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M. Sudzius, M. Langner, S. I. Hintschich, H. Fröb, V. G. Lyssenko, and K. Leo
Modern optical applications require the availability of miniaturized light sources capable of delivering coherent light over a broad spectrum of wavelengths. Organic materials are generally regarded as attractive candidates for laser applications due to their high optical gain and wide wavelength tunability. Optically pumped lasers made of organic molecules have already proven to show excellent lasing characteristics with the future potential of electrical pumping. We report on the control of the spatial, temporal, and spectral characteristics of low-threshold room-temperature organic dielectric microcavity lasers. For this, we use a tris-(8-hydroxyquinoline) aluminum and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran host-guest system (Alq3:DCM) as the active material. In contrast to semiconductor systems based on quantum dots, which are currently restricted to cryogenic temperatures, we employ the inhomogeneously broadened emission spectrum of small molecular weight organics to achieve room temperature inversion over a wide spectral range. The properties of this organic host-guest composite as an active laser medium are systematically investigated in planar (or slightly wedged) organic dielectric microcavities. These consist of two high-reflectivity SiO2/TiO2 distributed Bragg reflectors (DBRs) and the Alq3:DCM material sandwiched in between. We use tunable femtosecond or nanosecond pulses to optically excite the organic layer in the absorption band of either the host or the guest material. Thereby, we induce lasing from the emission band of the guest molecule. Since the intramolecular relaxation time in the DCM molecules is significantly shorter than the radiative time at the emission wavelength, the system is initially overpumped in a large spectral range under pulsed optical excitation. The lasing characteristics in such one-dimensional confined structures can be easily tuned to all wavelengths between 595 and 650 nm. It is not only the material properties of the active medium, but also the precise geometry of the microcavity, e.g. a further decrease of the dimensionality, which determines the unique features of the laser emission such as directionality, output power, and spectral properties. Via shadow mask evaporation we can deposit the organic cavity layer in the shape of micron-sized organic photonic dots. Here, the high optical quality of our bottom and top DBRs ensures a strong 3D confinement of light in the photonic dot which leads to a reduced number of cavity modes. We demonstrate experimentally that the coupling efficiency of the spontaneous emission to the mode - which will eventually lase - is enhanced orders of magnitude as compared to the planar structure. As an alternative to the above mechanical method, we also apply optical structuring of absorption, gain, and, consequently, the refractive index of the cavity layer simply by using a patterned optical excitation. In contrast to the photonic dots, where the laser characteristics are effectively modified due to the small active volume, we observe coherent coupling of the individual emitters across the entire pump pattern, which exceeds the mode volume of the planar microcavity many times. In this case, both the lasing angle and wavelength are tunable by all-optical means while the output power is substantially increased. |
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