Control of spontaneous and stimulated emission in conjugated polymers

We have been investigating the scope for fabricating microcavity structures, with mirrors above and below the polymer layers. The importance of microcavity effects has now been recognised for devices of this type, as we showed for polymer LEDs. In devices with high finesse cavities, formed with mirrors, substantial control of spectrum is possible, with line narrowing down to 4 nm. Refinements in polymer synthesis and processing have dramatically increased the quantum yield of EL and photoluminescence (PL), both of which result from recombination of singlet excitons, thus leading to improved efficiencies of polymer LEDs. Recently, we have demonstrated stimulated emission from films of PPV when they were excited by sub-picosecond UV pulses. This optical gain was enhanced when a similar PPV film was sandwiched between silver mirrors, making a microcavity structure, and it was found that this type of sample exhibited laser action when excited with UV pulses with energies of about one μJ, using microcavity structures of the type shown below. The transition from spontaneous to stimulated emission is shown by the collapse of intensity for two of the three resonant cavity modes above a threshold excitation intensity. It has also been shown recently that, for films of a variety of conjugated polymers prepared on glass substrates, the emission spectrum narrows with increasing optical-pump intensity when using pulsed excitation. There have been a number of suggestions for the origin of this phenomenon, including superradiance, and superfluorescence, but we have found recently very convincing evidence that the phenomenon arises because radiation is trapped inside the `slab waveguide´ formed by the high index polymer on the low index glass substrate, and this then allows ASE of the emission confined to these modes, and hence the associated line-narrowing. We will discuss recent developments in the investigation of devices of this type, with particular emphasis on the scope for generating high excitation densities by electrical injection in diode structures