Surface-induced ordering in thin uniaxial liquid crystal films

The interface localization transition in thin uniaxial liquid crystal films with competing surface fields has been studied using Metropolis Monte Carlo simulations. The model is constructed from a lattice of continuously orientable interacting spins, and the Hamiltonian contains both bilinear and biquadratic contributions. The biquadratic contribution to the Hamiltonian is familiar from the Lebwohl-Lasher model, and accounts for the particle anisotropy in a liquid crystal. The head-tail asymmetry of the molecules in a uniaxial liquid crystal is taken into account through a bilinear contribution familiar from the classical ferromagnetic Heisenberg model with exchange anisotropy Lambda. The critical temperature T(c), characterizing the interface localization transition within the uniaxial liquid crystal film, depends strongly on the relative magnitudes of the bilinear and biquadratic interactions between the spins. For systems dominated by the biquadratic interaction, T(c) is found to be close to the bulk critical temperature of the system. But as the biquadratic interaction strength is reduced, T(c) departs markedly from the bulk critical temperature of the system.     

Structures and phase transitions in thin free standing films of an antiferroelectric liquid crystal

The premises of a discrete mean-field model for polar smectic liquid crystals are tested by analyzing the ellipsometric experiments on two, three, and four-layer freestanding films of MHPOBC. The measured temperature dependences of the ellipsometric parameters in a weak dc external field are compared to the predictions of a simple clock model. A very good quantitative agreement is found indicating an odd-even effect: XY structures are stable for odd and Ising-like structures for an even number of layers.     

Spectroscopic ellipsometry study of liquid crystal and polymeric thin films in visible and near infrared

In this work, we propose spectroscopic ellipsometry as a suitable method for measuring optical properties in soft materials, polymers and liquid crystals, specially selected for use in photonics applications. We show the results of our measurements on some multilayered samples, in the range from visible to the near-IR region, of interest for telecom applications. We point out potentialities and limits of the technique and compare the obtained results with another experimental method, the m-lines spectroscopy, and/or with existing data in the literature. The results about the optical parameters for the analysed materials (the nematic liquid crystal 5CB, one commercial and one lab made optical polymer, and an Indium Tin Oxide film) are useful and interesting by themselves. In fact, as a paradigmatic example, we briefly discuss how an incomplete knowledge of this kind of data can lead to a wrong design of a Bragg grating device. However, more than in the provided data, we put the interest of the present analysis in the warnings about spectroscopic ellipsometry utilization and eventually the necessity of getting complementary information.Received: 13 May 2004, Published online: 29 June 2004PACS: 78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity) - 78.66.Qn Polymers; organic compounds - 07.60.Fs Polarimeters and ellipsometers     

Surface-surface interaction in smectic liquid crystal films

Null transmission ellipsometry was employed to study the field induced transition of the surface arrangements in freestanding films of smectic liquid crystals. The interlayer interaction between the two surfaces obtained from the threshold voltage for the transition is found to be antiferroelectric and is quasilong ranged. The possible microscopic origins of the measured interaction and its relevance to the interlayer interaction in antiferroelectric liquid crystal materials are discussed.     

On defects in aluminium single crystal thin films

Defects in Al single crystal thin films prepared by epitaxial growth on mica were investigated by transmission electron microscopy. Dislocations, subgrain boundaries, grain of different orientation and separated polycrystalline regions were observed in the single crystal matrix with density dependent on the pressure of residual gases.     

Poling of liquid crystals in thin films

We examine the effect of a strong DC electric field on the molecular orientational order and the nonlinear optical response of liquid crystals in thin films. We compare the results of second-harmonic generation measurements with the predictions of two models, one assuming that the dipoles carried by the molecules have no interactions (the isotropic model), and the other assuming that the dipoles evolve in a Maier-Saupe orienting field responsible for the liquid-crystalline order (the Maier-Saupe model). In both cases, we take into account the effect of surfaces and confinement on the behavior of the molecules. We find that the molecular dipoles behave as predicted by the isotropic model, but that their reorientation is correlated in such a way that the apparent dipole moment of the reorienting units is one order of magnitude larger than the molecular dipole moment. Received: 13 December 2002 / Accepted: 22 April 2003 / Published online: 21 May 2003 RID="a" ID="a"Also at FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands. RID="a" ID="a"Also at FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands. RID="b" ID="b"e-mail: jerome@science.uva.nl     

Polarity sensitive electro-optical response in liquid crystal devices due to ionic diffusion in WO3 thin films

The present work is aimed to a deeper understanding of the polarity sensitive electro-optical response observed in a liquid crystal cell with asymmetric insertion of a thin film of tungsten trioxide, deposited by r.f. sputtering on a transparent ITO substrate. After preliminary structural characterization of the films, the electro-optical response of the liquid crystal cells containing a WO₃ layer has been analyzed. The voltage thresholds of the Frédericksz transition are increased or decreased for the anodic or the cathodic charge of the tungsten trioxide film, respectively. In such a way a polarity-sensitive electro-optical response is generated, with the same frequency of the applied field. Impedance and cyclic voltammetry measurements have been performed on liquid crystal cells having WO₃ films inserted as electrode, for as-deposited films as well as for annealed films at increasing temperatures. A strong decrease of the ionic current is observed in the cell where the film underwent the highest temperature treatment, above 300 °C. All the measurements suggest that a charge redistribution occurs for the hydrogen ions, always present in the films, during the intercalation-deintercalation processes induced by the applied voltage, and it is responsible for the changes of the effective electric field acting on the liquid crystal layer. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Structural phase transitions in langmuir liquid crystal films

The isotherms of water molecule absorption on hyperfine Langmuir films made based on liquid crystals are investigated. A sharp increase in the adsorption capacity of the films at temperatures of 75°C (for a film with a thickness of ten monolayers) and 102°C (for a film with a thickness of five monolayers) has been revealed. This behavior is explained by structural phase transitions that take place at these temperatures.     

Photonic defect modes in cholesteric liquid crystal films

We discuss the properties of photonic defect modes in cholesteric liquid crystals. Twist defects, isotropic defect layers, and combinations of both are considered. After deriving the reflection and transmission properties of the defects, we study the effect of a finite sample thickness on the defect modes amplitude and on the required polarization of incident light to excite the defect mode.Received: 18 August 2003, Published online: 5 February 2004PACS: 42.70.Qs Photonic bandgap materials - 61.30.-v Liquid crystals     

Hexatic ordering in freely suspended liquid crystal films

In this paper, we report results from synchrotron X-ray scattering studies of thefluid/hexactic/solid phases and phase transitions in both very thick and very thin, freely suspended films of tilted hexatic liquid crystals. Contrary to the thick film case, the higher Fourier coefficients describing the bond orientational order are suppressed in very thin films. This suppression is consistent with a two-dimensional bond orientational order parameter, ₆, rather than the three-dimensional bond orientational order parameter found in very thick films. For a film containing twently-three (23) smectic layers we find that ₆ is two-dimensional whereas the positional order in the crystallineS _J phase is three-dimensional. We present an analysis of the thick film data in terms of the three-dimensionalXY-model and a new mean field theory model which incorporates explicitly the quasi two-dimensional nature of bulk smectic phases.     

Radiative decay of Wannier excitons in thin crystal films

The radiative decay of the Wannier exciton in thin crystal films is studied by the method of Heitler and Ma in the resonance approximation. It is shown that, while the broken crystal symmetry opens up the possibility of radiative decay, the correlation brought about by the interaction of the lattice atoms with a common radiation field leads to a superradiative enhancement of the decay rate. However, when it is compared with the corresponding case of a Frenkel exciton, the decay rate of the Wannier exciton is reduced due to the averaging of the dipole transition matrix element over the various possible distances between the widely separated electron and hole.     

Lasing mechanism in two-dimensional photonic crystal lasers

We conduct a comprehensive investigation of the lasing mechanism in a photonic crystal slab laser with a refractive index that is periodic in two dimensions. Experimental spectra of laser structures fabricated with organic gain media are presented. It is found that lasing frequencies can be explained in terms of Van Hove singularities in the density of modes. We also observe lasing spectra that cannot be obtained from structures with one-dimensional periodicity, such as traditional distributed feedback lasers. Lasing frequencies are computed using numerical techniques. Received: 7 April 1999 / Accepted: 12 April 1999 / Published online: 19 May 1999     

Optical reflection from cholesteric liquid crystal films

The interpretation of the reflection spectrum from the measurements of Fergason and Adams et al. by Subramanyam envisaging absorption of light by the material is shown to be incorrect. Some refined measurements of circular dichroism on the mixtures of the cholesteric substances as used by the above authors are reported which conclusively invalidate the explanation of Subramanyam.     

Computer simulations of adsorbed liquid crystal films

The structures adopted by adsorbed thin films of Gay-Beme particles in the presence of a coexisting vapour phase are investigated by molecular dynamics simulation. The films are adsorbed at a flat substrate which favours planar anchoring, whereas the nematic-vapour interface favours normal alignment. On cooling, a system with a high molecule-substrate interaction strength exhibits substrate-induced planar orientational ordering and considerable stratification is observed in the density profiles. In contrast, a system with weak molecule-substrate coupling adopts a director orientation orthogonal to the substrate plane, owing to the increased influence of the nematic-vapour interface. There are significant differences between the structures adopted at the two interfaces, in contrast with the predictions of density functional treatments of such systems.     

Contact line instabilities of thin liquid films

We present results of fully nonlinear time-dependent simulations of a thin liquid film flowing down an inclined plane. Within the lubrication approximation, and assuming complete wetting, we find that varying the inclination angle considerably modifies the shape of the emerging patterns (fingers versus sawtooth). Our results strongly suggest that the shape of the patterns is not necessarily related to either partial or complete coverage of the substrate, a technologically important feature of the flow. We find quantitative agreement with reported experiments and suggest new ones.     

Three-dimensional nonlinear dynamics of thin liquid films

Using a general two-body exponential parametrization for the wave function, the Nakatsuji two-particle density equation [Phys. Rev. A 14, 41 (1976)] is transformed into a set of nonlinear algebraic equations in which the number of unknowns precisely equals the number of equations. Since the Nakatsuji two-particle density equation is equivalent to the time-independent Schrodinger equation for Hamiltonians containing up to two-body interactions, the answer to the title question is affirmative, provided the equations have solutions. Practical implications of the parametrization and possible approximation schemes are briefly discussed.     

Lasing in thulium-doped polarizing photonic crystal fiber

We describe lasing of a thulium-doped polarizing photonic crystal fiber. A 4 m long fiber with 50 μm diameter core, 250 μm diameter cladding, and d/Λ ratio of 0.18 was pumped with a 793 nm diode and produced a polarized output with a polarization extinction ratio (PER) of 15 dB and an M(2) of <1.15. An intracavity polarizer and half-wave plate minimally increased the PER to 16 dB. The output power had 35% slope efficiency relative to the absorbed pump power. The maximum cw output power was limited to 4 W due to the quantum defect heating of the fiber.