Observation of long-lived population inversion in the antiferromagnet NiPS₃ under resonant excitation

Magnetic van der Waal materials offer a fascinating platform for the study of novel correlated physics. Tunable control of the properties of these materials could pave the way for applications in electronic and optical devices. Using time-resolved terahertz spectroscopy to probe the low energy optical response, we explore the intertwined electronic and magnetic excitations that have been discovered in the antiferromagnetic van der Waals insulator NiPS3. By pumping the material at several carefully selected, narrow spectral regions to disentangle the nonequilibrium behavior, we uncover a long-lived excited state expressing characteristics of population inversion at THz energies. Additionally, the optical response at THz frequencies illuminates photo-induced carrier mechanisms. Our results open up new routes to realize long-lived stimulated emission at THz energies as well as highlighting NiPS3 as a promising building block in electronic and optical heterostructure design.