Building ArCHitectures towards Electrically-pumped Lasers based on ORganic materials. – BACHELOR

"Within the past few years, there has been an increasing interest towards organic optoelectronics. Many devices have been developed and pushed towards commercialization: sensors, photodetectors, photovoltaic cells, and Organic Light Emitting Diodes (OLEDs). The latter have reached such a degree of maturity that they have now entered mass production. Notably absent from the list of electronic and optical devices that can be made from organic semiconductors is the laser diode. As an alternative to inorganic laser diodes (and spanning the entire visible spectrum), they would beneficiate from the cheap and universal fabrication techniques that pertain to other organic devices; which are also compatible with the Silicon realm. - Recent papers outlined the fact that an organic laser diode had to be considered however as a long term project, facing many, though well identified, challenges. These are: the presence of charge carriers (polarons) that have broad and intense absorption spectra overlapping the gain spectra of laser dyes; the detrimental losses brought by metallic contacts in a waveguide configuration. A third important issue is photostability. - Given theses challenges, it is reasonable to first study optically-pumped solid-state organic lasers. Because efficient solutions to the metallic contact losses problem have already been proposed, the present proposal aims at exploring two different ways : (1) : dealing with the photostability issue and increasing lifetime and performance (energy per pulse and beam quality) of optically pumped lasers and (2) : eliminating the polaron absorption problem thanks to the use of triplet emitters. - The first part of this project aims at making a Vertical External Cavity Surface emitting Organic Laser or VECSOL, which is nothing else that the organic equivalent of semiconductor VECSELs, and also resembles the "thin disk laser", a concept recently developed for Ytterbium-based high power lasers to overcome thermal issues. Indeed, it has been recently shown that photodegradation issues are for a large part thermally activated, showing that an efficient cooling of the laser material is of utmost importance. - This problem has not been studied extensively yet in the context of solid-state organic lasers, but will be however a key point since all the anticipated applications of optically as well as electrically-pumped devices require a beam in the CW regime, or at least with high repetition rate. - Numerical simulations (enclosed in this proposal) reveal that a thin film of organic semiconductor in a "classical" laser architecture (uncooled) undergoes a constant rise of its mean temperature when pumped by a 1-kHz repetition-rate laser, since the medium does not have time to relax between two subsequent pulses. We present in detail the VECSOL architecture and explain its advantages over existing resonator designs. Several structures will be realized, both with standard materials and with new materials, the latter being synthesised with a team of organic chemists, with whom we have been working for years. - In each case, we will study laser properties and gain provided by VECSOLs (in terms of lifetime, output power, beam quality...). An ASE (amplified spontaneous emission) measurement tool will be developed in order to investigate gain and passive losses properties of unknown materials. - The second part is more risky and exploratory: it aims at studying the potential of phosphorescent molecules as laser emitters. A laser effect between the first excited triplet state and the ground manifold has never been demonstrated, although it would represent a real breakthrough towards electrically-pumped organic diodes. - Indeed, by pumping the medium with a transient electrical pulse, the triplet level can be populated, and its lifetime is long enough to survive after charged polarons have disappeared. From the laser physicist point of view, this is a passively Q-switched laser, since laser emission build...