David Kreher, enseignant-chercheur
Innovating Polymers for Non Linear Optics Applications
Two years ago, we developed an approach based on Self-Doubled (SD) process to build organic lasers emitting in the blue-UV range of the spectrum, knowing that SD lasing procedure is the intracavity generation of a second harmonic signal by the active medium emitting the fundamental laser radiation.
We first experimentally established the validity of principle for operating such a system using a mixture of commercial compounds. But afterwards, a series of original molecules possessing both fluorescence and quadratic non linearities have been especially synthesized for the purpose of building a SD based laser. The specific requirements on the molecular absorption and emission bands for SD have been determined. Both numerical simulations and experimental measurements have been performed to ease the choice of the selected molecules. To sum up, for a single SD molecule, these requirements consist to synthesize a NLO fluorescent chromophore with a good compromise between fluorescence and quadratic hyperpolarizability efficiencies and to avoid the reabsorption of the doubling signal by the molecule itself. In addition, the molecules need to have a large permanent dipole moment for the orientation via an electric field and to possess a good solubility to provide a detectable signal.
We consequently followed a convergent tunable chemical strategy :
Among all the compounds designed and synthesized as novel non linear optical push-pull derivatives with a high fluorescence efficiency in a suitable wavelength domain for a possible self doubling emission in the blue-UV, we have selected the two compounds FL77 (left) and FL78 (right) shown below.
A special cell meeting the strict requirement needed to pump the solution while applying an electric field has been used and incorporated in an adapted laser cavity. This way, a self-doubled laser emission from a mixture composed of a dye and these non linear optical chromophores has been obtained in the blue-UV region. To conclude, this work proved the feasibility of the approach we developed for a self-doubling laser system and paves the way for a solid device where the solution would be replaced by a polymeric film functionalized by a self-doubling molecule assuming both functions, dye lasing and doubling of this initial lasing.
Consequently and to address the issue of the aggregation in second-order nonlinear optical (NLO) polymers we propose several approaches based on Controlled Radical Polymerization (PRC), acting as an alternative complementary way to minimize the chromophore-chromophore interactions.
First, to target well-defined second-order Nonlinear Optical Polymers, we developed an approach based on sequentially ordered polymers with NLO model chromophores (in red) in the main chain obtained by insertion polymerization of styrene monomer (blue) via a RAFT process. Our strategy seems to solve the aggregation problem in non polar matrices, and can be used in other fields where avoiding aggregation remains a key issue.
Well-Defined Second-Order Nonlinear Optical Polymers by Controlled Radical Polymerization, via Multifunctional Macromolecular Chain Transfer Agent : Design, Synthesis, and Characterizations. Anne-Lise Roy, Chong Bui, Ileana Rau, François Kajzar, Bernadette Charleux, Maud Save, David Kreher, André-Jean Attias, submitted to Macromolecules.
Second, we recently envisaged the combination of both ATRP and Click Chemistry, always starting with testing our strategy with a commercially available chromophore, the DR19 or DR1. Preliminary results are quite encouraging with the obtaining ofthe PMMA-modified materials described below.
Third, due to purification problems encountered, we will try soon to modify first the DR19 to directly couple him symmetrically before to initiate the controlled polymerization.
Fourth and not last, these strategies will be compared in the future to the one taking advantage of the unsymmetrical corresponding monomers... to reach this type of polymers and measure their NLO properties.