaxation rate of the Fe atoms in iron porphyrin monomers exhibits complex temperature dependence, resulting from the competing spin-spin and spin-lattice relaxation processes. The dimerization of iron porphyrin dramatically speeds up the magnetic relaxation. The Fe-Fe antiferromagnetic exchange coupling constant in Fe-O-Fe dimer is J≈−110 cm⁻¹. The complementary application of SQUID and the Mössbauer spectroscopy is proposed as a new precise quantitative analytical methodology for monitoring of the aggregation process of iron porphyrin.     

Plasmonic enhancement of linear birefringence and linear dichroism in anisotropic optical metamaterials

The resonant behavior of linear birefringence and linear dichroism spectra is found in anisotropic optical metamaterials made of noble metal thin films with stripes and rectangular hole nanoapertures forming one- or two-dimensional subwavelength gratings. Differences in effective refractive index and extinction coefficient for linearly polarized eigenstates are increased in spectral range of resonances of local and surface plasmon-polaritons at the normal incidence and reach the values of Δn ≃ 2.5 and Δκ ≃ 2.75, respectively.     

In silico design of chiral dimers to direct the synthesis of a chiral zeolite

ABSTRACT

We have applied in silico materials design in an effort to design chiral organic structure directing agents (OSDAs) for the synthesis of enantiomerically enriched STW zeolite. To do so, we have used both exhaustive virtual screening of a limited set of symmetrical imidazolium dimers and a multi-objective genetic algorithm to effectively search a larger virtual combinatorial chemistry space that is too vast for an exhaustive approach.     

Complexation in dense gases: concentrations of argon monomers,dimers and trimers

A classical statistical mechanical cluster formalism is presented for computing the thermodynamic properties and equilibrium state of aggregation of a fluid composed of structureless monomeric units which are capable of forming weakly bound van der Waals complexes. The general procedure is not restricted to any particular definition of the clusters, provided that the definition used satisfies certain reasonable criteria. The formalism is illustrated by applying it to two different cluster definitions. Calculations based on these definitions have been performed for the concentrations of argon monomers, dimers and trimers. It is found that gas imperfections due to cluster-cluster interactions can significantly affect the calculated values of these concentrations.     

EXAFS study of iron monomers and dimers isolated in solid argon

We report a study of local structure of iron monomers and dimers isolated in a rare gas Ar matrix using extended X-ray absorption fine structure measurements. The results have been compared with Mössbauer measurements on such systems. The interatomic distance found for Fe₂ in these measurements is 1.87 ± 0.13 Å.     

Metal-nitroxyl interactions. 18. Spin-labeled copper carboxylate dimers and monomers

EPR spectra of spin-labeled copper carboxylate dimers and monomers were obtained in room-temperature solutions and in frozen glasses at about −180°C. At room temperature the copper-nitroxyl interaction in the dimer results in severe broadening of the nitroxyl signal and a decrease in nitroxyl relaxation time. In the frozen-glass spectrum of the dimer the nitroxyl relaxation time is decreased by interaction with the copper but the lineshape changes are dominated by nitroxyl-nitroxyl interactions. In 1:1 pyridine: toluene solution the dimers dissociate into monomers. The spectra of the monomers in solution and in frozen glasses have g and A values which are averages of those expected for the copper and nitroxyl electrons, indicating that the electron-electron exchange interaction is large relative to the g-value differences and the nuclear hyperfine interactions.     

Plasmonic modes in periodic metal nanoparticle chains: a direct dynamic eigenmode analysis

We demonstrate an efficient eigendecomposition method for analyzing the guided modes in metal nanoparticle chains. The proposed method has the advantage of simultaneously showing the dispersion relation and the mode quality. It can also separate the material properties from the geometrical properties. Its efficiency therefore does not depend on the complexity of the material polarizability. We used the method to analyze the guided modes of a single and a pair of metal nanoparticle chains. The rigorous dynamic dipole polarizability typically gives a redshift compared with the results obtained from the broadly used quasi-static dipole polarizability with radiation correction.     

Coadsorption of monomers and dimers with first neighbour interactions on square lattices

We model surface coadsorption on a square lattice infinite in length and of finite width M. The adsorbed species occupy either one site (monomers) or two nearest neighbour sites (dimers) and have first neighbour interactions. The quantities computer per site at thermal equilibrium are the entropy, the number of each species, and the numbers of first neighbours (monomer-monomer, dimer-dimer, monomer-dimer). Numerical results are obtained for M ≤ 5. Structural orderings are observed and the detailed analysis of the corresponding phase diagrams is presented. Following our analysis, the boundary lines between phases are found to be linear for all M, and their exact analytic expressions are obtained for arbitrary values of the interaction energies. A number of extrapolations for any value of M is possible leading to predictions on the infinite two-dimensional lattice.     

Photoelectron spectroscopic studies of the monomers and dimers of acetic and trifluoracetic acids

Comparison of the He(I) and He(II) photoelectron spectra of acetic and trifluoracetic acids has clarified the assignments of their p-based ionization potentials. The He(I) spectra of the gas-phase homodimer of each molecule have been observed using a high-pressure nozzle inlet system operating at room temperature. Spectrum-stripping of the monomer—dimer mixed-spectra gives dimer spectra which are interpreted with the aid of molecular orbital calculations. Appropriate mixtures of the two acids have been analyzed to obtain the He(I) spectrum of the 1:1 heterodimer. Assignments of the three dimer spectra indicate that the inductive influences of the methyl and trifluoromethyl groups are transmitted across the hydrogen-bond bridges.     

Plasmonic abilities of gold and silver spherical nanoantennas in terms of size dependent multipolar resonance frequencies and plasmon damping rates

Absorbing and emitting optical properties of a spherical plasmonic nanoantenna are described in terms of the size dependent resonance frequencies and damping rates of the multipolar surface plasmons (SP). We provide the plasmon size characteristics for gold and silver spherical particles up to the large size retardation regime where the plasmon radiative damping is significant. We underline the role of the radiation damping in comparison with the energy dissipation damping in formation of receiving and transmitting properties of a plasmonic particle. The size dependence of both: the multipolar SP resonance frequencies and corresponding damping rates can be a convenient tool in tailoring the characteristics of plasmonic nanoantennas for given application. Such characteristics enable to control an operation frequency of a plasmonic nanoantenna and to change the operation range from the spectrally broad to spectrally narrow and vice versa. It is also possible to switch between particle receiving (enhanced absorption) and emitting (enhanced scattering) abilities. Changing the polarization geometry of observation it is possible to effectively separate the dipole and the quadrupole plasmon radiation from all the non-plasmonic contributions to the scattered light.     

Light scattering from spherical plasmonic nanoantennas: effects of nanoscale roughness

We have investigated the light scattering properties of smooth and roughened nanoshells with dipolar and quadrupolar plasmon resonances tuned to 830 nm. In the dipole resonant case small but measurable variations in the angle dependent light scattering (ADLS) due to the introduction of surface roughness are observed. In the quadrupole case, the distinctive side lobe scattering characteristic of quadrupolar emission is strongly quenched for roughened nanoshells. PACS 81.07.-b; 78.67.-n     

Plasmonic coupling from silver nanoparticle dimer array mediating surface plasmon resonant enhancement on the thin silver film

We investigate the optical absorption spectrum of a periodic array of silver nanoparticle dimer on a thin silver film using multiple-scattering formalism. Surface plasmon polariton mediated from silver nanoparticle dimer array is excited and enhanced by about four times compared with that from monomer array. This enhancement results from the coupling between the two nanoparticles’ plasmons of symmetry mode and anti-symmetry mode. We also illustrate the distance-dependent nanoparticle plasmonic coupling modes based on the polarized charge distribution in dimer geometry. The proposed silver nanoparticle dimer array can be used to enhance surface spectroscopy.     

An ionic force-field study of monomers, dimers and higher polymers in pentafluoride vapors

Pentafluoride compounds such as NbF₅ and TaF₅ have been reported in the literature to admit various states of polymerization coexisting with monomers in their vapor phase, in relative concentrations that vary with temperature and pressure. We construct a microscopic interionic force-field model for the molecular monomer of these compounds (including VF₅, SbF₅ and MoF₅ in addition to NbF₅ and TaF₅), the stable form of the monomer being in the shape of a D_3h trigonal bipyramid in all cases. The model emulates chemical bonds by allowing for electrical and short-range overlap polarizabilities of the fluorines, and is used to evaluate the structure and the stability of (MF₅)_n molecules with n running from 2 to 6. The dimer is formed by two distorted edge-sharing octahedral, while the trimer and the higher polymers can form rings of distorted corner-sharing octahedra. A chain-like configuration is also found for the trimer of NbF₅, which consists of a seven-fold coordinated Nb bonded to two distorted octahedra via edge sharing. Comparison of calculated vibrational frequencies and bond lengths with experimental data is made whenever possible. We find that there is a small net gain of energy in the formation of a dimer, while otherwise the static energy of the n-mer is very close to that of n separated monomers. High sensitivity of the state of molecular aggregation to the thermodynamic conditions of the vapor is clearly indicated by our calculations.     

Electromagnetic wormholes and virtual magnetic monopoles from metamaterials

We describe new configurations of electromagnetic (EM) material parameters, the electric permittivity epsilon and magnetic permeability micro, which allow one to construct devices that function as invisible tunnels. These allow EM wave propagation between the regions at the two ends of a tunnel, but the tunnels themselves and the regions they enclose are not detectable to lateral EM observations. Such devices act as wormholes with respect to Maxwell's equations and effectively change the topology of space vis-à-vis EM wave propagation. We suggest several applications, including devices behaving as virtual magnetic monopoles, invisible cables, and scopes for MRI-assisted surgery.     

Multiscale coarse graining of diblock copolymer self-assembly: from monomers to ordered micelles

Starting from a microscopic lattice model, we investigate clustering, micellization, and micelle ordering in semidilute solutions of AB diblock copolymers in a selective solvent. To bridge the gap in length scales, from monomers to ordered micellar structures, we implement a two-step coarse-graining strategy, whereby the AB copolymers are mapped onto ultrasoft dumbells with monomer-averaged effective interactions between the centers of mass of the blocks. Monte Carlo simulations of this coarse-grained model yield clear-cut evidence for self-assembly into micelles with a mean aggregation number n approximately 100 beyond a critical concentration. At a slightly higher concentration the micelles spontaneously undergo a disorder-order transition to a cubic phase. We determine the effective potential between these micelles from first principles.     

Broadband wave plates: Approach from one-dimensional photonic crystals containing metamaterials

Broadband wave plates working in subwavelength scale are realized by one-dimensional photonic crystals containing negative-index materials. It is demonstrated that the phase shift of reflected wave as a function of frequency changes smoothly within the stop band of the photonic crystal, while it changes sharply within the pass band. In the stop band, the difference between the phase of TE and that of TM reflected wave could remain constant in a rather wide frequency range. These properties are useful for designing compact wave plates or phase retarders which can be used in broad spectral bandwidth.     

Conversion from constitutive parameters to dispersive transmission line parameters for multi-band metamaterials

Abstract

In this study, we explain an approach including conversion from constitutive parameters to dispersive transmission line parameters using the double-band DNG (double-negative) properties of the circular type fishnet metamaterials. After designing the metamaterial structure, the numerical calculations and the composite right/left-handed (CRLH) modeling of circular-type metamaterials are realized in free space. Detailed dispersion characteristics give us the opportunity to explain the true behavior of the inclusions during the analysis stage. By combining the results coming from the standard retrieval procedure with the conventional CRLH theory, we calculate the actual values of the transmission line parameters for all frequency regimes. The constitutive parameters of an equivalent CRLH transmission line are derived and shown to be negative values. It is shown that the constitutive parameters present the same behavior for all negative refractive index regimes. The double-negative properties and the phase advance/lag behavior of metamaterials are observed based on the dispersive transmission line parameters.