Picosecond exciton and charge carrier dynamics in luminescent materials

Knowledge on energy transfer in insulator or semi-conductor materials to luminescence centers like transition metal (TM) and rare earth (RE) ions is of importance for development and understanding of new LED’s, ELD’s, radiation detector materials but also for improving the efficiency of photo-voltaic devices. Although materials absorb strongly above their bandgap energy, RE or TM-ion luminescence efficiencies are usually low because of the many possible alternative relaxation routes like charge trapping or the formation of excitonic species with subsequent radiative or non-radiative relaxation. Deeper insight in these picosecond timescale processes and how they are related to composition, structure and the impurity ion energies can be obtained by studying luminescence rise-time profiles. We have successfully built a setup with the ability of measuring very short rise times of slowly decaying states, characteristic for the 4f-levels RE ions and the 5d-states of TM-ions. The setup consists of a femtosecond laser with a streak camera with a custom made horizontal blanking unit that, with the help of streak camera gating, blanking and sequenced recording techniques result in an instrument response function better than 4 picoseconds.