Probing and Controlling Low Energy Excitations in Complex Materials with Using Terahertz Pulses*

Single–cycle terahertz (THz) pulses enable both the understanding and control of low-energy excitations in materials on ultrafast timescales. Combining excitation with THz pulses with a time-delayed probe based on second harmonic generation enables the observation of ultrafast structural changes with surface sensitivity. Using this technique, we investigate the dynamics of surface and bulk carriers, as well as nonlinear phonon dynamics, in the topological insulator Bi₂Se₃. We will also describe the use of THz pulses as a probe to reveal spin dynamics in insulating antiferromagnets.