M1 and E2 band structures in the Sn-Xe-Ba region

In the present talk I will discuss some ‘rare’ aspects of the E2 band structures and the novel features concerning the dipole bands in this mass region. Reliable and accurate lifetimes have been measured using coincidence recoil distance method. The results of ^129,130Ba will be discussed. In contrast to the predictions of the tilted axis cranking model, the dipole bands in Sb-Xe-Ba nuclei can be nicely described as high-K prolate bands. New data from multi-detector arrays has established extended bands structure, their decay to low lying states have been established and the angular correlation supports the predominant, ΔI=1 character. Finally the sensitive measures, i.e. B(M1) rates of the tilted axis model are compared with the high-K formula based on 1-dim cranking model.     

Photonic band effects in three dimensional lattices of macroscopic-sized dielectric spheres derived from microwave transmission spectroscopy

Several structures of dielectric spheres (glass) have been arranged and the electromagnetic absorption properties of the resulting materials have been measured in the microwave domain. In all cases strong rejected bands are present while keeping nearly transparent for other frequencies. The physical origin of the observed bands is studied by recording the spectra of the materials as they are grown layer by layer. These data show the appearance of a peak attributed to Bragg scattering generated by the spheres layers. Besides, higher order bands are developed over the resonances that are found even in the single layer of spheres. These are caused by a complex interaction of isolated sphere Mie modes and Bragg scattering in planes perpendicular (or nearly) to the incident radiation.     

Splitting of magnetic dipole modes in anisotropic TiO2 micro‐spheres

Monocrystalline titanium dioxide (TiO₂) micro‐spheres support two orthogonal magnetic dipole modes at terahertz (THz) frequencies due to strong dielectric anisotropy. For the first time, we experimentally detected the splitting of the first Mie mode in spheres of radii m through near‐field time‐domain THz spectroscopy. By fitting the Fano lineshape model to the experimentally obtained spectra of the electric field detected by the sub‐wavelength aperture probe, we found that the magnetic dipole resonances in TiO₂ spheres have narrow linewidths of only tens of gigahertz. Anisotropic TiO₂ micro‐resonators can be used to enhance the interplay of magnetic and electric dipole resonances in the emerging THz all‐dielectric metamaterial technology.

     

Ultra-directional forward scattering by a high refractive index dielectric T-shaped nanoantenna in the visible

In this work, we design and numerically demonstrate a touching dielectric nanoantenna with high directionality. This antenna consists of a dielectric cuboid dimer with different heights, and there are no gaps between the subunits of the dimer. Superior unidirectional scattering is achieved when the electric and magnetic dipolar modes inside the antenna satisfy the first Kerker condition. This unidirectional scattering is much more prominent than its components (i.e., the dielectric cuboid nanoantennas with different heights) in the considered spectral region. Furthermore, the radiation angle can be tailored in a 10-degree range by properly rotating the antenna along the out of axis. The off-normal scattering is due to the interference between one induced magnetic dipole and two electric dipoles inside the nanoantenna. Furthermore, we also demonstrate that similar unidirectional scattering effect can also be maintained when the antenna is close to an electric (or magnetic) dipole source, and the forward emission direction can be efficiently controlled by the relative position of the dipole source. Finally, we show that it is possible to further enhance the unidirectionality by arranging the antenna in an array and the main lobe angular beam width of the 2D far-field pattern can be reduced to 28 degree.     

Modulation of photonic band structures with dielectric anisotropy

By the multiple scattering method and the extended Mie theory, we have calculated the photonic band structure of the photonic crystals consisting of the dielectric spheres with uniaxial/biaxial anisotropy. The results demonstrate that for fcc lattice structure there exist two partial photonic band gaps which does not appear in the isotropic case. Among them, the lower one, lying between the second and the third bands, exists in one third of the first Brillouin zone, while the upper one, opening between the fourth and fifth bands, can appear simultaneously in the rest two thirds of the first Brillouin zone. The effects of anisotropy on the band structures are studied as well, which suggests the biaxial anisotropy are much more flexible than the uniaxial anisotropy in modulating the band structures.     

Switching mechanism due to the spontaneous emission cancellation in photonic band gap materials doped with nano-particles

We have investigated the switching mechanism due to the spontaneous emission cancellation in a photonic band gap (PBG) material doped with an ensemble of four-level nano-particles. The effect of the dipole–dipole interaction has also been studied. The linear susceptibility has been calculated in the mean field theory. Numerical simulations for the imaginary susceptibility are performed for a PBG material which is made from periodic dielectric spheres. It is predicted that the system can be switched between the absorbing state and the non-absorbing state by changing the resonance energy within the energy bands of the photonic band gap material.     

Smith-Purcell radiation from a charge running near the surface of a photonic crystal

Smith-Purcell radiation from a charge running near a photonic crystal is calculated for a slab system of a periodic array of dielectric spheres, with the photonic band effect taken into account exactly. The radiation spectrum has a series of resonantly enhanced structures, which are shown to arise accompanying the excitation of the photonic bands. It is also shown that the overall intensity of the emission band does not depend very strongly on the slab thickness but the height of the resonant peaks increases progressively with thickness due to the enhanced Q value of excited photonic bands.     

Dramatic transition between electric and magnetic rotations in 106Ag

The lifetimes for the states of magnetic dipole band in106Ag have been measured using the Doppler-shift attenuation method via the reaction of100Mo(11B,5n)106Ag at a beam energy of 60 MeV.The reduced transition strengths of the magnetic dipole band,the B(M1)/B(E2)ratios together with the signature of the level energy as a function of angular momentum for the positive parity states of106Ag show that a drastic change of excitation mode,that is,from electric rotation to magnetic rotation,occurs within one unit of spin at around Iπ=12+.Theoretical calculations based on the triaxial projected shell model consistently reproduce the experimental data and provide an explanation on the nature of observed phenomena such that the dynamical drift of the rotational axis suddenly from the principal axis to the tilted one,along the positive parity bands of106Ag.     

Multipole contributions to the effective dielectric constant of suspensions of spheres

We study multipole contributions to the effective dielectric constant of dilute suspensions of uniform spheres. It is shown that for frequencies near the plasma resoance multipole contributions are negligible in comparison with the dipole contributions. Outside resoance multipolar corrections to the Clausius-Mossotti formula are of the same order of magnitude as the dipolar ones. We argue that in resonance the dipole approximation should be valid also for more complicated sphere models.     

The validity range of the dipole approximation for a dielectric mixture

This paper investigates the validity range of the dipole approximation for a dielectric mixture with spherical inclusions through electric field analyses according to a new criterion: if the maximum perturbation on an arbitrarily selected sphere surface caused by the interactions among spheres is less than a given allowable error, the dipole approximation is considered acceptable. Based on the dipole-enhanced dipole approximation previously presented by us, a theoretical formula for calculating the validity range of the dipole approximation is derived. It shows that the validity range depends on three parameters, the dielectric mismatch, the volume fraction of inclusions, and the allowable error. The calculated results by this formula are compared with those numerically obtained by the FEM. The good agreement between them indicates the credibleness of this formula.     

Polarization of two close metal spheres in an external homogeneous electric field

The dipole moment of each of two uncharged conducting spheres with radius R in an external electric field was calculated. The distance between the centers of the spheres is 2l. It was shown that, if R/l≲0.8, the influence of higher multipole moments is negligible.     

Disentangled Higher-Orbital Bands and Chiral Symmetric Topology in Confined Mie Resonance Photonic Crystals

Topological phases based on tight-binding models have been extensively studied in recent decades. By mimicking the linear combination of atomic orbitals in tight-binding models based on the evanescent couplings between resonators in classical waves, numerous experimental demonstrations of topological phases have been successfully conducted. However, in dielectric photonic crystals, the Mie resonances' states decay too slowly as $1/r$, leading to intrinsically different physics between tight-binding models and dielectric photonic crystals. Here, a confined Mie resonance photonic crystal is proposed by embedding perfect electric conductors between dielectric rods, creating the chiral symmetric band structure which ideally matches tight-binding models with nearest-neighbour couplings. As a consequence, disentangled band structure spanned by higher atomic orbitals is observed. Moreover, the result provides an effective route to achieve a 3D photonic crystal with a complete photonic bandgap and third-order topology. The implementation offers a versatile platform for studying exotic higher-orbital bands and achieving tight-binding-like 3D topological photonic crystals.     

Phase diagram of dipolar hard and soft spheres: manipulation of colloidal crystal structures by an external field

Phase diagrams of hard and soft spheres with a fixed dipole moment are determined by calculating the Helmholtz free energy using simulations. The pair potential is given by a dipole-dipole interaction plus a hard-core and a repulsive Yukawa potential for soft spheres. Our system models colloids in an external electric or magnetic field, with hard spheres corresponding to uncharged and soft spheres to charged colloids. The phase diagram of dipolar hard spheres shows fluid, face-centered-cubic (fcc), hexagonal-close-packed (hcp), and body-centered-tetragonal (bct) phases. The phase diagram of dipolar soft spheres exhibits, in addition to the above mentioned phases, a body-centered-orthorhombic (bco) phase, and it agrees well with the experimental phase diagram [Nature (London) 421, 513 (2003)]. Our results show that bulk hcp, bct, and bco crystals can be realized experimentally by applying an external field.     

Photonic band structure and effect of ε and μ on the reflectivity of one-dimensional magnetic photonic crystal structure

In this paper, we study the photonic band structure and reflection properties in one-dimensional magnetic photonic crystals (MPCs). Investigation of dispersion characteristics shows that in the case of MPCs, photonic band gaps arise due to the contrast in the wave impedance, not due to the contrast in the refractive index, while contrast in the refractive index of the two layers decides the position and number of the band gaps. We also study the effect of permittivity and permeability on reflection bands, which shows that the structure that has larger values of magnetic permeability (μ) than dielectric permittivity (ε) have wider TM-reflection bands, whereas the structure for which ε is greater than μ has wider TE-reflection bands. But the gap to mid-gap frequency ratio for TM-reflection bands is larger than TE-reflection bands. Thus, magnetic permeability has greater impact on the reflectivity of MPCs than dielectric permittivity. Finally, the analysis of the omni-reflectance in MPCs has also been studied.     

Fluids of polarizable hard spheres with dipoles and tetrahedral quadrupoles Integral equation results with application to liquid water

The mean spherical, linearized hypernetted chain and quadratic hypernetted chain approximations are solved for a fluid of hard spheres with embedded point dipoles and tetrahedral quadrupoles and this system is shown to be quite similar to the dipole-linear quadrupole case previously studied. However, tetrahedral quadrupoles have a larger influence upon the structural and thermodynamic properties and are slightly more effective in decreasing the dielectric constant from the purely dipolar value. Also we describe a simple self-consistent mean field theory which allows molecular polarizability to be taken into account. This approximation together with the integral equation methods is applied to a polarizable dipole-tetrahedral quadrupole fluid with water-like parameters. The dielectric constant of this system is found to be in good agreement with the experimental results for liquid water for temperatures ranging from 25°C to 300°C. The influence of molecular polarizability is shown to be very large. At 25°C the mean dipole moment is ～2·56 D compared with ～1·85 D in the gas phase and the dielectric constant increases from ～25 for non-polarizable particles to ～80 for the polarizable model.     

Nonlinear dielectric effect of dipolar fluid mixtures

The linear and nonlinear dielectric effect for two- and three-component dipolar fluid mixtures are studied within the framework of the mean spherical approximation (MSA) of dipolar hard sphere mixtures. In our approach, equally sized dipolar hard spheres with different dipole moments are considered. Based on earlier results for the electric field dependence of the polarization our analytical equations show the so-called normal saturation effects, which are in good agreement with corresponding canonical ensemble Monte Carlo simulation data. Comparisons between the MSA based theoretical results and the corresponding Langevin and Debye-Weiss theories are also made.     

Light propagation in photonic crystal with gain: Applicability of the negative loss approximation

The applicability of the concept of permittivity with Im ?_gain < 0 to describe the light propagation in a metamaterial system with gain is discussed using the example of a 1D photonic crystal containing gain layers. It is shown that this approach is in agreement with the principle of causality, unless lasing is present. Though the lasing process itself requires nonlinear analysis, the lasing threshold can be determined by linear (negative loss) approximation. Connecting the onset of lasing with the passage of the transfer function pole into the upper half-plane of the complex frequency, we show that (i) if the pump frequency lies in a pass band then an increase in the number N of elementary cells will sooner or later lead to lasing; (ii) if the frequency of the pump lies in the band gap, then lasing at band gap frequencies may occur in a sample with a low N before the band gap has been formed. Nevertheless, lasing will be necessarily suppressed by a further increase in N. In any case, for sufficiently large number of layers due to a finite line width of ?_gain(ω), lasing appears in the pass band even if the pump frequency is in the band gap.     

THERMAL RADIATION PROPERTY OF A THREE DIMENSIONAL PHOTONIC CRYSTAL BASED ON MULTIPLE SCATTERING METHOD

We have studied absorption and thermal radiation for a three dimensional lossy photonic crystal of dielectric spheres by using a multiple scattering method. It is shown that such simple structures have excellent selective thermal radiative characteristics in different photonic bands. There is stronger thermal radiation at high frequency than that at low frequency. In comparison with the uniform slab without photonic crystal structure, in the photonic band gap, the thermal emission intensity is greatly suppressed and very weak, but not zero due to the penetration depth. Under the same lattice structure, the thermal emission of photonic crystal varies with the change of the radius of the spheres.     

Simulation of two dimensional systems with two dimensional electrostatics

Some of the problems associated with Monte Carlo and molecular dynamics simulations of three dimensional ionic and dipolar systems are discussed, with emphasis on the use of periodic boundary conditions (PBC). It is shown that analogous problems may arise in two dimensional systems provided that the interactions are two dimensional electrostatic interactions, that is, interactions derived from the two dimensional Laplace equation. The PBC hamiltonian is evaluated by considering the appropriate two dimensional lattice sums, and a computable form for the effective pair interactions in PBC developed. The idea of an external dielectric constant is introduced and its effects included in the PBC hamiltonian. Formulae for evaluating the dielectric constant from a simulation with any external dielectric constant are given. Perturbation formulae showing the effects on the structure and mean square dipole moment of a dipolar system which are caused by a change in external dielectric constant are derived. A formula for the shift in mean square dipole moment of an ionic system is also developed. The problem of interpreting results from such simulations is discussed briefly.     

Structure and dielectric properties of polar fluids with extended dipoles: results from numerical simulations

The strengths and shortcomings of the point dipole model for polar fluids of spherical molecules are illustrated by considering the physically more relevant case of extended dipoles formed by two opposite charges ±?q separated by a distance d (dipole moment μ=qd). Extensive molecular dynamics simulations on a high-density dipolar fluid are used to analyse the dependence of the pair structure, dielectric constant ε and dynamics as a function of the ratio d/σ (σ is the molecular diameter), for a fixed dipole moment μ. The point dipole model is found to agree well with the extended dipole model up to d/σ ? 0.3. Beyond that ratio, ε shows a non-trivial variation with d/σ. When d/σ > 0.6, a transition is observed towards a hexagonal columnar phase; the corresponding value of the dipole moment is found to be substantially lower than the value of the point dipole required to drive a similar transition.