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CdS microcrystals under weak confinement conditions embedded in a polyvinyl alcohol polymer film have been optically characterized by linear absorption and time-resolved luminescence spectroscopy.They are studied in the presence of an external electrical field at low and room temperature and different densities of photoexcited carriers. By applying a voltage of 400 V corresponding to an external electricfield strength of 5 X 10 V/cm, the observed absorption change in this material is Aa/a =0.07. The electrical field produces a redshift of the absorption band which is explained in terms of the quantumconfined Stark effect. In addition, a restoring of the oscillator strength is observed and explained by screening effects of internal fields in the interface region by the photogenerated electron-hole pairs. At high laser excitation an electric-field-induced change of the nonlinear absorption spectrum of ha/a=0. 25 is achieved, giving a considerable modulation of the absorption edge. The electrical field separates the laser excited carriers and the change in the absorption is attributed to a compensation of the many-particle interaction by the external electrical field.

The temperature distribution in a GaAsl(AI,Ga)As multiple quantum well structure was measured with high lateral and temporal resolution while passing through the characteristic hysteresis loop of a thermally induced absorptive optical bistability. In a pump and probe beam experiment, the local transmission spectra of the quantum well structure were recorded and the temperature distribution in the sample was deduced from the spectral positions of characteristic exciton resonances. These resonances remain sufficiently sharp even far above room temperature to guarantee an accuracy of better than 3 K The experiment was conducted under both steady-state and dynamical conditions. The steady-state resuits were compared with numerical calculations, showing good agreement. The dependence of the relative width of the characteristic hysteresis on the environmental temperature of the bistable sample was investigated. A simple relaxation-time model of the bistability was shown to describe nicely the ObSeNed behaviour t the bleaching of the lowest exciton resonance due to high laser intensities is considered. Based on this dependence, a miniature temperature-sensing device using an optical fibre to guide the laser beam is presented.

The optical properties are investigated of undoped and n-modulation doped multiple quantum well structures (MQWS) under quasistationary high excitation conditions. The characteristic nonlinear optical behaviour is due to an electron-hole plasma. Its density, temperature, and the renormalized bandgap are determined via photoluminescence and pump and probe beam measurements. A model is developed for the line shape analysis of the plasma luminescence taking into account the broadening of the single particle density of states with results deviating from those in literature, but very good agreement is achieved with the experimental findings from pump and probe beam spectroscopy. The study of the renormalization of the higher, unpopulated subbands reveals that the pure intersubband screening is weak with respect to the shift of the fundamental bandgap. The exciton resonances of the higher subbands are found to be stable against high plasma densities in the fundamental subbands and are mainly bleached by direct population. The idea of a rigid shift of the whole subband structure must be clearly rejected.Die optischen Eigenschaften von undotierten und n-modulationsdotierten Vielfachquantentrogstrukturen unter quasistationaren Hochanregungsbedingungen werden untersucht. Die Art der beobachteten optischen Nichtlinearitaten wird durch die Zahl der erzeugten Ladungstriiger bestimmt und uber ein Elektron-Loch-Plasma beschrieben. Dessen Dichte und Temperatur sowie die reduzierte Bandlucke werden iiber Photolumineszenz-und Pump-und Probestrahl-Messungen bestimmt. Ein zur Auswertung der Lumineszenzlinienform entwickeltes Model1 liefert zur Literatur kontrare Ergebnisse, die allerdings mit denen der Pump-und-Probe-Experimente iibereinstimmen. Die Untersuchung der Renormierung hoherer, unbesetzter Subbander zeigt, daR die reine Intersubband-Abschirmung schwach ist im Vergleich zur Verschiebung der fundamentalen Bandlucke. Die Exzitonen der hoheren Subbander sind stabil gegenuber einer Ladungstragerbesetzung in den fundamentalen Subbandern und bleichen nur durch direkte Besetzung aus. Das Konzept einer starren Subbandverschiebung muO fallengelassen werden.

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