devices needed for munication systems. РVarious new short-pulse-spectroscopy techniques such as degenerate-four-wave mixing Р(DFWM) [5], photon echo (PE) [6J or quantum beat (QB) experiments [7J allow stu•Рdies of these dynamics on a time scale where the elementary relaxation steps, such as Рscattering by phonons, defects or other quasi-particles, which lead to the decay of the Рinitially prepared coherent optical state, e directly observable. Р Particular attention has been given to the nonlinear optical properties and dyna•Рmics of excitons which are the dominating optical features in the region close to the Рband edge of a semiconductor. Many studies have revealed remarkable variations of the Рoptical nonlinearity and the relaxation dynamics with the dimension of the excitonic Рsystem.
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