Far- and midinfrared excitation of large amplitude spin precession in the ferromagnetic semiconductor InMnAs

Ultrafast laser excitation of the ferromagnetic semiconductor InMnAs is shown to trigger spin precession with the largest amplitude reported for magnetic semiconductors so far. To reveal the electronic transitions mediating the coupling between light and spins, we compared the spin dynamics triggered by short terahertz (photon energy 5 meV) and midinfrared (photon energy 500 meV) pulses. The experiments reveal that terahertz pump pulses excite qualitatively similar spin dynamics, but are 100 times more energy efficient than the mid-IR pulses. This finding shows that in a semiconductor with hole-mediated ferromagnetism intraband electronic transitions mediate ultrafast and the most efficient coupling between light and spins.