Determination of the Director Alignment in a Ferroelectric Liquid Crystal Device by Observation of Optical Modes

Abstract

By observing the angular dependence of the reflectivity of a ferroelectric liquid crystal (FLC) cell fabricated from glass pyramids with 60° evaporated SiO aligning layers it has been possible to sensitively probe the configuration of the optical dielectric tensor in the FLC layer. It is found that the optical data can best be explained by a uniaxial slab twisted from the alignment axis; this is consistent with the recently proposed chevron structure.     

The theoretical design and fabrication of a prism-coupled polarization conversion ferroelectric liquid crystal light modulator

It is possible to probe directly the optical dielectric tensor configuration within thin smectic layers (less than 6 μm thick) of ferroelectric liquid crystals (FLCs) by the propagation of optical prism-coupled leaky Fabry-Pérot modes. Incident polarized monochromatic light couples into the resonant modes of the system and may be coupled out of the cell in an orthogonal polarization. The observed reflectivity is a series of sharp peaks on a low background response at certain well-defined incident angles. These sharp resonant features make the prism-coupling technique a possible route for commercial fabrication of voltage-modulated devices. However, previous prism-coupled cells with sharp resonant guided mode features are not practical from a device point of view because they incorporate silver layers not used in conventional cell design. In this paper we demonstrate for the first time operating a leaky guided mode FLC cell with conventional surface layers, in a sp-mixed polarization mode of operation, allows sharp features to be observed which are modulated in intensity by an applied d.c. voltage. The prism-coupled cells used here are designed to be compatible with current FLC device technology.     

A Method for Computing the Optical Properties of a Smectic C∗ Liquid Crystal Cell with a Chevron Layer Structure

Abstract

In this paper a model developed previously for computing the director orientation in a cell containing chiral smectic C liquid crystal (SmC? LC) is extended to include a chevron layer structure in the relaxed (zero voltage) state. Results of the director orientation calculations are presented and used in a formalism developed elsewhere to compute the transmittance of a 7 μm thick cell for different azimuthal orientations of the cell between crossed polarisers. It is shown that the theoretical and measured orientations which give minimum transmittance are in good agreement over the entire voltage range considered (0–20 V). The computed average director tilt angle is found to be consistent with conoscopic measurements performed with a polarising microscope. Finally, on the basis of the computed director orientations, an explanation is given of why the zero-voltage minimum transmission orientation angle is so much smaller in thick (～7 μm) SmC? LC cells than it is in thin (～2 μm) cells.     

Evaporation heat transfer and pressure drop of ammonia in a mixed configuration chevron plate heat exchanger

Ammonia is a naturally occurring environment friendly refrigerant with attractive thermo-physical properties. Experimental investigation of heat transfer and pressure drop during steady state evaporation of ammonia in a commercial plate heat exchanger has been carried out for an un-symmetric 30°/60° chevron plate configuration. Experiments were conducted for saturation temperatures ranging from −25 °C to −2 °C. The heat flux was varied between 21 kW m⁻² and 44 kW m⁻². Experimental results show significant effect of saturation temperature, heat flux and exit vapor quality on heat transfer coefficient and pressure drop. Current mixed plate configuration data are compared with previous studies on the same heat exchanger with symmetric plate configurations. This comparison highlighted importance of optimization in selection of the heat exchangers. Correlations for two phase Nusselt number and friction factor for each chevron plate configuration considered are developed. A Nusselt number correlation generalized for a range of chevron angles is also proposed.     

The threshold voltage,dielectric and conductivity properties of C60-doped smectic A liquid crystal

Abstract

Influence of fullerenes C₆₀ on the threshold voltage, dielectric and conductivity properties of smectic A liquid crystal 4-hexyloxyphenyl ether 4′-hexyloxy 3′-nitrobenzoic acid is investigated. It is shown that the transverse component of the real part of dielectric permittivity increases at the additive of fullerenes while the longitudinal component decreases. In this case, a maximum of dielectric absorption shifts to the high-frequency region. Conductivity increases at low frequencies and decreases at the high ones. Experimental results are explained by location of fullerenes between liquid crystal molecules reducing their interaction. As a result, the order parameter and viscosity of the matrix decrease.     

Excitation of ray oscillations in quasioptical dielectric cavities with whispering gallery modes

It is observed that ray oscillations may be excited together with whispering gallery modes in quasioptical dielectric cavities. It is shown that as the excitation conditions change, the fields of these oscillations gradually “split off” from the fields of the whispering gallery modes whose characteristics deteriorate. The angular spectrum of ray oscillations in dielectric cavities is determined. Pis’ma Zh. Tekh. Fiz. 23, 25–29 (August 12, 1997)     

IR-stimulated second harmonic generation in Sb2Te2 Se-BaF2-PbCl2 glasses

Abstract

Theoretically predicted and experimentally observed infraredinduced second-harmonic generation of glasses in the mid-infrared spectral region can be described by fifth-order nonlinear optical susceptibility. The effect is observed in the mid-IR region when the value of the electronic energy gap is comparable to the energies of actual phonons participating in the anharmonic (non-centrosymmetric) electron-phonon interactions. As subjects for investigation, chalcohalide Sb₂Te₂Se-BaF₂-PbCl₂ glasses were chosen. They are transparent, over a spectral range of 1.1–10.9 μm. The second-harmonic generation (SHG) output signal within the 1.5–4.8 μm spectral range has significant spectral dependence. Correlation of the SHG spectral maxima positions with spectral positions of anharmonic phonon frequencies confirms that the fifth-order steady-state process occurs due to cascading processes and IR-induced charge density non-centrosymmetry. A maximum value of the SHG is achieved at a pump-probe delay time of 18–36 ps, which is typical for anharmonic electron-phonon interactions. As temperature rises, the values of the photoinduced SHG signal susceptibility increases up to 3 × 10^?39 m⁴/V⁴. The SHG signal reaches a saturation point for the IR-pump power densities of about 0.8 GW/cm², which corresponds to the output SHG intensities of about 6 × 10^?4 with respect to fundamental one. The values of the diagonal fifth-order tensor component χ^(2ω) _xxxxx are at least one order of magnitude larger than the off-diagonal tensor components.     

Temperature and voltage sensings by mode‐mode interference in polarization‐maintaining optical fibers

Abstract

Two optic fiber sensing systems for temperature and voltage have been developed which utilize the mode‐mode interference of the two orthogonally polarized modes, HE^x _II and HE^y _II, in two commencal polarization‐maintaining fibers (bow‐tie and elliptical core fibers). A package of controlled programs in a Macintosh computer, which can record and process all related data automatically, is established for temperature sensing. The signal drifting problem in voltage sensings has been investigated, and the elimination of signal drifting is obtained by the phase tracking with direct current technology The agreement between the sensing results for temperature and dynamic voltage and those predicted by experimental principles is satisfactory, which confirms the validity of the developed sensing systems.     

Optical and magneto-optical properties of the endohedral complex La@C82 crystal: a Hartree–Fock single-excitation configuration interaction model study

The dielectric constant tensor is used to describe the magneto-optical properties of crystals. Using the Hartree–Fock single-excitation configuration interaction model in conjunction with the local field method, the spin-dependent optical dielectric tensor, refractive index, reflectivity, circular dichroism (CD), and birefringence coefficient (θ) of the simple cubic (sc) phase of the La@C₈₂ crystal are calculated when the La atom is off-center. The calculations show that the spin-dependent optical dielectric tensor of the La@C₈₂ crystal is large. The displacement of the La atom from the center of the cage leads to molecular electric dipole moment which influences the magnetic and optical properties of the La@C₈₂ crystal. The Coulomb interactions and effective magnetic moment cause increasing dielectric tensor, CD, and θ. Our results show that the optical excitation in the region where the energy is lower than 2?eV has the strongest optical absorption bands and refractive index in the La@C₈₂ crystal. Also, our results show that CD and θ of the La@C₈₂ crystal are larger than those of the C₆₀ and C₇₀ crystals. We obtained the E_g of 0.27?eV similar to that of the obtained results of the La@C₈₂ thin film with a semiconducting behavior.     

Coupled-wave analysis of vector holograms. 2. Reflective gratings formed in photoanisotropic medium with uniaxial birefringence

The diffraction properties of reflective anisotropic gratings, which can be recorded in photoanisotropic media with uniaxial birefringence by three-dimensional vector holography, were characterized through the use of coupled-wave analysis (CWA). By investigating the perturbation of the dielectric tensor, we demonstrated that the gratings with sinusoidal distribution of the azimuthal angle of the optic axis diffract polarized light in which the ordinary and extraordinary components are converted for incident light. The polarization conversion was consistent with that calculated by a numerical method. In addition, it was shown that CWA enables highly accurate calculation of the diffraction efficiency with wavelength dispersion when the amplitude of the azimuthal angle is small.     

Dielectric losses in a polyethylene insulated superconducting cable between 4 and 22 K

The dielectric losses in a 5 m length of a superconducting cable, insulated with lapped polyethylene tape, have been determined between 4.3 K and 22 K with an inductively coupled ratio arms bridge which gave a resolution in tan δ of about 10^?6. The cable sample was built to a specification appropriate for power transmission at 132 kV and 6 kA; it was impregnated with helium at 0.4 MPa and cooled by a flow of cold helium circulated from a refrigerator.In the temperature range 4 to 5 K, applicable to a cable with niobium conductors, the dielectric loss increased slightly with electric field, and the observed tan δ (relative to tan δ of the reference capacitor) was 20 × 10^?6 at the maximum test stress of 7 MV m^?1. It is thought that a significant part of this loss can be attributed to the bedding layers and electrostatic screens between the dielectric and conductors.Tan δ showed a minimum at 18 K, which is encouraging for future developments of superconducting cables which might use hydrogen as a coolant and dielectric impregnant. The observed variation of tan δ with temperature is consistent with the existence of a low temperature relaxation mechanism with low activation energy.     

Localization of Light by a Set of Parallel Cylinders

Abstract

Using a rigorous theory of scattering, numerical evidence of the existence of localized modes in a one-dimensional or a two-dimensional set of circular dielectric rods is given. Particular attention is paid to the transition observed between the phenomena of propagation in periodic structures and localization phenomena in random structures. In particular, we show the strong connection between the phenomenon of a photonic band gap that appears in two-dimensional periodic sets of rods and the phenomenon of Anderson localization which is observed when the periodicity is broken     

The spin oscillations of 3He-B in the presence of dipole forces

The problem of the spin oscillations of ³He-B is investigated in the presence of dipole forces under the assumption that only the zeroth parameter of the Fermi-liquid interaction is significant. The solution of the gap equation gives the new thermodynamical phases. The spin susceptibility tensor is obtained in the most general form. It is shown that in addition to the longitudinal and transverse components some extra circular components can appear. These components are analogous to the components of the dielectric tensor that are responsible for the circular dichroism of optically active substances.     

Theory of orbital dynamics of the A phase of superfluid 3He. II. Orbital hydrodynamic equations

Orbital hydrodynamic equations in ³He-A are derived microscopically in the hydrodynamic regime near the transition temperature. Transport coefficients as well as reactive coefficients are evaluated as rigorously as possible. The expression for the time derivative of the phase of the order parameter is shown to contain l · rot _n with the Yosida function as a coefficient. It is shown that when the supercurrent is given by the usual definition which includes only the first-order space derivative of the order parameter, the stress tensor becomes antisymmetric and there is an additional contribution to the local angular momentum density of dynamical origin. The stress tensor can be made symmetric by changing the definition of the supercurrent.     

Theory and analysis of shells undergoing finite elastic-plastic strains and rotations

Summary In this paper theory and analysis of shells undergoing finite elastic and finite plastic strains and rotations are presented. The shell kinematics are based on a relaxed normality hypothesis allowing transverse normal material fibers to be stretched and bended, whereas shear deformations are neglected. Lagrangean logarithmic membrane and logarithmic bending strain measures are introduced, and it is shown that they can be additively decomposed into purely elastic and purely plastic parts for superposed moderately large strains and unrestricted rotations. The logarithmic strain measures are used to formulate thermodynamic-based constitutive equations for isotropic elastic and plastic material behavior with isotropic and kinematic hardening induced by continuous plastic flow. To analyse path-dependent elastic-plastic shell deformations by iterative procedures the application of logarithmic strain measures allows to realize load steps with corresponding moderate strains and unrestricted rotations. The moderate strain restriction for superposed deformations can be assured by an appropriate update procedure. Formulae are given to determine exactly the rotational change of the reference configuration during the update. Finally, the principle of virtual work with corresponding elastic-plastic material tensor is formulated and it is shown that the weak form of the virtual work leads to the Lagrangean equilibrium equations and boundary conditions well-known from the nonlinear theory of elastic shells.

Notation

3-D and 2-D continuum F deformation gradient - U right Cauchy-Green stretch tensor - R rotation tensor - E Green strain tensor - e Almansi strain tensor - H Lagrangean logarithmic strain tensor - Kirchhoff stress tensor in the 3-D continuum - Cauchy stress tensor - () ^e reference to an elastic deformation - () ^p reference to a plastic deformation - (^–), (⁺) reference to a first and a second superposed deformation, respectively - (^*) reference to an alternative decomposition of the superposed deformation - AB composition of the two tensorsA andB - A ^T transposed ofA - A ² square ofA - A ^–1 inverse ofA Shell theory u displacement field of the shell space - v displacement field of the middle surface - h, shell thickness in the initial and the current state - thickness coordinate, - g _i ,g ⁱ base vectors in the undeformed shell space,i{1, 2, 3} - base vectors in the deformed shell space - a ,a surface base vectors on the undeformed middle surface =0, {1, 2} - , surface base vectors on the deformed reference surface =0 - n, unit normal vector on the surface =0 in the initial and the deformed configuration, respectively - b, curvature tensor of the surface =0 in the initial and current state - Green membrane strain tensor - Green bending strain tensor - Green second order strain tensor - g logarithmic membrane strain tensor - k logarithmic bending strain tensor - p logarithmic second order strain tensor - T plane Kirchhoff stress tensor - N stress resultant tensor - stress couple tensor - second order stress resultant tensor     

用于非线性兰姆波检测的高灵敏度宽带光纤光栅传感器

光纤传感器因其灵敏度高,已逐渐应用于超声检测的研究中,但大多数光纤传感器的频带响应范围有限,约为几百k Hz,很难检测到更高频率的信号。所提出的光纤布拉格光栅(Fiber Bragg Grating,FBG)传感器的高频检测范围可以达到4 MHz左右,大大提高了其检测带宽范围。文中将传感器应用于304不锈钢板兰姆波的非线性检测,同时与传统超声换能器的检测结果做对比。实验结果表明,用脉冲波激励信号时,FBG传感器可以检测到钢板兰姆波的基频到五倍频信号,表明FBG在检测兰姆波非线性上是有很大潜力的。     

Estimation of fracture toughness of steel using chevron notched round bar specimens

A generalized methodology has been outlined in this paper for estimating the minimum normalized stress intensity factor (Y*_min) of chevron notched round bar specimens, subjected to three‐point bend loading. Using such specimens, a series of fracture toughness tests have been carried out for the first time on two steels. The major inferences drawn from this investigation are: (i) reproducible fracture toughness values can be achieved using chevron notched rod specimens of identical configuration and (ii) the estimated magnitudes of fracture toughness obtained by using chevron notched rod specimens are in good agreement with those achieved by using chevron notched rectangular bar specimens of the same material.     

Analysis of whispering-gallery superconducting dielectric resonator modes

The whispering-gallery (WG) modes of a superconducting dielectric resonator (SDR) based on a sapphire cylindrical dielectric resonator and a YBa₂Cu₃O_{7 –} shielding cylinder have been studied. A method for the determination of the resonant frequencies and the maximum quality factor of such modes is presented. Calculations have shown that most of the mode energy could be confined between the caustic surface of the WG modes provided the dimensions of the SDR are properly selected, and a magnitude of 10⁹ forQ of the SDR could be estimated. A phenomenal explanation is given to account for such outstanding microwave behavior.     

Novel promising crosslinkable tricyanopyrroline polymeric electro-optic materials

A tricyanopyrroline chromophore with methacrylate reactive group was synthesized to fabricate the side chain (polymer 1) and crosslinkable (polymer 2) EO (electro optic) polymers with enhanced electrooptical tensor coefficients. These type of polymers have demonstrated the large EO coefficients with values of 29 pm/V and 25 pm/V at wavelength 1,310 nm, respectively. Compared with polymer 1, t polymer 2 had demonstrated a better stability of polar structure. The thermal stability of polar structure was improved for about 100 °C. The temperatures of poling and crosslinking processes were compatible in polymer 2, which allowed obtaining the EO films with improved features. The experimental results have been interpreted with the aid of quantum-chemical calculations.