Abstract

In semiconductors, with increasing carrier densities, excitons are screened and eventually lose their oscillator strengths [1]. As a result, the excitonic absorption will be greatly reduced or bleached. One interesting question is: How strongly can excitons, which are electrically neutral but consist of electrons and holes, self-screen the exciton? In bulk GaAs, Fehrenbach et al. [2] observed that the screening of excitons by excitons is very weak, compared to the screening of excitons by free carriers. Recently, Knox et al. [3] found that the bleaching effect of excitons on excitons was stronger than that of the same density of "warm" free carriers in GaAs multiple quantum wells (MQWs). This behavior is qualitatively well explained by the theory of Schmitt-Rink et al. [4], in which only the effects of the Pauli exclusion principle (phase space filling and exchange interaction) are considered for the bleaching of excitons by both excitons and free electron-hole (e-h) pairs, assuming long-range Coulomb screening is very weak in quasi-two-dimensional (2D) QWs. This assumption could be best tested by measuring the bleaching strength of "cold" free e-h pairs which are supposed to screen the exciton most strongly in bulk [1].

© 1993 Optical Society of America