Lasing from chiral photonic band gap materials based on cholesteric glasses

Glass-forming cholesteric liquid crystals were studied as promising dye-doped lasing materials at different pumping energies and temperatures. Cholesteric samples doped with laser dye pyrromethane 597 were pumped by Nd:YAG laser. Lasing was found to depend strongly on the vitrification rate of cholesteric samples, their temperature and multidomain structure. The lasing threshold and intensity as a function of thickness of cholesteric resonator are determined by two competing factors: narrowing of the band edge modes and increasing disorder.     

Laser emission at the second-order photonic band gap in an electric-field-distorted cholesteric liquid crystal

ABSTRACT

We study the optical properties of a cholesteric liquid crystal doped with a fluorescent dye in the regime of highly distorted helix without full helix unwinding. The distortion was achieved by applying a pulsed AC electric field, perpendicular to the helix axis. If the pulse is in the millisecond range, the helix is deformed but keeps its original pitch even for electric fields higher than the theoretical critical field for helix unwinding. In this field regime, very pronounced high-order photonic band gaps are observed, in agreement with our calculations. We theoretically explore the possibility of obtaining viable laser emission at the second-order photonic band gap, and experimentally find that lasing is not only possible but has a figure of merit similar to that of the usual laser at the main-gap region. Therefore, electric-field-induced high-order photonic band gaps are potentially useful for multiline laser applications.     

Thermal tuning band gap in cholesteric liquid crystals

The phase of a liquid crystal (LC) changing from a nematic phase to a cholesteric (Ch) mesophase is achieved by adding different ratios of chiral dopants S811. By studying the transmission spectrum, we are able to measure the helical pitch in cholesteric phase. The pitch in the mixtures of nematic E7 and chiral dopants S811 as a function of the concentration of the dopant and temperature is investigated. The sensitivity of the selective reflection notch of the cholesteric phase to the thermal tuning depends strongly on the ratios of the chiral dopants. It reveals that the influence of temperature is more profound for those cholesteric liquid crystals (CLCs) which exhibit smectic A (SmA) at lower temperatures. When fitted using Keating's formula, the helical pitch calculated from our experimental results lies on the predicted curve. Optimised ratios of the mixture CLCs for the optimised reflection band with the specified wavelength ranging from 467 nm to 2123 nm are suggested.     

Tunable photonic band gap crystals based on a liquid crystal-infiltrated inverse opal structure

Composite materials comprised of nematic liquid crystals (LCs) and SiO(2) inverse opal films were fabricated. Their optical properties were quite different from those of inverse opal films without the LCs. The optical properties could be controlled by changing the refractive indices of the LCs, which vary with orientation, phase, and temperature. In particular, the optical properties were drastically changed by thermal or photoinduced isothermal phase transitions of the LCs. This means that the photonic band structure could be controlled, and tunable photonic crystals have been achieved, based on the inverse opal structure. The mechanism of this change was investigated by the evaluation of the effective refractive indices. As a result, it was found that the change in optical properties was derived from the orientation of the LC molecules in the voids in the inverse opal film. Furthermore, once the mechanism was understood, it was also possible to control the position of the reflection peak by changing the alignment of the LCs. Such materials have the possibility for practical use in optical devices and fundamental research systems.     

Tunable chiral materials for multicolour reflective cholesteric displays

A polymerizable tunable chiral material (TCM) has been prepared for the fabrication of multicolour reflective cholesteric displays. This photosensitive chiral material, whose chirality is adjustable upon UV irradiation, enables us to adjust the pitch of a cholesteric material and thus to produce the three reflected colours (red, green and blue) for a multicolour reflective cholesteric display. Furthermore, the possibility of linking this compound to a polymer network helped to solve the problem of colour diffusion. Reflection spectra of the corresponding cells show broad reflection peaks, because of scattering from the large amount of polymerizable compound. We also report the difference of response under UV irradiation between this polymerizable tunable chiral material and a non-polymerizable material.     

Full-field photonic biosensors based on tunable bio-doped sol-gel glasses

A full-field generic photonic biosensor approach, which relies on a bio-doped polymeric strip waveguide configuration, is described. We show the potential of tailor-made hybrid polymeric materials prepared by sol-gel technology for the fabrication of ultra-compact biosensor devices, where both the transducer and the recognition elements are merged into one single microstructure. Such devices were fabricated by micromolding in capillaries (MIMIC) soft lithographic technique. In contrast to evanescent field sensors, the sensor response does not only rely on the interaction of the evanescent wave with the recognition element, but on the interaction of the whole field, thus enabling a reduction of the sensor dimensions and/or a decrease of its limit of detection (LOD). The potential of this generic approach was demonstrated by developing a biosensor for the detection of H(2)O(2) using horseradish peroxidase (HRP) as the doping agent. Solutions containing 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic) acid (ABTS) and different concentrations of H(2)O(2) were dispensed over the waveguide and the green-coloured cation radical ABTS*(+) product was mainly obtained inside the photonic structure, resulting in a maximum absorption increase of 2.5 a.u. at a set working wavelength of 670 nm over the H(2)O(2) concentration range studied. The sensor exhibited a sensitivity of (3.1+/-0.2) x 10(3) a.u./mol L(-1) and a limit of detection (LOD) of 4.4 x 10(-5) mol L(-1) H(2)O(2). These results anticipate that full-field waveguide microstructures based on bio-doped sol-gel polymers will enable the fabrication of cost-effective photonic biosensors. Moreover, the ease of fabrication by a soft lithography technique and the use of such polymeric materials are fully compatible with their integration in compact automatic analysis systems.     

Focused ion beam micromachining and computation of polymeric photonic band gap

This article introduces a new approach to the achievement of low cost devices for optical telecommunications. For this purpose, the implementation of polymer components and of the WDM method show several advantages. A polymeric photonic band gap lattice can effectively assume demultiplexing at the wavelength of 1.55 μm without energy dissipation. First, the computation of the geometrical parameters designing a polymer photonic crystal by means of the finite‐difference‐time‐domain method, implemented with Bloch functions, is presented here. Second, the achievement of 2D polymer lattices by a dry process, the focused ion beam milling, is exposed. The experimental conditions for the achievement of a master are discussed on the basis of the milling performances and of the polymers physical properties. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2993–3002, 2007     

Photoswitching properties of photonic band gap material containing azo-polymer liquid crystal

A photochemically tunable photonic crystal was prepared by infiltration of the polymer liquid crystal (LC) having azo-chromophores in a SiO₂ inverse opal structure. The SiO₂ inverse opal film infiltrated with the polymer LC reflected a light corresponding to the periodicity as well as the refractive indices of the inverse opal structure. Linearly polarized light irradiation caused the shift of the reflection band to longer wavelength more than 15 nm. This is caused by the formation of uniaxially anisotropic molecular orientation of the polymer LC. The switched state was stable under interior light, and reversible switching of the reflection band can be achieved by the linearly and circularly polarized light irradiation. This photoswitching property will be suitable not only for various optical materials, but also for introduction of defects in the photonic crystals.     

Optoelectronic switches based on wide band gap semiconductors

Switching of photocurrent direction in semiconducting systems upon changes of the electrode potential or incident light wavelength was realized by a series of photoelectrodes covered with titania modified with pentacyanoferrate complexes, [Fe(CN)(5)L](n)(-) (L = NH(3), thiodiethanol, thiodipropanol). These materials were characterized by optical spectroscopy and electrochemistry. The structure of the surface complexes was modeled using simple quantum-chemical models. The electrodes described in this paper enable control of the photocurrent direction by two stimuli: Changing the wavelength or the photoelectrode potential easily switches the direction of photocurrent. The materials are different from those of similar characteristics studied by other authors: They are not composites comprising of two types of semiconductors but rather engineered uniform materials. The photocurrent switching phenomenon is an intrinsic feature resulting from a specific electronic structure of the surface-modified semiconductor.     

Anchoring effects on the electrically controlled optical band gap in twisted photonic liquid crystals

We study a one-dimensional twisted photonic liquid crystal (TPLC), consisting of various nematic liquid crystal cells adopting a twisted configuration intercalated by isotropic dielectric layers, submitted to a dc electric field (E^dc ) aligned along the periodicity axis. We write the corresponding Euler–Lagrange equations describing the nematic layer configuration. By assuming arbitrary anchoring quasi-planar boundary conditions, we calculate the equilibrium textures for the nematic, parametrized by the two types of strength of its interaction (polar and azimuthal) with the plane walls. We write the electromagnetic equations in a 4?×?4 matrix representation and using the transfer matrix formalism, we obtain the transmittance and reflectance coefficients for normal incidence as functions of the external electric field and anchoring strengths. We have observed a remarkable dependence of the electric field on the transmission and reflection spectra in opening and closing band gaps.     

Composite materials based on fluoropolymers and oxyfluoride glasses

It was shown that molded specimens of polymer composite materials can be obtained by an extrusion method by melt blending of Fluoroplast F-2 MB (modified poly(vinylidene difluoride)) and oxyfluoride glasses of the composition 3B₂O₃ (40SnF₂–30SnO–30P₂O₅). The compositions of the observed phases of the composites were determined. Conclusions were made on the incompatibility of the components, their dispersion distribution, and strong adhesion interaction. Data on the nano level of the blending of the components were obtained. The elongation and Brinell hardness were measured in the composites with various (0–50 vol %) oxyfluoride contents. It was concluded that it is possible to produce composites based on fluorinated hydrocarbon and fluoroxide polymers.     

Recent progress in chiral photonic band‐gap liquid crystals for laser applications

This article describes a brief review of recent research advances in chiral liquid crystals (CLCs) for laser applications. The CLC molecules have an intrinsic capability to spontaneously organize supramolecular helical assemblages consisting of liquid crystalline layers through their helical twisting power. Such CLC supramolecular helical structures can be regarded as one‐dimensional photonic crystals (PhCs). Owing to their supramolecular helical structures, the CLCs show negative birefringence along the helical axis. Selective reflection of circularly polarized light is the most unique and important optical property in order to generate internal distributed feedback effect for optically‐excited laser emission. When a fluorescent dye is embedded in the CLC medium, optical excitation gives rise to stimulated laser emission peak(s) at the band edge(s) and/or within the CLC selective reflection. Furthermore, the optically‐excited laser emission peaks can be controlled by external stimuli through the self‐organization of CLC molecules. This review introduces the research background of CLCs carried out on the PhC realm, and highlights intriguing precedents of various CLC materials for laser applications. It would be greatly advantageous to fabricate active CLC laser devices by controlling the supramolecular helical structures. Taking account of the peculiar features, we can envisage that a wide variety of supramolecular helical structures of CLC materials will play leading roles in next‐generation optoelectronic molecular devices. © 2010 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.201000013     

A class of supported membranes: formation of fluid phospholipid bilayers on photonic band gap colloidal crystals

We report the formation of a new class of supported membranes consisting of a fluid phospholipid bilayer coupled directly to a broadly tunable colloidal crystal with a well-defined photonic band gap. For nanoscale colloidal crystals exhibiting a band gap at the optical frequencies, substrate-induced vesicle fusion gives rise to a surface bilayer riding onto the crystal surface. The bilayer is two-dimensionally continuous, spanning multiple beads with lateral mobilities which reflect the coupling between the bilayer topography and the curvature of the supporting colloidal surface. In contrast, the spreading of vesicles on micrometer scale colloidal crystals results in the formation of bilayers wrapping individual colloidal beads. We show that simple UV photolithography of colloidal crystals produces binary patterns of crystal wettabilities, photonic stopbands, and corresponding patterns of lipid mono- and bilayer morphologies. We envisage that these approaches will be exploitable for the development of optical transduction assays and microarrays for many membrane-mediated processes, including transport and receptor-ligand interactions.     

Small band gap conjugated polymers based on thiophene–thienopyrazine copolymers

In this study, five small band gap thiophene ( TH )–thienopyrazine ( TP ) conjugated copolymers were synthesized by Stille‐coupling reaction. The polymer structures consisted of one to four thiophene rings with the TP of different side groups provided a systematical investigation on the structure–electronic property relationship. The absorption maxima of the polymer films decreased from 850 to 590 nm as the thiophene moieties increased from thiophene to quaterthiophene. The optical and electrochemical band gaps of the studied poly[2,3‐didodecyl‐5‐(thiophen‐2‐yl)thieno[3,4‐b]pyrazine] ( PTHTP‐C12 ) were 0.97 and 0.78 eV, respectively, indicating a significant intramolecular charge transfer. The theoretical geometry and electronic properties of the TH ‐ TP copolymers by the density functional theory at the B3LYP level and 6‐31G(d) basis set suggested that the bond length alternation enlarged with enhancing the thiophene content and resulted in the variation on the polymer band gap. The relatively small theoretical effective mass of poly( TH ‐alt‐ TP ) also indicated its potential applications for field transistor applications. Our study demonstrates the tunable electronic properties of small band gap copolymers by the thiophene content and the resulted geometry variation. Such polymers could be potentially used for near‐infrared electronic and optoelectronic devices. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5872–5883, 2007     

Double telecom band thermo-optic switch based on dual-line filled photonic liquid crystal fibres

We demonstrate a thermo-optic switch based on photonic liquid crystal fibres (PLCFs) in which two lines of air hole are selective filled with liquid crystal (LC), with a high extinction ratio of more than 20 dB around 1310 nm and 1550 nm. Only in the range of 2.0°C it can perform a turn off and on operation of transmitted light in the second telecom band around 1550 nm while the first telecom bands around 1310 nm is still on. Due to the splitting of the bandgap, the switching function is achieved in this kind of PLCFs. Before the cleaning point (CP) of LC, a broad bandgap from about 1120 nm to 1320 nm splits into two ones, which are continuing inducing huge bandgap extension to shorter wavelength and longer wavelength after the CP of LC, respectively. Moreover, the temperature responses around the CP of LC is also investigated. Its sensitivity is about ?92.32 nm/°C around the CP of LC. Therefore, such kind of selective-filled PLCFs could find potential applications as thermo-optic switch and temperature sensor in the telecom band.     

Germaindacenodithiophene based low band gap polymers for organic solar cells

We report the first synthesis of a fused germaindacenodithiophene monomer and its polymerisation with 2,1,3-benzothiadiazole by Suzuki polycondensation. The resulting polymer, PGeTPTBT, is semicrystalline, despite the presence of four bulky 2-ethylhexyl groups. Blends with P(70)CBM afford solar cells with efficiencies of 5.02%.     

Direct and indirect band gap types in one-dimensional conjugated or stacked organic materials

Whether a semiconductor has a direct or an indirect band gap is important in determining physical properties such as photoconductivity and electroluminescence. For one-dimensional conjugated polymeric semiconductors, as well as organic molecular crystals, we show how the band gap type (direct or indirect) is determined by the shapes of the HOMO and the LUMO of constituent monomeric conjugated molecules. The connectivity of the monomer units, and the topology of orbital interaction determine the band gap type. Pairing symmetry in the π electronic system of even alternant hydrocarbons allows the immediate prediction of the band gap type, direct or indirect, by examining only the structures of the monomer units and their connectivity in polymers or molecular stacks. Received: 29 June 1998 / Accepted: 3 September 1998 / Published online: 8 February 1999     

Advances in circularly polarized luminescent materials based on axially chiral compounds

Circularly polarized (CP) light, as a special form of polarized light, demonstrates potential application prospects in future displays and optoelectronic technologies. Circularly polarized luminescence (CPL) from chiral chromophores is an ideal method to directly generate CP light, but how to design efficient emitters is always a perplexing problem. Among various chiral materials, CPL materials with axial chirality can provide us with clear structural parameters and information to further explore the structure-activity relationship. Herein, we systematically summarize the development status of axially chiral compounds with CPL properties from two aspects of photoluminescence and electroluminescence, covering metal complexes, polymers, supramolecular assemblies, simple organic molecules, and liquid crystals systems. In addition, we initially explore the relationship between CPL performance and axially chiral configuration, and the current challenges and opportunities in this vibrant field are also discussed.     

Production of foamed materials from synthesized silicate glasses

Possibility of obtaining materials with cellular structure directly in syntheses of silicate glasses was demonstrated. It was confirmed that water released in glass formation can serve as a gas-producing agent. The necessity for a deep extent of the topochemical glass-formation process was revealed. A resource-saving solution was suggested for synthesis of foamed glasses, based on the replacement of a part of glass being synthesized from hydrosilicates with a finished sodium-calcium glass.     

Broadband reflection in polymer stabilized cholesteric liquid crystal cells with chiral monomers derived from cholesterol

In this study, a series of achiral monomers and chiral monomers of different flexible spacer chains based on cholesteryl moiety were synthesized. Polymer stabilized cholesteric liquid crystal (PSCLC) cells were then created by incorporation of the polymer networks. The influence of the nature of the monomers and the spacer length of chiral monomers on the reflectance properties of PSCLC was investigated as well as the polymerization condition. The results strongly suggest that the chirality of the polymer networks plays an integral role in the observed reflection spectra, and the chiral polymer networks with chiral centers separated well from the polymer backbone induce a greater change in the bulk helix pitch, and produce the broader reflection band in these LC composites. In addition, the temperature dependence of the pitch of the composites before and after polymerization was investigated. To broaden the reflection band further, the experimental processes of thermally induced pitch variation simultaneously with a UV crosslinking reaction of the composites were presented. The morphology of the polymer network in the composites was studied by scanning electron microscopy (SEM). Copyright © 2008 John Wiley & Sons, Ltd.