Bruno Darracq

Effect of chromophore–chromophore electrostatic interactions in the NLO response of functionalized organic–inorganic sol–gel materials ☆

Abstract In the last years, important non-linear optical (NLO) results on sol–gel and polymeric materials have been reported, with values comparable to those found in crystals. These new materials contain push–pull chromophores either incorporated as guest in a high T g polymeric matrix (doped polymers) or grafted onto the polymeric matrix. These systems present several advantages, however they require significant improvement at the molecular level—by designing optimized chromophores with very large molecular figure of merit, specific to each application targeted. Besides, it was recently stated in polymers that the chromophore–chromophore electrostatic interactions, which are dependent of chromophore concentration, have a strong effect into their NLO properties. This has not been explored at all in sol–gel systems. In this work, the sol–gel route was used to prepare hybrid organic–inorganic thin films with different NLO chromophores grafted into the skeleton matrix. Combining a molecular engineering strategy for getting a larger molecular figure of merit and by controlling the intermolecular dipole–dipole interactions through both: the tuning of the push–pull chromophore concentration and the control of tetraethoxysilane concentration, we have obtained a r 33 coefficient around 15 pm/V at 633 nm for the classical DR1 azo-chromophore and a r 33 around 50 pm/V at 831 nm for a new optimized chromophore structure.

Stable photorefractive memory effect in sol-gel materials

We report on the synthesis and the optical characterization of a photorefractive sol-gel material which contains nonlinear azo chromophore and carbazole charge transporting molecules. Both of these functional groups are covalently attached to the silica based backbone. Thin sol-gel films exhibit stable optically nonlinear properties: the electro-optic coefficient r13 measured at 633 nm is found to be 17 pm/V one day after corona poling and 15 pm/V after three weeks and over a period of several months. Photorefractivity is demonstrated by two beam coupling experiments without external applied electric field. Sol-gel films present a stable photorefractive memory effect with a net internal gain of 200 cm−1.

Novel photorefractive sol-gel materials

We have developed new photorefractive media based on hybrid organic-inorganic materials containing a charge transporting (CT) molecule either as side-chain or main-chain substituents on the silica backbone. Second order nonlinear optical (NLO) chromophores were introduced either as side chain or as guest units. These materials were prepared by the sol-gel process in the form of thin films of a few μm-thick. NLO and photorefractive properties have been evaluated using electro-optic measurements, two beam coupling experiments and photoconductivity measurements.

Surface and volume gratings investigated by the moving grating technique in sol-gel materials

Abstract We analyze the various contributions to optically induced gratings in an azo-hybrid sol–gel material. This is done by a time-resolved grating translation technique, by polarization and laser intensity dependent experiments, and by index matching with an oil film. We characterize surface relief and anisotropy (index and absorption) gratings. In particular, we measure the phase shift of the surface relief modulation with respect to the fringe interference pattern.

Hole mobilities in sol–gel materials

New silylated precursors with hole transporting units are prepared by modification of different active molecules (carbazole, oxadiazole and tetraphenylphenylenediamine derivatives) using sol-gel precursors. Absorption and photoluminescence spectra show that the electronic structures are not significantly modified by the functionalization. Field dependence of the hole mobility of the different sol-gel layers is measured using the time-of-flight technique. The highest hole mobility is observed for the layer having tetraphenylphenylenediamine units: 5.7 × 10—5 cm2.V—1.s—1 at a field strength of E = 5 × 105 V.cm—1. For the best carbazole compound, the mobility is found to be about twentyfold lower at the same field. Further experiments are required to test these new materials as hole transporting layers in photorefractive and electroluminescent devices. Copyright

Photochromic Sol-Gel Films Containing Dithienylethene and Azobenzene Derivatives: From the Design of Optical Components to the Optical Data Storage

Abstract This work is devoted to photoresponsive hybrid organic-inorganic materials prepared by the sol-gel method, using dithienylethene or azobenzene as photosensitive chromophores. These photochromic units were covalently attached to the silica backbone as part of the main-chain and as a side group respectively. The electrocyclization of dithienylethene or the isomerization of azobenzene under irradiation cause local modifications of the optical properties of the host sol-gel polymers We took advantage of either high refractive index changes (>10−2) or material deformation of the sol-gel films to design optical components and to store information.

Photochromism of Dithienylethene Derivatives Trapped in Sol-Gel Matrices

Abstract Solid-state materials containing photochromic dithienylethene derivatives were prepared by the sol-gel process in the form of thin films of a few μm-thick. The effect of substituents (pyridyl or methoxyphenyl) of the optically active molecules on photochromic properties was investigated. Thicknesses and refractive indices either at 633 nm or 785 nm were determined in the colored and in the discolored state by using the Attenuated Total Reflexion method. Sol-gel films containing methoxphenyl substituted molecules show a large refractive index change, Δn=3.10−3 at 785 nm. This variation is important considering the low doping level (0.7 wt%).

Functionalized dye-doped hybrid sol-gel materials: from solid state dye laser to nonlinear applications and organic photorefractivity

We optically functionalized transparent hybrid matrices prepared by the sol-gel process by doping them with organic chromophores. Highly dense and chemically stable bulk and film materials of good optical quality have been synthesized in this way. Such materials can exhibit very different optical properties, depending on the choice of the chromophores and the type of environment provided by the sol-gel matrices. On one hand, the optical properties conferred to the solid state material can be those for which the dyes have been known and optimized in solution; this is, for example, the case of solid state dye lasers which performances have dramatically improved over the past few years, from the possibility of emitting 1,000s of microjoules pulses to 1,000,000s of millijoule pulses, illustrating the huge optimization potential of this material synthesis route strong reverse saturable absorption for optical limiting applications. . . On another hand, new properties can be demonstrated in the solid form, in particular, due to the restricted motion of the dopants in rotation and translation; that is the case of all optical memory, of second order nonlinear activity ((chi) (2)), and, most recently, photorefractivity. We will especially detail the photorefractive properties of new media based on hybrid organic-inorganic materials containing charge transporting molecules and second order nonlinear optical chromophores introduced as side-chain units. These materials, prepared by the sol-gel process in the form of few micrometers thick films, were studied by two beam coupling (2BC), electronic absorption, electro-optic and photoconductivity experiments. The presence of a strong static electric field (30 V/micrometers ) inside poled films was evidenced, and can explain the photorefractive properties obtained without applying any external electric field. Finally, one common issue all organic-based materials have to solve to take advantage of their high efficiencies and have potential applications outside the laboratories, is the chromophore photodegradation which limits the operational lifetimes of devices made from such materials. This point is under investigation and initial results show that the quantum efficiencies of these photoinduced chemical reaction can vary by several orders of magnitude, within this class of hybrid materials.

Photorefractive Sol-Gel Materials

The authors report the synthesis and characterization of photorefractive sol-gel materials that possess covalently attached push-pull azobenzene and carbazole moieties. Molecular structural characterization of the modified silane monomers was achieved by {sup 1}H NMR and infra red spectroscopy. The second-order nonlinear optical properties of the organic-inorganic hybrid films prepared from modified silane monomers were evaluated by second-harmonic generation. The stabilized value of the second harmonic coefficient, d{sub 33}, of films poled by corona discharge, at 1,064 nm fundamental wavelength was found to be 107 pm/V. Photorefractivity was clearly displayed from a two beam coupling experiment.

Design of optical components and optical data storage in photochromic sol-gel films containing dithienylethene or azobenzene derivatives

We present results on photochromic properties of hybrid organic-inorganic host matrices in which dithienylethene or azobenzene derivatives are trapped. Materials were prepared by the sol-gel process in the form of thin films. To increase the photochrome content inside the silica-based gel, modified alkoxysilanes were synthesized. The photochromic functionalized monomers were copolymerized with methyltriethoxysilane or tetraethoxysilane. The study of sol-gel films containing dithienylethene both in the colored and discolored state shows that a large refractive index change, as high as 4.10-2 can be reached. Design of optical components was performed. Furthermore, from the colored state high remnant optical anisotropy can be photo induced by discoloration with linearly polarized visible light. This anisotropy induces a linear dichroism in the visible absorption band and an important birefringence in the near-IR transparency induces a linear dichroism in the visible absorption band and an important birefringence in the near-IR transparency region. Optical data storage is one of the most fascinating potential applications of this effect. Concerning azobenzene containing materials, we took advantage of material deformation, subsequent to the photoisomerization of the photochromes, to design optical components. Thus, stable surface relief gratings having high modulation depth and high diffraction efficiencies were inscribed on hybrid films. Experiments using near-field optical techniques were performed on the two system in order to improve density data storage.