Second-harmonic generation circular dichroism spectroscopy from tripod-like chiral molecular films

The second-harmonic generation (SHG) circular dichroism in the light of reflection from chiral films of tripod-like chiral molecules is investigated.The expressions of the second-harmonic generation circular dichroism are derived from our presented three-coupled-oscillator model for the tripod-like chiral molecules.Spectral dependence of the circular dichroism of SHG from film surface composed of tripod-like chiral molecules is simulated numerically and analysed.Influence of chiral parameters on the second-harmonic generation circular dichroism spectrum in chiral films is studied.The result shows that the second-harmonic generation circular dichroism is a sensitive method of detecting chirality compared with the ordinary circular dichroism in linear optics.All of our work indicates that the classical molecular models are very effective to explain the second-harmonic generation circular dichroism of chiral molecular system.The classical molecular model theory can give us a clear physical picture and brings us very instructive information about the link between the molecular configuration and the nonlinear processes.     

Optical second-harmonic generation in ferroelectric chiral smectic polysiloxane

The second-order nonlinear optical properties of ferroelectric liquid crystalline polymer have been studied. Angular phase-matched Second-Harmonic Generation (SHG) is observed in the chiral smectic C (SmC*) phase of siloxane copolymer with chiral mesogen in the side chain. The effective nonlinear optical coefficient d_eff under the phase-matching condition is determined. The intense SHG is observed even in the crystalline phase by cooling down from the SmC* phase under the electric field. The effective coefficient of SHG in the crystalline phase is more than ten times larger than that of the phase-matched SHG in the SmC* phase. The enhancement of SHG in the crystalline phase is observed only in a homeotropically aligned cell and maintained even in the non-biased state for at least several days. The angular dependence of the SHG in the crystalline phase is confirmed.     

手性薄膜表面二次谐波圆二向色性的研究

给出了螺旋型手性分子薄膜表面反射方向的圆二向色性的谐波强度表示式,分析了谐波的强度与手性分子的取向,分子的螺距和半径等结构参数的关系,给出了数值模拟结果,发现在入射光强一定的情况下,谐波强度在螺距和半径的取值范围内分别存在着最大值,以及螺距与半径之间满足的关系式随分子取向角的变化而不同。     

Dielectric Chiral Metasurfaces for Second-Harmonic Generation with Strong Circular Dichroism

Dielectric chiral metasurfaces can generate harmonic waves with high efficiency and strong circular dichroism (CD), when they are supported by metallic substrates. Numerical results show that the second-harmonic generation (SHG) efficiency of about 10⁻³% for a peak pump intensity of about 5 GW cm⁻² can be achieved in the blue-UV, and the SHG CD reaches up to about 1.8, from a metasurface of Z-shaped lithium niobate nanoantenna array supported by gold substrate. Highly efficient and strong circular dichroic nonlinear responses are attributed to the plasmon-assisted local field enhancement in the dielectric chiral nanoantennae adjacent to the metallic substrate.     

Two-dimensional model for mixtures of enantiomers,bent hard needles: a Monte Carlo simulation

We study the statistical mechanical conditions under which segregation of racemic mixtures of chiral molecules is possible in a two-dimensional fluid model. Motivated by experimental evidence indicating that chiral hydrophilic heads of amphiphilic molecules lying in a monolayer can crystallize undergoing a chiral phase separation, we propose a two-dimensional system to model the projection of the chiral head of amphiphilic molecules in a monolayer. The molecules of our model are infinitely hard and infinitely thin. We consider interactions with only a repulsive contribution where molecules have no effective area (two-dimensional volume). As a consequence all effects found are due to excluded area. The Monte Carlo Gibbs ensemble is used to study phase separation whereas constant pressure simulations are performed to obtain equations of state of pure and racemic systems. We find that for this simple model segregation is generally possible in the very high density regime.     

利用格点量子色动力学研究手征平滑过渡温度和手征相变温度

回顾了最近关于手征平滑过渡温度和手征相变温度的研究结果。首先给出了在零重子化学势能下的手征平滑过渡温度为156.5(1.5) MeV,其次,给出了在非零重子化学势能下手征相转变曲线的二阶及四阶曲率分别为0.012(4)和0.000(4)。接着讨论了在格点QCD中第一次得到的量子色动力学的手征相变温度。在热力学极限、连续极限及手征极限下,我们得到手征相变温度为132$^{+3}_{-6}$ MeV。     

Nonlinear optics of chiral surface systems

Differential SHG signals from chiral surfaces are recorded using right and left circularly polarized incident light. We show that chiral information from the surface is conveyed through this SHG process. This study focuses on the model protein system, cytochrome c, adsorbed on various self-assembled mono- and bi-layers at the solid/liquid interface. SHG-CD signals are shown to be related to the oxidation state of the protein and are utilized to observe changes in this feature of the protein in situ at surfaces of varying chemical properties. Received: 20 September 1998 / Revised version: 9 December 1998     

BSRF圆二色谱研究进展

圆二色（ＣＤ）谱是一种特殊的吸收谱，它对手性分子的构象十分敏感，因此它是最重要的光谱实验之一。北京同步辐射装置（ＢＳＲＦ）３Ｂ１Ｂ生物光谱实验站已经投入运行，它提供单色聚焦的同步辐射光，波长覆盖范围１７０ｎｍ～５００ｎｍ该站的主要设备是一台圆二色谱仪，用这台仪器成功地获取了活性生物样品～捕光色素（ＬｉｇｔＨａｒｖｅｓｔＣｈｌｏｒｏｐｈｙｌｌ）的ＣＤ谱，同时测得樟脑磺酸（ＣａｍｐｈｏｒＳｕｌｆｏｎｉ     

基于香蕉形液晶分子自组装的纳米螺旋丝有机凝胶及其流变特性

在香蕉形液晶分子B4相态中,非手性香蕉形液晶分子自组装形成层状结构,分子在层内倾斜,形成层手性和自发极化,并且造成层内不匹配,最终形成纳米螺旋丝.本文设计了NOBOW/十六烷混合体系,在高温时,香蕉形液晶分子溶解于十六烷,在低温时,香蕉形液晶分子自组装形成纳米螺旋丝,并最终形成三维网络,变成有机凝胶.为深入理解纳米螺旋丝有机凝胶的特性,拓展其在软物质领域的应用,本文通过流变实验对该有机凝胶的黏弹性质进行了系统研究.实验表明纳米螺旋丝有机凝胶与传统凝胶不同,纳米螺旋丝有机凝胶可以随温度变化形成凝胶-流体的可逆变化,并且通过测量NOBOW/十六烷混合体系在不同液晶分子浓度、温度、应变大小和应变速率下的流变特征,揭示了该有机凝胶的流变特性与纳米螺旋丝的性质密切相关.     

Interaction-Induced Chiral Quantum States of the Ultracold Polar Molecules

The ultracold polar molecules with the tunable dipole-dipole interaction, not only would enable explorations of a large class of exotic many-body physics phenomena, but also could be used for quantum information processing. In the present paper we demonstrate that this dipole-dipole interaction can generate the degenerate chiral quantum states acting as a qubit robust against noise when the ultracold polar molecules are confined by a triangular lattice. Moreover, we also find two first-order quantum phase transitions by controlling an external driving field. One is the transition with the change of the different degenerate chiral quantum states. The other is the transition with the breaking of the degenerate quantum chiral states to the nondegenerate state. In experiment, these first-order quantum phase transitions can be detected by measuring the collective molecular population.     

有机/无机薄膜凝胶动态过程的实时光学二次谐波产生研究

采用实时光学二次谐波产生(SHG)测量方法,以两亲性分子半花菁为掺杂探针分子,对溶胶-凝胶技术制备的玻璃薄膜的凝胶干燥的动态过程进行了研究.结合紫外—可见吸收光谱的测量,发现：因两亲性分子在膜中的自取向性可诱导出二阶光学非线性,溶胶-凝胶技术制备的有机/无机复合膜的凝胶干燥过程可通过薄膜SHG强度的变化半定量地反映.凝胶干燥收缩导致有序掺杂分子数减少,膜片的光学非线性减弱.该过程随薄膜处理温度的升高而急剧缩短;另外,有机分子在膜中的聚集程度随膜片处理温度的增加而很快减少. 关键词：     

Interaction-Induced Chiral Quantum States of the Ultracold Polar Molecules

The ultracold polar molecules with the tunable dipole-dipole interaction, not only would enable explorations of a large class of exotic many-body physics phenomena, but also could be used for quantum information processing. In the present paper we demonstrate that this dipole-dipole interaction can generate the degenerate chiral quantum states acting as a qubit robust against noise when the ultracold polar molecules are confined by a triangular lattice. Moreover, we also find two first-order quantum phase transitions by controlling an external driving field. One is the transition with the change of the different degenerate chiral quantum states. The other is the transition with the breaking of the degenerate quantum chiral states to the nondegenerate state. In experiment, these first-order quantum phase transitions can be detected by measuring the collective molecular population.     

Optically switchable, polarization-independent holographic polymer dispersed liquid crystal (H-PDLC) gratings

An optically switchable, polarization-independent holographic polymer dispersed liquid crystal (H-PDLC) transmission grating is demonstrated by adding azobenzene-LC and chiral molecules into the H-PDLC formulation. The optical switchable mechanism is from the trans-cis photoisomerization of the doped azobenzene-LC, which modulates the refractive index of the LC rich area. The dependence of the diffraction efficiency of the H-DPLC grating without chiral molecules on light polarization suggests that the orientation of LC directors within the droplet is ellipsoidal and uniaxial. However, the addition of chiral molecules into the H-PDLC formulation helps the formation of isotropic and non-uniaxial LC directors within the droplets. The polarization properties of the grating are investigated and analyzed by the coupled and modified coupled wave theory with a model of sinusoidal dielectric modulation. The results show that the addition of chiral molecules changes the LC phase from nematic to chiral-nematic, where the grating efficiency, which is modulated by the photoinduced phase transition, is independent of the polarization of incident light. Our findings may help improve optical systems that utilize non-polarized light.     

Switching behaviour of ferroelectric liquid crystals probed by optical second-harmonic generation

Ferroelectric properties of thin (1.5–4 μm planar cells of a ferroelectric liquid crystal (FLC) mixture are studied using electro-optic measurements, second-harmonic generation (SHG) and SHG interferometry. A switching behaviour of the FLC cells in external dc electric fields is observed. It is characterised by rotation of the polarisation plane of the transmitted light and by changes in the SHG intensity, phase and anisotropy dependences, which are attributed to a collective motion of the system as a ferroelectric uniform state with C₂ symmetry. Received: 16 October 2001 / Revised version: 18 March 2002 / Published online: 6 June 2002     

Chiral plasmonics and enhanced chiral light-matter interactions

Chirality, which describes the broken mirror symmetry in geometric structures, exists macroscopically in our daily life as well as microscopically down to molecular levels. Correspondingly, chiral molecules interact differently with circularly polarized light exhibiting opposite handedness(left-handed and right-handed). However, the interaction between chiral molecules and chiral light is very weak. In contrast, artificial chiral plasmonic structures can generate "super-chiral" plasmonic near-field, leading to enhanced chiral light-matter(or chiroptical) interactions. The "super-chiral" near-field presents different amplitude and phase under opposite handedness incidence, which can be utilized to engineer linear and nonlinear chiroptical interactions. Specifically,in the interaction between quantum emitters and chiral plasmonic structures, the chiral hot spots can favour the emission with a specific handedness. This article reviews the state-of-the-art research on the design, fabrication and chiroptical response of different chiral plasmonic nanostructures or metasurfaces. This review also discusses enhanced chiral light-matter interactions that are essential for applications like chirality sensing, chiral selective light emitting and harvesting. In the final part, the review ends with a perspective on future directions of chiral plasmonics.     

The synthesis of the chiral non-mesogenic d-seco estrone derivatives and their influence on the phase transitions of cholesteric liquid crystals

The synthesis of new chiral seco-estrone derivatives is presented, as well as their influence on the phase transition of binary mixtures of cholesteryc liquid crystals. The new chiral derivatives do not posses any liquid crystalline phases and were synthesized in several synthetic steps, starting from estrone. We have studied the mixtures of cholesteryl non-anoate (40%) with cholesteryl myristate (40%) and addition of new chiral derivatives 3 4, or 5 (20%). It was concluded that the addition of chiral derivative 3 to the binary mixture stabilizes smectic A and cholesteric phase and shifts the phase transition temperature with respect to pure binary mixture for about 5°C towards lower temperatures. The extension of the temperature range of the cholesteric phase from 5°C to 15°C was established in the case when the derivatives 3 and 4 are added to the binary mixture of cholesteryl nonanoate with cholesteryl myristate. The phase diagrams of investigated compounds are formed on the basis of data obtained by the optical microscopy. Using X-ray diffraction on the crystalline powder of unoriented samples we have determined the molecular parameters: the thickness of smectic and cholesteric layers and average distance between the long axes of neighboring molecules.     

Meeting Reports: BioCD-2005 Workshop

An intensive weeklong course on Circular Dichroism (CD) was presented at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL), from June 20-24, 2005. CD is the difference in absorption of left and right circularly polarized light. It is observed in the spectral regions of absorption of molecules that are chiral, i.e., lacking a center of symmetry. While most CD spectra are small—only a tiny fraction of the total absorption biopolymers with helical structures—such as proteins, DNA, RNA and polysaccharides, exhibit informative CD spectra. The objective of BioCD-2005 was to acquaint the participants with the theory of ultraviolet CD spectroscopy.     

用非共线二次谐波法实现对锁模激光器超短脉冲宽度的测定

用非共线自相关SHG方法测量了Nd:YAG被动锁模激光器的脉冲宽度。非线性晶体ADP的取向是使仅在两束光都存在时才有位相匹配的SHG。两束寻常光之间的夹角在晶体中是20°50′。SHG出现在它们的角分线方向上;倍频输出是非寻常光。所测得的自相关函数的宽度对应着Nd:YAG被动锁模激光器的脉冲宽度为24ps。 关键词：     

Distinctive thermal behavior and nanoscale phase separation in the heterogeneous liquid-crystal B4 matrix of bent-core molecules

By means of high-resolution calorimetry, we studied thermodynamic properties of the liquid-crystal B(4) phase where bent-core molecules form a helical nanofilament structure. Distinctive thermal behavior characterizing the growth process of the B(4) phase was obtained in undergoing the phase transition with many sharp peaks, indicating a highly heterogeneous structure. It has been demonstrated that such unusual behavior is commonly seen for two types of rodlike molecules as well as for various mixture compositions. We speculate that mixture systems involve a nanoscale phase-separated structure due to the remarkable aggregation effect in the bent-core molecules and that the helical nanofilament structure independently grows in the isotropic state of rodlike molecules. We also propose that the asymmetry in viscoelastic property plays a role in yielding unusual behavior.     

Effect of the chiral chain length on structural and phase properties of ferroelectric liquid crystals

Studies of structural and phase properties obtained on several ferroelectric liquid crystalline materials with 2-alkoxypropionate group used as a chiral centre and without any lateral substitution are presented. In dependence on the chiral chain length these compounds exhibit the cholesteric N* phase, the ferroelectric smectic C* and a low-temperature SmX phase. Values of the spontaneous polarization and spontaneous tilt angle have been determined within the whole range of the SmC* phase. A low-temperature SmX phase has been identified as the orthogonal hexatic SmB* phase. The molecular parameters, namely the layer spacing in the SmC* and SmB* phases and the average intermolecular distances (D) between neighbouring parallel molecules in all investigated phases have been determined using the results of the X-ray diffraction obtained on non-oriented samples. The effect of the chiral chain length on mesomorphic, structural and physical properties of the studied ferroelectric liquid crystalline materials is discussed.