Jean-Michel Nunzi

Anisotropy of the photo-induced translation diffusion of azobenzene dyes in polymer matrices

Large-amplitude negative holographic surface-relief gratings are optically induced by resonant visible exposure in azobenzene dye-containing polymers. We propose a simple diffusion model accounting for the essential dynamic features of surface-relief grating formation. It accounts for intensity and polarization dependences. Understanding of the process appears to be essential for holographic grating applications. It opens new perspectives associated with optically controlled anisotropic diffusion.

A nonvolatile memory element based on an organic field-effect transistor

Organic field-effect transistors were fabricated with pentacene as the active material and a ferroelectric copolymer poly(vinylidene fluoride–trifluoroethylene) as the gate insulator. As-prepared devices showed normal p-type transistor operation. The ON- and OFF-states could be written to the device by applying appropriate voltages to the gate with respect to short-circuited source and drain electrodes. The devices exhibited excellent memory retention properties.

Light-induced second-harmonic generation in azo-dye polymers.

The permanent all-optical poling of an azo-aromatic acrylic copolymer is experimentally demonstrated by seeding preparation in a backward phase-conjugation geometry. The microscopic mechanism involves an orientational hole burning followed by orientational redistribution caused by trans-to-cis isomerization of the azo-dye chromophores. The characteristic kinetics of monitored by second-harmonic generation.

Quasi-permanent all-optical encoding of noncentrosymmetry in azo-dye polymers

We present an original poling technique that uses a purely optical excitation process. The experiment consists of a seeding-type process. Writing and probing periods are alternated. Writing periods correspond to simultaneous irradiation of the sample by the coherent superposition of the 1064-nm fundamental and the 532-nm second-harmonic light of a picosecond-pulsed Nd:YAG laser. The sample is a spin-coated film of a poly(methyl methacrylate) copolymer onto which the azo-dye molecule Disperse Red 1 is grafted. We demonstrate efficient and quasi-permanent poling of the molecules with a spatial period that satisfies the phase-matching condition for second-harmonic generation. The influence of seeding parameters such as the relative phase and the relative intensities between the writing beams is studied both theoretically and experimentally. Tensorial properties and the spatial profile of the photoinduced χ(2) are analyzed from a microscopic point of view. Dark and photostimulated relaxation processes are investigated from a chemical-physics point of view. The physical origin of the photoinduced molecular orientation process is discussed. A minimal model involving the relevant experimental parameters is developed. Numerical simulations are in agreement with the experiment.

Efficient polymer-based interpenetrated network photovoltaic cells

Organic solar cells based on an interpenetrated network of conjugated polymer as donor and fullerene derivative as acceptor materials have a great potential for improving efficiency. We fabricated a device based on a composite of poly(2-methoxy-5-(2′-ethylhexyloxy)-1, 4-phenylenevinylene and [6,6]-phenyl C60 butyric acid methyl ester. Surface treatment, insertion of interfacial layers, and improvement of the morphology of the active layer significantly increase the photovoltaic performances of the structure. We obtain an open circuit voltage of 0.87 V and short circuit current density of 8.4 mA/cm2 under 100 mW/cm2 air-mass 1.5 solar simulator illumination, yielding a 2.9% power conversion efficiency.

Anisotropy of the photoinduced translation diffusion of azo-dyes

Large-amplitude holographic surface relief gratings are optically induced by resonant visible exposure in azobenzene dye-containing polymers. We propose a simple diffusion model accounting for the essential features of surface relief grating formation. It is based on the anisotropic diffusion of azobenzene-dyes in polymer matrices further to their photoinduced trans-cis isomerization cycles. The essential feature of the model is that dye-molecules undergo a 1D-random-walk along their excitation direction. The model accounts for intensity, grating-pitch and polarization dependencies. Understanding of this process appears essential for holography applications. It opens new perspectives associated with optically controlled anisotropic diffusion.

Laser emission in periodically modulated polymer films

We report on the realization of a compact distributed feedback laser using luminescent polymer films where the optical feedback is provided by Bragg diffraction on an index grating. Permanent modulation of the polymer refractive index is achieved using an original technique for photoinduced patterning of surface relief grating, using laser-controlled mass-transport in azoaromatic polymers. We describe the fabrication of such surface gratings and show the laser emission properties resulting from a transversal one-photon pumping of the sinusoidally modulated polymer films upcovered with a luminescent-dye-doped film. Control of the laser wavelength by the grating pitch is evidenced.

Molecular migration mechanism for laser induced surface relief grating formation

Direct and reversible holographic recording of surface-relief gratings in azo-dye polymers was recently evidenced using atomic force microscopy. Irradiation with an interference pattern of polarized laser beams was observed to lead to substantial mass-transport. In typical experiments, the wavelength of the laser was chosen to be near the absorption of the chromophores. The surface gratings have a negative amplitude which can be as large as twice the polymer film thickness. The origin of such photo-driven mass transport is still unclear. We provide here experimental evidence of a migration of the chromophores from high to low intensity regions. It indicates that, in such doped or grafted guest-host systems, polymer chain migration is a result of the guest chromophores photoinduced movements. This is in good agreement with our recently developed model of azo-dye photoinduced translation diffusion.

Picosecond photoinduced dichroism in solutions of thiophene oligomers

Abstract The picosecond photoinduced absorption of a series of α-coupled thiophene oligomers in solution have been measured using the Kerr ellipsometry configuration. We present spectral and dynamic studies of the sharp transient absorption peak observed in the transparency region of these oligomers. Its origin and the reasons for its narrow line shape are discussed. The conversion towards the triplet state is observed in terthiophene.

Electrode interface effects on indium–tin–oxide polymer/metal light emitting diodes

A spin‐coated doped‐polymer light emitting diode is studied. Blue electroluminescence decays within 1 h. X‐ray reflectivity analysis of the aged diode shows the formation of an interfacial layer made of the ITO semitransparent electrode indiffused into the polymer. X‐ray reflectivity stands as a powerful tool for aging studies of organic semiconducting devices.