Stephane Mery

External stimulus driven variable-step grating in a nematic elastomer.

We report on the creation of micro-patterns in an oriented nematic elastomer (an artificial muscle material) by photopolymerization of surface aligned nematic liquid crystal monomers. We demonstrate that microscopic techniques are able to create accurate patterns in rubber-like liquid crystal materials. Two approaches, based on one and two-photon excitations respectively, are implemented using a microscope-based setup. Due to its high spatial selectivity, the two-photon excitation mode yields finer patterns. Benefitting from the intrinsic, thermally-induced, contractile properties of the material, gratings with variable steps in response to temperature changes were fabricated.

Influence of the average molecular weight and the concentration of plasticizer on the orientational dynamics of chromophores in guest-host polymers

We report experimental results obtained from dielectric spectroscopy, electro-optic, and second harmonic generation dynamics measurements on different polymers doped with nonlinear optical chromophores. The polymers considered in this work are polyvinylcarbazole plasticized by ethylcarbazole and polystyrenes with different average molecular weights. Altogether the influence of temperature, average molecular weight of the polymer host, and concentration of plasticizer on the orientational processes of chromophores is investigated and described by polymer rheology laws. Finally, these results, which identify the major role played by the average molecular weight of polymers on the orientational dynamics of chromophores, suggest another possible way of optimization for low glass transition temperature photorefractive polymers, where fast orientational response times are required.

Photostructuration of nonlinear optical properties in doped photopolymers

We present novel substrates for the elaboration of organic active devices for use in integrated optics. The selected materials are based on photopolymerizable matrices doped with optically active molecules. In such organic complexes, photoinduced chemical reactions initiate the polymerization of small monomers inducing an increase in density as well as in viscosity. Since these reactions are limited to the illuminated regions, the properties of these materials are easily patterned. Indeed, a refractive index increase occurs with the densification. Thus, it is possible to create a spatial modulation of the refractive index which can be used to make optical waveguides or phase holograms. Moreover, we induce quadratic non linear optical (NLO) properties by doping the photopolymers with push-pull chromophores. For this purpose, we need a non centro-symmetric alignment of the quadratic optical chromophores. This is obtained by orienting these polar entities with an external static electric field followed by the freeze of their orientation through the viscosity increase associated with the polymerization process. The non linear properties can then be patterned by irradiating the samples through appropriate masks. Long life-time periodically poled structures obtained with NLO chromophores doped photopolymers will be presented.

Chromophore doped photopolymers for integrated optics

Polymers doped with nonlinear optical (NLO) chromophores are attractive candidates for manufacturing optical integrated devices, especially for applications based on second order non-linearities. Such optical devices require to select accurately the areas where the NLO push-pull molecules embedded in the matrix are oriented. This is usually achieved using patterned electrodes deposited onto the samples or microlithography techniques. We suggest a novel approach to control the spatial distribution of the oriented chromophores, based on the use of photopolymerizable mixtures. The formulation developed, which consists of a triacrylate monomer and a photoinitiator, is polymerized upon a visible light irradiation. It is doped with chromophores specially synthesized for this purpose, transparent at the actinic wavelength and highly soluble in the acrylate monomer. The doped photopolymerizable films are poled with a static electric field before irradiation and the orientation of the chromophores is then frozen in the desired areas by irradiating the samples with an appropriate pattern of light. We observed the ability of the polymerized medium to slow down the chromophore relaxation compared to the unexposed solutions. The influence of the formulation composition and irradiation parameters on the temporal stability of the chromophore orientation was studied through second harmonic generation measurements. Infrared spectroscopy measurements enabled to draw a correlation with the monomer conversion degree of the polymer host.

Optical patterning of the quadratic optical properties of doped photopolymers for optical devices

Photopolymers are already well known as holographic materials. The modulation of the refractive index is patterned by the distribution of the cross-linking rate which depends on the illumination conditions. More recently, it has been demonstrated that the quadratic non linear optical (NLO) properties can be patterned in photopolymerized materials doped with push-pull chromophores. In that case, one takes advantage of the huge increase of the viscosity during the polymerization process to freeze the chromophores orientation. By using adapted sequences of applied electric field to orientate the polar NLO molecules combined with appropriate illumination conditions, it is then possible to create periodic poled structures in such doped photopolymers. The technique is therefore especially adapted for the realization of quasi phase matching structures in organic materials.

Role of the polymer viscoelasticity on the orientational processes of chromophores and on the photorefractive performances in low-Tg-doped polymers

The dynamics of the photorefractive properties in low glass transition temperature (Tg) doped polymers essentially depend on the photoconductivity of the host and on the orientational dynamics of nonlinear optical chromophores imbedded in the matrix. A high rotational mobility of push-pull chromophores is required to observe the so-called orientational enhancement. The influence of Tg on the photorefractive performances of guest-host polymers has been previously pointed out. However, the effects of the viscoelastic properties of polymers on the orientational processes of chromophores are neglected in most of the studies devoted to the optimization of photorefractive dynamics. In the present work, the orientational dynamics of chromophores are investigated by dielectric spectroscopy and ellipsometric dynamical measurements in various low Tg doped polymers. The experimental results show the role of different physical parameters (temperature, applied electric field magnitude, amount of plasticizer, average molecular weight of polymer) on the rotational mobility of chromophores. These data underline the necessity to take into account the viscoelastic behavior to improve the dynamics of photorefractive polymers.

Optimization of the efficiencies of photorefractive polymers: correlations between visoelastic properties and electro-optical responses

The photorefractive properties of low glass transition temperature (Tg) doped polymers are essentially ruled by the ability of the push-pull chromophores to align along the electric field. Therefore,a high rotational mobility is therefore needed for these non linear optical chromophores incorporated in the matrix. However, even if the influence of Tg on the photorefractive performances of guest-host polymers has been previously pointed out, this unique parameter is not sufficient to take into account the viscoelastic properties of the matrices. A complete study of the orientational dynamics of chromophores in various low Tg doped polymers, investigated by dielectric spectroscopy, second harmonic generation and electrooptical dynamical measurements, is presented. The results are fully interpreted and modeled by rheologic laws used to describe the viscoelastic behavior of polymers. Finally, the influence of the average molecular weight is also analyzed and then demonstrates the interest of using low molecular weight polymers.

New materials for integrated optics based on functionalized photopolymers

Organic materials are of increasing interest for optical devices, especially in applications based on quadratic optical non linearities. In this context, we have studied photopolymerizable mixtures doped with non linear optical push-pull chromophores. The doped photopolymerizable films are poled with a static electric field and polymerized with a spatially controled irradiation in order to freeze the orientation of the chromophores in the desired areas only. The temporal stability of the chromophore orientation is checked by second harmonic measurements. The photopolymerizable system was chosen by taking into account the possibilities of microstructuration under visible illumination. This property was demonstrated by recording permanent thick phase gratings. Simultaneously, optimization of the polymer matrix from the viewpoint of chemical composition, conditioning and functionalization of the chromophore was performed in order to improve the stability of the chromophore orientation. Formulating of the material requires a fair knowledge of the coupling between photochemical conversion, mass transport, stiffening of the polymer matrix. Photostructuration together with stable non linear optical properties allows now to consider realistic applications.

Bifunctional dimer and polymers for photorefractive applications

We present the electro-optic, photoconductive and photorefractive characterization of a novel organic photorefractive structure. It is composed of a bifunctional dimeric molecule combining both photoconduction and electro-optic properties. This low molecular weight material is a glass, showing a glass transition temperature (Tg) at 27 degrees C. Doped with (2,4,7-trinitro-9-fluorenylidene)malononitrile (TNFDM) as photosensitizer (1% wt), this dimer exhibits 200 cm-1 photorefractive gain at 633 urn for 50 V μm-1 electric field applied. Compared to usual doped polymers which generally show inhomogeneities in the mixtures, this material presents good optical quality thanks to its monophasic character. A serie of functionalized copolymer has also been synthesized, in which the ratio of the chromophore moieties over the charge carrier moieties was varied. These copolymers show photoconductivity but no photorefractivity.