Separating terahertz spin and charge contributions from ultrathin antiferromagnetic heterostructures

Femtosecond laser excitation of nanometer thin heterostructures comprising a heavy metal and a magnetically ordered material is known to result in the emission of terahertz radiation. However, the nature of the emitted radiation from heavy metal~/~antiferromagnet heterostructures has sparked debates and controversies in the literature. Here, we unambiguously separate spin and charge contributions from Pt~/~NiO heterostructures by introducing an unprecedented methodology combining high external magnetic fields with a symmetry analysis of the emitted terahertz polarization. We observe two distinct mechanisms of terahertz emission which we identify as optical difference frequency generation and ultrafast laser-induced quenching of the magnetization. We emphasize the absence of spin transport effects and signatures of coherent magnons. Overall, our work provides a general experimental methodology to separate spin and charge contributions to the laser-induced terahertz emission from heterostructures comprising a magnetically ordered material thus holding great potential for advancing terahertz spintronics and establishing terahertz orbitronics.