Emission of coherent THz magnons in an antiferromagnetic insulator triggered by ultrafast spin-phonon interactions

Antiferromagnetic (AFM) materials have been proposed for new types of narrowband THz spintronic devices owing to their ultrafast spin dynamics. Although manipulating coherently their spin dynamics remains a key challenge, this can be potentially accomplished by spin-orbit torques or direct optical excitations. Here, we demonstrate the combined generation of broadband THz (incoherent) magnons and narrowband (coherent) magnons at 1 THz in ultra-thin films of AFM - heavy metal (NiO/Pt). We show, experimentally and through modeling, two excitation processes of the spin dynamics in NiO, an off-resonant instantaneous optical spin torque in (111) oriented films and a strain-wave induced THz torque induced by ultrafast Pt excitation in (001) oriented films. Both phenomena lead to THz emission through the inverse spin Hall effect in the adjacent heavy metal layer, opening new routes towards the development of fast opto-spintronic devices based on AFM materials.