Defects in a TGBA phase: A theoretical approach

Following our experimental observations of disclination lines in freely suspended droplets and free-standing films (Yu.A. Nastishin et al., Eur. Phys. J. E 5 353 (2001)), topological defects of the twist grain boundary (TGBA) phase are considered according to two aspects: topological and energetical. There are two classes of line defects, disclinations (as in the cholesteric (N*) phase and the liquid vortices phase (NL*), relating to the directors tripod symmetries) and dispirations (relating to the translation-rotation symmetries); there are no topological point defects. Differences between N*, NL* and TGBA disclinations are physical, not topological. The absence of focal conic domains in the TGBA phase is an immediate consequence of the materialization of the helical axis (along the χ-director); the same feature, coupled to the trend to parallelism of the smectic layers, accounts for the predominance of λ-lines. Finally, the presence of defects akin to developable domains is explained in the frame of the leastcurvaturemodel, that requires the introduction of a third type of defects: the densitiesofedgedislocations of the smectic layers. Received 20 February 2002     

Laser nanosources based on planar photonic crystals as new platforms for nanophotonic devices

Two-dimensional photonic crystal lasers have been fabricated on III–V semiconductor slabs. Tuning of the spontaneous emission in micro and nanocavities has been achieved by accurate control of the slab thickness. Different structures, some of them of new application to photonic crystal lasers, have been fabricated like the Suzuki-phase or the coupled-cavity ring-like resonators. Laser emission has been obtained by pulsed optical pumping. Optical characterization of the lasing modes have been performed showing one or more laser peaks centred around 1.55 μm. Far field characterization of the emission pattern has been realized showing different patterns depending on the geometrical shape of the structures. These kinds of devices may be used as efficient nanolaser sources for optical communications or optical sensors.     

Structural analysis of Fe3+ centers in Tl2MgF4 and Tl2ZnF4 fluorine compounds

Theoretical analysis of the Fe³⁺ centers observed in Tl₂MgF₄ fluorine crystals have been carried out by means of semi-empirical approaches. The most appropriate models are proposed by matching the theoretically predicted zero-field splitting parameters (ZFSPs) with the experimental ones obtained by EPR spectroscopy. Compression on the MF₆ octahedron of tetragonal (TE) center I is indicated in both Tl₂MgF₄ and Tl₂ZnF₄. A structural model for monoclinic (MO) center II and orthorhombic (OR) Fe³⁺ center IV in Tl₂ZnF₄ is proposed by assuming that the substitution of Fe³⁺ induces both ligand length and angular distortions.     

IR Absorption and Raman spectroscopic study of reentrant-nematic liquid crystalline N-[4-(4-Octyloxy-benzoyloxy) Benzylidene]-4-Cyanoaniline (OBBC)

Abstract

The IR absorption and Raman scattering of OBBC have been investigated in the solid, ReN, SmA and nematic phases. The intensity of the 2229 cm^?1 band assigned to the C[tbnd]N stretching mode decreases with increasing temperature in the reentrant nematic phase; this is attributed to a change in the overlap of the molecules. While the IR bands at 1728 and 841 cm^?1 also show a remarkable temperature dependence, Raman bands do not show significant temperature dependence in the liquid-crystal phases.     

High-pressure spectroscopic studies of free and self-trapped excitons in alkali halides at low temperatures

Abstract

A brief survey of our studies of free and self-trapped excitons (FE and STE) in alkali halide crystals under hydrostatic pressure up to 12.5 kbar at 4.2–140 K is presented. Main attention is paid to the following effects observed: (1) the strong coupling of three energy levels of FE in CsI revealing itself as an exciton analog of pressure-scanned Fermi resonance; (2) emergence of a new emission band of STE in CsI under pressure; (3) a large pressure shift of the thermal quenching curve for STE emission in NaCl.     

Tunable local surface plasmon resonance in liquid-crystal-coated Ag nanoparticles

The far-field and near-field properties of a spherical nematic liquid crystal (NLC) coated metal nanoparticle (NPs) have been investigated in an external field, basing on the quasistatic theory. The resonant wavelength is tunable by varying metallic material of core, anisotropy extent and thickness of liquid crystals (LCs). The field enhancement is along the incident polarization near the outer surface of the shell. The direction of field is reverse in the inner surface comparing with the one if outer shell. In contrast to isotropy shell, the surface plasmon resonance (SPR) shows an obvious red shift and field enhancement near outer surface of the shell always is stronger.     

Effects induced by 𝛄-irradiation on intraband transitions in Sr2+-doped ammonium zinc chloride crystals

Considerable changes were observed in the X-ray diffraction pattern of ammonium zinc chloride after doping with Sr²⁺ in different concentrations and after irradiating with γ rays at different doses. The effect of γ-radiation and Sr²⁺ content on optical parameters of (NH₄)₂ZnCl₄: x Sr²⁺ single crystals (x?=?0.00, 0.020, 0.039, 0.087 or 0.144?wt.%) has been investigated. The transmittance near the absorption edge of unirradiated crystals and crystals irradiated with different γ-doses has been measured, hence the absorption coefficient (α) was calculated. The values of α at room temperature increased under the influence of γ-irradiation. The rate by which α increases with photon energy just before the absorption edge is strongly inhibited by higher γ-doses. The type of intraband transition in (NH₄)₂ZnCl₄: x Sr²⁺ single crystals was found to be of the allowed indirect transition, and γ-irradiation had no effect on the type of transition. The values of the optical energy gap (E _g ) were calculated as a function of γ-dose. The effect of γ-irradiation was found to be more pronounced on samples doped with x?=?0.087 or 0.144?wt.% Sr²⁺. The results can be discussed on the basis of γ-irradiation-induced defects and Sr²⁺ concentration. A diagram representing probable transitions to the created bands due to irradiation could be constructed.     

Electro-optic properties of nematic and ferroelectric liquid crystalline nanocolloids doped with partially reduced graphene oxide

Flakes of partially reduced graphene oxide (PRGO) were doped in nematic liquid crystals (NLCs) and ferroelectric liquid crystals (FLCs), respectively. The dielectric and electro-optical properties of NLCs doped with those flakes have been investigated. Threshold voltage and switching times are reduced by 30%–50%. This is primarily due to the decrease of the elastic properties of the nanocolloids compared to the non-doped nematics. The influence of the PRGO flakes on the spontaneous polarization, tilt angle and switching time of FLCs was investigated too. Such flakes reduce the response time by 40%–60%, increases spontaneous polarization by 20%–25% and increase the tilt angle by 15%–20%.     

Complementary interference method for determining optical parameters of liquid crystals

ABSTRACT

Wedge cells of small apex angle, filled with liquid crystals, were used to determining optical characteristics as functions of temperature for three liquid crystalline mixtures recently produced and a reference nematic. The interference fringes appearing in polarised monochromatic light (of sodium yellow line) normally incident on the cell were exploited to measure the ordinary and extraordinary refractive indices in the reflection mode and birefringence in the transmission mode. The measurements were repeated using Abbe's refractometer for 6CHBT as the reference to verifying the precision. Additionally the order parameter was computed from birefringence as a function of temperature. The results confirm the usefulness of the method and provide the properties of two nematic liquid crystals of small and large birefringence and one smectic liquid crystal of medium birefringence, recently produced. The experimental systems served also to investigating phase transition between the liquid crystals and the isotropic liquid at near-clearing temperature.     

Low-amplitude Screening-photovoltaic Spatial Solitons in Biased Photovoltaic Photorefractive Crystals

The low-amplitude spatial solitons in biased photovoltaic photorefractive crystals are investigated theoretically. The analytical solutions for both the bright and the dark low-amplitude screening-photovoltaic spatial solitons in photorefractive crystals are obtained. The expressions for the width of these solitons are given. The explicit expressions for the spatial deflection and angular deviation of the bright low-amplitude screening-photovoltaic spatial soliton are also presented by taking into account the effect of diffusion.