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Metal ion mediated mesomorphism and thin film behaviour of amphitropic tetraazaporphyrin complexes
(2001)
The influence of the azobenzene concentration on the photo-induced surface relief grating (SRG) formation in polymer films was investigated. Two series of polymers with 4-alkoxy-4'-cyanoazobenzene side groups were synthesized. In series A, the degree of substitution was varied, while in series B, azobenzene and biphenyl groups were introduced in varying composition, but the concentration of non-reacted HEMA-groups was kept constant. Photo-induction of the dichroism and the SRG was studied as function of the azobenzene concentration. An optimum was found for the SRG formation (76%), while the highest dichroism was induced at the lowest azobenzene concentration of 20%. The restriction of rotational and translational molecular motions observed at higher azobenzene concentration was explained by pi-pi stacking of the azobenzene moieties and interaction of unreacted HEMA groups
Three series of semiflexible and rigid main-chain polyesters containing photoreactive mesogenic units derived from p-phenylenediacrylic acid (PDA) and cinnamic acid have been synthesized by high-temperature polycondensation. The thermal and mesomorphic properties of the polymers have been determined. The photochemical behavior of polymer P-[1]-T, which contains a PDA unit, has been studied both in solution and in films. In solution, [2+2] photocycloaddition, E/Z photoisomerization, and photo-Fries rearrangement can take place. In contrast, the dominant process in spin-coated films is the [2+2] photocycloaddition reaction, which causes crosslinking of the polymer. In films, the photochemistry and induction of anisotropy are strongly influenced by the aggregation of the PDA phenylester unit. A dichroism of about 0.2 has been induced in films by irradiation with linearly polarized UV light, and thus the capability of these films to induce optical anisotropy and align liquid crystals has been demonstrated. Liquid-crystalline cells have been made with polarized irradiated films of P[1]-T as aligning layers. A commercial liquid-crystalline mixture has been used for this study, and a similar liquid-crystalline order determined by polarized Fourier transform infrared to a commercial cell with rubbed polyimide as an aligning layer has been detected. Because of crosslinking of the irradiated P[1]-T photoaligning layer, the photoinduced anisotropy is stable at high temperatures, and the liquid-crystalline molecules are insoluble in the irradiated polymer. (c) 2005 Wiley Periodicals, Inc
The influence of molecular architecture on light-induced SRG formation was investigated. Polymers with different degree of branching were synthesized by ATRP and functionalized with azobenzene chromophores. The polymers differ only in their architecture - linear, 4-, 6-, or 12-arms stars. The photo-induced dichroism as well as the efficiency of SRG formation was similar for all polymers of this series. New consideration for the origin of the driving force was used to explain this behavior. The comparable SRG inscription rate in differently branched polymers can be rationalized by assuming that azobenzene acts as an internal molecular motor and can cause a non-turbulent motion on a scale smaller than that on which normal entanglement restriction forces act.
Effective volume holographic structures based on organic-inorganic photopolymer nanocomposites
(2009)
We demonstrate a practical approach for the development of a broad range of nanocomposites based on acrylate polymers and organically capped inorganic nanoparticles (NPs). The submicrometer scale volume patterning of the nanocomposites using holographic photopolymerization was investigated. The specific adjustment of both the material parameters (core-shell of the NP, monomer mixture, concentrations) and the patterning conditions led to materials that were suitable for the fabrication of effective optical diffractive elements and specific functional microdevices with light-emissive and nonlinear optical (NLO) properties. The nanocomposite preparation and properties, their holographic performance and some examples of functional polymer-NP structures are reported.
This Letter reports on new methods and a consistent model for voltage tunable optical transmission gratings. Elastomeric gratings were molded from holographically written surface relief gratings in an azobenzene sol-gel material. These were placed on top of a transparent electroactive elastomeric substrate. Two different electro-active substrate elastomers were employed, with a large range of prestretches. A novel finite-deformation theory was found to match the device response excellently, without fitting parameters. The results clearly show that the grating underwent pure-shear deformation, and more surprisingly, that the mechanical properties of the electro-active substrate did not affect device actuation. (C) 2011 Optical Society of America
Reversible structuring of photosensitive polymer films by surface plasmon near field radiation
(2011)
We report on the fabrication and characterisation of a novel type of hybrid azo-modified photosensitive polymer film with a nanoscale metallic structuring integrated into the substrate. The metal structures permit to generate surface plasmon near fields when irradiated by UV-light from the rear without directly illuminating the polymer. This allows establishment of a localized, complex-shape intensity distribution at sub-wavelength resolution with a corresponding impact on the photosensitive polymer. The possibilities of exploiting this setup are manifold. We find that just by using the change of polarization of the incident light as means of control, the topography can be driven to change between various patterns reversibly. These results are confirmed by numerical simulations and compared with in situ recorded topography changes.
The structure and morphology on different length scales dictate both the electrical and optical properties of organic semiconductor thin films. Using a combination of spectroscopic methods, including scanning near-field optical microscopy, we study the domain structure and packing quality of highly crystalline thin films of oligomeric PQT-12 with 100 nanometer spatial resolution. The pronounced optical anisotropy of these layers measured by polarized light microscopy facilitates the identification of regions with uniform molecular orientation. We find that a hierarchical order on three different length scales exists in these layers, made up of distinct well-ordered dichroic areas at the ten-micrometer-scale, which are sub-divided into domains with different molecular in-plane orientation. These serve as a template for the formation of smaller needle-like crystallites at the layer surface. A high degree of crystalline order is believed to be the cause of the rather high field-effect mobility of these layers of 10(-3) cm 2 V(-1) s(-1), whereas it is limited by the presence of domain boundaries at macroscopic distances.
Synthesis, self-organization, and optical properties of supermolecular tripedal liquid crystals incorporating various prototypical mesogenic units such as alkoxy-azobenzene (AZB), alkoxy-biphenylene (BPH) or alkoxy-cyanobiphenyl (OCB) derivatives are reported. Different molecular systems were designed in order to sequentially incorporate the smectogenic-like alkoxy-azobenzene-based chromophore within the molecular structure, whose relative proportion is selectively varied by exchanging with the other mesogens. A divergent synthetic mode was elaborated for their synthesis, starting from the regioselective functionalization of the phloroglucinol-based (PG) inner core. This methodology allowed the preparation of several sets of unconventional tripedal oligomers with conjugated heterolithic structures (made of different blocks, e.g. PG(6)AZB(x)BPH(3-x) and PG(6)AZB(x)OCB(3-x), x = 1 or 2) along the homolithic parents (all identical blocks, e.g. PG(z)AZB(3), z = 6 or 11, z is the number of methylene in the spacer between PG and the protomesogen, PG(6)BPH(3), and PG(6)OCB(3)), respectively. Essentially all the synthesized systems behave as thermotropic liquid crystals and show various types of highly segregated multilayered smectic phases, or, in one case, a nematic phase, depending on the nature of the constitutive anisotropic blocks and on the molecular topology (homolithic versus heterolithic, mesogenic ratio x : 3 - x). The effects of these structural modifications on the mesomorphism (mesophase structures, temperature ranges, and thermodynamic stability) have been investigated by differential scanning calorimetry and small-angle X-ray diffraction experiments combined with dilatometric measurements. Models describing the various supramolecular organizations of these tripedes into such multilayered structures are proposed and discussed. Preliminary results of the investigations of their optical properties will also be presented.
We studied peculiarities of the structural reconstruction within holographically recorded gratings on the surface of several different amorphous azobenzene-containing polymers. Under illumination with a light interference pattern, two processes take place in this type of polymer. The first process is the light-induced orientation of azobenzene units perpendicular to the polarization plane of the incident light. The second one is a transfer of macromolecules along the grating vector (i.e. perpendicular to the grating lines). These two processes result in the creation of a volume orientation grating (alternating regions of different direction or degree of molecular orientation) and a surface relief grating (SRG)-i.e. modulation of film thickness. One can assume that both orientation of molecules and their movement might change the local mechanical properties of the material. Therefore, formation of the SRG is expected to result also in modulation of the local stiffness of the polymer film. To reveal and investigate these stiffness changes within the grating, spin-coated polymer films were prepared and the gratings were recorded on them in two different ways: with an orthogonal circular or orthogonal linear polarization of two recording light beams. A combination of atomic force microscopy (AFM) and ultrasonic force microscopy (UFM) techniques was applied for SRG development monitoring. We demonstrate that formation of the phase gratings depends on the chemical structure of polymers being used, polymer film thickness, and recording parameters, with the height of grating structures (depth of modulation) increasing with both the exposure time and the film thickness. UFM images suggest that the slopes of the topographic peaks in the phase gratings exhibit an increased stiffness with respect to the grating depressions.
A facile one-pot synthesis for preparing thermosensitive, luminescent gold nanodots with diameters of 1-2 nm is presented. The influence of the alkyl chain length of the surface ligands (alkyl thiols) on the optical properties of the gold nanodots was investigated. The synthesized gold nanodots show strong thermosensitive photoluminescence. A photoluminescence quantum yield of 16.6% was observed at room temperature, which could be improved to a value of 28.6% when cooling the gold nanodot solutions to -7 degrees C. The synthesized thermosensitive, luminescent gold nanodots are interesting candidates for optoelectronic devices, medical imaging, sensing, or security labels.
Threshold reduction and emission enhancement are reported for a gold nanoparticle-based waveguided random laser, exploiting the localized surface plasmon resonance excitation. It was experimentally found that a proper thickness of the spacer layer between the gold nanoparticles and the gain layer enhances the random laser performance. It tunes the coupling between the gain polymer and the gold nanoparticles and avoids the quenching of emission in close contact to the gold nanoparticles which is considered as one of the main sources of loss in the current laser system. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4800776]
We report on the influence of localized surface plasmon resonance excitation of Au@SiO2 core-shell nanoparticles on the amplified spontaneous emission of a semiconductor polymer composite (F8BT/MEH-PPV). Au@SiO2 nanoparticles are compatible with the donor-acceptor polymer matrix and get uniformly distributed within the whole polymer film. The plasmon resonance band of the nanoparticles correlates with both the emission and excitation spectra of the polymer composite, as well as with the donor emission and acceptor excitation spectra. We demonstrate that resonantly excited Au@SiO2 nanoparticles enhance the amplified spontaneous emission and the modal gain of the polymer films. The measurement of influential factors reveals that the emission is enhanced predominantly by the increase of acceptor excitation rate, which is accompanied by depletion of the FRET efficiency and increase of quantum yield. The enhancement factor is increased by both introducing a higher loading of plasmonic nanoparticles in the polymer film and increasing the excitation energy. This work shows that these plasmonic nanoantennas are able to enhance the stimulated emission of semiconductor polymers by improving the size mismatch between the excitation light and the emitting polymer.
A new class of star-shaped, liquid crystalline, low-molecular weight compounds functionalized with photochromic azobenzene and mesogenic groups was investigated in terms of light-induced anisotropy. The behaviour of the materials under the action of light with simultaneous or subsequent thermal treatment was examined with respect to the induction of anisotropy. The unconventional UV light treatment prior to the irradiation with linearly polarized light allowed induction of very high values of anisotropy (D = 0.77) at room temperature. Moreover, the simultaneous action of light and temperature led to the induction of higher values of dichroism in comparison with anisotropy generated by the standard procedure. Subsequent thermal treatment led to dewetting and the formation of 3D macroscopic stripe- and dome-like structures for one of the investigated compounds. Despite photoinduction of anisotropy by a single beam, the formation of polarization and surface relief gratings by two-beam interference pattern was also investigated.
Passive and active polarization elements were created by surface and bulk photo-alignment of LCs, reactive LCs, photo-sensitive LCP and photo-curable monomer/LC composites. The use of different photo-sensitive liquid crystalline materials for the development of highly anisotropic elements with high spatial resolution and stability or, alternatively, fast switch ability will be discussed. Photo-active and voltage tunable polarization and diffraction elements are presented. For active micro-optic application a photo-addressed patterned retarder was created. Electrically switchable diffraction gratings were generated by interference exposure of photo-curable LC composites at room temperature characterized by droplet free morphology. These polarization sensitive diffraction elements are characterized be excellent optical properties and low switching times.
The approach of deriving new photoresponsive active supramolecular azopolymers based on the hydrogen bonds is described. Polymers with imide rings, i.e., poly(esterimide)s and poly(etherimide)s, with phenolic hydroxyl or carboxylic groups were applied as matrixes for the polymer dye supramolecular systems. Supramolecular films were built on the basis of the hydrogen bonds between the functional groups of the polymers and various azochromophores, that is, 4-phenylazophenol, 4-[4-(6-hydroxyhexy loxy)phenylazo]benzene, 4[4-(6-hexadecaneoxy)phenylazo]pyridine and 4-(4-hydroxyphenylazo)-pyridine. The hydrogen bonding interaction in azo-systems were studied by Fourier transform infrared spectroscopy and for selected assembles by H-1 NMR technique. The obtained polyimide azo-assembles were characterized by X-ray diffraction and DSC measurements. H-bonds allow attaching a chromophore to each repeating unit of the polymer, thereby suppressing the macroscopic phase separation except for the systems based on 4-[4-(6-hydroxyhexyloxy)phenylazo]benzene. H-bonds systems were amorphous and revealed glass transition temperatures lower than for the polyimide matrixes (170-260 degrees C). The photoresponsive behavior of the azo-assemblies was tasted in holographic recording experiment. (C) 2015 Elsevier B.V. All rights reserved.
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.
The past two decades witnessed 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 photo-optical behavior 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: 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.
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.
We demonstrate plasmonically nano-engineered coherent random lasing and stimulated emission enhancement in a hybrid gainmedium of organic semiconductors doped with core-shell plasmonic nanoparticles. The gain medium is composed of a 300 +/- 2 nm thin waveguide of an organic semiconductor, doped with 53 nm gold nanoparticle cores, isolated within silica shells. Upon loading the nanoparticles, the threshold of amplified spontaneous emission is reduced from 1.75 mu J cm(-2) x 10(2) for an undoped gain medium, to 0.35 mu J cm(-2) x 10(2) for a highly concentrated gain medium, and lasing spikes narrower than 0.1 nm are obtained. Most importantly, selection of silica shells with thicknesses of 10, 17 and 21 nm enables engineering of the plasmon-exciton energy coupling and consequently tuning of the laser slope efficiency. With this approach, the slope efficiency is increased by two times by decreasing the silica shell from 21 nm down to 10 nm, due to the enhancement of the localized electric field.
We present an optically addressed non-pixelated spatial light modulator. The system is based on reversible photoalignment of a LC cell using a red light sensitive novel azobenzene photoalignment layer. It is an electrode-free device that manipulates the liquid crystal orientation and consequently the polarization via light without artifacts caused by electrodes. The capability to miniaturize the spatial light modulator allows the integration into a microscope objective. This includes a miniaturized 200 channel optical addressing system based on a VCSEL array and hybrid refractive-diffractive beam shapers. As an application example, the utilization as a microscope objective integrated analog phase contrast modulator is shown. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
We present an optically addressed non-pixelated spatial light modulator. The system is based on reversible photoalignment of a LC cell using a red light sensitive novel azobenzene photoalignment layer. It is an electrode-free device that manipulates the liquid crystal orientation and consequently the polarization via light without artifacts caused by electrodes. The capability to miniaturize the spatial light modulator allows the integration into a microscope objective. This includes a miniaturized 200 channel optical addressing system based on a VCSEL array and hybrid refractive-diffractive beam shapers. As an application example, the utilization as a microscope objective integrated analog phase contrast modulator is shown. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement