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Time evolution of diffraction efficiency for homoand copolymer films. Light power was 215 mW; sample temperature 110uC. The arrows show the laser beam switching off.
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For the first time a comparative study of holographic recording in planarly oriented films of nematic and cholesteric azobenzene‐containing polymers was performed. The influence of temperature and light intensity on the values of diffraction efficiencies of holographic gratings was investigated. The kinetics of grating relaxation at different tempe...
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... seen from figure 1, the magnitude of the diffrac- tion efficiency is about one order of magnitude higher for the homopolymer as compared to the one for the copolymer. This fact reflects a significant influence of the helical supramolecular organization within the cholesteric copolymer on the light-induced optical modulations. The presence of the helical structure seems to prevent the reorientation of azobenzene moieties induced by polarized light to a significant extent. After switching off the recording laser beams, the diffraction efficiency rapidly drops down to zero. Figure 2 shows microphotos of the texture of the copolymer before and after holographic recording followed by switching the recording beam off. Initially the copolymer film forms a planar texture with typical cholesteric disclination lines (oily strikes) (figure 2a). After irradiation and switching off the laser the texture transforms to a non-oriented scattering state (figure ...
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... first we will consider the peculiarities of holographic recording for the homopolymer and copolymer at high temperatures, corresponding to the temperature range of the nematic or the chiral nematic phases, respectively. As can be clearly seen from figure 1, the irradiation of the homo-and copolymer using the holographic set up leads to a fast increase in diffraction efficiency. Most probably, the grating formation in such polymers is caused by the photo-orientation of azobenzene frag- ments into a direction perpendicular to electric field vector of light as described previously ...
Citations
... Recording media have been a key issue in realizing holographic data storage. Among different materials [1][2][3][4][5], the photopolymer has been considered as a promising medium due to several advantages, such as it being easy to modify the compositions, cheap, simple fabrication and large refractive index change. Several techniques have been 4 Authors to whom any correspondence should be addressed. ...
... In addition, to improve the sensitivity, radical and cationic photopolymerization with different additives and photosensitizers has been applied to speed up the photoinitiation process, such as sol-gel [19,20] and twostep thermo-polymerization [22][23][24][25]. Neutral components such as bromonaphthalene [3], liquid crystals [4,5,26,27] or nanoparticles [28][29][30][31] are introduced into the photopolymer to enhance the diffraction efficiency. This research is aimed at developing a material with large refractive index change, high sensitivity and good dimensional stability. ...
In this research, we fabricate and characterize lanthanide organometallic compounds and 9,10-phenanthrenequinone (PQ) co-doped poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) photopolymers for holographic recording. Five different lanthanoid (Ce3+, Nd3+, Er3+, Yb3+ and Lu3+) organometallic compounds, one at a time, is co-doped with PQ in a poly(HEMA-co-MMA) matrix, respectively. Holographic experiments demonstrate that lutetium organometallic compounds and a PQ co-doped poly(HEMA-co-MMA) photopolymer can greatly enhance the volume holographic characteristics. The degree of improvement in hologram diffraction efficiency and the recording dynamic range for these co-doped photopolymers follows the order: Lu3+>Yb3+>Er3+>Nd3+>Ce3+.
Modern optics and photonics constantly require break‐through materials and designs in order to achieve miniature, lightweight, highly tunable, and effective optical devices. One of the basic optical components is the diffraction grating (DG), widely used for the dispersion of light, beam steering, etc. This review gathers research efforts on diffractive optical elements based on cholesteric liquid crystal (CLC) materials with a supramolecular helical architecture. All main types and fabrication approaches of periodic diffractive structures from CLCs are classified and described. Key optical properties of DGs, their advantages and drawbacks are considered. Special attention is paid on the tunability of DGs including design principles and prospective chiral materials. The review consists of three parts divided according to the formation mechanism of diffractive structures: i) the spontaneously formed periodic structures from CLCs confined in cells with hybrid or homeotropic boundary conditions; ii) DGs generated by external electric field applied to CLCs layers; iii) light‐generated DGs (e.g., obtained by holography, mask exposure, photoalignment). The review also aims to initiate and gain collaborations between physicists, engineers and organic chemists to combine novel chiral photoswitches and molecular motors with sophisticated optical design paving the way towards novel smart optical materials.
A new type of self-organized materials based on cholesteric networks filled with photoactive side-chain copolymer is being developed. Supramolecular helical structure of cholesteric polymer network resulting in the selective reflection is used as a photonic scaffold. Photochromic azobenzene-containing nematic copolymer is embedded in cholesteric scaffold and utilized as a photoactive media for optical pattering. 1D and 2D transmission diffraction gratings are successfully recorded in composite films by holographic technique. For the first time the possibility to create selective reflection gratings in cholesteric material mimicking the natural optical properties of cholesteric mesophase is demonstrated. That enables the coexistence of two selective gratings, where one has an intrinsic cholesteric periodic helical structure and the other is a holographic grating generated in photochromic polymer. The full-polymer composites provide high light-induced optical anisotropy due to effective photo-orientation of side-chain fragments of the azobenzene-containing liquid crystalline polymer, and prevent the degradation of the helical superstructure maintaining all optical properties of cholesteric mesophase. The proposed class of optical materials could be easily applied to a broad range of polymeric materials with specific functionality. The versatility of the adjustment and material preprogramming combined with high optical performance makes these materials a highly promising candidate for modern optical and photonic applications.
The results of studies on thermotropic liquid crystalline polymers containing mesogenic groups in the main chains of linear macromolecules or in pendant side-chain branches of comb-shaped polymers are analyzed and summarized. The concept for the preparation of liquid crystalline polymers via introduction of molecules of low-molecular-mass liquid crystals into the macromolecules is outlined. The presence of rodlike anisometric fragments of liquid crystals in the main chains of macromolecules is shown to control the high level of orientational order in melts and solutions of liquid crystalline polymers. The structure and photooptical properties of photochromic comb-shaped liquid crystalline polymers are considered. The mechanism of light-induced structural chemical transformations in photoactive liquid crystalline compounds is addressed. Examples illustrating the development of photocontrollable liquid crystalline polymers and related composites are discussed. Structural optical properties of binary and ternary liquid crystalline photochromic block copolymers with independent modulation of photoalignment of photochromic and non-photochromic subblocks are analyzed. The feasibility of preparation of light-controlled liquid crystalline gels is considered. Special attention is given to mass transfer processes in liquid crystalline polymers, which allow the development of nanostructured surfaces and formation of diffraction gratings as well as enable preparation of diverse supramolecular structures. This review covers the challenges concerning the preparation of light-controlled liquid crystalline dendrimers and holographic media as well as the problems related to the nonlinear optical properties of liquid crystalline polymers. The roadmap for the practical applications of liquid crystalline polymers and their composites as photoactive media in photonics, optics, display technology, and in the systems for data recording and storage is outlined.
The past two decades witnessed by tremendous progress in the field of creation of different types of responsive materials. Cholesteric polymer networks present a very promising class of smart materials due to the combination of the unique optical properties of cholesteric mesophase and high mechanical properties of polymer networks. In the present work we demonstrate the possibility of fast and reversible photocontrol of the optical properties of cholesteric polymer networks. Several cholesteric photopolymerizable mixtures are prepared and porous cholesteric network films with different helix pitches are produced by polymerization of these mixtures. An effective and simple method of the introduction of photochromic azobenzene-containing nematic mixture capable of isothermal photoinducing the nematic-isotropic phase transition into the porous polymer matrix is developed. It is found, that cross-linking density and degree of polymer network filling with a photochromic nematic mixture strongly influence the photooptical behaviour of the obtained composite films. In particular, the densely cross-linked films are characterized by a decrease in selective light reflection bandwidth, whereas weakly cross-linked systems display two processes that are the shift of selective light reflection peak and decrease of its width. It is noteworthy, that the obtained cholesteric materials are shown to be very promising for the variety applications in optoelectronics and photonics.
For the first time the cholesteric mixture containing nematic polymer with small amount of chiral-photochromic dopant is used for electroinduced diffraction gratings production. The gratings are obtained by applying electric field to the planar-aligned cholesteric polymer layer causing its periodical distortion. Material developed permits manipulating supramolecular helical structure by means of UV exposure resulting in helix untwisting. Photo-controlling of helix pitch brings to change parameters of the electroinduced gratings. Due to macromolecular “nature” of the material one can easily stabilize electroinduced gratings by fast sample cooling. All-known cholesteric grating types are realized in the studied polymer material. It is observed that the grating vector can be oriented along or perpendicular to the rubbing direction of the cell. It is shown that the diffraction efficiency is dictated by grating type and the amplitude of the applied electric field and can achieve about 80%. Moreover, the period of gratings can be tuned upon UV light illumination. The possibility of 2D gratings creation is also demonstrated. The described material and approach gives an opportunity to easily fabricate a variety of diffraction gratings with flexibly controllable parameters. Such gratings can be potentially applied in optics, optoelectronics, and photonics as intelligent diffraction elements.
A cholesteric mixture based on the nematic liquid crystalline (LC) side-chain polymer doped with chiral-photochromic compound was prepared and used as an active medium for creation of the stable polarization selective gratings by phototunable modulation of the helix pitch. Such modulation was fabricated in the polymer mixture by a non-polarized 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 polymer film under study forms the right-handed helical structure and, correspondingly, the diffraction of the right-circularly polarized light was only found in the transmittance mode. The maximal diffraction efficiencies were found for the wavelengths laying between the maxima of selective light reflection. The obtained films open very interesting possibilities for development of materials with stable gratings in the entire visible spectra range. Both position and width of the spectral range of an efficient diffraction can be easily controlled by the UV exposure and concentration of the dopant. The obtained materials and developed methods can be used for creation of specific diffraction elements for optics and photonics.
Using a novel experimental method combining of polarizing optical microscopy (POM) and atomic force microscopy (AFM), the surface topography and optical properties of chiral- photochromic LC systems were studied. For this purpose, a mixture of cholesteric cyclosiloxanes with azobenzene-containing dopant was prepared. Correlations between the features of surface topography of mixture films and POM images of planarly oriented texture were found. Polarized light action (532 nm) leads to the formation of partially aligned surface structure features. The observed photooptical effects are associated with E-Z, Z-E isomerization cycles of azobenzene groups, anisotropic cooperative photoinduced rotational diffusion and directional mass-transport of chromophores and mesogens in the films.
Important parameters of hydroxybutyl 4-aroyloxybenzoate ligands were calculated and correlations were made between theoretical and experimental results. In addition, supramolecular side-chain liquid crystalline polymers of the ligands with poly(4-vinylpyridine) were synthesized. Hydrogen bond formation was ascertained by Fourier tranform infra-red spectroscopy. The stabilities of the complexes were determined by thermogravimetric analysis. The mesomorphic behavior was investigated using differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction analysis. Semiempirical study revealed that a ratio of ∼1.2 in the lengths of flexible and rigid parts of the molecules is the most favorable for a wider liquid crystal (LC) temperature range.
The recording of polarization gratings in films of a cholesteric liquid crystalline polymer with different helix pitch was studied in detail. For this purpose, the cholesteric mixture of the nematic azobenzene-containing copolymer with a chiral-photochromic dopant was prepared. The utilization of such mixture has made possible to realize dual optical photorecording in one system, first due to the phototuning of the helix pitch by UV light and second the polarization grating recording process by exposure with polarized visible light. The diffraction efficiency strongly depends on the cholesteric helix pitch and films thickness: the increase of the confinement ratio d/p (where d, film thickness; p, helix pitch) results in growth of the diffraction efficiency. Comparison of the induction of polarization gratings in cholesteric, nematic (copolymer without chiral dopant), and amorphous (nonannealed) cholesteric films revealed that only the cholesteric films were characterized by significant oscillations in the diffraction efficiency signal as well as by the presence of the maximum in the first-order diffraction efficiency in the initial stage of the grating recording process. It was found that in addition to the polarization grating surface relief gratings (SRGs) were also formed in the studied systems, however, the amplitude of the SRG inscribed in the cholesteric films was lower (∼20 nm) compared to the grating amplitude obtained in nematic films (∼60 nm). Moreover, increasing helix pitch resulted in a decrease of the SRG amplitude. The obtained experimental data demonstrate the great potential of cholesteric LC mixtures of such type for different applications as photoactive materials for photonics. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014
