Search

—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.

A depressed cladding waveguide composed of 36 parallel tracks is inscribed in high purity 70TeO2-22WO3-8Bi2O3 glass by femtosecond laser operating at 1030 nm. The Sellmeier formula for the refractive index of the investigated glass is obtained for the range of 1170–2350 nm. The module of the negative refractive index change in a track is found to increase with increasing wavelength in the range of 640–2000 nm. The waveguide shows single mode behavior in the wavelength range of 1700–2100 nm with mode field diameter (MFD) of 20 μm. The propagation loss is predominantly due to scattering in the waveguide. Spectrum broadening as large as 503 nm was demonstrated in a 14 mm long waveguide pumped by femtosecond pulses at 1950 nm. In the low input pulse energy region, the spectrum broadening is well described by generalized nonlinear Shrodinger equation (GNLSE) taking into account Kerr and the Raman nonlinearities under the assumption of constant MFD along the waveguide. Under higher energy pumping, an extra broadening was observed that is attributed to the focusing effect of Kerr lens in the waveguide mode.

We demonstrate the advantages of application of an optical vortex for direct femtosecond laser writing of crystalline tracks with substantially improved morphology in lanthanum borogermanate glass, i.e. crystalline tracks possessing a solid cross-section with an aspect ratio of close to 1 and a highly oriented crystal structure can be obtained.

In this paper, we have studied the crystallization behavior of amorphous Ge2Sb2Te5 thin films upon irradiation with femtosecond laser pulses. Crystalline and melt-quenched amorphous regions were produced by exposure to laser multipulses, and were characterized by the optical microscopy and by the micro-Raman spectroscopy. Using irradiation with single laser pulses of varying fluence it was verified that crystallization was not possible, requiring minimum ten pulses for triggering partial crystallization. We obtained laser-induced periodic surface structures in the form of near-subwavelength ripples of two types with different periods. It was found that the fraction of the crystalline phase is higher in the ridges of the ripples compared to the valleys. We used the model of heterogeneous crystallization for bodies with different curvatures of the boundary surface to explain the observed effect of the phase transformation in laser-induced periodic surface structures.

Tellurium dioxide based glasses with a low hydroxyl groups concentration were synthesized in an atmosphere of dried oxygen from initial high purity oxides. Large high-quality optical elements were manufactured and investigated from the point of view of nonlinear propagation of ultrashort pulses in the mid-IR range. Optical homogeneity, thermophysical, optical, including nonlinear properties were studied.

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.

Sub-surface femtosecond laser waveguide writing in ZnS is being investigated using both experimental and numerical simulations. We show that non-linear absorption and self-focusing play a critical role in the creation of the sub-surface modifications. The wavelength- and intensity dependence of the non-linear optical parameters change the strength of the sub-surface modifications when using lasers operating at different wavelengths. We investigate several wavelength ranges of interest, covering the wavelength peaks of the different non-linear processes. Furthermore, we compare the results of the numerical simulations to several different experiments and show a close correlation between the experimentally obtained results and the numerically obtained results. Finally, we also show that in the investigated wavelength range between 800nm and 1000nm there is no significant difference between the commonly used wavelengths for femtosecond laser processing, provided the other processing parameters are the same.

High-purity tungstate-tellurite glasses doped with molybdenum, lanthanum or bismuth oxides were investigated from a point of view promising medium for nonlinear optical devices fabrication. A tubular-like architecture was employed for fabrication of waveguides in the of tellurite glasses, in which refractive index change is negative under femtosecond laser writing at 1028 nm. Thermal, nonlinear optical properties and optimal laser writing conditions were determined. The glasses revealed a very high stability against crystallization, high transparency in the visible, near- and mid-IR regions, large values of Kerr nonlinear coefficient that are within the range of (4.3 ÷ 5.4)·10− 6 cm2/GW. The highest values of the nonlinear coefficient were found for bismuth containing glasses. The single mode waveguide with propagation loss of 0.15 dB/cm at 1064 nm was inscribed in tungstate-tellurite glass.

Three-segment microtracks were for the first time inscribed in bulk fluorite in the scanning mode via multishot exposure by tightly-focused 515-nm ultrashort laser pulses of variable pulse energy and pulsewidth (0.3–3.8 ps). The local micro/nanoscale material modification and related refractive-index changes within the microstructures, envisioned by optical and cross-sectional electron microscopy, result in their form birefringence acquired by polarimetric microscopy. Retardance and thickness of the microstructures were measured by optical polarimetry and confocal scanning Raman/photoluminescence microscopy, respectively, exhibiting the intensity- and pulsewidth-dependent trends, with the maximal effect for ≈1-ps laser pulses. These and complementary optical self-reflectance studies enabled for the first time to consider ultrafine, pulsewidth-dependent interferential longitudinal sub-structure of the birefringent microtracks. Such regular longitudinal sub-structure was proposed to emerge via back-reflection and interference of laser radiation prior near-critical laser-generated plasma in the focus, and the following transverse plasmon propagation in each nanosheet of ionized fluorite in the resulting sub-wavelength interference pattern. Our findings open the way for ultrashort-pulse laser inscription of embedded optical microstructures in non-oxygen fluoride materials and shed light onto the underlying laser-dielectric interactions.

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.

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.

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.

The nature of crystallization in telluride based phase change memory materials is being actively discussed, and we present new experimental results on the effect of a pulsed laser treatment on the amorphous thin film crystallization of composition Ge2Sb2Te5 (GST225). It is shown that there are two ways of crystalline phase formation under femtosecond pulses excitation. At the moderate value of laser fluence, crystalline fraction forms inside the solid amorphous phase. This process is apparently two-stage, and this leads to formation of fine-grained polycrystalline material. The second way of crystallization proceeds at higher levels of the laser fluence, closer to the beam axis. In this case, larger grains are produced closer to the beam axis, and eventually no new nuclei can be formed due to high cooling velocity leading to inhibition of nucleation. The article also presents the results on the effect of the substrate type (metallic or oxide) and film thickness on crystallization, which is due to the light penetration depth and reflection from the film-substrate interface.

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.

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.

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.

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.

We report the first direct laser-writing of buried channel waveguides in ZnS. A depressed cladding waveguide with propagation loss of 0.62 dB/cm at 1030 nm allowed to obtain spectral broadening under femtosecond-pumping at 1030 nm. © 2018 The Author(s). One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.

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.

One of the most prospective electrical and optical nonvolatile memory types is the phase change memory based on chalcogenide materials, particularly Ge2Sb2Te5. Introduction of dopants is an effective method for the purposeful change of Ge2Sb2Te5 thin film properties. In this work, we used the ion implantation method for the introduction of In and Sn into Ge2Sb2Te5 thin films by a Multipurpose Test Bench (MTB) at the National Research Center "Kurchatov Institute"-Institute for Theoretical and Experimental Physics. For Sn and In ion implantation into Ge2Sb2Te5, the following MTB elements were used: a vacuum arc ion source, an electrostatic focusing system, and a system for current and beam profile measurements. The MTB parameters for Sn and In ion implantation and its effect on the material properties are presented. Implanted Ge2Sb2Te5 thin films were irradiated by femtosecond laser pulses. It was shown that the ion implantation resulted in a decrease in the threshold laser fluence necessary for crystallization compared to the undoped Ge2Sb2Te5.

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 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.

A large mode area depressed cladding waveguide was inscribed by a femtosecond laser beam in bulk of high purity 70TeO2-22WO3-8Bi2O3 glass. Non-linear propagation of femtosecond pulses in the single mode waveguide with mode field diameter of 12 μm was investigated under input at 1030 nm wavelength. The highest peak value of a Raman-gain coefficient of all the tellurite glasses was measured (1.2x10−9 cm/W). The dispersion of the refractive index was obtained in the range of 1170-2300 nm. Spectrum broadening of up to 160 nm at the −10 dB level was observed under input pulse energy as low as 84 nJ in a 14 mm long waveguide. The spectrum broadening was numerically simulated, and it was found that both the Kerr and the Raman nonlinearities result to comparable contributions to the spectrum transformation.

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.

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 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.

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.

Nonlinear-optical properties of nematic liquid crystal 5CB doped with a small amount of high-molecular polymer MEH-PPV (poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]) have been studied. This liquid crystalline system was shown to exhibit a negative nonlinearity due to the light-induced director reorientation away from the light-field direction.

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.

Light interaction with nematic liquid crystals doped with high-molecular polymer MEH-PPV (poly[2-methoxy-5-(2'-ethylhexyloxy)-l,4-phenylenevinylene]) has been studied. The director reorientation away from the light field was found within the absorption band of the polymer. The reorientation is explained by the appearance of the torque due to changing the intermolecular forces between the polymer macromolecules and nematic-host molecules under polymer excitation by light absorption.

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.