Optical pump-terahertz emission probe of ultrafast magnetization dynamics

Understanding spin dynamics on ultrafast timescales offers not only fundamental insights into the coupling of both electrons and phonons with spins but also opportunities for faster and more efficient spintronic devices. However, reliable access to ultrafast spin dynamics in materials and devices under realistic device-operation conditions remains a challenge. Here, we demonstrate a spectroscopic method of optical pump-terahertz (THz) emission (OPTE), which gives direct access to the ultrafast demagnetization and re-magnetization dynamics in a contact-free method under ambient conditions. The observation time window is not limited by the pulse width of THz radiation emitted by the sample. We measure the ultrafast spin dynamics in a laser-excited Fe monolayer. Our measurements disentangle distinct components originating from (i) the ultrafast magnetization quenching that occurs in less than 0.5 ps and (ii) the fast and slow magnetization recoveries that correspond to the spin-lattice coupling and heat diffusion from the sample into the substrate/surroundings, respectively. The OPTE can be a platform to probe and optimize the performance of magneto-optical recording materials and THz emission applications.