Fluorescence evidence for cholesterol regular distribution in phosphatidylcholine and in sphingomyelin lipid bilayers

Our previous studies indicated that sterols (including cholesterol and dehydroergosterol) can be regularly distributed into hexagonal superlattices in the plane of liquid-crystalline phosphatidylcholine bilayers. It was suggested that regular and irregular regions coexist in the membrane. In the present study, we report supporting evidence for our sterol regular distribution model. We have examined the fractional concentration dependencies of dehydroergosterol (a naturally occurring cholesterol analogue) fluorescence intensity and lifetime in various phosphatidylcholine and sphingomyelin bilayers. Fluorescence intensity and lifetime dips have been observed at specific sterol mole fractions. At those mole fractions, the acrylamide quenching rate constant of dehydroergosterol fluorescence reaches a local maximum. Those mole fractions match the critical sterol mole fractions at which sterol molecules are expected to be regularly distributed into hexagonal superlattices. The results support the idea that the sterols in the regular region are embedded in the bilayer less deep than those in the irregular regions. We have also examined the fractional cholesterol concentration dependencies of diphenylhexatriene (DPH) fluorescence intensity, lifetime, and polarization in DMPC vesicles. DPH fluorescence intensity and polarization also exhibit distinct dips and peaks, respectively, at critical sterol mole fractions for hexagonal superlattices. However, DPH lifetime changes little with sterol mole fraction. As a comparison, the fluorescence properties of DHE and DPH behave differently in response to the formation of sterol regular distribution. Furthermore, finding evidence for sterol regular distribution in both phosphatidylcholine and sphingomyelin membranes raises the possibility that sterol regular distribution may occur within phospholipid/cholesterol enriched domains of real biological membranes.     

Elastic properties of polymer-doped dilute lamellar phases: A small-angle neutron scattering study

We investigate experimentally, using small-angle neutron scattering the elastic properties of polymer-doped dilute lamellar phases. In our system the polymer is water-soluble but nevertheless partially adsorbs onto the negatively charged surfactant bilayers. The effective polymer-mediated interaction between bilayers is less repulsive than the weakly screened electrostatic interaction that prevails at zero polymer content. It even becomes attractive in some regions of the phase diagram. Small-angle neutron scattering allows us to measure directly the Caillé exponent η characterizing the bilayer fluctuations in lamellar (smectic A) phases, and thus indirectly estimate the compression modulus as a measure of the strength of the bilayer-bilayer interactions. The compression modulus appears to be vanishing at a point located on the lamellar-lamellar phase separation boundary, a candidate critical point. Received: 7 August 2000     

The net charge at interfaces between insulators

The issue of the net charge at insulating oxide interfaces is briefly reviewed with the ambition of dispelling myths of such charges being affected by covalency and related charge density effects. For electrostatic analysis purposes, the net charge at such interfaces is defined by the counting of discrete electrons and core ion charges, and by the definition of the reference polarization of the separate, unperturbed bulk materials. The arguments are illustrated for the case of a thin film of LaAlO(3) over SrTiO(3) in the absence of free carriers, for which the net charge is exactly 0.5e per interface formula unit, if the polarization response in both materials is referred to zero bulk values. Further consequences of the argument are extracted for structural and chemical alterations of such interfaces, in which internal rearrangements are distinguished from extrinsic alterations (changes of stoichiometry, redox processes), only the latter affecting the interfacial net charge. The arguments are reviewed alongside the proposal of Stengel and Vanderbilt (2009 Phys. Rev. B 80 241103) of using formal polarization values instead of net interfacial charges, based on the interface theorem of Vanderbilt and King-Smith (1993 Phys. Rev. B 48 4442-55). Implications for non-centrosymmetric materials are discussed, as well as for interfaces for which the charge mismatch is an integer number of polarization quanta.     

Spin polarization in carbon nanostructures with disclinations

We performed ab initio calculations, using density functional theory, to study spin polarization in carbon nanostructures with disclinations. The results indicate that compounds with positive and negative Gaussian curvature may exhibit a net magnetic moment in the ground state. Additionally, we can conclude that, carbon compounds that display an odd number of pentagons and heptagons, present polarization in the ground state.     

The tilted-multifoil hyperfine interaction and nuclear spectroscopy

Ions emerging from a stretched foil with their velocity vectorv at an oblique angle to the normaln (tilted foil geometry) are known to be polarized along the axisv×n. The electronic polarization of atomic configurations can influence the nucleus during flight in vacuum via the hyperfine interaction. For a large number of polarizing foils and for very short interaction times, the resulting effects resemble a pure precession in an external magnetic field and have been used to measureg-factors of short-lived nuclear high-spin levels. For long interaction times, a net nuclear polarization is induced and has so far been utilized to determine signs of nuclear quadrupole moments of high-spin isomers and to investigate parity nonconservation effects in⁹³Tc.     

快速可调谐电光聚合物波导光栅

提出并设计了快速可调谐电光聚合物波导光栅.该波导光栅通过极化聚合物的线性电光效应 可实现谐振波长的纳秒级快速调谐，调谐灵敏度为61pm/V.研究了该波导光栅的反射谱和透 射谱特性与光栅周期,周期数,折射率调制函数及其调制大小的关系.讨论了波导光栅的材料 选择,制备工艺,快速可调谐性和偏振相关性.该波导光栅不仅克服了光纤光栅调谐速度慢和不利于大规模集成的不足，而且具有调谐灵敏度高,制备工艺与半导体工艺兼容和偏振无关 等优点. 关键词： 光通信器件 光波导 电光效应 聚合物 波导光栅 光纤光栅     

Piezoelectric properties of polymer-piezoelectric ceramic composites crystallized under the action of an electric-discharge plasma

The crystallization of a polymer-piezoelectric ceramic composite under the action of electric-discharge temperature and plasma is shown to cause strong oxidation of polymer chains, which increases the concentration of charge localization centers or the number of local levels in the quasi-bandgap of the polymer phase in the composite. In turn, this oxidation is accompanied by an increase in the interphase charges during electrothermal polarization, which favor effective polarization of piezophase domains and, hence, an increase in the piezoelectric properties. The degree of oxidation of the polymer phase is found to be controlled by the nature of the electric discharges used for composite crystallization. It is experimentally shown that piezoelectric composites should be crystallized under the action of a microdischarge that does not reach the streamer phase of its development. The optimum crystallization conditions are determined.     

Carrier-envelope phase measurement by all-optical poling with a polar side-chain polymer

Dependence of all-optical poling efficiency on carrier-envelope phase (CEP) could be measured using photoisomerization of dye molecules which are covalently bound to a polymer main chain and have large difference in static dipole moment between the ground state and excited state. Increased chromophore density leads to an order of magnitude reduction in signal-detection time from a dye doped polymer. Analysis of all-optical poling experiments with CEP changes clearly showed the presence of polarization restoring force to zero polarization. This enables resetting of SH activity in the all-optical poling process to be used for fast response loop of CEP stabilization. Phenomenological model could explain well the difference in the growth-and-decay dynamics of poling between sample of dye doped in polymer studied previously and that grafted to a polymer main chain used in the present paper.     

Confined multilamellae prefer cylindrical morphology

By evaporating a drop of lipid dispersion we generate the myelin morphology often seen in dissolving surfactant powders. We explain these puzzling nonequilibrium structures using a geometric argument: the bilayer repeat spacing increases and thus the repulsion between bilayers decreases when a multilamellar disk is converted into a myelin without gain or loss of material and with number of bilayers unchanged. Sufficient reduction in bilayer repulsion can compensate for the cost in curvature energy, leading to a net stability of the myelin structure. A numerical estimate predicts the degree of dehydration required to favor myelin structures over flat lamellae.     

Interlayer coupling between out-of-plane magnetized multilayers across a thin antiferromagnetic spacer

The interlayer exchange coupling between Co/Pt perpendicular-to-plane magnetized layers across a thin IrMn spacer layer was experimentally studied. In contrast to earlier studies on interlayer coupling through antiferromagnetic NiO, which revealed an oscillatory coupling behavior as a function of NiO thickness, a ferromagnetic coupling was observed here in the range of IrMn thickness between 0.6 and 1.5 nm and antiferromagnetic between 1.5 and 2.5 nm. The antiferromagnetic coupling is attributed to an orange peel magnetostatic mechanism whereas the ferromagnetic coupling is attributed to an out-of-plane polarization of the antiferromagnetic IrMn layer induced by the interfacial exchange interaction with the adjacent out-of-plane ferromagnetic layers. Measurements of hysteresis loops versus temperature show that the coupling vanishes at 510 K for t_IrMn=1 nm. This critical temperature is far below the Néel temperature of bulk IrMn, but above the blocking temperature of IrMn/Co bilayers at such thickness. Using a one-dimensional model describing a partial domain wall in the antiferromagnet, we explain the coupling in terms of an out-of-plane tilt of the Mn moments at the IrMn/(Co/Pt) interfaces yielding a weak net polarization of the IrMn. Finally, the non-oscillatory decay of the coupling was attributed to the compensated spin structure of the IrMn in the parallel to the interfaces.     

室温单分子偶极取向与量子化再取向动力学实验研究

对室温下染料单分子进行了偶极取向和偶极再取向动力学的实验研究.利用共焦扫描显微镜光学系统与荧光偏振探测分析相结合的方法分别测量了聚合体薄膜中的单分子和无聚合体薄膜的单分子偶极方向变化特性,经采样统计测量镶嵌于聚合体薄膜中的单分子发生偶极再取向的概率约为5%—9%,无聚合体薄膜的单分子发生偶极再取向的概率约为26%.通过测量单分子荧光的偏振度轨迹曲线发现,偶极再取向存在着在多个偏振态之间的量子化跳跃行为. 关键词： 单分子 偶极取向 再取向 量子化跳跃     

Circular-polarization-dependent study of the microwave photoconductivity in a two-dimensional electron system

The polarization dependence of the low field microwave photoconductivity and absorption of a two-dimensional electron system has been investigated in a quasioptical setup in which linear and any circular polarization can be produced in situ. The microwave induced resistance oscillations and the zero resistance regions are notably immune to the sense of circular polarization. This observation is discrepant with a number of proposed theories. Deviations between different polarizations occur only near the cyclotron resonance where an unprecedented large resistance response is observed.     

Driving algorithms for an endless polarization controller using two wave-plates

We present in this paper an endless polarization controller using two wave-plates with rotating axes and variable birefringence. We demonstrate the characteristics of the polymer network liquid crystal wave-plate and a polarization control method with only two degrees of freedom. Finally, some experimental attempts toward endless polarization control have been performed.     

Defect mode emission of a dye doped cholesteric polymer network

We have observed enhanced fluorescence and laser emission due to a photonic defect mode in a dye doped cholesteric polymer network. The defect is caused by a phase jump of the cholesteric helix at the interface of two stacked layers of a cholesteric polymer film. Fluorescence spectra show an additional resonant mode inside the photonic stop band. Pulsed excitation gives rise to laser emission of the defect mode, with an exceptionally low lasing threshold. The defect mode emission has a circular polarization whose sense of rotation is opposite to that of the cholesteric helix.     

Transition from unilamellar to bilamellar vesicles induced by an amphiphilic biopolymer

We report some unusual structural transitions upon the addition of an amphiphilic biopolymer to unilamellar surfactant vesicles. The polymer is a hydrophobically modified chitosan and it embeds its hydrophobes in vesicle bilayers. We study vesicle-polymer mixtures using small-angle neutron scattering (SANS) and cryotransmission electron microscopy (cryo-TEM). When low amounts of the polymer are added to unilamellar vesicles of ca. 120 nm diameter, the vesicle size decreases by about 50%. Upon further addition of polymer, lamellar peaks are observed in the SANS spectra at high scattering vectors. We show that these spectra correspond to a co-existence of unilamellar and bilamellar vesicles. The transition to bilamellar vesicles as well as the changes in unilamellar vesicle size are further confirmed by cryo-TEM. A mechanism for the polymer-induced transitions in vesicle morphology is proposed.     

Vibrational and impedance spectroscopic analysis of poly(vinyl alcohol)-based solid polymer electrolytes

Solid polymer electrolytes based on poly(vinyl alcohol) (PVA) doped with NH₄Br have been prepared by the solution-casting method. The complex formation between the polymer and the salt has been confirmed by Fourier transform infrared spectroscopy. The highest conductivity at 303 K has been found to be of the order of 10⁻⁴ Scm⁻¹ for 25 mol% NH₄Br-doped PVA system. The ionic transference number of polymer electrolyte has been estimated by Wagner’s polarization method, and the results reveal that the conducting species are predominantly ions. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006.     

Polarization properties of elliptical core non-hexagonal symmetry polymer photonic crystal fibre

In this paper, polarization properties and propagation characteristics of polymer photonic crystal fibres with elliptical core and non-hexagonal symmetry structure are investigated by using the full vectorial plane wave method. The results show that the birefringence of the fibre is induced by asymmetries of both the cladding and the core. Moreover, by adjusting the non-symmetrical ratio factor of cladding $\eta$ from 0.4 to 1 in step 0.1, we find the optimized design parameters of the fibre with high birefringence and limited polarization mode dispersion, operating in a single mode regime at an appropriate wavelength range. The range of wavelength approaches the visible and near-infrared which is consistent with the communication windows of polymer optical fibres.     

Structuring of photosensitive material below diffraction limit using far field irradiation

In this paper, we report on in-situ atomic force microscopy (AFM) studies of topographical changes in azobenzene-containing photosensitive polymer films that are irradiated with light interference patterns. We have developed an experimental setup consisting of an AFM combined with two-beam interferometry that permits us to switch between different polarization states of the two interfering beams while scanning the illuminated area of the polymer film, acquiring corresponding changes in topography in-situ. This way, we are able to analyze how the change in topography is related to the variation of the electrical field vector within the interference pattern. It is for the first time that with a rather simple experimental approach a rigorous assignment can be achieved. By performing in-situ measurements we found that for a certain polarization combination of two interfering beams [namely for the SP (?, ?) polarization pattern] the topography forms surface relief grating with only half the period of the interference patterns. Exploiting this phenomenon we are able to fabricate surface relief structures with characteristic features measuring only 140 nm, by using far field optics with a wavelength of 491 nm. We believe that this relatively simple method could be extremely valuable to, for instance, produce structural features below the diffraction limit at high-throughput, and this could significantly contribute to the search of new fabrication strategies in electronics and photonics industry.     

Polarization bremsstrahlung at collisions of fast ions with multiatomic targets

The processes of polarization bremsstrahlung at collisions of fast ions with linear chains consisting of isolated atoms have been considered. The intensities and angular distributions of radiation spectra have been obtained for an arbitrary number of atoms in a chain. It has been shown that the interference of the photon emission amplitudes leads to a noticeable change in the spectral angular distributions of polarization bremsstrahlung as compared to the distributions at collisions with an isolated atom. The results allow standard generalization to the cases of polarization bremsstrahlung at channeling of fast ions over surfaces and in solid lattices.