Page 293 - Organic Electronics in Sensors and Biotechnology

270 Chapter Seven

Due to their short pulses and high energy densities, diode-
pumped solid-state (DPSS) lasers were the first compact laser
sources used for pumping organic semiconductor lasers. 10, 28 With
steady advances in developing low-threshold organic lasers, a gal-
lium nitride laser diode-pumped organic laser was presented in
2006. 29–31 In 2008 the first organic laser pumped by a blue light-
emitting diode was demonstrated. 32

7.2.4 Prospects for Organic Laser Diodes
With various optoelectronic components based on organic semicon-
ductors being implemented successfully during the last decade, an
organic laser diode device has become of particular interest. How-
ever, it could not be realized so far. Electrically pumped organic lasers
are expected to provide numerous advantages over their inorganic
counterparts such as tunability over the whole visible spectrum and
the promise of low-cost fabrication.
The implementation of such a device seems to be straightforward.
Electrodes are put on both sides of the slab waveguide formed by the
organic laser material to inject charge carriers. The problem, however,
are the strong optical losses which are introduced by coating the
waveguide with a metal layer or even with a transparent metal oxide
layer such as indium tin oxide (ITO).
To deal with this problem, the thickness of the organic layer can
be increased in order to minimize the overlap of the guided mode
with the absorbing electrodes. However, this results in very high
operation voltages of the device.
Although high current densities for thin-film devices can be achieved
without damaging the organic layer, 33–35 neither an electrically pumped
organic laser nor an electrically pumped organic amplifier has been
shown yet. The main reason for this behavior is the rising charge car-
rier density which causes additional losses such as bimolecular anni-
hilation, induced absorption by polarons and triplet excitons, and
field-induced exciton dissociation. 36

7.3 Fabrication
There are basically two methods to realize DFB resonator gratings:
either the lower or the upper active layer boundary has to be pat-
terned with the requested topography of the distributed feedback
grating. As shown in Fig. 7.4, this may be achieved either by direct
patterning of the active laser material or by structuring the substrate
beneath the active layer. The quality of the resonator structure affects
the threshold and efficiency of the laser. For patterning of the active
layer, photoisomerization, direct embossing, 38, 39 or direct laser inter-
37
ference ablation (see Sec. 7.3.4) can be used.