Coherent Acoustic Phonon Generation in Ferromagnetic GaMnAs Films*

Ferromagnetic semiconductors such GaMnAs exhibit strong coupling between their magnetic and elastic properties. This allows their magnetic anisotropy to be manipulated by applying external strain, electric or magnetic fields, as well as optical fields. In this talk, we report on the ultrafast optical processes in ferromagnetic (III,Mn)V semiconductor films induced by femtosecond laser pulses. We present two-color time-resolved spectroscopy on MBE grown GaMnAs (T_C = 110 K). These measurements allow us to observe coherent phonon dynamics. Here we focus on the generation of coherent acoustic phonon wavepackets, between 40.9 – 42.9 GHz, which are generated in the ferromagnetic layer which display strong tunability in the presence of external magnetic fields. We compare our experimental observations to theoretical calculations on an effective mass approximation, using an eight-band Pidgeon-Brown model to calculate magnetic field induced changes to the electronic structure, absorption and index of refraction, and coherent phonon amplitude.