  Nonlinear optical response and optical coherence of excitons in semiconductor
dense quantum dots in GaAs single quantum wells has been studied by using
photon echo techniques. At low temperatures, the optical coherence is estimated
to be 2 ns from the decay curve in the photon echo signal. Rabi oscillations
are also observed. Unlike the results for a single quantum dot in the
literature to date, the oscillation shows nonlinear behavior affected strongly
by the pre-excited carrier density. The results can be explained by the theory
of the local field correction originating from exciton-exciton interaction
between the inter dots.
