Magnetism in a 2D magnet: THz gap is one critical energy range that bridges between electronics (DC – 10 GHz) and optics (10 THz – 1000 THz). Searching for systems that host THz excitations could provide novel ways for processing information in devices at THz frequencies, a couple of orders quick than the state-of-the-art electronics. This work on a honeycomb magnet CrI3 aims to look for THz spin waves – i.e., collective spin excitations that propagates like waves in magnetic materials. Contributors: Xiangpeng Luo, Liuyan Zhao