Lasing in a three-dimensional photonic crystal of the liquid crystal blue phase II

Photonic-bandgap materials, with periodicity in one, two or three dimensions, offer control of spontaneous emission and photon localization. Low-threshold lasing has been demonstrated in two-dimensional photonic-bandgap materials, both with distributed feedback and defect modes. Liquid crystals with chiral constituents exhibit mesophases with modulated ground states. Helical cholesterics are one-dimensional, whereas blue phases are three-dimensional self-assembled photonic-bandgap structures. Although mirrorless lasing was predicted and observed in one-dimensional helical cholesteric materials and chiral ferroelectric smectic materials, it is of great interest to probe light confinement in three dimensions. Here, we report the first observations of lasing in three-dimensional photonic crystals, in the cholesteric blue phase II. Our results show that distributed feedback is realized in three dimensions, resulting in almost diffraction-limited lasing with significantly lower thresholds than in one dimension. In addition to mirrorless lasing, these self-assembled soft photonic-bandgap materials may also be useful for waveguiding, switching and sensing applications.     

Dynamic Spirals of Nanoparticles in Light‐Responsive Polygonal Fields

Nanoparticles tend to aggregate once integrated into soft matter and consequently, self‐assembling nanoparticles into large‐scale, regular, well‐defined, and ultimately chiral patterns remains an ongoing challenge toward the design and realization of organized superstructures of nanoparticles. The patterns of nanoparticles that are reported in liquid crystals so far are all static, and this lack of responsiveness extends to assemblies of nanoparticles formed in topological singularities and other localized structures of anisotropic matter. Here, it is shown that gold nanoparticles form spiral superstructures in polygonal fields of cholesteric liquid crystals. Moreover, when the cholesteric liquid crystals incorporate molecular photoswitches in their composition, the pitch of the nanoparticulate spirals follows the light‐induced reorganization of the cholesteric liquid crystals. These experimental findings indicate that chiral liquid crystals can be used as chiral and dynamic templates for soft photonic nanomaterials. Controlling the geometry of these spirals of nanoparticles will ultimately allow modulating the plasmonic signature of hybrid and chiral systems.     

Light‐Driven Chiral Molecular Switches or Motors in Liquid Crystals

The ability to tune molecular self‐organization with an external stimulus is a main driving force in the bottom‐up nanofabrication of molecular devices. Light‐driven chiral molecular switches or motors in liquid crystals that are capable of self‐organizing into optically tunable helical superstructures undoubtedly represent a striking example, owing to their unique property of selective light reflection and which may lead to applications in the future. In this review, we focus on different classes of light‐driven chiral molecular switches or motors in liquid crystal media for the induction and manipulation of photoresponsive cholesteric liquid crystal systems and their consequent applications. Moreover, the change of helical twisting powers of chiral dopants and their capability of helix inversion in the induced cholesteric phases are highlighted and discussed in the light of their molecular geometric changes.     

Helix Inversion Controlled by Molecular Motors in Multistate Liquid Crystals

Unravelling the rules of molecular motion is a contemporary challenge that promises to support the development of responsive materials and is likely to enhance the understanding of functional motion. Advances in integrating light-driven molecular motors in soft matter have led to the design and realization of chiral nematic (cholesteric) liquid crystals that can respond to light with modification of their helical pitch, and also with helix inversion. Under illumination, these chiral liquid crystals convert from one helical geometry to another. Here, a series of light-driven molecular motors that feature a rich configurational landscape is presented, specifically which involves three stable chiral states. The succession of chiral structures involved in the motor cycle is transmitted at higher structural levels, as the cholesteric liquid crystals that are formed can interconvert between helices of opposite handedness, reversibly. In these materials, the dynamic features of the motors are thus expressed at the near-macroscopic, functional level, into addressable colors that can be used in advanced materials for tunable optics and photonics.     

Visible‐Light‐Induced Self‐Organized Helical Superstructure in Orientationally Ordered Fluids

Light‐induced phenomena occurring in nature and in synthetic materials are fascinating and have been exploited for technological applications. Here visible‐light‐induced formation of a helical superstructure is reported, i.e., a cholesteric liquid crystal phase, in orientationally ordered fluids, i.e., nematic liquid crystals, enabled by a visible‐light‐driven chiral molecular switch. The cyclic‐azobenzene‐based chiral molecular switch exhibits reversible photoisomerization in response to visible light of different wavelengths due to the band separation of n–π* transitions of its trans‐ and cis‐isomers. Green light (530 nm) drives the trans‐to‐cis photoisomerization whereas the cis‐to‐trans isomerization process of the chiral molecular switch can be caused by blue light (440 nm). It is observed that the helical twisting power of this chiral molecular switch increases upon irradiation with green light, which enables reversible induction of helical superstructure in nematic liquid crystals containing a very small quantity of the molecular switch. The occurrence of the light‐induced helical superstructure enables the formation of diffraction gratings in cholesteric films.     

Wide‐Range Tuning and Enhancement of Organic Long‐Persistent Luminescence Using Emitter Dopants

Most long‐persistent luminescent (LPL) materials, which slowly release energy absorbed from ambient light, are based on inorganic compounds. Organic long‐persistent luminescent (OLPL) systems have advantages over inorganic LPL materials in terms of solubility, transparency, and flexibility. Here, the characteristics of OLPL emission are improved by doping emitter molecules into an OLPL matrix. Greenish‐blue to red and even warm white emission are achieved by energy transfer from exciplex in the OLPL matrix to the emitter dopants. The dopants also improve brightness and emission duration through efficient radiative decay and the trapping of electrons, respectively. This technique will enable the development of a wide range of organic glow‐in‐the‐dark paints.     

Circularly polarized laser emission induced by supramolecular chirality in cholesteric liquid crystals

This paper describes the circularly polarized spectroscopic studies on absorption and emission of an achiral fluorescent dye embedded in cholesteric liquid crystals (CLCs). Optical excitation of the dye-doped CLC cell with a linearly polarized laser brought about the two laser emission peaks at longer and shorter reflection band edges of the CLC host through the internal laser feedback effect of the one-dimensional CLC photonic band-gap. At this stage, the optically excited laser emissions showed circularly polarized characteristic, even though the excitation beam was linearly polarized. The circularly polarized direction of the laser emission was determined by molecular chirality of only few mol% of the enantiomeric chiral dopant in this molecular system.     

Polarization characteristics of phase retardation defect mode lasing in polymeric cholesteric liquid crystals

We have studied the lasing characteristics of a dye-doped nematic layer sandwiched by two polymeric cholesteric liquid crystal (CLC) films as photonic band gap (PBG) materials. The nematic layer acts as a defect layer, the anisotropy of which brings about the following remarkable optical characteristics: (1) reflectance in the PBG region exceeds 50% due to the retardation effect, being unpredictable from a single CLC film; (2) efficient lasing occurs either at the defect mode wavelength or at the photonic band edge; and (3) the lasing emission due to both the defect mode and the photonic band edge mode contains both right- and left-circular polarizations, while the lasing emission from a dye-doped single CLC layer with a left-handed helix is left-circularly polarized.     

Optically Rewritable Transparent Liquid Crystal Displays Enabled by Light‐Driven Chiral Fluorescent Molecular Switches

Functional soft materials exhibiting distinct functionalities in response to a specific stimulus are highly desirable towards the fabrication of advanced devices with superior dynamic performances. Herein, two novel light‐driven chiral fluorescent molecular switches have been designed and synthesized that are able to exhibit unprecedented reversible Z/E photoisomerization behavior along with tunable fluorescence intensity in both isotropic and anisotropic media. Cholesteric liquid crystals fabricated using these new fluorescent molecular switches as chiral dopants exhibit reversible reflection color tuning spanning the visible and infrared region of the spectrum. Transparent display devices have been fabricated using both low chirality and high chirality cholesteric films that operate either exclusively in fluorescent mode or in both fluorescent and reflection mode, respectively. The dual mode display device employing short pitch cholesteric film is able to function on demand under all ambient light conditions including daylight and darkness with fast response and high resolution. Moreover, the proof‐of‐concept for a “remote‐writing board” using cholesteric films containing one of the light‐driven chiral fluorescent molecular switches with ease of fabrication and operation is disclosed herein. Such optically rewritable transparent display devices enabled by light‐driven chiral fluorescent molecular switches pave a new way for developing novel display technology under different lighting conditions.     

Nonlinear transmission and excited-state absorption in fluorene derivatives

The nonlinear transmission and the excited-state absorption spectra of three fluorene derivatives exhibiting large two-photon absorptivity were measured by the third harmonic of a picosecond Nd:YAG laser. We analyzed their capability for exhibiting stimulated emission in polar solvents and found that asymmetrical fluorene compounds with a diphenylamino substituent exhibited large Stokes shifts (approximately 8000 cm(-1)), high quantum yields (approximately 0.9-1.0), and no optical gain over their entire fluorescence spectral region. In contrast, a symmetrical fluorene derivative with vinylphenylbenzothiazole substituents in positions 2 and 7 underwent lasing under one-photon excitation by use of picosecond pulsed irradiation.     

Liquid Crystals for Charge Transport,Luminescence, and Photonics

Ordered molecular materials are increasingly used in active electronic and photonic organic devices. In this progress report we discuss whether the self‐assembling properties and supramolecular structures of liquid crystals can be tailored to improve such devices. Recent developments in charge‐transporting and luminescent liquid crystals are discussed in the context of material requirements for organic light‐emitting devices, photovoltaics, and thin film transistors. We identify high carrier mobility, polarized emission, and enhanced output‐coupling as the key advantages of nematic and smectic liquid crystals for electroluminescence. The formation of anisotropic polymer networks gives the added benefits of multilayer capability and photopatternability. The anisotropic transport and high carrier mobilities of columnar liquid crystals make them promising candidates for photovoltaics and transistors. We also outline some of the issues in material design and processing that these applications demand. The photonic properties of chiral liquid crystals and their use as mirror‐less lasers are also discussed.     

Helical twisting power of three-ring chiral molecules and polymerization in cholesteric electrolyte solutions

A series of chiral three-ring type compounds with rigid shape was employed as chiral inducers for induction of chiral cholesteric liquid crystal (cholesteric LC) from achiral nematic LC. Helical twisting power of the chiral compounds was estimated with the Cano wedge method. Cholesteric LC electrolyte solution was prepared by adding the chiral compounds. Subsequently, polymerization in the cholesteric LC was carried out to produce chiroptically active polymer films. This method is different from conventional methods for synthesizing chiral polymers because neither chiral monomers nor asymmetric catalysts are employed. Surface structure and optical properties of the polymer thus prepared were examined.     

Cholesteric Liquid Crystals for Color Information Technology

Fixation of the molecular ordering of cholesteric liquid crystals is essential for use of these materials in color information technology. Methods to achieve this as well as thermal and photochemical control of the cholesteric pitch of the liquid crystals are addressed and applications in rewritable color recording are highlighted. The Figure shows a sample recorded using a laser.     

Synthesis and properties of trifluoromethylated chiral dopants for ferroelectric liquid crystals

The synthesis and specific properties of a new family of chiral dopants for ferroelectric liquid crystals, which are derived from a series of trifluorinated dihydrofuranones and tetrahydrofurans, are described. On the basis of the structural features of designed materials and the MOPAC-PM 3 calculations, a discussion of the response time-structure relationships is given. Optically active trifluoromethylated materials with a tetrahydrofuran tail unit were found to be superior chiral dopants for preparing ferroelectric liquid crystalline compositions.     

Tunable lasing from a cholesteric liquid crystal film embedded with a liquid crystal nanopore network

Continuous tuning of lasing wavelength is achieved in cholesteric liquid crystal lasers by embedding a network of nanopores with an average size of 10 nm filled with liquid crystals inside a polymerized matrix with helical order. The device possesses both high transparency and a fast response time because the tuning is driven by local reorientation of the liquid crystal molecules in the nanopores.     

Defects in liquid crystals (II)

Defects in the disordered (uniaxial) liquid crystals, nematic, smecticA and cholesteric, and the use of topological analysis in classifying them, are discussed. While the latter is very successful in classifying defects in nematics, it fails to do so in the case of smecticA and cholesteric liquid crystals because of geometrical constraints. However, topological arguments have been partially successful in predicting some of the defects in cholesterics. The known features of the isotropic (cubic and amorphous) cholesteric blue phases are summarised and the various theoretical models picturing them as defect lattices, are also discussed briefly.     

Hydrodynamics of cholesteric liquid crystals in the coarse-grained limit

The hydrodynamical behaviour of cholesteric liquid crystals has been considered in the limit of low amplitude and low frequency distortions and motions. It is shown that there are interesting analogies with superfluid-hydrodynamics, such as the fountain effect, thermal superconductivity and temperature wave propagation. In certain situations, there is an unusual formation of a boundary layer at low velocities, and in certain others the properties resemble those of percolation in porous media. Results concerning some special phenomena peculiar to cholesteric liquid crystals are also presented. Finally it is pointed out that there should be two types of second sound in chiral smectic C.     

Nature-Inspired Emerging Chiral Liquid Crystal Nanostructures: From Molecular Self-Assembly to DNA Mesophase and Nanocolloids

Liquid crystals (LCs) are omnipresent in living matter, whose chirality is an elegant and distinct feature in certain plant tissues, the cuticles of crabs, beetles, arthropods, and beyond. Taking inspiration from nature, researchers have recently devoted extensive efforts toward developing chiral liquid crystalline materials with self-organized nanostructures and exploring their potential applications in diverse fields ranging from dynamic photonics to energy and safety issues. In this review, an account on the state of the art of emerging chiral liquid crystalline nanostructured materials and their technological applications is provided. First, an overview on the significance of chiral liquid crystalline architectures in various living systems is given. Then, the recent significant progress in different chiral liquid crystalline systems including thermotropic LCs (cholesteric LCs, cubic blue phases, achiral bent-core LCs, etc.) and lyotropic LCs (DNA LCs, nanocellulose LCs, and graphene oxide LCs) is showcased. The review concludes with a perspective on the future scope, opportunities, and challenges in these truly advanced functional soft materials and their promising applications.     

Controlled synthesis of carbon-based alumina nanophosphors with tunable blue-green luminescence

Carbon-based alumina nanophosphors were prepared by a facile liquid process. The nanophosphors showed high photoluminescence properties, and tunable color emission from blue to green. The luminescent intensity and color were controlled by simply manipulating of polyethylene glycol and urea amounts, respectively. Oxygen deficiency, surface defects, and carboxylic impurities are considered to be possible emission mechanisms.     

Thermally tunable random laser in dye-doped liquid crystals

A thermally tunable random laser in dye-doped liquid crystals (DDLCs) is reported. The gain medium of PM597 dye-doped E7 nematic LC is injected into a glass cell. The experimental results show that the random lasing is still obtained when the cell temperature becomes higher, even above the nematic–isotropic transition, and that its polarization changes at the same time. Temperature has little effect on the full width at half maxiumum of the random lasing. The center wavelength of the random lasing shifts from 575.69?nm to 593.43?nm when the temperature increases from 25.5°C to 148°C. Meanwhile, a random laser based on a solution of laser dye is first reported in this article. The reasons are possibly that nanoparticles consisting of dye molecules provide a new scattering mechanism in both solution and isotropic phase.