Lasers UV organiques – LUVORGAN

1. Scientific context and objective Many applications (data storage, sensing) demand laser sources emitting in the blue and near UV part of the spectrum, which are compact, easy to use and low-cost. In this context, organic materials are good candidates for the realization of this kind of sources, thanks to the fact that their optical properties can be tailored via molecular engineering and because of the electronic origin of their nonlinear optical properties. For second harmonic generation in the blue and near UV, organics can compete with inorganic crystals of the borate family. The latter are generally problematic to produce and have low nonlinear coefficients, operating at best in the visible range (the exception being yttrium and aluminium borates doped with neodymium, which nevertheless have low crystalline quality). As of today, research on light sources based on organic materials have focused mainly on organic electroluminescent diodes OLEDs (small molecules and and electrolumiscent conjugate polymers) or organic lasers (conjugate polymer and polymers doped with dyes). The technological progress achieved since the first work of Tang and Burroughes, in particular regarding the lifetime of the devices, have resulted in a growing commercialization of products based on OLEDs which are starting to compete with liquid crystals for applications requiring a high emissivity of the source. More recently, research has advanced on organic distributed feedback lasers (DFDL). In these devices the Bragg grating is obtained through patterning of the index of refraction of the sample, either through mass transport by azo-dye-like compounds (molecular displacement through repeated cis-trans photoisomerization) or by one- or two- photon photopolymerization. These lasers emit at the wavelength of the fluorescence band of the doping dye, generally situated in the visible part of the spectrum. 2. Description of the project Our objective, in the present multidisciplinary project LUVORGAN, is the fabrication of organic laser sources, compact and low-cost, emitting in the blue and near-UV part of the spectrum by using a novel class of push-pull molecules which provide both fluorescence in the visible range as well as optical nonlinearity to frequency-double their own laser emission. Push-pull chromophores (or charge transfer molecules) have been largely studied in the last years with the aim of improving their quadratic hyperpolarizability for applications like frequency-doubling and electro-optic modulation. Significant progress has been obtained both in the understanding of the nonlinear phenomena as well as the synthesis strategy of molecular compounds of good efficiency. This progress has provided the basis for the fabrication of the first organic opto-electronic modulators.Our project is based on the long-time experience of our teams in the domain of charge transfer molecules for nonlinear optics. It consists in using push-pull molecules which can perform second-harmonic generation starting from a fundamental wavelength corresponding to their own laser emission when optically pumped by an external source (e.g. a Nd:YAG laser). This self-doubling process, where the same molecule presents both quadratic non linear optical (NLO) properties and fluorescence, will allow us to produce compact laser sources by introducing such NLO fluorophores into a polymer. 3. Expected results Preliminary studies have already been undertaken at IPCMS in Strasbourg (Partner 2) using a prototype fluorophore that has been synthesized at LCP in Paris (Partner 1). The first results, obtained in the liquid phase using an innovative cell constructed for this purpose, have validated the approach we propose through the detection of a frequency doubled coherent emission from a mixture containing a laser dye and a NLO chromophore. Using the same cell, we have also verified the lasing activity of the fluorophore itself in solution. The low self-doubling efficiency of the prototype compound and the lack of optical confinement in the liquid cell are the two handicaps that this project will overcome. First, the synthesis of optimized molecular structures will be carried out in Paris while their NLO performances will be measured in Strasbourg. Second, the new compounds will be incorporated into polymeric matrices where the optical confinement of the beam will lead to a higher lasing efficiency. Finally, the efforts to improve the fabrication process of the self doubling active medium will be the starting point of the development of a solid organic laser device emitting in the blue and / or near UV.