Ultrafast chiral precession of spin and orbital angular momentum induced by circularly polarized laser pulse

Abstract: Despite spin (SAM) and orbital (OAM) angular momentum dynamics are well-studied in demagnetization processes, their components receive less focus. Here, we utilize the non-collinear spin version of real-time time-dependent density functional theory (rt-TDDFT) to unveil significant x and y components of SAM and OAM induced by circularly left and right polarized laser pulse in Ferromagnetic Fe, Co and Ni. Our results show that the magnitude of OAM is an order of magnitude larger than that of SAM, highlighting a stronger optical response from the orbital degrees of freedom of electrons compared to spin. Additionally, we observe a marked dependency of the oscillations of the x and y components on the helicity of the pulse. Intriguingly, left and right polarized pulses induce chirality in the precession of SAM and OAM, respectively, with clear associations with laser frequency and duration. Finally, we demonstrate that the time scale of OAM and SAM precession occurs even earlier than that of the demagnetization process and OISTR effect. Our investigation provides detailed insight into the dynamics of SAM and OAM of electrons during and shortly after a circularly polarized laser excitation.