Conjugaison de phase en régime picoseconde d'absorption à deux photons dans les polydiacétylènes

Polydiacetylenes exhibit large two-photon absorption at 1064nm wavelength. Its different effects on phase-conjugation nonlinearity are described in the framework of picosecond experiments.In solutions, gels, and films (optically thin media), third-order susceptibility appears as an increasing intensity dependent function. Phase measurements by nonlinear interferometry with the substrate or with the solvent are compared with predictions of a resonantly driven three level system. Phase-conjugation response exhibits a multi-exponential decay. Polarization symmetry analysis shows a one-dimensional effect. Study under strong static electric field action reveals that we face charged species bound to photoconductive polymer chains.In PTS single crystals (optically thick media), response saturates and cancels out at high light intensity. This is well accounted for by propagation equations solved in large two-photon absorption conditions. The effect is exploited in a phase conjugation experiment under external optical pump excitation. We thus demonstrate that enhanced nonlinearity is a two-photon absorption relayed and amplified by mid-gap absorbing species which have been created by this two-photon absorption. We formally face a four-photon absorption described by a positive imaginary seventh-order nonlinearity.Les polydiacétylènes possèdent une forte absorption à deux photons à 1064 nm. Ses différents effets sur la non-linéarité de conjugaison de phase sont décrits dans le cadre d'expériences réalisées en régime d'impulsions picosecondes. Dans les solutions, les gels et les couches évaporées (milieux optiquement minces), la susceptibilité du troisième ordre apparaît comme une fonction croissante de l'intensité d'excitation. L'analyse de sa phase par interférométrie avec le substrat ou avec le solvant permet de comparer l'effet d'amplification avec les prédictions d'un modèle de niveaux atomiques peuplés par absorption à deux photons et déplacés par effet Stark. La réponse conjuguée en phase présente une mémoire à décroissance multi-exponentielle. L'analyse de ses symétries de polarisation permet de l'attribuer à un effet uni-dimensionnel. L'étude de l'action d'un champ électrique statique intense révèle qu'il s'agit d'espèces chargées liées aux chaînes photo-conductrices. Dans les mono-cristaux de PTS (milieux optiquement épais), la réponse sature à haute intensité lumineuse. Ceci est bien pris en compte par les équations de propagation traitées en conditions de forte absorption à deux photons. L'effet est mis à profit dans une expérience de conjugaison de phase sous excitation par une pompe optique externe. On démontre ainsi que la non-linéarité amplifiée est une absorption à deux photons relayée par des espèces absorbant en milieu de bande, elles-même excitées par cette absorption à deux photons. Il s'agit formellement d'une absorption à quatre photons décrite par une non-linéarité du septième ordre imaginaire positiv

ADVANCED MATERIALS Communications Molecular Rectification in Oriented Polymer Structures""

International audiencePolymeric semiconductor devices are receiving increasing attention in view of potential applications requiring low-cost processing over large areas.'"*] In this respect, unlike with evaporated molecules, the wet-processing capability of polymers offers total compatibility with other complemen-tary technologies. The concepts from which organic-semi-conductor devices are designed are mostly derived from in-organic-semiconductor physics and technology.''] In order to build efficient organic-semiconductor devices such as electroluminescent or photovoltaic solar cells, a rectifying junction is required. Such junction can be of two main typed4] Schottky junctions between an organic semi-conductor and a metal, and p-n junctions between two p-and n-type organic-semiconductors. The Schottky junctions often suffer from degradation problems originating from electrochemical potential differences at the rectifying con-tacts, which induce diffusion of electrode material into the polymer.'" Such a drawback is corrected with p-n junctions in which metal to polymer contacts are ohmic. However, p-n junctions require the use of two polymer layers with wet-processing compatibility. Moreover n-type organic-semiconductors are less common than p-type ones owing to their lower stability under oxygen which is a consequence of their reductive behavior. We report here a different approach in which oriented diode-like molecules, the so-called push-pull molecules used in nonlinear optics,[6] are contained inside a polymer binder. Oriented molecules induce a rectifying effect, be-having as a distributed homojunction within a single poly-mer thin film. In order to demonstrate the principle, we start with an initially symmetric structure: the polymer film is sandwiched between two identical electrodes. The cur-rent-voltage characteristics of the device are also symme-trical. By application of a static electric field through the polymer film, while heating near the glass transition tem-perature (T,,) dopant molecules are oriented in the field with an order parameter (cosO).['] The current-voltage characteristics then become strongly asymmetric. Experimentally, molecular order is controlled using sec-ond harmonic generation (SHG): it appears tha

Growth and organization of (3-Trimethoxysilylpropyl) diethylenetriamine within reactive amino-terminated self-assembled monolayer on silica

Alkane chains are the most commonly used molecules for monolayer fabrication. Long chains are used for their strong van der Waals interactions inducing good layer organization. Amine function-terminated alkyl chains are of great interest and are widely used for further surface functionalization. Since it is mandatory that such layers be organized to provide amine moieties at the surface, the present study deals with exploring amine-terminated SAM formation as an alternative to the usual aminopropylalkylsilane SAM. Additionally, using a long NH2terminated alkyl chain allows the formation of hydrogen bonding thanks to the two NH moieties born along the chain. Furthermore, such hydrogen bonding makes possible to shorten the molecule length while preserving a well-organized monolayer. For this purpose we performed a complete study of the grafting of (3-Trimethoxysilylpropyl) diethylenetriamine (DETAS) on native silicon oxide using various solvents, relative humidity and temperature values. Grafting kinetics was monitored by ellipsometry and goniometry, and SAM structure and organization using AFM and ATR-FTIR spectroscopy. Hydrogen bonding was evidenced within the SAM growth process and in the final complete SAM. We believe such study enables a better control of good quality DETAS SAM in order to improve their efficiency in further surface functionalization applications

Cognitive ability experiment with photosensitive organic molecular thin films

We present an optical experiment which permits to evaluate the information exchange necessary to self-induce cooperatively a well-organized pattern in a randomly activated molecular assembly. A low-power coherent beam carrying polarization and wavelength information is used to organize a surface relief grating on a photochromic polymer thin film which is photo-activated by a powerful incoherent beam. We demonstrate experimentally that less than 1% of the molecules possessing information cooperatively transmit it to the entire photo-activated polymer film.Comment: 16 pages, 4 figure

Up-conversion injection in Rubrene/Perylene-diimide-heterostructure electroluminescent diodes

We implement and demonstrate a scheme that permits to drive electroluminescence with an extremely low turn-on voltage. The device behaves like compound semiconductors, in which the electroluminescence turn-on voltage is about the same as the open circuit voltage for the photovoltaic effect. However, the electroluminescence turn-on voltage is about half of the band gap of the emitting material, that cannot be explained using current models of charge injection in organic semiconductors. We hereby propose explanation through an Auger-type two-step injection mechanism (Auger-fountain).Comment: we report the first evidence of an Auger-fountain electroluminescence in a semiconductor heterojunctio