Résumé
We show that femtosecond time-resolved optical techniques are ideally suited to excite and probe collective spin organization and collective spin modes, in diluted magnetic semiconductors based quantum wells. The spin systems that we consider are low concentration CdMnTe quantum wells, which exhibit various spin phenomena under optical pulsed excitation, such as spontaneous magnetization patterning, soft mode of magnetization precession in p-doped quantum wells, and mixed electron-Mn precession in n-doped quantum wells. We examine also how the carrier-carrier Coulomb interactions affect these collective spin excitations.