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.