Optical gain in Coumarin 545T-doped Tris(8-hydroxy-chinolinato)aluminium thin films

In this paper we investigate the optical gain in organic thin film waveguides using the variable stripe length method (VSL). As active medium the guest-host system containing Tris-(8-hydroxy-chinolinato)-aluminium (Alq3) doped by 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H- (1)-benzopyropyrano- (6,7-8-i,j)quinolizin-11-one (C545T) is studied. The doping concentration is varied over a wide range and the gain coefficient is measured at different excitation densities to analyze the behavior of the differential gain. The F¨orster energy transfer is responsible for the occupation of the exited state of the coumarin molecules. For low doping concentrations with an inefficient host-guest energy transfer a low stimulated cross section can be observed. At optimal doping concentrations (3.7-6.4 wt%) we obtain a cross section of σ =6.8x10-17 cm2 and a high material gain of gmat ≈500 cm-1 at an excitation density Eex of Eex ≈300 μJ/cm2. A further increased doping concentration (15 wt%) leads to a reduced cross section, due the onset of concentration quenching in the guest-host system. Furthermore, at high excitation densities we observe a strong saturation effect of the maximum gain which depends strongly on the doping concentration.

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