Organic semiconductor lasers with two-dimensional distributed feedback

The work at hand presents a comprehensive investigation of solid-state lasers based on thin films of a disordered organic semiconductors (OS). The high luminescence yield of OS together with their conductivity and the ease of processing them into sub-micron thick films on substrates of virtually any shape and size open vast opportunities both for the realization of novel devices and for the exploration of innovative resonator geometries. Within the course of this work optically pumped lasers with spatially distributed feedback were fabricated by deposition of a thin film of an OS on a nanopatterned substrate. Upon optical excitation with femtosecond laser pulses laser operation is observed. It is characterized using time-integrated and time-resolved spectroscopy. As active medium two of the most promising electroluminescent organic materials were employed, namely the conjugated polymer MeLPPP and the composite molecular system Alq3:DCM. Owing to their large gain coefficient low-threshold light-amplification by stimulated emission can be observed from slab waveguides formed by a thin layer of the OS on a planar substrate. In periodically modulated waveguides Bragg scattering gives rise to distributed feedback. Utilizing UV-embossed plastic substrates with an appropriate onedimensional surface corrugation, edge- and surface-emitting lasers were fabricated. Excellent agreement is found between the performance of these 1D-DFB lasers and the predictions of a quantitative numerical modeling. Unless further precautions are taken these lasers operate on a continuum of lateral modes, characterized by a broad emission spectrum and a large beam divergence. A significantly improved performance is achieved by application of a two-dimensional corrugation. The specific properties of light-propagation in the resulting 2D photonic crystal give rise to monomode laser operation, accompanied by a reduction of the laser threshold, an increased differential efficiency, and by the emission of a circularly shaped, diffractionlimited laser beam. It is demonstrated that the emission linewidth is (time-bandwidth) transform limited by the duration of the laser pulse. The mechanism leading to monomode operation at low excitation density as well as the photonic band structure at high excitation density is revealed by a Laue model for the feedback in such a 2D photonic crystal laser. By use of a self-assembly technique waveguide structures with stochastically distributed scatter centers were fabricated. Lasing with resonant feedback is observed and attributed to the formation of random closed loop cavities (random lasing). The underlying process of recurrent light-scattering is made possible by the strong scattering, the reduced dimensionality of the waveguide geometry, and the large gain of the organic semiconductor material. In order to explore the technological potential of organic semiconductor lasers two alternative concepts for electrical operation were examined. In a hybrid concept an OS DFB laser is merely used as a tunable solid-state laser source. Thanks to the low laser threshold a very compact setup can be realized utilizing a microchip pump laser with a repetition rate of several kilohertz. In the approach of making an organic diode laser the low mobility of disordered organic semiconductors imposes severe constraints on the device architecture. Several structures were developed and fabricated that concurrently permit large current densities and possess a low laser threshold. As a result, optically pumped lasing is for the first time demonstrated in structures that are suitable for electrical excitation.