Photoinduced effects in hybrid sol-gel materials

Hybrid silica-based sol-gel films containing Disperse Red 1 (DR1), carbazole units (CBZ) and 2,4,7-trinitro-9-fluorenone (TNF) at different concentrations show reversible photoinduced birefringence. Polarization sensitive holographic gratings have been produced and characterized. Dichroism measurements give information on the orientation mechanisms and on the aggregation of the dye molecules. The alignment of the DR1 molecules by corona poling is verified by second harmonic generation (SHG) and a moving grating technique is used to investigate an asymmetric energy exchange in a two-beam coupling experiment, typical of photorefractive gratings.     

Small dosage of holographic exposure via a He-Ne laser to fabricate tunable liquid crystal phase gratings operated with low electric voltages

The fabrication processes of tunable liquid crystal (LC) phase gratings via small dosage of holographic exposure of a He-Ne laser beam were investigated. The initial LC cells were filled with various ratios of ingredients mainly including LCs and photocurable monomers. The fabricated LC phase gratings shown in the Raman–Nath regime possessed a maximum value of first-order diffraction efficiency close to 33.3% at 5.8 V_rms. Furthermore, optimised grating was demonstrated and used as an interference recorder in holography.     

Comparative study of holographic recording in cholesteric and nematic azo-containing side-chain polymers

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 temperatures was studied. It was shown that the helical supramolecular structure of cholesteric copolymer causes a significant decrease of the diffraction efficiency in comparison with the one observed for the nematic state of the homopolymer.     

Polymer network liquid crystal grating/Fresnel lens fabricated by holography

In this article, polymer network liquid crystal (PNLC) grating/Fresnel lens is fabricated by holography. The exposure light pattern for the grating is obtained by interfering two planar wave fronts, while the Fresnel pattern is achieved by interfering a planar wave front and a spherical wave front. Owing to the alignment effect and anchoring power of polymer network, the holographic PNLC grating achieves improved diffraction efficiency, and remarkably reduced operation voltage (reduced by 80%) compared with holographic polymer-dispersed-liquid-crystal and holographic polymer-stabilised blue-phase liquid-crystal gratings, while maintaining submillisecond response. Moreover, it achieves high spatial frequency with a 2-μm grating period, thanks to the holographic fabrication. The holographic PNLC Fresnel lens also exhibits attractive electro-optical properties.     

Liquid crystal orientation by holographic phase gratings recorded on photosensitive Langmuir-Blodgett films

Holographic gratings were recorded on photosensitive Langmuir-Blodgett films characterized by UV spectroscopy, birefringence measurements and atomic force microscopy. Different polarizations of Ar laser writing beams create particular patterns of chromophore orientation in the diffraction spots. The gratings were shown to orient a nematic liquid crystal with the director parallel to the axes of the chromophores predetermined by film irradiation. In the case of the sp grating (recorded with laser beams polarized perpendicular to each other), two equivalent easy directions for the liquid crystal orientations at 90degree with respect to each other were observed; that is a quasi-bistable anchoring interface had been prepared. Measurements of the pretilt angles theta1s and anchoring energy Ws of 5CB on different holographic gratings show that this orientation technique is very promising for display technology.     

New developments in the dye-doped polymer dispersed liquid crystals gratings: A review

In this study, the dye-doped polymer dispersed liquid crystals (PDLC) gratings techniques performed by the various research groups or being developed are briefly reviewed. Especially, the electrically switched diffraction and holographic gratings, have attained much attention by various research groups working in the PDLC-related display studies. The fabrication methodologies used for such grating texture, include like the conventional dye-doped PDLC grating, Azo-dye doped PDLC gratings, and lasing techniques etc., adopted for dye-doped PDLC gratings. The useful features and characteristics of their fabrication process of such gratings are discussed. Finally, some of the future perspectives on this particular research field are presented.     

Optical grating recording in highly organized thin films of Disperse Red 1

Optical grating recording with submicrometer spatial resolution, which can handle grey-level patterns, has been investigated in photochromic material made of Disperse Red 1 (DR1) molecules vacuum-deposited on a glass substrate. Holographic gratings of periods Λ within the range of 0.6 μm - 12 μm were recorded by 514.5 nm light from cw Ar⁺ laser using a degenerate two-wave mixing technique. Despite the very small DR1 layer thickness (∼ 0.1 μm), the diffraction efficiency measured in a Raman-Nath scattering regime reached 2 %. The obtained amplitude gratings were analysed with an optical microscope and Fourier transforms. Grating profiles were analysed in relation to exposure conditions and in correlation with molecular organisation. Polarising microscopy studies revealed the presence of light-induced optical anisotropy. Following that, we have checked the possibility of polarisation-sensitive recording in this medium.     

Dynamics of biphotonic intensity holographic gratings based on dye-doped liquid crystal films

The dynamics of biphotonic intensity holographic gratings (BIHGs) based on dye-doped liquid crystal (DDLC) films, including optical and thermal effects, are studied. Experimental results indicate that the formation of a BIHG involves bulk reorientation and surface adsorption. The former yields a transient biphotonic grating; the latter results in a persistent biphotonic grating. Additionally, the dynamic behaviours of the biphotonic diffraction signals are different from those of a conventional one-photonic diffraction signal, and depend on the intensity(polarization) of the green(red) pump-beam. The effect of ambient temperature on the diffraction efficiency of a BIHG is also studied: a higher ambient temperature prevents more dye molecules from being adsorbed on the substrate.     

Holographic grating recording in azobenzene functionalized polymers

A group of 22 polymers have been synthesized to test their suitability for recording holographic gratings. Polyamides, polyimides, polyesters and their combinations were functionalized with pendant azobenzene groups containing single or double N=N. The polymers were studied using a standard degenerate two-wave mixing technique, which enables measurement of light-induced periodic modification of polymer refractive index and absorption coefficient by analysis of the diffracted light. Two qualitatively different configurations of the holographic polarization recording were used, s-s and s-p. The relationship between structural properties of polymer matrix and azobenzene groups and the holographic grating recording kinetics and light diffraction efficiency was investigated.     

Analysis of the kinetics of diffraction efficiency during the holographic grating recording in azobenzene functionalized polymers

The laser-assisted holographic grating recording process in films of azobenzene functionalized polymers is usually studied by observation of the efficiency of light scattering on a developing in time diffraction grating. Various possible mechanisms contributing to grating formation as well as the bulk or surface origin (bulk refractive index and/or relief grating) of light scattering make the analysis of kinetics of grating recording, from the light scattering data only, difficult and ambiguous. To fully explain experimentally observed various and complex (frequently nonexponential) kinetics of the first-order light diffraction intensity, we considered a simple single-exponential growth of the two phase gratings in the same polymer film. In modeling we assumed that the bulk refractive index grating Deltan(t) and the surface relief grating Deltad(t) differ considerably in their growth rates and we allowed for a nonstationary phase shift Deltaphi(t) between them which was experimentally observed during the recording process. The origin of the nonstationary phase shift is a result of a slow shift of interference pattern due to delicate symmetry breaking in illumination conditions (e.g., difference in beam intensities and deviation of exact symmetrical beam incidence angles on the sample). Changing only such parameters as stationary amplitudes of refractive index and relief gratings for a span of phase shifts (0-pi) between them, we obtained a series of kinetic responses which we discuss and interpret. The various examples of temporal evolution of diffraction efficiency for the same grating formation kinetics, modeled in our work, supply evidence that great care must be taken to properly interpret the experimental results.     

Diffusion kinetics investigations of Nano Ag-doped holographic polymer dispersed liquid crystal gratings

ABSTRACT

In this paper, we use multicomponent mutual diffusion method to derive a one-dimensional non-local diffusion dynamic model to describe the diffusion kinetics of a dynamic holographic polymer dispersed liquid crystal grating (H-PDLC) doped with nano-silver. The physical mechanism of diffusion between monomer and liquid crystal, monomer and nano-silver particles is analysed using this model. Using coupled-wave theory, the H-PDLC’s diffraction efficiency curve with the expose time are simulated due to the vivid changing of effective refractive index modulation caused by the movement of concentration of each component with the expose time. Correspondingly, in the experiment, the diffraction efficiency of the grating is measured in real time, which shows the improvement for the holographic properties because of nano-silver doped H-PDLC. The simulation results have a good agreement with experimental data by fitting the corresponding parameters of the model. In addition, through comparing with simulation and experimental results with doping different concentrations of nano-silver particles, the recipe and diffraction characteristics of H-PDLC grating can be improved. Thus, the diffusion Kinetics model can be used to optimise the phase separation of the PDLC grating, and finally to improve the opto-electrical properties of H-PDLC gratings.     

Dynamic holographic liquid crystal device containing nanoscale CuPc film

In this paper, CuPc-film-containing nematic liquid crystal (NLC) cells were fabricated, and dynamic holographic gratings recorded in the NLC cells were studied. The dependence of the diffraction efficiency on the applied voltage, light intensity, and the thicknesses of CuPc and NLC film was investigated. The high diffraction efficiency about 30% and the moderate build-up and erasing time in the order of magnitude of 0.1 s were achieved. The asymmetric energy transferring in two-beam coupling indicates photorefractive (PR) nature of the grating. The results of the photoinduced change of birefringence experiments suggest that the surface charge at the interface between the CuPc film and the NLC film plays a crucial role in the PR effect and the CuPc–NLC interface is mainly responsible for the formation of modulated surface charge layer and enables the photoelectric function of the NLC device. Our research may provide a new material option to optimise and develop NLC device for optical information processing and storage.     

Preparation of holographic gratings of liquid crystals dispersed in polyurethane acrylates under controlled reaction conditions.

Transmission gratings of holographic polymer-dispersed liquid crystals (HPDLC) are prepared under controlled reaction conditions by adding various amounts of chain-transfer agent (CTA). The resulting films have a higher gel content with lower cross-link density, less dark reaction, less grating shrinkage with much smoother LC/polymer interfaces, smaller induction period, and fast saturation of diffraction efficiency, which shows a maximum of 95 % with 1 % CTA. An optimum LC content of 35 % is verified on the basis of morphology and reaction kinetics.     

Enhanced wavelength-sensitivity of volume holographic grating with dual-photoinitiation system in polymer-dispersed liquid crystal

We propose a method to improve the wavelength-sensitivity of a volume holographic grating by mixing dual-photoinitiation dopants, namely, rose bengal (RB)/N-phenylglycine (NPG) system and methylene blue (MB)/p-toluenesulfonic acid (PTSA) system, which enable the fabrication of gratings upon both green and red light illumination. The RB/NPG green light photoinitiation system is chosen in combination with the red-active MB/PTSA system because RB and MB absorb light near 567 and 665 nm independently. In this case, the holographic gratings, recorded in polymer-dispersed liquid crystal, can be fabricated simultaneously by two different visible laser lights with an output wavelength of 532 and 632.8 nm. The electro-optical performances of the gratings exposed upon 532 and 632.8 nm have been implemented: the diffraction efficiencies (DE) are 75% and 57% respectively when the exposure time is optimised to 2 min and 17 min; the threshold voltage is approximately 2.9 V/μm. The enhanced wavelength-sensitivity of holographic gratings allows for the storage of three-dimensional (3D) images on the same hologram plate, and these 3D images are easily reconstructed by both red and green light.     

A multiple grating flame photometer for the simultaneous determination of five elements

A flame emission spectrometer for the simultaneous determination of five elements has been constructed. The instrument is built around a special holographic diffraction grating, built up from several smaller gratings with different ruling characteristics, each covering a different spectral range. It is provided with automatic background correction and a photon counting system. Sensitivity and precision are good. The instrument is compared with other multichannel flame emission systems. Construction of the spectrometer including electronics is described.     

A novel process for holographic polymer dispersed liquid crystal system via simultaneous photo-polymerization and siloxane network formation

Formation of transmission holographic polymer dispersed liquid crystal gratings was studied for matrix components of trimethylolpropane triacrylate:trimethoxysilylmethyl methacrylate:1-vinyl-2-pyrrolidone (reactive diluent) in the range from 80:10:10,wt% to 10:80:10,wt% and E7 as nematic liquid crystal under the irradiation with Nd-YAG laser (532,nm). The optimum concentration of E7 in the recording solution was 35,wt% (65,wt% of matrix components) in presence of small amounts of radical photo-initiator system (Rose Bengal 0.05,wt%, NPG 0.1,wt%). When the concentration of methacrylate in matrix components was low (< 30%), gratings with apparently high diffraction efficiency could be fabricated both for trimethoxysilylmethyl methacrylate (cross-linkable by hydrolysis) and trimethylsilylmethyl methacrylate (non-cross-linkable by hydrolysis), although transiently high initial diffraction efficiency was observed by the non-equilibrium initial photo-polymerization of cross-linking components. Distinct difference was seen at higher concentration (> 50,wt%) of the methacrylates. Contrary to that gratings with reasonably high and stable diffraction efficiency were successfully fabricated with trimethoxysilylmethyl methacrylate, gratings with only low diffraction efficiency were obtained for non-cross-linkable trimethylsilylmethyl methacrylate. With higher concentration of photo-sensitizer and photo-initiator (0.2,wt%; 1.0,wt%), and shorter irradiation time, gratings with high diffraction efficiency could be fabricated only for the photo-initiator system of 3,3′-carbonylbis(7-diethylaminocoumarin) and diphenyliodonium hexafluorophosphate with shorter induction period (∼174,s). Grating with diffraction efficiency of 72% was obtained with trimethoxysilylpropyl acrylate (80,wt% in the matrix component) and 35,wt% E7. By increasing the concentration of diphenyliodonium hexafluorophosphate to 2,wt%, diffraction efficiency increased to 85% and induction period was shortened to 129,s, and low volume shrinkage of 8% was attained via simultaneous radical cross-linking of trimethylolpropane triacrylate and siloxane network formation of trimethoxysilyl groups of trimethoxysilylmethyl acrylate by atmospheric moisture catalyzed by cationic species produced from the initiator system. In SEM morphology, although gratings formed with high concentration of trimethoxysilylpropyl acrylate had some cracks in polymer matrix, the largest grating spacing indicating the lowest volume shrinkage and very regular and well-defined gratings were observed.     

Conducting polymer diffraction gratings on gold surfaces created by microcontact printing and electropolymerization at submicron length scales

Conducting polymer diffraction gratings on Au substrates have been created using microcontact printing of C18-alkanethiols, followed by electropolymerization of either poly(aniline) (PANI) or poly(3,4-ethylenedioxythiophene) (PEDOT). Soft-polymer replicas of simple diffraction grating masters (1200 lines/mm) were used to define the alkanethiol template for polymer growth. Growth of PANI and PEDOT diffraction gratings was followed in real time, through in situ tapping-mode atomic force microscopy, and by monitoring diffraction efficiency (DE) as a function of grating depth. DE increased as grating depth increased, up to a limiting efficiency (13-26%, with white light illumination), defined by the combined optical properties of the grating and the Au substrate, and ultimately limited by the loss of resolution due to coalescence of the polymer films. Grating efficiency is strongly dependent upon the grating depth and the refractive index contrast between the grating material and the surrounding solutions. Both PEDOT and PANI gratings show refractive index changes as a function of applied potential, consistent with changes in refractive index brought about by the doping/dedoping of the conducting polymer. The DE of PANI gratings are strongly dependent on the pH of the superstrate solution; the maximum sensitivity (DeltaDE/DeltapH) is achieved with PANI gratings held at +0.4 V versus Ag/AgCl, where the redox chemistry is dominated by the acid-base equilibrium between the protonated (emeraldine salt) and deprotonated (emeraldine base) forms of PANI. Simulations of DE were conducted for various combinations of conducting polymer refractive index and grating depth, to compute sensitivity parameters, which are maximized when the grating depth is ca. 50% of its maximum obtainable depth.     

Fast and stable recording of birefringence and holographic gratings in an azo-polymethacrylate using a single nanosecond light pulse

Pulsed light-induced recording in azobenzene polymers has recently been studied due to its potential use in optical storage applications. In this paper we study the photoinduced birefringence (Deltan) and holographic grating recording in an azobenzene side chain liquid-crystalline polymethacrylate irradiating with a single 4 ns light pulse at 532 nm. For some irradiation conditions, Deltan grows in less than 50 ns reaching an essentially stable value of about 10(-2). Holographic gratings have been registered using intensity and polarization patterns. Fast response and stability, similar to those of Deltan, was observed in the holographic recording process. Both light-induced anisotropy and relief contributions have been found in the case of gratings recorded using intensity patterns, relief being the dominant contribution at high recording energies. Polarization gratings have been recorded using two orthogonally circularly polarized beams. The resultant gratings showed stable efficiencies up to 0.8% (measured at 633 nm in 1-mum-thick films) and no measurable relief was observed.     

Holographic method for determining the spatial extent of photochemistry: Room-temperature photopolymerization of diacetylene TS6

An experimental arrangement is described that makes possible the production of holographic gratings with lasers that have low temporal and spatial coherence. This scheme is incorporated into a strategy for measuring the spatial spread of photochemistry. In preliminary experiments on single crystals of diacetylene TS-6 (diacetylene-bis-(p-toluenesulfonate)) we show that high-quality gratings can be produced directly with a XeCl laser and that it is possible to read out from these gratings data from which the refractive index profile for the grating stripes can be directly determined.     

Optical storage in a smectic mesophase: thermal amplification of light-induced chromophore orientations and surface relief gratings

This contribution is concerned with the exceptionally strong influence of smectic mesophase formation on the optically induced orientation and on the surface relief grating formation in a liquid crystalline melamine derivative containing azobenzene groups. We demonstrate that the strong amplification of optically induced holographic gratings by thermal treatment is not only caused by the enhancement of photo-orientation through the mesophase formation, but is furthermore a consequence of significant amplification of the amplitude of surface relief gratings during the thermal development.