Abstract

Charged excitons, also called trions, are bound states of electron-hole pairs with an additional charge.¹ Their observation is possible in quantum wells (QWs) where a gas of charged carriers is present and where the 2-D confinement increases the trion binding energy. The stability of excitons and trions embedded in such a gas has been explained by a localization of the carriers in the fluctuations of the electrostatic potential induced by the distribution of the remote dopants.^2,3 We have probed the depth of these spatial fluctuations and their length scale by performing both ps and fs degenerate four-wave mixing (FWM) experiments in p-type modulation doped CdTe/Cd_0.69Mg_0.23Zn_0.08Te QWs according to the following idea: On the one hand, due to the Coulomb interaction, free trions and holes should have a strongly enhanced scattering efficiency compared to that of free but uncharged excitons. A localization of trions and holes, however, should drastically weaken this scattering. On the other hand, an important part of the dephasing of excitons and trions is caused by these scattering processes.

© 1999 Optical Society of America