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 electric-field strength of 5×${10}^4$ V/cm, the observed absorption change in this material is Δα/α≊0.07. The electrical field produces a redshift of the absorption band which is explained in terms of the quantum-confined 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 Δα/α=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.

• Received 16 February 1993

DOI:https://doi.org/10.1103/PhysRevB.48.11979