Final Report - Faculty of Physics

Final Report CEMOS G5MA-CT-2002-04062 Page 20/43
Exciton-exciton interaction and biexcitons in the presence of spin-polarized carriers by
P.Kossacki et al.
An absorption study of the neutral exciton, biexciton and positively charged exciton
(trion) in a (Cd,Mn)Te quantum well under the influence of a femtosecond, circularly
polarized, resonant pump pulse has been performed (see Fig. 1) . By using spin-dependent
selection rules in a quantum well containing a strongly diluted magnetic semiconductor, we
could unambiguously separate the biexcitonic transitions from the charged exciton ones, and
we report on their simultaneous observation for a well chosen configuration of the pump and
probe polarization. The influence of the hole gas on the biexciton line is described. In a
polarization such as the charged exciton line is not present in the transmission spectrum, the
sum of the neutral exciton and biexciton intensities is conserved, whatever the density of
preexisting holes. However, when both the charged exciton and the biexciton coexist, we
observed some intensity stealing from the biexciton by the charged exciton. [20]
FIG. 1 (a), (b) Transmission of the σ+ (a) and σ − (b) polarized probe pulse 10 ns after the cross-polarized
pump pulse (“negative delay”), (c)-(f) Difference transmission spectra for zero delay (black solid lines), 2 ps
positive delay (red dashed lines), and 4 ps, positive delay (green dotted lines). The magnetic field was B=0.5 T,
the hole density p=0.5x10 10 cm −2 . from [20]