New organic nonlinear optical materials: dithienylethylenes
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New organic nonlinear optical materials: dithienylethylenes B. Sahraoui1, K.J.Pluciński, I. V. Kityk2, B. Paci3, P. Baldeck4 , J-M. Nunzi5, P. Frère6, and J. Roncali6 Military University of Technology, 00-908 Warsaw, POLAND. 1 Laboratoire des Propriétés Optiques des Matériaux et Applications, UMR CNRS 6136 Université d'Angers, 2 Boulevard Lavoisier, 49045 Angers, FRANCE. 2 Inst. of Physics, WSP, Częstochowa, POLAND. 3 CEA (LETI-Technologies Avancées), DEIN/SPE, Gif sur Yvette, FRANCE. 4 Laboratoire de Spectrométrie Physique, Université J. Fourier, Saint Martin d’Hères, FRANCE. 5 CEA (LETI-Technologies Avancées), Gif sur Yvette, FRANCE. 6 Laboratoire d’Ingénierie Moléculaire et Matériaux Organiques, Université d'Angers, Angers,FRANCE. ABSTRACT Measurements of the third-order susceptibilities of new dithienylethylenes in solutions using a degenerate four wave mixing technique at λ=532 nm in picosecond regime were made. From these measurements we deduced the second order hyperpolarizabilities. Optical power limiting properties of the mentioned compounds using nanosecond laser pulses in the visible range are evaluated. The dispersion of nonlinear absorption coefficients is obtained. The appropriate quantum chemical calculations are carried out. INTRODUCTION Research into new optical materials which are promising for non-linear optical devices is currently an interesting field of investigation [1,2]. Considerable attention has been focused over the last years in materials which have optical limiting properties making it possible to control the intensity or the pulse energy of laser beams for optical protection of eyes and photo-detectors . For these reasons we looked for colorless materials, transparent at low energy and absorbing in the whole visible range for microjoule pulse energy. Materials which exhibit two-photon absorption satisfy the first requirement, but their nonlinear absorption is rather low. It has been possible to approach the above specifications in the nanosecond regime, but in a restricted wavelength range [3] by combining two-photon absorption with reabsorption toward excited states of higher energy. In this paper, we report a systematic study of the third order nonlinear optical properties of two new dithienylethylenes: (a) (E,)-1,2-bis[2-(3,4-dibutylthienyl)]ethylene, (b) (E,E)-1,4bis[2-(3,4-diHexylthienyl)buta-1,3-diene (see Figure1). They were synthesized using a procedure described in Reference [4]. The trans configuration of ethylenic units were established from NMR spectra for (b) and from X-ray structure for (a). The X-ray structure of (a) reveals good planarity of the π-conjugated systems (see Figure 2). These organic compounds are dissolved in chloroform. The optical absorption spectra of the molecules presented in Figure 3 were performed with a cell 1 cm thick and at a very weak concentration ( 4⋅ 10-5 mol/ l). They show a well-resolved fine-structure with a maximum absorption (λ max ) at 360 nm for molecule (a) and 378 nm for (b). As expected, the chain extension leads to a red shift of th
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