Light-controlled change in the helical pitch and broadband tunable cholesteric liquid-crystal lasers

This paper presents an overview of the results obtained in recent investigations of the control over the helical pitch under irradiation and the use of these data for the design of compact broadband tunable lasers based on dye-doped cholesteric liquid crystals. It is demonstrated that a reversible change in the lasing frequency can be achieved upon exposure to two low-power light-emitting diodes. Another alternative approach to the generation of tunable laser radiation in the visible and ultraviolet spectral regions (370–680 nm) in a specially designed cell with a cholesteric liquid crystal that is doped with several dyes and possesses a helical pitch gradient is considered.     

Photonics of liquid-crystal structures: A review

The original results of studies of the electro-optical and laser effects which have been performed at the Laboratory of Liquid Crystals of the Institute of Crystallography, Russian Academy of Sciences, over the last few years are reviewed. Cholesteric liquid crystals as vivid representatives of photonic structures and their behavior in an electric field are considered in detail. The formation of higher harmonics in the periodic distribution of the director field in a helical liquid crystal structure and, correspondingly, the new (anharmonic) mode of electro-optical effects are discussed. Another group of studies is devoted to bistable light switching by an electric field in chiral nematics. Polarization diffraction gratings controlled by an electric field are also considered. The results of studies devoted to microlasers on various photonic structures with cholesteric and nematic liquid crystals are considered in detail. Particular attention is given to the new regime: leaky-mode lasing. Designs of liquid crystal light amplifiers and their polarization, field, and spectral characteristics are considered in the last section.     

Photo- and thermo-induced variation of photonic properties of cholesteric liquid crystal containing azobenzene-based chiral dopant

Photooptical properties of a new cholesteric mixture containing azobenzene-based chiral dopant were investigated. Presence of a photosensitive dopant in the investigated liquid crystal enabled to effectively manipulate optical characteristics of the photonic band upon temperature change and illumination. Temperature dependence of the optical anisotropy and the orientational order parameter of the liquid crystal were determined. The obtained results are compared with existing theories. Discrete multistable change of the spectral position of the photonic band on illumination is realized.     

Differential etching of chalcogenides for infrared photonic waveguide structures

Chemical etching rates for two different chalcogenide glass compositions, As₄₀S₆₀ and As₂₄S₃₈Se₃₈, were studied using sodium hydroxide based etchant solutions. Etching was performed using a variation of standard photolithographic masking and wet-etching techniques. Variations in etch rate with NaOH concentration and glass composition were observed. The depth of etch was characterized using an optical profilometer. Etch rate differences as large as three orders of magnitude between these two glasses were observed at low NaOH concentration (0.053 M). We present a single variable etch rate curve of etch depth per time (nm/s) versus NaOH overall solution concentration (in M) for these two different chalcogenide glasses. This technology shows promise for fabricating photonic structures and has potential applications in fabricating novel photonic bandgap structures that will function in the long-wave infrared (LWIR) regime.     

Design of photonic structures by sol–gel-derived silica nanospheres

Sol–gel processing was used to obtain monosized silica spheres of 270 nm in diameter. Starting from these spheres, two different systems have been fabricated: (i) 3D Photonic Crystals by means of vertical deposition and evaporation-assisted sedimentation deposition methods; (ii) core-shell-like Er³⁺-activated silica spheres, where the core is the silica sphere and the shell is an Er₂O₃–SiO₂ coating. Optical and spectroscopic assessment, as well as morphological and structural characterization of the systems, have been performed.     

Three-dimensional circular spiral photonic crystal structures recorded by femtosecond pulses

Photonic crystal structures having three-dimensional (3D) circular spiral architecture have large theoretical photonic band-gap but cannot be implemented by most of the existing microfabrication techniques. We have successfully used a highly versatile 3D microstructuring technique called femtosecond laser microfabrication to prepare templates of the circular spiral photonic crystals in a commercially available photoresist SU-8. The structures fabricated have good structural quality, are highly periodic and exhibit spectral signatures of photonic bands dispersion in the short infrared wavelength region.     

An alternative method to obtain direct opal photonic crystal structures

We describe the protocol that we have elaborated in order to obtain monosize polystyrene spheres. Starting from these spheres a simple and effective method, based on spin-coating technique, was developed to realize colloidal photonic crystal structures. The process produces compact 3D arrays of polystyrene microspheres (opals) that are organized into crystalline lattices. This process offers the ability to rapidly form 3D photonic crystals using inexpensive instrumentation, which makes it attractive for an array of applications. Process parameters, fabricated structures, and their experimental characterization are presented.     

High-temperature lasing in ZnO microcrystallites

High-temperature studies of the luminescence and lasing spectra parameters of the main UV bands (spontaneous luminescence and lines of stimulated-emission mode structure) and the recombination band of electron-hole plasma in zinc oxide microcrystallites have been performed. ZnO microcrystallites of tetrapod morphology, characterized by type-II lasing (microlasers), have been chosen as objects of study [1]. The temperature behavior of a number of key parameters related to specific features of microlasing is investigated.     

Formation of photonic structures in Sm2+-doped aluminosilicate glasses through phase separation

Sm²⁺-doped Al₂O₃–SiO₂ glasses with three-dimensionally interconnected macroporous morphology have been prepared via the alkoxide-derived sol–gel process containing poly(ethylene oxide) and SmCl₃ · 6H₂O. The macroporous morphology is obtained by concurrently inducing the phase separation and sol–gel transition. When the macroporous aluminosilicate glasses doped with Sm²⁺ are irradiated with a visible light laser at the wavelength of 488 nm, a hole or a dip appears in the plot of fluorescence intensity versus the incident angle of laser beam, indicating that the valence state of Sm²⁺ is spatially modulated through the interference of multiply scattered light. The hole profile can be controlled by adjusting the macroporous morphology.     

Steroid motor: dynamics of cholesteric helix induction in the nematic droplet

A nematic liquid crystal can be converted into a cholesteric phase by a chiral dopant, and the cholesteric pitch can be changed by its photochemical transformations [1]. For the first time we investigate the dynamics of the cholesteric phase induction using seven steroids (vitamin D isomers and related compounds) as chiral dopants. Here we report the new effect of rotation of a rod-like steroid crystal (0.1-1 mm length) when it is placed at the surface of a nematic drop and its dissolution course was followed with a polarizing microscope. For all the compounds univocal correspondence was noticed between the crystal rotation direction, the helicity of the molecular steroid ring system [2] and the sign of the cholesteric macrohelix determined by the Cano-Grandjean method [3]. No rotation was observed in the isotropic phase.     

Time evolution of cholesteric fingers of the second species in an electric field

Different growth modes of isolated cholesteric fingers of the second species (CF‐2) in an electric field at voltages near a coexistence line (V₂) between cholesteric and nematic mesophases are in detail described. Videomicroscopy and computer image analysis were used for investigation of the pattern in polarized light. It is shown how a drift, a lengthening and a shape of fingers depend on the voltage at which the growth sets in and three typical scenarios are distinguished. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The investigation of the kinking process with help of cholesteric liquid crystals

Process of kinking in CsI crystals has been studied with help of CLC (cholesteric liquid crystals) sensitive to deformation and temperature. It is shown that kinking formation is accompanied by local heat in the kink band. The velocity of deformation is the more the larger the quantity of heat in the kink band.     

Effect of higher protonation on lasing performance of Rhodamine-B in sol-gel glasses

Three types of Rhodamine-B (Rh-B) impregnated sol-gel samples (mentioned as type-A, type-B, type-C) were prepared. After studying photophysical properties, the dried samples were subjected to laser study under nitrogen laser pumping in a transverse dye laser cavity. Type-B and type-C samples were found to be lasing, while type-A did not show any lasing action. The photostability study of these samples under nitrogen laser pumping showed that Rhodamine-B was more photostable in type-C sol-gel host than in type-B. These results have been explained by taking into account the molecular changes in Rh-B in the sol-gel host matrix.     

Acousto-optic effect in a nematic liquid-crystal layer under the binary effect of sound and viscous waves

The optical effect in a liquid crystal cell containing a homeotropic layer of nematic liquid crystal (NLC) is analyzed. An NLC layer, located between crossed polaroids and opaque in the absence of external effect, is cleared after irradiation by an ultrasonic beam with a sharp spatial boundary. This enlightenment is suggested to be caused by the reorientation of crystal molecules in the acoustic flows that arise under the binary effect of the layer compression in the irradiated region and the viscous waves propagating from the layer boundaries. The flows were calculated taking into account the stress caused by the velocity convection and crystal structure relaxation. An expression is derived for the cell transparency, and the relative role of the convection and relaxation processes in the effect is determined.     

Effect of various external factors and pretransitional phenomena on structural transformations in cholesteric liquid crystals

The publications on low-molecular thermotropic liquid crystals with cholesteric structures have been reviewed. The effect of an applied electric field on cholesteric structures is studied. Bistability of the cholesteric-nematic transition, electric field-induced color textures of cholesteric mixtures, and electro-optics of amorphous cholesteric structures are considered as well as pretransitional (blue and TGB) phases, discotic cholesterics, PDLC films, flexoelectric electro-optics, and photostimulated switching in cholesterics.     

Structural investigation of photonic materials at the nanolevel using XPS

This work deals with X-ray photoemitted spectra (XPS) from materials which are of interest for photonic applications. In particular xHfO₂ ? (100 ? x) SiO₂ (x = 10, 20, 30 mol%) glass–ceramics planar waveguides and silver ion-exchanged (0.5, 1.5, 5 mol%) sodalime glasses are investigated. The aim of the work is to explore the material structural changes occurring at the nanometric scale which are produced during the fabrication process in order to enlighten the formation of the nanostructures. The results show that XPS is sufficiently sensitive to detect the formation of nanostructures in the analyzed materials providing at the same time also chemical information. Both these inputs are important to tune the production processes to increase the efficiency of the optical devices.     

Peculiarities of the phase behavior of liquid-crystal mixtures at the Cholesteric → Smectic (Ch → Sm) transition boundary

It is found that an interphase state, characterized by a large Bragg reflection coefficient, is formed in smectogenic cholesteric liquid-crystal mixtures at the Ch-Sm transition boundary. The interrelation between the mixture composition and the optical and spectral characteristics is studied. A possible structure of the interphase state is proposed.