Controlling photoexcited electron spin by light polarization in ultrafast-pumped altermagnets

Altermagnets (AMs) constitute a novel class of spin-compensated materials in which the symmetry connecting opposite-spin sublattices involves a spatial rotation. Here, we uncover a set of unique nonlinear, light-driven properties that set AMs apart from traditional ferro-and antiferromagnets. We demonstrate theoretically that the polarization of an electromagnetic pulse that photo-excites electrons and holes in an AM, controls the spin orientation of these nonequilibrium charge carriers. For a d-wave AM model and a prototype material, a RuO2 bilayer, we show that very large post-pump spin polarizations may be attained by exploiting resonances. We show that this protocol also allows, in an AM, to directly probe the spin splitting of the electronic states in energy and momentum space. Thus, it can be used to identify and characterize altermagnetic materials via ultrafast pump-probe Kerr/Faraday spectroscopy or spin-and time-resolved ARPES. This opens up the possibility of devising ultrafast optical switches of nonequilibrium spin-polarization, finely tunable by adjusting the pump-pulse characteristics.