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An empty polymeric structure has been realized by combining a high precision level optical holographic setup and a selective microfluidic etching process. The distinctive features of the realized periodic microstructure enabled aligning several kinds of liquid crystal (LC) compounds, without the need of any kind of surface chemistry or functionalization. In particular, it has been possible to exploit light sensitive LCs for the fabrication of all-optical devices, cholesteric and ferroelectric LCs for ultrafast electro-optical switches, and a common LC for a two-dimensional periodic structure with high anisotropy. All-optical and electro-optical experiments, performed for investigating the samples in terms of switching voltages and response times, confirm good performances of the realized devices.

Common fluids cannot sustain static mechanical stresses at the macroscopic scale because they lack molecular order. Conversely, crystalline solids exhibit long-range order and mechanical strength at the macroscopic scale. Combining the properties of fluids and solids, liquid crystal films respond to mechanical confinement by both flowing and generating static forces. The elastic response, however, is very weak for film thicknesses exceeding 10 nm. In this study, the mechanical strength of a fluid film was enhanced by introducing topological defects in a cholesteric liquid crystal, producing unique viscoelastic and optomechanical properties. The cholesteric was confined under strong planar anchoring conditions between two curved surfaces with sphere–sphere contact geometry similar to that of large colloidal particles, creating concentric dislocation loops. During surface retraction, the loops shrank and periodically disappeared at the surface contact point, where the cholesteric helix underwent discontinuous twist transitions, producing weak oscillatory surface forces. On the other hand, new loop nucleation was frustrated by a topological barrier during fluid compression, creating a metastable state. This generated exceptionally large forces with a range exceeding 100 nm as well as extended blueshifts of the photonic bandgap. The metastable cholesteric helix eventually collapsed under a high compressive load, triggering a stick-slip–like cascade of defect nucleation and twist reconstruction events. These findings were explained using a simple theoretical model and suggest a general approach to enhance the mechanical strength of one-dimensional periodic materials, particularly cholesteric colloid mixtures. [ABSTRACT FROM AUTHOR]

Spontaneous breaking of symmetry in liquid crystal (LC) films often reveals itself as a microscopic pattern of molecular alignment. In a smectic-A LC, the emergence of positional order at the transition from the nematic phase leads to periodic textures that can be used as optical microarrays, templates for soft lithography, and ordering matrices for the organization and manipulation of functional nanoparticles. While both 1d and 2d patterns have been obtained as a function of the LC film thickness and applied fields, the connection has not been made between pattern formation and the peculiar critical behavior of LCs at the nematic–smectic transition, still eluding a comprehensive theoretical explanation. In this article, we demonstrate that an intense bend distortion applied to the LC molecular director while cooling from the nematic phase produces a frustrated smectic phase with depressed transition temperature, and the characteristic 1d periodic texture previously observed in thin films and under applied electric fields. In light of De Gennes' analogy with the normal-superconductor transition of a metal, we identify the 1d texture as the equivalent of the intermediate state in type I superconductors. The bend distortion is analog to the magnetic field in metals and penetrates in the frustrated phase as an array of undercooled nematic domains, periodically intermixed with bend-free smectic-A domains. Our findings provide fundamental evidence for theories of the nematic–smectic transition, highlighting the deep connection between phase frustration and pattern formation, and perspectives on the design of functional smectic microarrays. [ABSTRACT FROM AUTHOR]

Using nematic liquid crystal cells aligned by conductive polymers, like polyaniline or doped polypyrrole, a very fast electro-optic response is observed. We show that when a switch that interrupts the voltage across the cell is placed between the cell and the ground, the largest voltage drop is on the switch in its open position. The voltage distribution among the nematic cell and the switch is evaluated. The role played in this very fast electro-optic response of the ionic charges built on the interfaces together with the redox processes among the free charges in the polymer-liquid crystal interface is also described. For an asymmetric cell (only one side covered with a conductive polymer) a rectifying effect appears. In this case a circuital model is used to mimic the steplike behavior of the transmitted light during the relaxation of the system. [ABSTRACT FROM AUTHOR]

We present a review of polymer-liquid crystal-based devices for optical applications. Starting from a particular fabrication technique, which enables to obtain the POLICRYPS (POlymer LIquid CRYstal Polymer Slices) structure, we illustrate different realizations, along with their working principle and main features and performances. The name POLICRYPS indicates a structure made of parallel slices of pure polymeric material alternated to films of well-aligned nematic liquid crystal (NLC), with a spatial periodicity that can be settled in the range 0.2÷15 μm. Suitably designed samples can be utilized as optical devices with a high efficiency, which can be switched on and off both by applying an electric field of a few V/μm or by irradiating samples with a suitable light beam. In different geometries, POLICRYPS can be specialized to operate as switchable diffraction grating, switchable optical phase modulator, switchable beam splitter, or tunable Bragg filter. The POLICRYPS framework can be also used as a soft matter template for aligning different types of LCs or to create an array of tunable microlasers. Finally, we present a POLICRYPS structure with a polar symmetry of the director alignment, which enables local shaping of light polarization, allowing to convert circularly polarized beams into cylindrical vector beams. [ABSTRACT FROM AUTHOR]

Localized plasmon resonance (LPR) of noble Metal Nanoparticles (MNPs) opens up a new horizon for nanoscale materials able to convert light into heat, since the strong electric field generated around the MNPs can transform them into original heat nanosources. Thus, investigation of the heat transport mechanism, from the heated MNPs to their surrounding medium, is fundamental for realizing applications in nanotechnology and thermal-based therapies, and a challenge is definitely represented by the possibility of measuring temperature variations at the surface of the MNPs undergoing optical illumination. In this framework, we show that an ingenious combination of characteristics of short pitch liquid crystalline compounds and MNPs has demonstrated effective to provide an advanced tool to monitor nanoscale temperature variations. [ABSTRACT FROM PUBLISHER]

Large-area periodic defect patterns are produced in smectic A liquid crystals confined between rigid plate electrodes that impose conflicting parallel and normal anchoring conditions, inducing the formation of topological defects. Highly oriented stripe patterns are created in samples thinner than 2 μm due to self-assembly of linear defect domains with period smaller than 4 μm, whereas hexagonal lattices of focal conic domains appear for thicker samples. The pattern type (1d/2d) and period can be controlled at the nematic-smectic phase transition by applying an electric field, which confines the defect domains to a thin surface layer with thickness comparable to the nematic coherence length. The pattern morphology persists in the smectic phase even after varying the field or switching it off. Bistable, non-equilibrium patterns are stabilized by topological constraints of the smectic phase that hinder the rearrangement of defects in response to field variations. [ABSTRACT FROM AUTHOR]

In this paper we report on the realization and characterization of a polymer template sculptured in photosensitive material on a chemical inert surface, devoted to micro/nano-confinement of a wide range of organic components, with self-arrangement properties at the nanoscale. The high quality morphology of the polymeric micropattern arrays is obtained by combining a nano-precision level optical holographic setup and a multi-step chemico-physical process. The general purpose template represents the basic platform to be filled with different soft composite materials: Due to their self organization capabilities, light responsive Liquid Crystals (LC) and short pitch Cholesteric LC have been investigated. [ABSTRACT FROM PUBLISHER]

The chiral flexo-electro-optic effect produces a sub-millisecond, temperature independent in-plane rotation of the optical axis and is potentially interesting for the display industry. The main drawback in the exploitation of this effect is that it relies on a texture, the Uniform Lying Helix (ULH), which is intrinsically unstable since neither planar nor homeotropic surface conditions are compatible with it. We present a method, based on the use of periodic polymeric micro-channels, to create highly ordered and stable ULH structures. We show that the periodic structure, which is created holographically, naturally aligns the cholesteric helical superstructure along the micro-channels, without requiring any elaborate ad-hoc procedure, even when the size Λ of the micro-channels is much larger than the pitch P (Λ > 20P). Electro-optic measurements performed on the test-device show a large contrast ratio between bright and dark states (better then 100:1), short switching time (200μs) and large optical rotation (>30°). [ABSTRACT FROM AUTHOR]

Thin films of lead zirconate titanate (PZT) were obtained by a modified sol-gel route on float glass and indium tin oxide (ITO)-covered float glass substrates. Different thermal treatments were performed on the deposited films in the temperature range 100-700°C. Spectroscopic ellipsometry was used to investigate the optical properties of the deposited films, and the changing optical absorption spectra were interpreted in terms of the growth of two different crystal phases, pyrochlore and ferroelectric perovskite, as a function of annealing temperature. Moreover, a specific resonance at 1.9 nm was detected when thin PZT films are deposited on ITO substrates and was attributed to a particular charge distribution at the interface. Finally, the performance in rectifying the electro-optical response of asymmetric nematic liquid crystal cells was tested for some of the films undergoing different thermal treatments. [ABSTRACT FROM AUTHOR]

Quasicontinuous tuning of a dye doped cholesteric liquid crystal (CLC) mirrorless laser in the ultraviolet-visible wavelength range is demonstrated using a single original device based on CLC as resonator and several resonant dyes. The thought is to combine the CLC pitch gradient and the distribution of different dyes. In the same cell, six dyes are combined in order to nearly cover the whole wavelength range from ultraviolet (370 nm) to red (680 nm). Some of the used dyes work as emitter, while others work in the Förster regime to decouple the excitation and emission processes. The relevant aspect of the device is that a simple translation of the cell respect to the same pump beam enables fine tuning of the laser wavelength in almost all the visible range, up to the ultraviolet. [ABSTRACT FROM AUTHOR]

We report the observation of laser emission from a luminescent dye-doped cholesteric liquid crystal (CLC) excited by another dye-doped CLC laser. The idea is based on the cascade of two CLC cells containing two different dyes where the emission band of the first overlaps the absorption band of the second. This system of low threshold mirrorless lasers emphasizes the main advantages of these organic materials for lasing applications and identifies a simple laser device. Preliminary characterizations of these CLC laser systems produce evidence of remarkable features that suggest fascinating developments. The main aspects are related to the wavelength tunability of the laser emission and to the miniaturization of the device. [ABSTRACT FROM AUTHOR]

In this letter, we report the results of phototunable lasing in dye-doped cholesteric liquid crystals (DD–CLC). Photoexcitation of DD–CLC films gives rise to laser emission in the violet-UV range. Control of the structure of the chiral dopant driven by UV phototransformation is exploited in order to obtain a permanent variation of the cholesteric pitch. Laser emission wavelength tuning, by means of photoinduced shifting of the selective reflection band of the cholesteric liquid crystals is established. A tuning interval of about 35 nm, in the wavelength range of 385–415 nm, is observed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A preface for the February 2008 issue of the journal "Molecular Crystals & Liquid Crystals" is presented.

Three photosensitive cholesteric liquid crystal mixtures have been investigated as UV sensors. Each of them is sensitive to a well definite part of the solar UV radiation spectrum, the UV A or the UV B or the UV C range, with suitable absorption spectra. The photosensitive elements in the mixtures are either photoisomerizable nematic hosts or photoisomerizable optically active dopants. The selective reflection peaks of these cholesteric mixtures are in the visible part of the light spectrum. The UV exposure changes the cholesteric pitch and hence the position of the selective reflection peak. The consequence is that the colour reflected by each mixture varies under influence of the UV and it can be used as an indicator of the UV exposure itself. We present the mechanisms of the observed effects and possible application features. [ABSTRACT FROM AUTHOR]

We have developed a noninvasive,all-optical, holographic techniquefor permanently aligning liquid crystalline DNA filaments in a microperiodictemplate realized in soft-composite (polymeric) materials. By combiningoptical intensity holography with a selective microfluidic etchingprocess, a channelled microstructure has been realized which enablesself-assembly of DNA. The striking chemicophysical properties of thestructure immobilize the DNA filaments within the microchannels withoutthe need of any kind of surface chemistry or functionalization. Polarizedoptical, confocal, and electronic microscopies have been used forcharacterizing the DNA geometry inside the microchannels in termsof birefringence, fluorescence, and nanoscale organization properties.In particular, observation of a far-field diffraction pattern confirmsa periodic organization of the DNA filaments inside the polymerictemplate. [ABSTRACT FROM AUTHOR]

Using an atomic force microscope (AFM) we confined a smectic-A liquid crystal (LC) between a flat glass plate and a 10-μm glass sphere attached to the free end of the AFM cantilever. Both surfaces were treated with a surfactant that induces normal alignment of the LC molecules. We measured the force F acting on the cantilever while varying the plate-sphere distance D with subnanometer precision. For D < 50 nm, the force was periodically oscillating and decayed as D was increased. Analyzing the force in the framework of a simple model of elastic deformation of the smectic layers, we have evaluated the undeformed layer thickness α0 and compressibility modulus B. Compared to other techniques used to determine α0 and B, AFM measurements are faster and require a much smaller amount (microliters) of LC. Moreover, they are based on purely mechanical deformations of the LC structure and do not require any static or radiative electromagnetic field. [ABSTRACT FROM AUTHOR]