Mesomorphic metal alkanoates is very promising yet overlooked class of nonlinear-optical materials. Metal alkanoates can exhibit a broad variety of condensed states of matter including solid crystals, plastic crystals, lyotropic and thermotropic ionic liquid crystals, liquids, mesomorphic glasses, and Langmuir–Blodgett films. Glass-forming properties of metal alkanoates combined with their use as nano-reactors and anisotropic host open up simple and efficient way to design various photonic nanomaterials. Despite very interesting physics, the experimental data on optical and nonlinearoptical properties of such materials are scarce. The goal of the present paper is to fill the gap by discussing recent advances in the field of photonic materials made of metal alkanoates, organic dyes, and nanoparticles. Optical and nonlinear-optical properties of the following materials are reviewed: (i) mesomorphic glass doped with organic dyes; (ii) smectic glass composed of cobalt alkanoates; (iii) semiconductor nanoparticles embedded in a glassy host; (iv) metal nanoparticles - glass (the cobalt octanoate) nanocomposites.
Dynamic holographic properties under the action of nanosecond laser pulses are investigated in novel composites based on lyotropic ionic liquid crystals, ionic smectic glasses with dye impurities and colored ionic smectic glasses of cobalt decanoate. Different mechanisms of optical susceptibility are found for different composites. The holographic gratings are characterized by fast time of recording, fast time grating relaxation, high spacial resolution, negligible small thermal grating. Oriented smectic glasses are novel matrixes to operate with "pure" optical nonlinearity of used impurities. Switching on/off holographic recording by dc voltage is obtained in lyotropic ionic liquid crystals with electro-chromic impurities. The materials are perspective for dynamic photonic elements operated with pulsed laser excitation.
Dynamic holographic recording is investigated in novel class of liquid crystals namely in ionic metal-alcanoic smectics, which contain photosensitive impurities. A design of a heterostructure cell that consists of at least two layers: the first layer is a solid dye film and a second layer is an ionic smectic matrix, is proposed for dynamic holographic applications. Fast temporal relaxation of dynamic holograms in investigated cells is provided due to high values of both thermal and electric conductivities in the ionic smectic matrix. The high aggregation of molecules in the dye film affords strong light
absorption and changes of polarizability in the cells. The structural features of lyotropic both potassium caproate and potassium caprilate are investigated by small angle X-ray scattering. Proposed LC cells can find applications in ultra-fast all-optical switchers.
In this work we investigated structural, optical and non-linear optical properties of lyotropic liquid crystal (LC) composition with the electrochromic admixture of viologen. X-ray investigations showed the presence of smectic A liquid crystal phase in samples. We calculated the main values, which characterize samples' structure, and made conclusions about the influence of the admixture at LC matrix Under the action of static electric field we observed colouring of the samples, which depends on the value of the voltage applied and influences the absorption spectra. We provided an analysis of the dependence of colour intensity of LC composition on the admixture concentration. Basing on our previous works we interpreted processes running inside the samples under the action of the electric field and leading to colour change. We registered holographic recording for the composition of LC-viologen and made suggestions about the mechanism of recording.
Smectic glasses of ionic thermotropic liquid crystals (system based on cobalt (11) decanoate) were investigated. Holographic properties of these systems were observed. Electronic absorption spectra of Co2+ in the isotropic melt, ionic smectic mesophase and smectic glass phase were studied as a function of temperature. On the base of small angle X-ray scattering was determined structure of cobalt decanoate mesophase.
Metal-organic ionic smectices (MOIS) have investigated to find their photorefractive properties. For the first time the multiplexing holographic recording of permanent gratings is obtained in pure lyotropic MOIS. It is proposed new mechanism of holographic recording in ionic smectics involving photo-chemical processes associated with multi-photon absorption. Dynamic grating recording has obtained in MOIS doped by electrochromic impurity (viologen) and in ionic smectic glasses composed by decanoate Co2+. A regeneration of viologen from radical state to initial molecule state becomes the main recording mechanism in MOIS with viologen impurity. Electronic absorption of Co2+ determines the recording of dynamic gratings in the ionic smectic glasses.
Metal alkanoates (systems based on lead decanoate with Co2+) under their melting were investigated. Electronic absorption spectra of Co2+ in the isotropic melt, ionic smectic mesophase and crystal phase were studied as a function of temperature. Dielectric properties and conductivity of ILC also were observed.
In this work we provided the investigations of electrooptical properties of liotropic ionic liquid crystals (LILC) doped with electrochromic admixture, namely with viologen. It was shown that under applying of electric field to LILC-viologen the electrochromic effect is observed. It manifests itself in colour change and in appearance of absorption bands on optical
spectra. The action of the electric field also leads to the change of electric properties of the samples and to the change of parameters ofdouble electric layers existing near electrodes.
For the first time the multi-gratings holographic recording has been obtained in novel class of liquid crystals namely in ionic lyotropic metal-organic smectics formed on the base of metal alkanoates. It was shown that alkali metal alkanoates form smectic structures. They self-organized in hydrophobic bi-layers of alkanoate chaines with electrostatic conductive layers. The gratings are recorded by the action of pulsed laser radiation both picosecond and nanosecond duration from double frequency Nd-YAG laser. The multi-gratings formation can be connected with the recording process in micro-domains of smectic matrix.
Electric and optical properties of ionic lyotropic liquid crystals (ILLC) with different ionic polymethine dyes (cation- and anion-types) as well as neutral fluorescein (DILLC) are studied. Dielectric characteristics of the ILLC and DILLC are investigated before and after applied stationary electric field. The comparison analysis of the electric and electro-optical properties of different DILLC has been carried out. Stationary electric field, which exceeded a certain voltage (~6V), causes DILLC irreversible electrochemical changes and bleaching films. It has been established that the influence of electrodes, DILLC boundary layers and near electrode electrochemical processes give the main contributions to the electro-optical effects in DILLC.
Two-wave mixing holography experiments with Raman-Nath diffraction in ionic lyotropic liquid crystals (ILLC) pure and doped by polymethine dyes is observed for the first time. The laser generation in ILLC films doped by polymethine dyes is demonstrated. Pure and doped ILLC is considered as new perspective photorefractive material for applications in signal and information processing.
Ionic metal-organic liquid crystals are calling the growing attention of researches in view of variety of their optical, electro-optical and nonlinear optical properties. The typical for this class of mesogenes are metal alkanoates, the structure of which corresponds to smectic A. Salts of alkanoate cacids being mixed with water at room temperature form ionic lyotropic liquid crystals (ILLC), the mesophase structure of which is analogous to that of a thermotropic smectic A. In this work we investigated absorption, luminescence spectra, electrooptical and nonlinear optical properties of ILLC doped with different ionic polymethine dyes. Obtained spectral and optical characteristics of doped ILLC allowed the holography grating recroding. ELectrical investigation has shown that in ILLC and double electrical layer took place near the electrodes. The charge transfer in this area can be described by the Schottky emission through a thin dielectric layer. The comparison with usual liquid crystals shows that the conductivity thin ILLC is more than by three orders higher. Specific spectral, electrical and optical properites of pure and doped ILLC were explained due to the peculiarties of interionic interaction.
Electro-optics of smectic liquid crystals has been drawing an increasing interest, especially for new classes of compounds, such as metal alkanoates, which manifest as thermotropic as lyotropic smectic mesophases. In the present work electric and electro-optic properties were studied of pure and dye-doped (polymethine dye) potassium kaproate, a water mixture of which is forming a lyotropic Sm A type liquid crystalline phase at room temperature. As follows from experimental data, for frequencies - f < 3 X 104 Hz, dispersion of the components of complex dielectric permittivity ε' and ε" is observed. Appropriate relaxation process is described by the Debye equation. On the basis of the analysis of frequency dependencies of (epsilon) ', (epsilon)" for the frequencies f > 3 X 104 Hz the conductivity of samples on an alternating current was found. The introduction of dye in ionic lyotropic liquid crystals (ILLC) results in an increase of conductivity (by 1,5 times) and reduction of relaxation time. The bleaching of the dye in ILLC under the action of a direct voltage (U > 5V) was observed. The mechanism explaining threshold character of such process was proposed.
Water solutions of metal alkanoates were shown to exhibit a certain range of concentration the liquid crystalline mesophase referred to as ionic lyotropic liquid crystalline phase. By means of introduction of different dyes the mechanisms of light interaction with dye-doped ionic liquid crystals were studied. Absorption and luminescence spectra of a number of dye-doped lyotropic liquid crystals on the base of caprilat-K water solutions were obtained and analyzed, their nonlinear optical properties were investigated also. Some conclusions concerning photorefractive properties of the studied systems were made and possibilities for their development and application are discussed as well.
According to the papers, liquid crystalline mixtures of achiral mesogen matrix and chiral mesogen additive area of great interest for the investigations of structural and physical properties of twist-grain-boundary (TGB) phases. Such systems allow studying the changes of the structure and properties of TGB phases resulting from the difference in the chiral additive concentration. We used liquid crystalline systems, polymorphism, structure and some physical properties of which had been investigated earlier. Of particular interest was to investigate selective reflection spectra and optical activity of helical smectic A (SmA*) in comparison with the same properties of the neighboring mesophases--a chiral nematic liquid crystal (N*) and a smectic C (SmC*)--with the different temperatures and chiral agent's concentrations. A comparative analysis of obtained spectral and optical characteristics of described mesophases and some conclusions on the peculiarities of the packing structure of these phases have been made in this paper.
The study of photorefractive materials attracts a big interest due to large variety of their optical as well as electro-optical effects. As was shown by our investigations dye-doped ionic liquid crystal (DILC) appeared to be a promising medium for the said processes. Doping of ILC with electro-chromic additives leads to significant changes in both electrical and optical properties of ILC with electro- chromic additives leads to significant changes in both electrical and optical properties of ILC with respect to pure substances. Doped ILC was used as a medium for dynamic holography grating recording. Electric current passing through the DILC film induces the appearance of an absorption band in different spectral regions.Existence of self-diffraction during a single pulse as soon as its rather high efficiency pointed out that the mechanism of grating creation might include optical and electro-optical nonlinearity.
Investigations of the features of thickness dependence of optical activity of dye-doped chiral liquid crystals (DCLC) were accomplished on the wedge and parallel cells filled with either pure or dye-doped nematic chiral mixtures. A method of precise calculation of spectral dependences for both angle of polarization plane rotation and ellipticity for the light passed through the DCLC film was developed. Nonlinear thickness dependence of the angle of polarization plane rotation was established with differing proportionality coefficients for the bigger and smaller LC layer thickness. Changes of polarization state of light while passing through the CLC film were observed for both types of circular light polarization. The influence of the dopant absorbing within selective reflection band on different optical characteristics is discussed.
Due to its diffractive nature the light polarization plane rotation in cholesterical liquid crystals (CLC) is highly dependent on the parameters of a selective reflection (SR) band of liquid crystal matrix and possesses a number of qualitative peculiarities in contrast to usual liquids, solutions or crystals. The aim of the present work is to study the influence of such parameters as cell thickness, temperature and dye-doping on the optical activity of CLC. A method of precise calculation of the cell gyrotropy and elliptisity of light passed through CLC film is described. The square dependence of polarization plane rotation upon CLC film thickness was established. Abnormally strong light absorption of dye-doped CLC in the 'red' edge of SR band was observed for small dopant concentrations resulting in an asymmetry of polarization plain rotation with respect to the maximum of SR band. On the base of experimental data the possibility to create an electrically controlled two-color CLC light indicator with high color contrast is discussed.
Rotation of the light polarization plane in both pure and dye-doped chiral liquid crystals (CLC) was investigated close to the CLC selective reflection region. Spectral dependences of the angle of the polarization plane rotation and circular dichroism were obtained for a number of CLC-compounds. Selection of an appropriate dye-stuff and liquid crystal compound allows to design electrically controlled two-color filter with good color alliance and contrast. Gyrotropy of dye-doped CLC was studied using Nd-laser radiation with varying light intensity. Under the light intensities above 250 kW/sq. cm a polarization nonlinearity was observed as a variation of polarization plane rotation of the light passes through the cell.
Resonant optical nonlinearity caused by absorption saturation of dye-stuffs in the selective reflection spectra region of chiral liquid crystals (CLC) was for the first time used for dynamic holography grating recording. Such a type of nonlinearity was accompanied with low recording times (about 10 picoseconds) and a rather high diffractive efficiency of 10 - 20%.
Dynamic grating recording was investigated in doped induced cholesterical liquid crystals. The recording time is some picoseconds, diffractive efficiency is about 10%. Self-diffraction was observed with different polarization of input beams (linear or circular). Vector self-diffraction was obtained with diffractive intensity about 2%.
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