Molecular orbital topology and optical properties of gallium arsenide clusters

The oscillator strength is expressed in terms of a density matrix formulation for optical absorption and is calculated for a GaAs cluster by using the self-consistent-field Xα-scattered wave cluster molecular orbital method. The discrete value of oscillator strength due to optical excitation defined by the selection rule leads to a model of the fundamental nature of the excitonic absorption behavior in GaAs bulk material and quantum well structure. Calculated results of band-to-band transition and exciton states are found to agree well with absorption spectra published in literature. The implication of the excitonic states for the nonlinear-optical behavior observed in GaAs quantum well structures is discussed. The presence of excitonic states may cause a `saturation´ phenomenon which will lead to the optical nonlinearity