Optical selection rules and structures of cholesteric blue phases

By deriving optical selection rules for cholesteric blue phases and considering the nature of the phase transitions between them, a structural assignment for the three experimentally observed phases of Johnson et al. is obtained. All three are argued to be bcc. A method of testing this structure assignment directly by polarized light scattering studies is also presented.     

Optical activity measurements in the smectic blue phases

Optical activity measurements have been performed on the smectic blue phases ( BP _Sm), which are a new kind of chiral liquid crystal. BP _Sm exhibit both three-dimensional orientational order, like the classical blue phases, and smectic positional order. Thus, they can be viewed as the three-dimensional counterpart of the twist grain boundary phases. A comparison with the optical activity of the classical blue phases is carried out, and an estimation of the BP _Sm lattice parameter is reported for the first time. Received 25 May 2001     

Structural investigations on smectic blue phases

Smectic blue phases ( BP(Sm)) are thermotropic liquid crystalline phases which exhibit both three-dimensional orientational order, such as classical blue phases, and smectic positional order. BP(Sm) appear as the three-dimensional counterpart of twist grain boundary phases. X-ray scattering and optical polarizing microscopy provide information on the hexagonal and cubic symmetries of these new phases.     

计算机红青滤色立体图对的制作

彩色立体显示是光学成像技术中的一个十分重要的研究方向 ,文中简述了红青滤色立体图的原理 ,提出并介绍了作者用计算机PHOTOSHOP软件制作红青滤色立体图的几种方法。并对各种立体图的效果进行了比较。     

On the structure of the microscopic dielectric tensor in incommensurate crystals

A microscopic model for calculating the optical response of incommensurately modulated phases in insulating crystals is presented. The dominant contribution to the dielectric permittivity tensor is shown to originate from the lowest-index reciprocal lattice vectors, thus proving the validity of the mesoscopic approach developed in several earlier studies. The expression for the mesoscopic Fourier component of the dielectric tensor is obtained. These results may be useful in relation to the controversial problem of the optical activity observed in the incommensurate phases of some A₂BX₄ family crystals.     

Blue laser phase change recording system

The migration paths from DVD phase change recording with red laser to the next generation optical disk formats with blue laser and high NA optics are discussed with respect to optical aberration margins and disc capacities. A test system for the evaluation of phase change disks with more than 20 GB capacity is presented and first results of the recording performance are shown.     

Elastic light scattering and birefringence measurements in the coexistence state of α and β quartz

New optical measurements of light scattering in the heterophase structure produced by the coexistence of α and β phases in quartz around 573°C, with simultaneous observations by optical microscopy, are presented. The heterophase structure variations are described in a heating and cooling temperature cycle with the two phases always remaining in the crystal.Nucleations of one phase in the other are observed, and the associated effects on right angle and small angle scatterings are presented. Birefringence in the plane perpendicular to the optical axis was measured as a function of temperature and position in the crystal, showing clearly the existence of induced stresses in the heterophase state.     

Improvement of the blue phase stability in chiral nematic liquid crystal mixtures

ABSTRACT

The method of widening of the blue phase (BP) temperature range is presented. By means of polarising optical microscopy, we demonstrate that the temperature regime in which BP is stable is greatly enhanced by the cell thickness changes in mixtures of nematic liquid crystal (LC) and chiral dopant.     

Effect of nanometer sized Ni dot/Ag/Pt reflection metal of InGaN light emitting diodes on conversion efficiency of yellow phosphor

Nanometer sized Ni-dot/Ag/Pt metal coating films with different Ni-dot sizes were incorporated in InGaN based blue light emitting diodes as p-type reflection metal to enhance the extraction efficiency of blue light. For the reflectivity change from 84% to 93.7%, the optical output power of blue light before phosphor coating and the luminous flux of white light after phosphor deposition have improved by 49.3% and 58.2% at 350 mA, respectively. For the presented white light devices, the blue and yellow luminescence respectively has been improved relatively up to 1.52 and 1.67 times. The improved extraction efficiency of yellow light from the phosphor integrated blue light emitting diode is thought to be attributed to the reduction of round trip loss of blue and yellow light inside the device due to the enhanced reflectivity of nanometer sized Ni/Ag/Pt p-type reflection metal.     

Structural and optical investigations of TGBA and TGBC mesophases exhibiting cylindrical and cone-like domain textures

The characterization by optical microscopy, X-ray diffraction and differential scanning calorimetry of two new liquid crystalline compounds is presented. The compounds under consideration incorporate in their molecules two chiral centres (one asymmetric carbon and one asymmetric sulphur of a sulphinate group). While the carbon chirality is fixed in the R configuration the sulphur chirality may be either in the S or in the R configuration. A mesophase in a large temperature domain is evidenced for one of the two diastereomers, while both and mesophases are observed in a similar temperature domain for the other one. For both diastereomers and in both mesophases it is possible to observe the coexistence of two different types of optical textures, namely planar cholesteric textures and developable domains. The latter are coiled in a different way than proposed in an earlier publication. More precisely, they form cylindrical or cone-like domains with double-twist properties as observed in the blue phases. The pitch of the helix in the TGB mesophases is evaluated by means of optical reflection observations in samples exhibiting the planar cholesteric textures. For both diastereomers it is also possible to evaluate structural parameters related to the organization within the mesophases. Finally, the induced phase transition - is studied by the application of an AC electric field. Received: 16 June 1997 / Revised: 17 October 1997 / Accepted: 20 November 1997     

Exploration of Induced Blue Phases and Orthogonal Smectic Batonnets in Hydrogen-Bonded Ferroelectric Nanoliquid Crystalline Complexes for Optoelectronic Devices

A novel blue phase hydrogen-bonded ferroelectric liquid crystal (HBFLC) series has been synthesized from cholesteryl stearate (CHS) and p-n-alkyloxybenzoic acid (nOBA, where n?=?2 to12). Blue phase (BP) liquid crystalline complex is a high potent material for next-generation optoelectronic devices. The structural, optical, and thermal properties of present HBFLC complexes have been characterized by X-ray diffraction techniques (XRD), Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), polarizing optical microscope (POM), and differential scanning calorimetry (DSC) techniques. The band gap energy of the present HBFLC complex (4.6 eV) is estimated by UV-Visible spectrometer. X-ray diffraction and scanning electron microscope (SEM) studies which confirm the monoclinic nature and morphology of the present complex. Phase diagram of the present CHS + nOBA is constructed from DSC data and the same is discussed. The lower homologous of CHS + nOBA HBFLC (n?=?2 to 6) complex shows BP_sm1, BP_sm2, BP_sm3 while higher homologous (n?=?7 to 12) exhibiting orthogonal smectic A* (SmA*) character with blue phases. A noteworthy observation is that the identification of extended thermal span of smectic blue phases and induced SmA* phase in the present HBFLCs. Another important observation is that the widest BP range is reported in the present hydrogen-bonded ferroelectric nanoliquid crystalline (HBFNLC) complex which is more suitable for photonic devices. In addition, LC parameters such as, phase width, thermal stability factor, phase transition temperature with enthalpy value, and the origination of orthogonal phases in CHS + nOBA HBFLC complex is also reported.     

Phase polymorphism and electro-optical properties of a ferroelectric liquid crystal containing the biphenyl system

ABSTRACT

In this article we present results concerning phase transitions and physical properties of the ferroelectric phase of the compound (S)-4-(1-methylheptyloxy)biphenyl–4'-(heptyloxy phenyl)-4-carboxylate (MHOBOPO7). The compound has the following phases: smectic ferroelectric C (SmC*), chiral nematic N*, and two defected phases, TGB_C and blue phase. The mesomorphic properties were investigated by means of three complementary methods: differential scanning calorimetry, polarizing light optical microscopy, and transmitted light intensity. The electro-optical measurements were carried out on an ordered sample in a middle electric field during very slow cooling from the nematic phase to the ferroelectric phase.     

A possible similarity between lyotropic cubic phases and cholesteric blue phases

Summary The common features in structures and properties of lyotropic cubic phases (LCP) and cholesteric blue phases (BP) are discussed. In both systems the cubic phases have nontrivial structures governed by frustrations. The frustration is connected with the double-twist in BPs and with the layer curvatures in LCPs. The structure of BP can be quantitatively described using the minimization of the Landau free energy with tensor order parameter. The order parameter of LCP is unknown till now; however, some minimization procedure can also be used for the solution of the phase problem. It is noted that in both systems the stable amorphous phases are observed (the fog phase or BPIII in BPs andL ₃ phase in lyotropic crystals). The importance of the observation of ?forbidden? reflections for the unequivocal identification of the space groups is emphasized. Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988.     

Photoluminescence from C ion-implanted and electrochemical etched Si layers

The microstructural and optical analysis of Si layers emitting blue luminescence at about 431 nm is reported. These structures have been synthesized by C⁺ ion implantation and high-temperature annealing in hydrogen atmosphere and electrochemical etching sequentially. With the increasing etching time, the intensity of the blue peak increases at first, decreases then and is substituted by a new red peak at 716 nm at last, which shows characteristics of the emission of porous silicon. CO compounds are induced during C⁺ implantation and nanometer silicon with embedded structure is formed during annealing, which contributes to the blue emission. The possible mechanism of photoluminescence is presented.     

基于半导体光放大器瞬态交叉相位调制效应的高速反相和同相波长转换的研究

采用半导体光放大器(SOA)中的瞬态交叉相位调制效应是实现高速全光信号处理的有效途径.利用SOA和带宽为0.4 nm的窄带滤波器同时实现了重复频率为10 GHz、脉冲宽度为10 ps的同相和反相全光波长转换.当滤波器的中心波长相对于探测光载波波长蓝移0.25 nm或者红移0.05 nm时，得到反相波长转换；当滤波器的中心波长相对于探测光波长蓝移0.29 nm或者红移0.25 nm时，得到同相的波长转换.同时用数值模拟了从同相到反相波长转换的极性演化过程，理论分析和实验结果基本相符. 关键词： 半导体光放大器 波长转换 瞬态交叉相位调制     

Compact laser system for atom interferometry

We describe an optical bench in which we lock the relative frequencies or phases of a set of three lasers in order to use them in a cold atom interferometry experiment. As a new feature, the same two lasers serve alternately to cool atoms and to realize the atomic interferometer. This requires a fast change of the optical frequencies over a few GHz. The number of required independent laser sources is then only three, which enables the construction of the whole laser system on a single transportable optical bench. Recent results obtained with this optical setup are also presented. PACS 32.80.Pj; 42.50.Vk; 39.20.+q     

Tuning of the electronic and optical properties of single-layer indium nitride by strain and stress

Using first principles calculations, electronic and optical properties of indium nitride graphene-like structure have been studied under various stress and strain values. The results exhibit that this compound in the range of ±6 applied biaxial strain remains a direct band gap semiconductor. Also, exerting stress and strain reduces the energy band gap of the considered materials. The optical calculations illustrate that applying stress and strain on system results in blue and red shift in optical spectra. All obtained results presented that we can tune the optoelectronic properties of indium nitride by applying stress and strain.     

Two-dimensional,blue phase tactoids

ABSTRACT

We use full nematohydrodynamic simulations to study the statics and dynamics of monolayers of cholesteric liquid crystals. Using chirality and temperature as control parameters, we show that we can recover the two-dimensional blue phases recently observed in chiral nematics, where hexagonal lattices of half-skyrmion topological excitations are interleaved by lattices of trefoil topological defects. Furthermore, we characterise the transient dynamics during the quench from isotropic to blue phase. We then proceed by confining cholesteric stripes and blue phases within finite-sized tactoids and show that it is possible to access a wealth of reconfigurable droplet shapes including disk-like, elongated and star-shaped morphologies. Our results demonstrate a potential for constructing controllable, stable structures of liquid crystals by constraining 2D blue phases and varying the chirality, surface tension and elastic constants.     

Electrical and structural characterization of inhomogeneously doped semiconductors by infrared spectroscopy

An optical method is presented for thenon-destructive electrical and structural characterisation of multilayer structures. The presented concept yields the complex refractive index profiles within an arbitrary composed layer system by using the symmetrical transmission and the asymmetrical reflectivity caused by the optical inhomogeneity of a material. This method includes the superimposition of coherent multiple reflections within the refractive index profile as well as incoherent multiple reflections within the whole wafer. This analysis yields the spacial distribution of free carrier density profiles within the sample as well as the transitions between different crystal phases by using the least squares fit procedures. The results of the optical analysis are compared with other methods used to quantitatively determine density profiles and phase transitions such as SIMS, spreading resistance, stripping Hall and RBS.     

Investigation of the optical limiting properties of acid blue29 in various solvents

The optical limiting properties of acid blue29 solutions in different solvents are investigated. Experiments are performed using a CW He-Ne Laser beam at 632.8 nm wavelength and 35 mW power. The strength of the optical limiting action is dramatically influenced by the solvent. Multiple diffraction rings were observed when the samples were exposed to laser radiation. The effect of concentration, solvent and laser intensity on the diffraction rings are studied experimentally. Our results show that the nonlinear refraction is the dominant mechanism to explain the optical limiting behavior in acid blue29.