Laser-induced orbital and spin excitations in ferromagnets: insights from a two-level system

A recent time-resolved measurement showed that laser-induced orbital and spin excitations proceed in unison and the spin-orbit ratio is held constant during demagnetization. Here a two-level model shows that these orbital and spin excitations originate from state population and state interference effects. For an addressed state, spin and orbital dynamics are solely from the state interference, where the spin and orbital momenta oscillate with the laser frequency and match the dipole moment exactly, an unambiguous test case for the time-resolved magneto-optical Kerr effect. For an undressed state, the interference effect introduces a rapid beating in orbital momentum, which is observed in the first-principles calculation in fcc Ni. The state population change leads to a constant spin-orbit ratio, which explains the linear dependence between spin and orbital momentum changes within 2 ps upon the arrival of a pump pulse in Fe.