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The contrast of an interference pattern during its registration with an 8-bit digital photodetector has been determined and a method for correcting distortions of the extrema of the interference fringes that are caused by speckle noise and the nonlinearity of the camera has been proposed. Frosted glasses were used as test objects to solve the problem. This method allows a significant extension of the dynamic range of the measured extrema, up to 1 : 50 (34 dB) and can be used to analyze interference patterns using low-cost 8-bit webcams with a measurement accuracy at the level of a linear (10–12)-bit converter.

—The properties of the aberrational pattern resulting from the self-action of the light radiation mode TEM01 in liquid-crystal systems are studied. It is found that the interference of light rays corresponding to two peaks of intensity and nonlinear phase shift leads to the formation of a system of fringes in the Fraunhofer region; their angular period is controlled by the distance between peaks. These fringes fill the entire aberrational pattern (concentric ring pattern) or only its part, depending on the type of the profile of the light-induced phase shift. The possibilities of using the properties of the aberrational pattern in the TEM01 mode for studying the light-induced reorientation of the nematic liquid crystal director are shown.

Dye-doped nematic liquid-crystalline polymers exhibit much higher orientational optical response than the systems based on low-molar-mass liquid crystals. In addition, polymer materials possess the glassy state, which allows one to record their supramolecular structure. We study the effect of light field inhomogeneity on the director deformation in the geometry of Freedericksz transition and recording of corresponding phase structures. The relationship of the light beam width with the threshold light intensity and the size of the induced and recorded deformation region was determined. An influence of the beam waist radius on the polymer director relaxation time was detected. The results of numerical calculations based on the continuum theory and the independence of the enhancement factor of the optical torque on light–polymer interaction geometry agree with the experimental data.

The influence of the coherence of far-red (730 nm) light on the functional activity of plants was studied. Blackberry explants cultivated in vitro on an artificial nutrient medium served as a biological model. The explants were irradiated with light beams with different spatial and temporal coherence. The average cell size $D$ was taken as the discrimination threshold for the coherence length ${L}_{\mathrm{coh}}$ and the correlation radius ${r}_{\mathrm{cor}}$. The results of irradiation were judged by the length and number of shoots formed on each explant. The greatest photoinduced effect was observed when the conditions ${L}_{\mathrm{coh}}$, ${r}_{\mathrm{cor}}gD$ were fulfilled, i.e., when the cell fit completely in the coherence volume of the light wave field. Significant differences in growth parameters were also observed in the variants of the experiment with a constant frequency spectrum of radiation (fixed ${L}_{\mathrm{coh}}$), but different ${r}_{\mathrm{cor}}$. It is concluded that the correlation properties of radiation affect photoregulatory processes.

Dye-doped nematic side-chain liquid-crystalline polymers possess extraordinary large optical nonlinearity and ability to store the induced orientational deformations in a glassy state, which makes them a very promising material for photonic applications. In this study, the phase structures were generated and recorded in the bulk of a 50-&mu, m layer of a nematic liquid-crystalline side-chain polymer, containing polyacrylate backbone, spacer having five methylene groups, and phenyl benzoate mesogenic fragment. The polymer was doped with KD-1 azodye. The director field deformations induced by the light beam close to the TEM01 mode were studied for different geometries of light&ndash, polymer interaction. The phase modulation depth of 2&pi, was obtained for the 18-&mu, m spacing between intensity peaks. The experimental data were analyzed based on the elastic continuum theory of nematics. The possibility to induce and record positive and negative microlenses in the polymer bulk was shown experimentally.

A comparative study of optical orientation in nematic liquid crystals (NLCs) doped with a low-molar-mass bis-azobenzene dye (monomer) and a comb-shaped polymer with side fragments similar in structure to the monomer has been carried out. Both types of bis-azobenzene dopants induced a sign-alternating nonlinearity in the NLC: the positive when the angle Ψ between the light field and the director is less than a certain critical value Ψc and the negative in the opposite case of Ψc The magnitudes of the optical-torque enhancement factors due to trans- and cis-isomers (ηT and ηC), and the ratios ηT/ηC for the low- and high-molar-mass dopants are compared. The results obtained show the possibilities of increasing the orientational optical nonlinearity when passing from low-molar bis-azobenzene dye dopants to the corresponding polymers and are important for elucidating the optimal architecture of absorbing additives.

The dynamics of the light-induced director rotation has been studied for a comb-shaped polymers in nematic phase with a small amount (0.05 wt %) of azobenzene-dye additive. The effect of temperature on the rotational viscosity and nonlinear optical response of the liquid crystalline polymers was investigated. The orientational processes occured at the beam intensity ∼0.5 W/cm2. Cooling the samples below the glass temperature results in the memory effect.

By the example of plants, fungi and bacteria, we consider the possibility of controlling the interaction of cells, being in competitive, antagonistic, or parasitic relations. For this aim we used short-time irradiation (a few seconds or minutes) with the red (633 nm) quasi-monochromatic light having different spatiotemporal coherence. It is shown that the functional activity is mostly increased in the cells whose size does not exceed the coherence length and the correlation radius of the light field. Thus, in the case of cells essentially differing in size, it is possible to increase the activity of smaller cells, avoiding the stimulation of larger ones. For example, the radiation having relatively low coherence (Lcoh, rcor ≤ 10 μm) facilitates mainly the damage of large-size plant cells by pathogen fungi, while the exposure to light with less statistical regularity (Lcoh = 4 μm, rcor = 5 μm) inhibits the growth of the Fusarium microcera fungus, infected by the bacterium of the Pseudomonas species. The quasi-monochromatic radiation with sufficiently high spatiotemporal coherence stimulated all interacting species (bacteria, fungi, plants). In the considered biocenosis, the equilibrium was shifted towards the favour of organisms having the highest rate of cell division or the ones better using their adaptation potential.

The formation of beams with wavefront screw dislocation appeared when circularly polarized light passes through a light-induced orientation defect formed in a homeotropically oriented NLC at the surface photorefractive effect is calculated. NLCs with negative and positive low-frequency dielectric anisotropy are considered. The calculated data are compared with the results of previously performed experiments.

The generation of optical vortices in nematic liquid crystals (NLCs) due to the photorefractive effect has been studied. The vortices can appear in the NLCs with both positive and negative dielectric anisotropy. The efficiency of the Gaussian beam (TEM00) to vortex conversion approached about 30%.

The effect of light on the conformational composition of low- and high-molecular (fifth-generation dendrimer) azobenzene dyes in a nematic host is studied. The orientational optical response of the nematic liquid crystal (NLC), caused by the presence of these compounds in the nematic host, is measured. The contributions of isomers to the orientational optical torque affecting the NLC director are determined. The results obtained suggest that the increase in the optical orientation efficiency in NLCs when using high-molecular dopants instead of low-molecular ones is unrelated to the difference in their conformational composition in the light field, but is caused by deceleration of orientational motion of chromophores.

Optical orientation in transparent nematic liquid crystalline polymer (NLCP) was observed for the first time. In NLCP with dye dopant, the light-induced second-order orientation transition being an analogue of the Freedericksz transition in low-frequency fields is found. The transition threshold is ~10 μW which is lower than the threshold of the light-induced Freedericksz transition in low-molecular nematics by four orders ofmagnitude and is lower than the characteristic value for low-molecular nematics with dye dopant by two orders of magnitude.

A light beam with wavefront screw dislocation is generated using a nematic liquid crystal under a dc electric field. Axisymmetric deformation of the director field required for the formation of such a beam appears due to light-induced removal of dc field screening by surface electric charges.

Diffraction of the focused circularly polarized Gaussian beam passed through an undeformed homeotropic nematic liquid crystal is considered. The parameters of two coaxial Gaussian beams and two optical vortices formed behind the liquid-crystal layer are determined. The difference in the positions of foci of Gaussian beams and vortices appears in the experiment as interference ring patterns. The efficiency of the beam transformation to optical vortices is calculated.

It is demonstrated that an optical beam acquires a component with spiral dislocation of wave front (optical vortex) due to passage through a layer of homeotropically aligned nematic liquid crystal with light-absorbing admixture. The vortex is formed owing to the heating of liquid crystal and transition to isotropic phase in the irradiated region, which leads to the generation of axisymmetric field distribution of director at the interface of the isotropic channel and nematic liquid crystal.

Light-induced reorientation of the dye-doped liquid-crystal polymer director is found and studied. The director rotates from the light field direction, which corresponds to the negative optical nonlinearity. The nonlinearity coefficient is several times larger than that of a low-molecular liquid crystal doped with the same dye.

By the example of vegetable organisms differing in structure and functional properties it is shown that their response to the action of quasi-monochromatic light from laser sources does not obey the Bunsen – Roscoe dose law. The dependence of biological effect on the irradiation time has the multimodal (multiextremal) form with alternating maxima and minima of the stimulating effect. Such a property manifests itself in the spectral ranges, corresponding to photoinduced conversion of chromoproteins of photocontrol systems and is probably related to the cyclic variations of metabolic activity in vegetable cells.

The problem of the light coherence effect on the magnitude of the photoinduced cell response is discussed. The origins of ambiguous interpretation of the known experimental results are considered. Using the biological models, essentially differing in anatomy, morphology and biological functions (acrospires of radish, blackberry microsprouts cultivated in vitro, plum pollen), the effect of statistical properties of quasi-monochromatic light (λmax = 633 nm) on the magnitude of the photoinduced cell response is shown. It is found that for relatively low spatial coherence, the cell functional activity changes insignificantly. The maximal enhancement of growing processes (stimulating effect) is observed when the coherence length Lcoh and the correlation radius rcor are greater than the cell size, i.e., the entire cell fits into the field coherence volume. In this case, the representative indicators (germination of seeds and pollen, the spears length) exceeds those of non-irradiated objects by 1.7 – 3.9 times. For more correct assessment of the effect of light statistical properties on photocontrol processes, it is proposed to replace the qualitative description (coherent – incoherent) with the quantitative one, using the determination of spatial and temporal correlation functions and comparing them with the characteristic dimensions of the biological structures, e.g., the cell size.

Visualization of the phase object with the use of a phase-contrast scheme incorporating a nonlinear Zernike filter was carried out. A liquid crystal or liquid crystalline polymer doped with azobenzene dye inducing orientational optical nonlinearity was used as the filter. Inversion of the image contrast depending on the angle of incidence of the light beam on a nematic liquid crystal cell was implemented.

Light-induced director orientation of polymeric liquid-crystalline systems was investigated. The materials under study were composed of a nematic liquid-crystalline polymer (NLCP) and a small amount (0.05-0.5 wt.%) of conformationally active (azobenzene) or stable (anthraquinone) dye impurity. Light action on the homogeneously aligned polymer films above glass transition temperature leads to the director reorientation and, consequently, to a change in the extraordinary refractive index. The effect is associated with the dye molecule excitation and related change of intermolecular forces. In the case of NLCP with conformationally active dye dopant, an extremely high orientational optical response was detected (nonlinear coefficient is n_{2}∼0.1cm^{2}/W). In contrast, the efficiency of orientational light action on NLCP with conformationally stable dye dopant is of the same order of magnitude as that of dye-doped low-molar-mass liquid crystals. At the normal light incidence on the NLCP doped with azo-dye, the threshold director orientation is observed which is similar to the Fréedericksz transition under the action of magnetic and electric fields. The obtained high-orientational optical response of NLCP caused by azo-dye dopant in combination with the possibility of the recording of deformed structure in the glassy state, typical for polymer compounds, reveals new opportunities in photonics applications.

The orienting effect of light on nematic liquid crystals (NLCs) doped with comb-shaped polymers with different spatial distributions of side absorbing azobenzene fragments, i.e., a homopolymer (containing only azofragments), a block copolymer (containing additionally a block of non-absorbing fragments), and a statistical copolymer (containing randomly arranged absorbing and non-absorbing fragments) is experimentally studied. The light-induced Freedericksz transition threshold for the block copolymer is two times lower than that for the homopolymer. For NLC with statistical copolymer dopant, the first-order orientation transition with an extremely wide optical bistability region is observed.

It is proposed to apply the birefringence method to measure the threshold of the light-induced Freedericksz transition and the nonlinearity enhancement factor of nematic liquid crystals by director relaxation dynamics in the light field. A birefringence change is recorded by interference of ordinary and extraordinary components of the light beamaffecting the liquid crystal director orientation. The Freedericksz transition thresholds and nonlinearity enhancement factors are measured for the samples doped with comb-shaped polymers with various degrees of polymerization. The determined thresholds are in agreement with the results obtained by the aberrational self-action method.

The effect of the statistical properties of light on the value of the photoinduced reaction of the biological objects, which differ in the morphological and physiological characteristics, the optical properties, and the size of cells, was studied. The fruit of apple trees, the pollen of cherries, the microcuttings of blackberries in vitro, and the spores and the mycelium of fungi were irradiated by quasimonochromatic light fluxes with identical energy parameters but different values of coherence length and radius of correlation. In all cases, the greatest stimulation effect occurred when the cells completely fit in the volume of the coherence of the field, while both temporal and spatial coherence have a significant and mathematically certain impact on the physiological activity of cells. It was concluded that not only the spectral, but also the statistical (coherent) properties of the acting light play an important role in the photoregulation process.

Optical nonlinearity of nematic liquid crystalline polymer caused by molecular reorientation was found and investigated. Under the action of light on the polymer film, director of polymer reorients to light field increasing the refractive index. If the polymer is doped with an azobenzene compound, nonlinear response dramatically increases; in this case, the light-induced refractive index becomes negative.

The Zernike method using a nematic liquid crystal (NLC) filter with orientational optical nonlinearity was applied to visualize phase objects. It was shown that the contrast sign of the image switches by means of varying the incidence angle of a light beam on liquid crystal cell. The comparison of a contrast sign change of opaque object in the schemes based on NLC-filter and on liquid filter with thermal optical nonlinearity was performed.

Optical vortices were generated by means of photoinduced point defects in orientation of the nematic liquid crystal (NLC). The axisymmetric distribution of NLC director field was produced due to photorefractive effect in NLC or due to isotropic channel formation in light absorbing NLC.

The light-induced director reorientation in NLCs doped with the comb-shaped azopolymers with different degrees of polymerization (the number of chromophore groups) has been studied. The negative optical nonlinearity (a decrease in the refractive index of the extraordinary wave due to the director rotation away from the light field) was observed for all dopants. It was found that the threshold power of the Freedericksz transition decreases with the polymerization degree. The light-induced Freedericksz transition of the first order was observed for the dopants with lower degrees of polymerization.

Processes of light-induced reorientation of nematic liquid-crystalline molecules induced by the addition of low concentrations (0.1–2.0 wt %) of comb-shaped polymers and carbosilane dendrimers containing azobenzene fragments are studied. When the molecular structure of the above compounds becomes more complicated, the induced orientational nonlinearity increases. The introduction of 2G and 3G dendrimers into a nematic has for the first time made it possible to visualize and study a purely optical first-order Freedericksz transition in the field of a linearly polarized wave.

It was found that the second-generation carbosilane dendrimers with statistically distributed terminal aliphatic and azobenzene fragments induce negative orientational nonlinearity in nematic liquid crystals (NLC) (the NLC director turns perpendicular to the light field, decreasing the refractive index of an extraordinary wave). The light-induced Freedericksz transition in planar oriented NLCs is the first-order orientational phase transition characterized by a wide bistability region.

The results of experimental study of the light beam self-action in a nematic liquid crystal placed in a dc electric field are presented, and a theory of this effect is developed. This self-action of a light beam is shown to cause a hyperbolic umbilic caustic. The intensity distribution and caustics calculated in the far diffraction zone of the light beam agree well with the experimental data.

Photocurrents in liquid crystal cells of different structures were experimentally studied. The results of the study confirm the photorefractive mechanism of the light beam self-action in nematic liquid crystals, which consists in a decrease in the density of surface charges screening an external electric field upon light exposure. The data obtained from photoelectric and optical measurements suggest that the photorefractive effect appears in cells with different orientants and in the absence of orientant. In a planar cell oriented by rubbed polyimide, the crucial role is played by the interface between the liquid crystal and polyimide deposited on the anode.

The interaction of light with a nematic liquid crystal (NLC) doped with dendrimers containing terminal light-absorbing azobenzene fragments and a low-molar-mass dye identical to the fragments was studied in detail. Both the dendrimers and the dye induce orientational nonlinearity in a transparent nematic host. For the fifth-generation dendrimer, the nonlinearity is negative (director is rotated away from the light field direction). For the first-generation dendrimer and the dye, the nonlinearity sign depends on the light propagation direction with respect to the NLC director, light polarisation and external electric field. The results obtained show that complication of the molecular structure of the dopant, i.e. passing from the ‘free’ dye molecule to the first- and fifth-generation dendrimers, results in an increase of the efficiency of the orienting effect of the light.

The results of the studies of the orientational optical nonlinearity of a nematic host doped with high-molar-mass compounds of different molecular architecture such as a comb-like polymer and a dendrimer containing azobenzene chromophores are presented. The nonlinearity induced by these compounds was found to be higher than that induced by the low-molar-mass dyes similar in structure to the light-absorbing constituents of the polymer and dendrimer. Possible reasons for the nonlinearity enhancement for macromolecular dopants are discussed.

Light-beam interaction with nematic liquid crystals doped with stilbene dye DEANS (4-diethylamino-4′-nitrostilbene) has been studied experimentally. For the homeotropic sample, a conventional ring-shaped aberration self-focusing pattern (related to the dye-induced orientational nonlinearity) and the memory effect were observed. For the planar sample, a complex asymmetric aberration pattern qualitatively different from the concentric-ring system was observed for the first time. The study of the phenomenon allowed us to assume that this pattern is due to the bulk photorefractive effect.

The effect of optical orientation in nematic liquid crystals containing small additions of high-molecular compounds, i.e., comb-shaped polymers with light-absorbing azobenzene side fragments, was studied. The effects of light-induced reorientation of the director of nematic liquid crystals caused by light absorption of polymers and a low-molecular compound with a structure similar to side fragments of the polymers were compared in detail. An explanation was proposed for large values of the orientational nonlinearity induced by polymers.

The properties of a light beam passed through an NLC layer with positive orientational nonlinearity and then reflected from the NLC–substrate interface have been experimentally studied. The divergence and the intensity profile of this beam were found to be quite different from those of the beam transmitted through the NLC cell. It is shown that the aberrational pattern in the cross-section of the reflected beam and the dynamics of its formation can be used to study the orientational nonlinearity even in the presence of thermal effects.

Orienting influence of femtosecond laser pulses on nematic liquid crystals has been studied. We have shown experimentally that in both a nematic liquid crystal doped with anthraquinone dye D4 and nematic matrix “mixture A” (consisting of azoxymolecules) the efficiency of the liquid-crystal director reorientation under the influence of femtosecond pulses is less than for continuous wave radiation. The mechanism responsible for this difference is associated with light-induced variation of the anchoring conditions.

The presence of light-absorbing dopants in nematic liquid crystals (NLCs) can induce orientation nonlinearities conditioned by the torque due to changing the intermolecular forces at dopant excitation. It was shown that large nonlinearities are caused by high-molar-mass substances (comb-shaped polymers [1] and the homodendrimers with the generation number m = 1,5 [2]) containing azobenzene chromophores. The nonlinearities induced by polymers and the dendriemer with m=5 were negative (director n rotates perpendicularly to the light field E, thus decreasing the refractive index of the e-wave) in contrast to the low-molar-mass dyes similar in structure to the azobenzene chromophores and the dendrimer with m = 1, in which case the nonlinearity sign changes with the angle between E and n.

Interaction of light and ac electric fields with a nematic liquid crystal (NLC) doped with nanosized second-generation carbosilane codendrimers containing terminal azobenzene fragments has been studied. A first-order Freedericksz transition in the linearly polarized light, accompanied by an intrinsic bistability in a wide region, was observed. An additional ac electric field decreases the light-induced Freedericksz transition threshold and narrows the bistability region. Light illumination transforms the second-order electric-field-induced Freedericksz transition to a first-order one. The width of the bistability region increases with the light wave intensity. The theory of the interaction of light and ac electric fields with the dendrimer-doped NLCs is developed taking into account an additional (with respect to the undoped nematic host) dependence of the optical torque on the angle between the director and the light field.

The orientational self-action of a light beam in (1) transparent planar nematic liquid crystal (NLC) subjected to external DC field and (2) planar NLC doped with stilbene dye in the absence of external field has been studied. It is shown that the shapes of the aberrational patterns developed in the far-field diffraction zone are highly asymmetric, unlike the common system of concentric rings. In case (1) the aberrational pattern exhibits complex dynamics of its formation. The shape of the pattern is independent of the polarization of the incident light. In case (2), the divergence of the pattern does depend on the incident-light polarization. In both cases, the light-induced director deformation in the field of the ordinary wave was found. The physical mechanisms of the director reorientation responsible for the asymmetric aberrational patterns are discussed.

Director deformation in a planar nematic liquid crystal (NLC) with negative optical nonlinearity (director rotates away from the light field) has been studied. First-order orientation transition and a wide hysteresis loop were observed at changing the light intensity in the presence of an ac field that is non-perpendicular to the undistorted director of liquid crystal due to pretilt. The theory of light beam interaction with NLC under ac field was constructed.

The orientational optical nonlinearity induced in NLCs by light-absorbing carbosilane codendrimers with statistical distribution of terminal aliphatic and azobenzene fragments has been studied comparatively for different (from the second to the fourth) codendrimer generations. It was found that all the codendrimers induce the negative nonlinearity (director rotates away from the light field thus decreasing the refractive index of the extraordinary wave). The orientational response increases with the generation number. The optical bistability is manifested for NLCs doped with the codendrimers of the second and third generations.

Orientational interaction of a light beam and nematic liquid crystals subjected to an external electrical dc field has been studied. It is shown that the intensity distribution in the aberrational pattern appeared in the cross section of the transmitted beam can be highly asymmetric, unlike the conventional, rather centrally symmetric, ring system. The director-field distortion produced by the combined action of light and dc fields is independent of the light polarization. The results are explained by the surface photorefractivity.