Twist Defects Research Articles

Planewave remittances of an axially excited chiral sculptured thin film with gain

Reflectances and transmittances of chiral sculptured thin films (STFs) with gain qualitatively differ from those of lossless and lossy chiral STFs, chiefly at the edges of the Bragg regime, for normally incident plane waves. The difference is explained in terms of the photonic density of states. A chiral STF with a central twist defect exhibits a reflection hole if there is no gain and emits narrowband radiation if there is gain, provided the thickness is not large. This narrowband feature diminishes as the thickness increases, and is replaced by an ultranarrowband transmission hole only if the chiral STF has neither gain nor loss.

Yielding of polyethylene through propagation of chain twist defects: Temperature, stem length and strain-rate dependence

The temperature, the stem length and the strain-rate dependence of the yield stress of polyethylene (PE) is investigated via a modified crystal plasticity approach. Yielding is considered in terms of nucleation and propagation of [001] screw dislocations with Burgers vector c/2 due to migration of 180° chain twist defects. The stress-induced twist motion within the dislocation cores is modeled as an Eyring activated rate process. This gives an inelastic contribution to the dislocation core energy depending on the stem length and the strain rate and results in improved predictions of the crystal plasticity approach. The model is compared to available experimental data as well as to the predictions of the modified crystal plasticity approach proposed by Brooks and Mukhtar.

Double-handed circular Bragg phenomena in polygonal helix thin films

Oblique-incidence physical vapor deposition has been used to create optical thin films with a polygonal helix-shaped nanostructure. A series of titanium dioxide thin films are investigated, including triangle, square, pentagon, and star-shaped polygonal helices. Experimental optical measurements reveal a double-handed circular Bragg response: at one frequency band a polygonal helix reflects left-handed circularly polarized light, and at a second frequency band reflects right-handed circularly polarized light. The relative wavelength dependence of each reflection band is determined by the physical structure of the polygonal helix, a property that is set during the thin-film deposition process. Spectral-hole polarization filters, produced by adding twist and spacing layer defects to polygonal helix thin films, are also reported.

Multiplet structure of the defect modes in 1D helical photonic crystals with twist defects

We theoretically analyse the defect modes generated by equispaced twist defects in 1D helical (cholesteric-like) structures within their frequency gap which is such that only the first two of the four eigenwaves 1±, 2± are exponentially attenuated. n0 identical defects generate n0 different defect modes, each one represented by a linear combination of the four eigenwaves. The components 1+ and 1− are by far the dominant ones and they are localized near the defect planes. We give exact analytic expressions for the elements of the transfer and scattering matrices of the defect planes, for the functions defining the defect mode when n0 = 1, and for the defect frequencies when n0 = 1, 2, 3. In the particular case n0 = 2 and twist angle θ = π/2, the difference between the two defect wavelengths λd2, λd1 depends exponentially on the distance z1 between the defect planes, going to zero for z1 → ∞ and becoming as large as the entire frequency gap for z1 → 0.

Optical crossover phenomenon due to a central 90°-twist defect in a chiral sculptured thin film or chiral liquid crystal

We present a theoretical analysis of a remarkable phenomenon evinced by a periodic structurally chiral material—for example, a chiral sculptured thin film (STF) or a chiral liquid crystal—with a central 90°-twist defect illuminated with normally incident, circularly polarized light. Based on the coupled-wave theory (CWT), an approximate but closed-form solution of the relevant boundary-value problem is obtained in terms of a 4 × 4 CWT transmission matrix. The CWT transmission matrix is decomposed into two terms. The first term favours total transmission in the central part of the Bragg regime of the axially excited, structurally chiral material, while the second term favours total reflection in the whole Bragg regime. When the thickness of the structurally chiral material is relatively small, the second term is dominated by the first, which gives rise to a co-handed transmittance peak in the centre of the Bragg regime. As the thickness increases, the second term becomes significant and interferes with the first term such that the transmission matrix is isomorphic to that of a defect-free structurally chiral material—except in a tiny wavelength-regime wherein the L ∞ -norms of the two terms become identical to engender the total-reflection feature. Hence, the co-handed transmittance peak diminishes (and eventually vanishes) as the thickness increases and is replaced by a cross-handed reflectance peak. The bandwidths of the two peaks depend, in different ways, on the local birefringence of the structurally chiral material.

Twist defect in chiral photonic structures with spatially varying pitch

The properties of photonic defect modes in a chiral photonic structure were investigated using the finite element method. By stacking two cholesteric liquid crystal (ChLC) films, the defect mode due to the introduction of a twist defect was considered in both cases of chiral structures with constant pitch and spatially varying pitch. Two types of linear pitch gradients for achieving a broadband reflection were analysed, and the number of chiral pitches required for establishing the stop band was simulated. The effect of a finite sample thickness on the energy density distribution of the defect mode and on the required polarization of the incident light to excite the defect mode was studied. In both cases of constant pitch and spatially varying pitch, an unusual crossover behaviour in reflection at the defect resonance wavelength of a single circularly polarized mode appears when the structure thickness increases beyond a specific value. The energy distribution inside the sample also reveals the unusual distribution. Two different resonance wavelengths can be created by a twist defect in the ChLC composite film with linearly varying pitch, while only one resonance wavelength can be created in the identical film with constant pitch.

Third method for generation of spectral holes in chiral sculptured thin films

The introduction of either a central layer defect or a central twist defect in a periodic structurally chiral material generates circular-polarization-sensitive spectral holes in the remittance spectrums for normally incident plane waves. We propose and theoretically establish here the third method to generate such spectral holes using two-section chiral sculptured thin films (STFs). Both sections of the proposed device have the same periodicity and handedness, but their dielectric properties are different and related in a specific way. The concept of pseudoisotropy is highly relevant for the production of the proposed device.

Defect modes in multisection helical photonic crystals

We examine defect modes in four-section helical hotonic crystals (HPCs) that comprise three twist defects located at the intersections. The three twist defects are introduced by a single angle phit , but they are quantified differently by the jump angles across the successive sections. The two defect modes are localized at the different defect sites and can be either coupled or uncoupled to each other, depending on the value of phit . The two defect modes are excited by normally incident plane waves of different circular polarization states as the HPC thickness increases. When the two defect modes are uncoupled to each other, two co-handed reflection holes are present in the Bragg regime for small thickness, but they evolve into two stable cross-handed transmission holes for sufficiently large thickness. When the two defect modes are coupled to each other, however, three co-handed reflection holes appear around the center of the Bragg regime for small thickness, and they evolve into three cross-handed transmission holes as thickness increases, and eventually all three co-handed transmission holes coalesce into one stable cross-handed transmission hole for sufficiently large thickness. Finally, the simultaneous occurrence of the two types of spectral holes at a single resonance wavelength can be realized for specific values of sample thickness when the two defect modes are uncoupled to each other.

Theory of Nucleosome Corkscrew Sliding in the Presence of Synthetic DNA Ligands

Histone octamers show a heat-induced mobility along DNA. Recent theoretical studies have established two mechanisms that are qualitatively and quantitatively compatible with in vitro experiments on nucleosome sliding: octamer repositioning through one-base-pair twist defects and through ten-base-pair bulge defects. A recent experiment demonstrated that the repositioning is strongly suppressed in the presence of minor-groove binding DNA ligands. In the present study, we give a quantitative theory for nucleosome repositioning in the presence of such ligands. We show that the experimentally observed octamer mobilities are consistent with the picture of bound ligands blocking the passage of twist defects through the nucleosome. This strongly supports the model of twist defects inducing a corkscrew motion of the nucleosome as the underlying mechanism of nucleosome sliding. We provide a theoretical estimate of the nucleosomal mobility without adjustable parameters, as a function of ligand concentration, binding affinity, binding site orientation, temperature and DNA anisotropy. Having this mobility in hand, we speculate on the interaction between a nucleosome and a transcribing RNA polymerase, and suggest a novel mechanism that might account for polymerase-induced nucleosome repositioning on short DNA templates.

Defect modes in helical photonic crystals: An analytic approach

Considering helically periodic structures with a twist defect, we define the defect mode in unbounded media and the scattering properties of bounded samples. The mode frequency, the lifetime of the defect mode, and the spectral width of the transmittance and reflectance peaks are expressed by fully analytic and very simple equations, the simplest ones appearing in the literature of photonic band gap materials. Our approach provides a clear explanation of the interesting optical properties of lossless samples with a twist defect and shows that the absorption greatly changes such properties, giving rise to unexpected effects. In particular, the total absorption within thick samples can decrease drastically when the absorption coefficient is increased beyond a well defined value.

Supermodes of chiral photonic filters with combined twist and layer defects.

We consider the circularly polarized localized modes of chiral photonic structures with combined central twist and isotropic layer defects. The general filter is shown to suffer from anomalous remittance and saturation of linewidth as the thickness of the structure is increased. However, by choosing parameters that phase match the elements of the round-trip matrix of the isotropic layer defect, we demonstrate the existence of supermodes that maintain exceptional purity of polarization state and exponential decrease in linewidth as the thickness is increased.

Photonic defect modes in cholesteric liquid crystal films.

We discuss the properties of photonic defect modes in cholesteric liquid crystals. Twist defects, isotropic defect layers, and combinations of both are considered. After deriving the reflection and transmission properties of the defects, we study the effect of a finite sample thickness on the defect mode's amplitude and on the required polarization of incident light to excite the defect mode.

1A19 コレステリック液晶への捻れ欠陥の導入と欠陥モードレーザー発振(物理・物性)

1A19 コレステリック液晶への捻れ欠陥の導入と欠陥モードレーザー発振(物理・物性)

Chromatin dynamics: nucleosomes go mobile through twist defects.

We study the spontaneous "sliding" of histone spools (nucleosomes) along DNA as a result of thermally activated single base pair twist defects. To this end we map the system onto a suitably extended Frenkel-Kontorova model. Combining results from several recent experiments we are able to estimate the nucleosome mobility without adjustable parameters. Our model shows also how the local mobility is intimately linked to the underlying base pair sequence.

Optical second-harmonic generation enhanced by a twist defect in ferroelectric liquid crystals.

Second-harmonic generation (SHG) spectra were numerically calculated in ferroelectric liquid crystals with a twist defect. It is shown that SHG is enhanced when the SHG wavelength is close to the defect mode. The spectral width of the enhanced peak becomes sharper with increasing the sample thickness at the same rate for the width of the defect mode peak. The SHG peak intensity increases with about seventh power of the sample thickness.

Twist defects in helical sonic structures.

We analyze theoretically both the acoustic wave propagation in periodic media made of anisotropic materials whose stiffness tensor is uniformly rotating along a given axis x(3) and the defect mode produced by twisting about x(3) one part of the helical structure with respect to the other. Within the Bragg band of the periodic structure, the twist defect gives rise to a resonant mode that is a superposition of two standing waves: one localized with exp(-gamma|x(3)|) dependence centered at the defect and the other extended over the whole sample. The ratio between the amplitudes of the localized and nonlocalized waves depends sharply on both the twist angle and the elastic anisotropy, and can assume huge values. The defect mode and the resonance frequency omega(0) are defined by fully analytical and very simple expressions. Finally, we discuss how around omega(0), a finite sample acts as a frequency filter for circularly polarized shear waves, whose bandwidth can be changed by many orders of magnitude by varying the sample thickness, the twist angle, or the elastic anisotropy.

Transmission through chiral twist defects in anisotropic periodic structures.

A long-lived photonic state is observed in measurements of microwave transmission through a helical stack of anisotropic overhead transparencies with various twist defects in the center of the structure. Once account is taken of absorption and of the angular spread of the source, computer simulations of transmission through a polarized localized state are in agreement with measurements. Unlike for isotropic one-dimensional bandgaps, the intensity of the localized mode is not modulated in space on a wavelength scale.

Mechanisms for nucleosome mobilization

Nucleosomes are the ubiquitous and fundamental packaging for eukaryotic genomes, and are the substrate for many processes in the nucleus. Nucleosomes are not static entities but can readily be moved by thermal energy and ATP-dependent chromatin remodeling complexes in a process known as sliding or shifting. We summarize from a mechanical perspective the twist defect and bulge diffusion mechanisms proposed as the most likely pathway for nucleosome mobilization. We then consider the elastic properties of DNA and how this affects the potential for each mechanism, concentrating on kinetic aspects of twist diffusion and possible planar bulge sizes and summarize the experimental evidence reflecting on each. Either, or both, mechanisms could occur, and careful experimentation focusing on their uniquely distinguishing features will be required to determine their relative contributions to chromatin dynamics.

Specular and nonspecular, thickness-dependent, spectral holes in a slanted chiral sculptured thin film with a central twist defect

Abstract We demonstrate analytically that two types of spectral holes located approximately in the center of the Bragg regime could be exhibited by a slanted chiral sculptured thin film (STF) containing a 90° twist defect midway through its thickness. One is a nonspecular spectral reflection hole excited by an incident circularly polarized plane wave of the same handedness as the chiral slanted STF, and the other a specular transmission hole excited by an incident circularly polarized plane wave of the opposite handedness. The occurrences of these holes depend on the device thickness, and a crossover thickness can be defined. The existence of both types of spectral holes is sensitive to the dual-periodicity of the slanted chiral STF, and can be completely subverted by the Rayleigh–Wood phenomenon.

Twist defect in chiral photonic structures.

We demonstrate that twisting one part of a chiral photonic structure about its helical axis produces a single circularly polarized localized mode that gives rise to an anomalous crossover in propagation. Up to a crossover thickness, this defect results in a peak in transmission and exponential scaling of the linewidth for a circularly polarized wave with the same handedness as structure. Above the crossover, however, the linewidth saturates and the defect mode can be excited only by the oppositely polarized wave, resulting in a peak in reflection instead of transmission.