Study of Lasing in Liquid-Crystal Systems with Microgratings

Waveguide lasing in a layer of nematic liquid crystal (NLC) doped with a dye has been experimentally observed. A chromium micrograting with a period of 1.76 μm has been deposited on the surface one of the cell substrates to provide a distributed feedback (DFB) and partially extract laser radiation to a quartz substrate. The DFB provides lasing in the ninth diffraction order for waveguide TM modes. Laser radiation is observed at the output of the substrate end face at an angle of 67.0° ± 1.5° with respect to the normal to the waveguide plane. An increase in voltage across the micrograting electrodes leads to red-shifted multimode lasing. A numerical simulation of a structure imitating the experimental cell has shown good agreement between the calculation results and experimental data.     

Amplification of the Fluorescence Propagating in the Waveguide Regime in a Planar Layer of NLC

Crystallography Reports - The amplification of spontaneous fluorescence in a planar layer of nematic liquid crystal doped with DCM dye in the mode of waveguide light propagation has been studied....     

Linear Optical Properties in Multilayers of Polydiacetylenes

Abstract

The linear optical properties in multilayers of polydiacetylenes (PDA) have been investigated by studying both the transmission/absorption coefficients and the waveguide properties of the thin PDA films (?5000 A). The PDA films were deposited one monolayer at a time by the Langmuir Blodgett technique. The waveguide modes of the PDA waveguide (in an air - PDA film - silver configuration) were coupled via a grating at the air-PDA and/or PDA-Ag interface. Since the diacetylene chains are parallel to the interface, the TE wave/ waveguide modes (i.e., the electronic field is parallel to the PDA film), the system can be treated simply as an isotropic case. For TM wave/waveguide modes, the system has to be treated as an anisotropic one. The results of wavelength dependent linear optical property of PDA films and their impact to the waveguide properties as well as the grating coupling effects in the waveguide system will be discussed.     

Surface Order Parameter and Polar Anchoring Energy in Nematic Liquid Crystals

The trends of temperature dependence of the surface order parameter and of the anchoring strength coefficient are theoretically evaluated in the Landau-de Gennes phenomenological theory for nematic liquid crystals. The cases of planar, homeotropic and tilted alignment at a planar limiting surface are considered. It is shown that the cases with surface order parameter greater than the bulk one or smaller than this can occur in function of the phenomenological coefficients in the surface free energy expression. The anchoring strength coefficient, W, diminishes when temperature approaches the nematic-isotropic point, after passing through a maximum in most of cases. The possibility of a non-zero anchoring strength at temperatures above the transition temperature is revealed. The obtained trends of W(T) describe well a large variety of experimental results from literature.     

Modification of the malus law for the torsional deformation of lyotropic nematics in magnetic field on the basis of statistical approach

A method based on the statistical approach is proposed to calculate the light intensity for the torsional deformation of lyotropic nematic liquid crystals at violated Mauguin adiabatic approximation. Theoretical dependences of the light intensity on the magnetic field strength are obtained for two limiting cases of lyotropic nematic anchoring with bearing surfaces: infinite and low anchoring energies.     

Dynamic photorefractivity guided by single-pulse voltage

The dynamic photorefractivity in a cell with photoconducting orienting layers, filled with a nematic liquid crystal (LC) 6CHBT and a mixture of anthraquinone dyes AD1 and AD2, has been investigated. The single-pulse mode, in which the polarity and amplitude of a dc electric field applied to an LC cell are switched for a fixed time interval, has been used. The scheme of dynamic self-diffraction of low-power laser beams was used in the experiment. The dependences of the width and intensity of diffraction pulses on the bias and switching voltages have been investigated. It is established that the width and intensity of the diffraction pulse arising after initial voltage recovery depends also on the switching pulse width. At the optimal width of the control pulse, the diffraction efficiency increases by two orders of magnitude.     

Optical properties of a nematic liquid crystal under the binary effect of periodic shear and compression

The optical response (clearing) of a liquid crystal cell, containing a layer of a nematic crystal with homeotropic alignment, under the binary effect of periodic shear and compression has been theoretically described. The effect was calculated taking into account the presence of orientational waves arising at displacement of one of the boundary plates in its plane, and the mobility of this plate along the normal upon statement of boundary conditions. The influence on the effect of the alignment energy of molecules at the layer boundaries is analyzed. The calculation results are compared with the experimental data.     

Chain Ordering in the Nematic Phase of 40ba

From the intensity profile of the X-ray diffraction pattern in the nematic phase of 40BA it is inferred that the alkoxy chains have an orientational order.     

Holographic Diffraction Grating Controlled by Means of Nematic Liquid Crystal

Results are reported on the investigations on an electrically controlled phase holographic grating. It has a nearly sinusoidal profile with 1,4.μm deep grooves spaced at 12,5μm. The control voltage applied to the nematic liquid crystal has an amplitude of 0-25V and frequency 1-10 kHz. A light beam from a He-Ne laser (0,633μm) with a palarization parallel, perpendicular and at 45° the grooves of the grating is used.     

Influence of magnetic field on the director profile in nematic layer with curved surface

A nematic liquid crystal layer with a curved upper boundary exposed to a magnetic field directed along the layer plane has been studied. Strong nematic anchoring at the lower boundary surface is assumed. A phenomenological expression for the director profile is proposed to solve the problem on the director profile distribution in this system and its stability with respect to the rotational deformations in relation to the degree of the upper-surface roughness and the magnetic field value. The distribution parameters are found by optimizing the expression for the free energy of the liquid crystal system.     

Design of Optical Directional Couplers Made of Polydimethysiloxane Liquid Crystal Channel Waveguides

We present numerical simulations of a directional coupler based on three-dimensional waveguides made of a nematic liquid crystal, acting as the waveguide core, infiltrated in polydimethysiloxane channels. Modeling is based on the combination of minimization of Oseen-Frank energy of the liquid crystal molecules with a beam propagation algorithm. Design of the coupler waveguides is optimized to minimize coupling lengths and maximise efficiencies. Such components can be made at low cost on flexible plastic substrates and can be also integrated with optofluidic devices for biomedical applications.     

Photorefractive Properties of Co-Doped Potassium Sodium Strontium Barium Niobate Crystals

The transmissivity and the results of two-beam coupling and self-pumped phase conjugation (SPPC) experiments on Co-doped potassium sodium strontium barium niobate (KNSBN) single crystals are reported. The Co ions doped into KNSBN can form a deep energy level near 2.25 eV. The two-beam coupling gain coefficients of the Co-doped KNSBN are nearly 20 cm^-1 at 2 teta = 37⁰ for the transmission photorefractive index grating and larger than 29.2 cm^-1 for the reflecting photorefractive index grating. A striking single-beam asymmetric transmission was observed. The maximum SPPC reflectivity is larger than 70% at teta = 49.7⁰ for an intensity W = 77 mW, which is the largest value measured in the family of KNSBN crystals.     

Nonlinear Optics of Nematic and Blue Phase Liquid Crystals

We present a critical reexamination of individual electronic and collective optical nonlinearities of nematic liquid crystals (NLC) in the context of all-optical transmission switching of femtoseconds – CW lasers, and report recent observations of similar optical nonlinearities in Blue-phase liquid crystals (BPLC) and their utilization in feasibility demonstrations of polarization-free nonlinear grating diffraction and transmission switching of CW as well as pulsed lasers.     

Phase diagram of growth mode for the SiGe/Si heterostructure system with misfit dislocations

The strain, surface and interface energies of the SiGe/Si (SiGe grown on Si) heterostructure system with and without misfit dislocations were calculated for the Frank–van der Merwe (FM), Stranski–Krastanov (SK) and Volmer–Weber (VW) growth modes essentially based on the three kinds of fundamental and simple structures. The free energies for each growth mode were derived from these energies, and it was determined as a function of the composition and layer thickness of SiGe on Si. By comparison of the free energies, the phase diagrams of the FM, SK and VW growth modes for the SiGe/Si system were determined. The (1 1 1) and (1 0 0) reconstructed surfaces were selected for this calculation. From the phase diagrams, it was found for the growth of SiGe on Si that the layer-by-layer growth such as the FM mode was easy to be obtained when the Ge composition is small, and the island growth on a wetting layer such as the SK mode was easy to be obtained when the Ge composition is large. The VW mode is energetically stable in the Ge-rich compositional range, but it is difficult for the VW mode to appear in the actual growth of SiGe on Si because the VW region is right above the SK region. The regions of the SK and VW modes for the (1 1 1) heterostructure are larger than those for the (1 0 0) one because the strain energy of the (1 1 1) face is larger than that of the (1 0 0) face. The regions of the SK and VW modes for the heterostructure with misfit dislocations are narrower than those for the one without misfit dislocations because the strain energy is much released by misfit dislocations. The phase diagrams roughly explain the behavior of the FM and SK growth modes of SiGe on Si.     

Electron diffraction study of lizardite 1<Emphasis Type="Italic">T</Emphasis> with stacking faults

The stacking faults observed in the structure of the mineral lizardite 1T belonging to the polytype group A are investigated using the digital oblique-texture electron diffraction patterns, difference Fourier-potential maps, and model diffraction patterns obtained for this compound. Numerical simulation of the diffraction profiles along the first (the 02l and 11l reflections) and second (the 20l and 13l reflections) ellipses in the oblique-texture electron diffraction patterns is performed for finite sequences of ten layers with the use of the Markovian statistical model in the quasi-homogeneous approximation. The specific features of the intensity distributions along the first and second ellipses are associated with the manifestation of translational (displacements of the layers by ±b/3) and orientational (rotations of the layers through an angle of 180°) defects of the layer stacking, respectively. For both ellipses, the experimentally observed intensity distributions are in the best agreement with the diffraction profiles calculated for stacking faults at a content of approximately 25%, the short-range order parameter S = 1, and the maximum degree of ordering in the layer alternation. It is demonstrated that the irregularities revealed in the layer alternation in the structure of lizardite 1T (which is characterized by an identical orientation of the adjacent layers) arise from layer displacements by ±b/3 and, to a considerable extent, from the formation of sequences with opposite orientations of the adjacent layers. As a result, the structure of lizardite 1T nanocrystals involves a combination of layer sequences that are typical of structures belonging to the polytype groups A and D.     

The influence of the boundary conditions on the character of the phase transition nematic-smectic C

The possibility is investigated for inducing the intermediate smectic A structure in substances with nematic and smectic C phases by the boundary forces. Different geometries are used: homeotropic, homogeneous and tilted under a different angle toward the supporting substrates of the liquid crystal cell. At homeotropic and at tilted orientation where the molecules tilt toward the supporting substrates is different from the molecule tilt in the smectic layer, an intermediate smectic A structure is observed at the transition nematicsmectic C in a narrow temperature interval. In this case the phase transition nematicsmectic C extends and transforms into phase transition of second order. At homogeneous orientation and at tilted orientation of an angle equal to the molecule tilt in the smectic layer, the transition nematic-smetic C carried out in two stages, as partially it begins on the surface still in the nematic phase and terminates at the temperature of the phase transition N-Sc. The influence of the boundary forces depends essentially on the energy of the phase transition N-S_C of the substances.     

Paracrystalline Nature of Poly (p-Phenylene)

The electron diffraction pattern of oriented, annealed poly (p-phenylene) is explained on the basis of an orthorhombic unit cell: a = 0.7781, b = 0.5520 and c(fiber axis) = 0.4300 nm. and exhibits the following features:

(1)The reflections on the equator and meridian are sharp. Only a few weak diagonal reflections can be observed.

(2) Strong diffuse scattering is seen on the third layer line.

(3)The reflections on the meridian (00l-reflections) have a characteristic line profile. in which the intensity increases suddenly at the Bragg angle and decreases gradually.

From (1) to (3), it can be concluded that the material is paracrystalline like a nematic structure in liquid crystals. Molecular chains are laterally packed in a rather regular way. However, the disorder by the shift of chains in the direction parallel to their axes is remarkable.     

Influence of annealing temperature and Au concentration on the electrical properties of multilayered a-Ge/Au films

Multilayered a-Ge/Au was investigated to realize low-temperature formation of poly-Ge films on insulator. Increasing Au layer thicknesses, peak intensity of crystalline Ge–Ge TO mode increased and amorphous Ge–Ge TO mode decreased. When Au composition ratio is high, the samples are crystallized under eutectic temperature. Annealing temperature at 673 K, all samples with Au layer are crystallized. Crystalline Ge has no strain evaluated by X-ray diffraction and Raman scattering spectroscopy. However, the Au is compressed or expanded in the films with various annealing temperature. It is thought that this phenomenon changes depending on the size of the space in the film. The behavior of electrical resistivity is changed at eutectic temperature.     

Organic light-emitting transistors with a thin metal layer covering a diffraction grating

We fabricated organic light-emitting transistors (OLETs) characterized by an Ag layer deposited on a one-dimensional (1D) or two-dimensional (2D) diffraction grating that acts as a combined gate insulator with SiO₂. The Ag layer was entirely covered with an organic crystal. Upon photoexcitation that crystal showed narrow linewidth emissions (NLEs) parallel to the substrate plane. The narrowed lines were either redshifted or blueshifted with rotation of the crystal around a normal to its surface with respect to the grating wave vector. Strong emissions (～10⁴–10⁶ cd m^?2) accompanied by current-injected NLEs were observed from the 1D and 2D grating OLETs.     

Quasi-Rayleigh waves in sandwich structures: Dispersion equation, eigenmodes, and resonance reflection

The dispersion equation for quasi-Rayleigh acoustic modes in transversely isotropic sandwich-type structures (an elastic layer between two substrates) has been obtained in the analytical form. The equation is analyzed for three types of structures—a layer between two soft substrates, a layer between a soft and a hard substrate, and a structure consisting of two contacting substrates with markedly different values of hardness. Eigenmodes are determined for all three structures. Depending on the combination of the material parameters in these structures, these eigenwaves are either Rayleigh-type waves localized at interfaces in the substrates and accompanying a Lamb wave in the interlayer (in the absence of an interlayer, forming a Stoneley wave) or the leaky modes characterized by a small imaginary addition to the phase velocity and the presence of bulk partial waves of leakage providing some energy removal from the interfaces. Without any calculations, simple criteria were established that allow one to predict the existence of a leaky mode in the given structure and the number of leakage fluxes it contains. The resonance reflection is analized for the case where the incidence angle of the wave in the substrate corresponding to the velocity v of the wave-front propagation along the interface is close to the value of the real part of the phase velocity of the leaky mode.