The molecular character of organics allows the description of their macroscopic properties through the properties of their microscopic compounds. The understanding of the mechanisms governing the quadratic NLO properties is an important requirement when developing optical devices based on second harmonic generation or electro-optical effects. In particular, a work of optimization of molecular properties is critical for their performance, which is the key to their possible application.
When looking at molecules, two parameters are of particular importance :
- their ability to provide a nonlinear response (an intrinsic property of the molecule, called hyperpolarizability)
- their ability to be aligned in a specific direction, which allows the individual nonlinear responses to interfere constructively
Alignment of polar molecules can be done by a static external electric field : efficiency of the alignment is then dependent on the dipolar moment of the molecule. The optimisation of the hyperpolarizability, together with a high value for the dipole moment are therefore the essential conditions for the elaboration of a molecule serving as a base for an efficient quadratic NLO material.
Charge transfer molecules (“push-pull molecules”), with electron donor D and electron acceptor A moieties, related through a conjugation path, are highly polarizable and may possess very high quadratic hyperpolarizabilities. The experimental measurement of the product of dipole and hyperpolarizability is performed using an EFISH (Electric Field Induced Second Harmonic) set-up for molecules in solution. The experiment consists in measuring the intensity of the second harmonic as a function of the optical path length in the nonlinear media. The source we employ is a Nd+:YAG laser used as a pump for an OPO or a Raman cell emitting a wavelength at 1907 nm.
We collaborate with several groups of chemists active in the synthesis of new nonlinear chromophores, and with the (LINK TO ALEX) team for molecular simulations. Some examples of compounds we investigate are :
- Disubstituted zwitterions with dyaryl as conjugation path. The interesting parameter is the torsion angle between the phenyl groups, controlled by steric hindering. In order to experimentally demonstrate the possibility to optimize the quadratic NLO properties with respect to this parameter, we have undertaken a systematic study on a series of compounds with distinct torsion angles, in a collaboration with chemists from Mulhouse both for the synthesis (COB, UMR 7015) and for the solvatochromic work (DPG, UMR 7525) and from Bordeaux for the IR, Raman et hyper Rayleigh studies (ISM, UMR 5255).
- Donor-acceptor-substituted biphenyl derivatives. The NLO properties are investigated as a function of the torsion angle between the two phenyl rings, which is controlled by an additional interphenyl alkyl bridge of varying length. The work is done in collaboration with the Department of Chemistry, University of Basel and the Institute of Nanotechnology, Karlsruhe Institute of Technology.
- Dicarboxamide pyridine based dyes. The advantages of these molecules lie in the fact that the dialkylamino donor end-group can be easily functionalised with hydrosolubilising fragments and the nature of the conjugated bridge can tune the optical properties in order to get optimal two-photon emission fluorescence and resonance enhanced SHG signal. Collaboration with the Laboratoire de Chimie of the ENS Lyon (UMR 5182).
- Chromophores with multidimensional charge transfer : symmetrical derivatives of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)-styryl]-4H-pyran (DCM). These molecules are part of the class of organic chromophores with multidimensional charge transfer, offering potential advantages on linear push-pull molecules such as increased hyperpolarizability without undesirable loss of visible transparency. Both theoretical and experimental works have proved the good NLO activity of Y-shaped chromophores based on DCM with two donor segments. Collaboration with the Dipartimento di Chimica, Università di Napoli and Dipartimento di Fisica, Università di Roma “Tor Vergata”.
- Transition metal diimine-dithiolato complexes. The interest in these complexes is related to their applications in the areas of conducting and magnetic molecular materials. The effect on nonlinear optical properties of the terminal groups and of the metal are under investigation. Collaboration with the Dipartimento di Chimica Inorganica ed Analitica, Università di Cagliari.