Alexander Ryabchun

Novel Generation of Liquid Crystalline Photo-Actuators Based on Stretched Porous Polyethylene Films

The preparation of photo-actuators based on stretched porous polyethylene and an azobenzene-containing liquid crystalline polymer network is reported for the first time. It is revealed that this kind of photo-actuator possesses the following advantages: the lack of a need for using aligning coatings and cells preparation, high deformation of the actuator and its complete reversibility, good mechanical properties, and relatively low cost of fabrication. In addition some kinetic and thermodynamic features of the bending and unbending processes have been studied.

Dual photorecording on cholesteric azobenzene-containing LC polymer films using helix pitch phototuning and holographic grating recording

For the first time the possibility to perform dual recording in cholesteric LC polymer films was demonstrated by means of helix pitch photo-tuning and holographic grating recording, as far as these two photo-processes take place under the exposure to light with different wavelengths. For this purpose the cholesteric mixture of the nematic azobenzene-containing copolymer doped with the chiral-photochromic dopant was prepared and the photo-optical properties of thin planarly-oriented films of this mixture were investigated. It was found that the holographic grating recording over ∼1 min allows high diffraction efficiency values to be reached but does not disrupt the supramolecular helical structure that makes possible a two independent images recording.

A novel type of crown ether-containing metal ions optical sensors based on polymer-stabilized cholesteric liquid crystalline films.

For the first time, the films based on polymer-stabilized cholesteric composites containing crown ether fragments with the optical properties sensitive to the complexation with potassium and barium ions were obtained. The complexation with these ions leads to blue spectral shift of the selective light reflection of planar cholesteric texture of composite films. Peculiarities of spectral changes and kinetics of selective light reflection shift were studied. The proposed approach can be used for the creation of the effective and selective sensor materials for different ions or groups of ions.

New azobenzene-based chiral-photochromic substances with thermally stable Z-isomers and their use for the induction of a cholesteric mesophase with a phototunable helix pitch

Novel photosensitive azobenzene-containing substances with sorbitol- and lactate-based chiral fragments were synthesized to serve as chiral-photochromic dopants in cholesteric mixtures. Owing to the presence of chlorine or methyl lateral substituents in the azobenzene fragment of the dopants, the photoinduced Z-form of their molecules is highly stable. The photooptical properties of the prepared cholesteric mixtures were studied and it was shown that UV-irradiation of the planarly-oriented films results in a shift of the selective light reflection wavelength to a long-wavelength spectral region due to E–Z isomerization of the azobenzene moieties. This process is thermally and photooptically reversible and the rate of blue-light-induced helix twisting is comparable with the rate of the untwisting process, whereas the thermal relaxation completes only after ca. 40 days at room temperature (for a chlorine-substituted chiral-photochromic dopant). It was found that this value is almost two orders of magnitude less in comparison with the non-substituted chiral-photochromic dopant also studied in this work. The obtained results demonstrate the promising properties of the synthesized compounds for creation of novel photoswitchable cholesteric materials.

Stable Selective Gratings in LC Polymer by Photoinduced Helix Pitch Modulation

A cholesteric mixture based on the nematic liquid crystalline side-chain polymer doped with a chiral-photochromic compound was prepared and used as an active medium for creation of stable polarization selective gratings by phototunable modulation of the helix pitch. Such modulation was fabricated in the polymer mixture by a nonpolarized UV-irradiation with spatially modulated intensity that causes E-Z isomerization of a chiral-photochromic dopant, decreasing its helical twisting power. It was shown that the gratings recorded by UV-exposure through a mask are strongly selective to the handedness of circular polarized light. The studied polymer film forms a right-handed helical structure and, correspondingly, the diffraction of only the right-circularly polarized light was found in the transmittance mode. The maximum diffraction efficiencies were found for the wavelength values between the maxima of selective light reflection. The films obtained open very interesting possibilities for further development of materials with stable gratings operating in the entire visible spectral range. Both the position and the width of the spectral range of an efficient diffraction can be easily controlled by the UV exposure and concentration of the dopant. The materials obtained and methods developed can be used for creation of specific diffraction elements for optics and photonics.

Influence of the cation type on the DFB lasing performance of dye-doped azobenzene-containing polyelectrolytes

The well-known commercially available azobenzene-containing polyelectrolyte poly{1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt} (PAZO-Na) has been easily modified by converting it into carboxylic acid and different salts. These new materials, similar to original PAZO-Na, were used as matrices to create DFB lasers based on polarization gratings. Such matrix modification allowed us to obtain laser generation with the laser dye 1,2,3,5,6,7-hexamethyl-8-cyanopyrromethene-difluoroborate complex (pyrromethene 650), to increase the laser device stability (up to a half-life time of 4 × 104 pulses) and to considerably increase the laser efficiency (up to 9%). Therefore, these polymer composites can be considered as cheap and promising materials for photonics, optoelectronics, and sensor applications.

Photochromic Composite for Random Lasing Based on Porous Polypropylene Infiltrated with Azobenzene-Containing Liquid Crystalline Mixture

We report on a new low-cost and easily fabricated type of liquid crystalline polymer composites demonstrating low threshold random lasing, which can be used as a cheap and simple mirror-less laser source. The composite is based on mass-producible commercially available porous polypropylene (Celgard 2500) infiltrated with low-molar-mass liquid crystal material doped with Rhodamine 800 laser dye. Excitation with red nanosecond laser (630 nm) induces random lasing with the emission peak in NIR spectral range (804 nm) with noticeable degree of linear polarization. The possibility to control the lasing threshold and polarization of the output light with UV radiation through photoswitching of liquid crystal phase from nematic to isotropic is demonstrated. The photocontrollable phase switching is achieved by reversible E/Z isomerization of the azobenzene dopant introduced to the nematic host matrix. It is revealed that the isotropic state of liquid crystal provides more efficient random lasing with lower threshold due to significant scattering of the ordinary wave.

Conventional elastomers doped with benzophenone derivatives as effective media for all-optical fabrication of tunable diffraction elements

Simple and effective approach for the fabrication of volume diffraction gratings (VDGs) using different widely used elastic materials is demonstrated for the first time. The method consists in the introduction of UV-photoactive compounds into the ready-to-use elastomeric films with following gratings inscription by a holographic technique. As photoactive dopants a number of methyl-substituted benzophenone derivatives have been tested. Polydimethylsiloxane- and acrylate-based elastic polymers as well as styrene-ethylene-co-butylene-styrene copolymer are exploited as elastomeric matrices. The diffraction gratings of different geometrical parameters are successfully fabricated in all developed elastic materials. The highest value of the refractive index modulation of about 1 × 10−3 is achieved. The technique enables to create also slanted gratings and multiple gratings. The use of such gratings as strain-controllable elements for managing of light is demonstrated. Moreover, elastic reflection gratings have been produced for the first time using conventional polydimethylsiloxane (PDMS) material. The proposed approach provides a very flexible, low-cost and effective tool for all-optical fabrication of complex diffraction structures in the broad class of commercially available elastomeric materials that undoubtedly gives a strong impact to the field of soft tunable optics, photonics, and spectroscopy.

Holographic Structuring of Elastomer Actuator: First True Monolithic Tunable Elastomer Optics.

Volume diffraction gratings (VDGs) are inscribed selectively by diffusive introduction of benzophenone and subsequent UV-holographic structuring into an electroactive dielectric elastomer actuator (DEA), to afford a continuous voltage-controlled grating shift of 17%. The internal stress coupling of DEA and optical domain allows for a new generation of true monolithic tunable elastomer optics with voltage controlled properties.

Fluorescent and photo-optical properties of hydrogen-bonded polymer liquid-crystalline composites based on derivatives of stilbazole and crown ethers

New hydrogen-bonded photochromic stilbazole- and crown-ether-containing polymer LC composites are prepared and characterized. A smectic homopolymer with p-oxybenzoic groups, a nematic homopolymer carrying phenylmethoxy benzoate groups, and a nematic copolymer containing p-oxybenzoic and phenylmethoxy benzoate groups are synthesized and used as polymer matrices for these composites. The phase behavior and photo-optical properties of LC composites of various compositions are studied. A marked difference is observed in the fluorescence spectra for hydrogen-stabilized LC mixtures and model mixtures not forming hydrogen bonds. This effect is explained by the reversible transfer of proton of carboxyl groups involved in hydrogen bonding. It is shown that the complexation of crown-ether-containing groups of LC composites with potassium ions leads to a dramatic reduction in the intensity of fluorescence. This phenomenon may be used for creation of a new generation of sensors for metal ions.