The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Photoinduced motions in azo-containing polymers.

An overview on the recent developments in the field of liquid crystalline bent-core molecules (so-called banana liquid crystals) is given. After some basic issues, dealing with general aspects of the systematisation of the mesophases, development of polar order and chirality in this class of LC systems and explaining some general structure–property relationships, we focus on fascinating new developments in this field, such as modulated, undulated and columnar phases, so-called B7 phases, phase biaxiality, ferroelectric and antiferroelectric polar order in smectic and columnar phases, amplification and switching of chirality and the spontaneous formation of superstructural and supramolecular chirality.

/pdf/bent-core-liquid-crystals-polar-order-superstructural-17m3bst2sc.pdf

Bent-core liquid crystals: polar order, superstructural chirality and spontaneous desymmetrisation in soft matter systems

Structures and properties of liquid crystalline phases formed by bent-core molecules are reviewed. At least eight phases designated as B1–B8 have been found, being unambiguously distinguished from phases formed by usual calamitic molecules due to a number of remarkable peculiarities. In addition to B1–B8 phases, smectic A-like phases and biaxial nematic phases formed by bent-core molecules are also reviewed. The most attractive aspects of this new class of liquid crystals are in polarity and chirality, despite being formed from achiral molecules. The bent-core mesogens are the first ferroelectric and antiferroelectric liquid crystals realized without introducing chirality. Spontaneous chiral deracemization at microscopic and macroscopic levels occurs and is controllable. Moreover, achiral bent-core molecules enhance system chirality. The interplay between polarity and chirality provides chiral nonlinear optic effects. Further interesting phenomena related to polarity and chirality are also reviewed.

https://iopscience.iop.org/article/10.1143/JJAP.45.597/pdf

Bent-Core Liquid Crystals: Their Mysterious and Attractive World

Light-driven phenomena both in living systems and nonliving materials have enabled truly fascinating and incredible dynamic architectures with terrific forms and functions. Recently, liquid crystalline materials endowed with photoresponsive capability have emerged as enticing systems. In this Review, we focus on the developments of light-driven liquid crystalline materials containing photochromic components over the past decade. Design and synthesis of photochromic liquid crystals (LCs), photoinduced phase transitions in LC, and photoalignment and photoorientation of LCs have been covered. Photomodulation of pitch, polarization, lattice constant and handedness inversion of chiral LCs is discussed. Light-driven phenomena and properties of liquid crystalline polymers, elastomers, and networks have also been analyzed. The applications of photoinduced phase transitions, photoalignment, photomodulation of chiral LCs, and photomobile polymers have been highlighted wherever appropriate. The combination of photoc...

Light-Driven Liquid Crystalline Materials: From Photo-Induced Phase Transitions and Property Modulations to Applications.

The behaviour of the anisotropic electrical conductivity of liquid crystal–gold nanoparticle (LC‐GNP) composites consisting of a commercially available room temperature nematic compound doped with alkylthiol‐capped GNPs has been investigated. The nematic–isotropic transition of the composite decreases nearly linearly with increasing X, the concentration of GNP (in weight %) at a rate of about 1°C /weight %. The inclusion of GNPs increases the electrical conductivity of the system with the value increasing by more than two orders of magnitude for X = 5%. However, the anisotropy in conductivity, defined as the ratio of the conductivity along (σ∥) and orthogonal (σ⊥) to the director shows a much smaller but definite decrease as X increases.

Electrical conductivity and dielectric constant measurements of liquid crystal–gold nanoparticle composites

A "peelable banana" is formed when a bent-core molecule is linked to a rodlike mesogen through a flexible aliphatic spacer. This is an appropriate description of this novel low-molar-mass organic system, which displays a transition from a biaxial nematic (N b ) phase to a biaxial smectic A phase. The illustration gives a schematic representation of the dimeric molecules in the N b phase as well as the corresponding textural and conoscopic patterns obtained.

A Low‐Molar‐Mass, Monodispersive, Bent‐Rod Dimer Exhibiting Biaxial Nematic and Smectic A Phases

Although the liquid-crystal research is well established in science, there are newly emerging exciting systems, that deserve extensive basic studies One of these areas is the research of the bent-shaped molecules (so-called "banana liquid crystals"), which have delicate chirality and polarity properties In this paper we show that these materials also have very unusual rheological features, such as the formation of stable fluid fibers and bridges Under electric fields, these objects present striking mechanical effects, such as horizontal and transversal vibrations Studies indicate that the research of banana-liquid-crystal fibers may lead to new type of artificial muscle systems

Liquid crystal fibers of bent-core molecules.

Evidence of a first-order smectic-A--smectic-${C}^{\mathrm{*}}$ (SmA-Sm${C}^{\mathrm{*}}$) transition is presented. Detailed x-ray studies on 4-(3-methyl-2-chloropentanoyloxy)4'-heptyloxybiphenyl show that the SmA-Sm${C}^{\mathrm{*}}$ transition in this material is characterized by a discontinuous jump in the layer spacing (and hence in the tilt angle) and by a narrow two-phase region in which the density modulations due to the smectic-A and the smectic-${C}^{\mathrm{*}}$ phases coexist. Addition of a second compound drives the SmA-Sm${C}^{\mathrm{*}}$ transition towards second order.

Evidence of a first-order smectic-A-smectic-C transition and its approach to tricritical behavior.

Controllable manipulation of self-organized dynamic superstructures of functional molecular materials by external stimuli is an enabling enterprise. Herein, we have developed a thermally driven, self-organized helical superstructure, i.e., thermoresponsive cholesteric liquid crystal (CLC), by integrating a judiciously chosen thermoresponsive chiral molecular switch into an achiral liquid crystalline medium. The CLC in lying state, in both planar and twisted nematic cells, exhibits reversible in-plane orthogonal switching of its helical axis in response to the combined effect of temperature and electric field. Consequently, the direction of the cholesteric grating has been observed to undergo 90° switching in a single cell, enabling non-mechanical beam steering along two orthogonal directions. The ability to reversibly switch the cholesteric gartings along perpendicular directions by appropriately adjusting temperature and electric field strength could facilitate their applications in 2D beam steering, spectrum scanning, optoelectronics and beyond.

Geetha G. Nair

Papers

Electrical conductivity and dielectric constant measurements of liquid crystal–gold nanoparticle composites

A Low‐Molar‐Mass, Monodispersive, Bent‐Rod Dimer Exhibiting Biaxial Nematic and Smectic A Phases

Liquid crystal fibers of bent-core molecules.

Evidence of a first-order smectic-A-smectic-C transition and its approach to tricritical behavior.

Dynamic Orthogonal Switching of a Thermoresponsive Self-Organized Helical Superstructure