Petr Shibaev

Mechanically tunable microlasers based on highly viscous chiral liquid crystals

Chiral composition is designed for highly viscous lasing microemitters. The composition forms cholesteric liquid crystal and after doping with pyrromethene 597 was used as an active lasing media in stretchable aluminized silicone cavities. Optical pumping of the system led to lasing at the wavelengths defined by a degree of cavity deformation. Lasing thresholds were lower in aluminized cavity than in transparent cavity. A simple model allowing to predict the shift of lasing wavelength as a function of deformation is developed.

Novel Color Changing pH Sensors Based on Cholesteric Polymers

Color changing cholesteric hydrogen bonded polymer films responding to pH changes induced by basic amino acids or inorganic bases were synthesized and studied. The films with a higher concentration of donor/acceptor groups display a larger shift of the selective reflection band and faster response. This effect is explained in terms of the breakage of hydrogen bonds that occurs between the components of a cholesteric polymer, immersed in various basic aqueous solutions including amino acids.

Cholesteric materials with photonic band gap sensitive to shear deformation and mechanical sensors

Shear deformation of highly viscous liquid crystals obtained as mixtures of high and low molar weight liquid crystals results in dramatic reversible changes of the selective reflection band and colour. The colour of the material changed instantaneously during the deformation; the time of its restoration increases with the viscosity of the polymer mixture. A qualitative model considering viscoelasticity of the liquid crystals is proposed to explain changes in the reflection spectra and microscopic observations of the texture.

Mechanism of colour changes in stretchable cholesteric films

Large shifts of the selective reflection band and colour changes are achieved in highly viscous mixtures of cholesteric polymers and low molar mass liquid crystals subject to mechanical deformation. The liquid crystal mixture was sandwiched between two silicone strips and stretched. The colour of the material changed instantaneously during stretching, and the time for the colour to be completely restored increased with the viscosity of the polymer mixture. A quantitative model considering the material as an incompressible viscoelastic fluid is proposed. The model accurately describes colour changes (shift of the selective reflection band) and its relaxation back to the original spectral location.

Rebirth of Liquid Crystals for Sensoric Applications: Environmental and Gas Sensors

Films and droplets of liquid crystals may soon become an essential part of sensitive environmental sensors and detectors of volatile organic compounds (VOCs) in the air. In this paper a short overview of recent progress in the area of sensors based on liquid crystals is presented, along with the studies of low molar mass liquid crystals as gas sensors. The detection of VOCs in the air may rely on each of the following effects sequentially observed one after the other: (i) slight changes in orientation and order parameter of liquid crystal, (ii) formation of bubbles on the top of the liquid crystalline droplet, and (iii) complete isotropisation of the liquid crystal. These three stages can be easily monitored by a photo camera and/or optical microscopy. Detection limits corresponding to the first stage are typically lower by a factor of at least 3–6 than detection limits corresponding to isotropisation. The qualitative model taking into account the reorientation of liquid crystals is presented to account for the observed changes.

Remote optical detection of bending deformations by means of cholesteric paint

Remote optical detection of bending deformations is suggested and realised by means of using a combination of patterned silicone elastomers painted with thin viscoelastic cholesteric layer. The presence of highly viscous cholesteric liquid crystal is crucial as it results in a long relaxation time of deformation and allows to detect it. Detection of bending deformation is also analysed theoretically for the suggested geometry and proved to be experimentally an advantageous method in remote detection of bending.

Distant mechanical sensors based on cholesteric liquid crystals

The improved performance is demonstrated for mechanical sensors based on cholesteric paint covering elastic silicone substrate with imprinted grooves. Due to the presence of the grooves, the deformation of the substrate is translated to the deformation of the cholesteric paint differently along the grooves and perpendicular to them. The model is suggested to describe the anisotropy of deformations and color changes.

Light‐induced structural changes in chiral liquid crystals

Novel light‐sensitive chiral dopants are studied as a light‐sensitive component in chiral liquid crystals which may be used in tunable optical devices. Light‐induced cis–trans‐ isomerization of chiral dopants results in changes of helical twisting power which translates into variations of helical pitch. Due to the light absorption in the liquid crystal cell the pitch variation is non‐uniform across the cell, which leads, at first, to a deformation of cholesteric layers, and then to the formation of cholesteric bubbles. The sequence of structural changes has a distinct visual pattern and occurs at the surface close to the UV light source. Small deformations of cholesteric layers and bubbles are unstable and disappear after removing UV irradiation. The increasing size of the cholesteric bubbles results in better stability; large bubbles do not disappear after removing UV light. A theoretical model is suggested to describe the undulations of cholesteric layers.

Distant optical detection of small rotations and displacements by means of chiral liquid crystals

The paper describes novel chiral viscoelastic liquid crystalline mixtures and their application for the detection of small rotational displacements of two plates confining cholesteric liquid crystals (CLC). The mixtures are characterized by extremely high viscosities and stability of the selective reflection band (SRB) at ambient temperatures. Even a small rotation applied to the chiral liquid crystal (CLC) cell results in dramatic changes of the reflective properties of sandwiched CLC films. The angle and direction of rotation as well as the magnitude of CLCs shear deformation can be determined for a variety of experimental geometries, each of which is characterized by its own response function. The proposed model explains changes in the reflection spectra for different experimental geometries and relates them to the angle of rotation and magnitude of shear. The method was tested for a detection of small rotations from a distance of up to 50 m and allows for resolving small rotations of the order of fra...

Surface reconstruction of chiral glassy oligomers under the action of volatile organic compounds (VOCs)

ABSTRACT The response of chiral polymers to volatile organic compounds (VOCs) was studied by atomic force microscopy. It was found that under the influence of toluene vapour, the focal conic domains on the surface of chiral polymer undergo remarkable reconstruction that is at least partially reversible. This opens new ways for building VOC sensors and using surface reconstruction in creating new surface patterns for nanotechnology. GRAPHICAL ABSTRACT