Probing and controlling terahertz-driven structural dynamics with surface sensitivity

Intense, single-cycle terahertz (THz) pulses are powerful tools to understand and control material properties through low-energy resonances, such as phonons. Combining this with optical second harmonic generation (SHG) makes it possible to observe the resulting ultrafast structural changes with surface sensitivity. This makes SHG an ideal method to probe phonon dynamics in topological insulators (TI), materials with unique surface transport properties. Here, we resonantly excite a phonon mode in the TI ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with THz pulses and use SHG to separate the resulting symmetry changes at the surface from the bulk. Furthermore, we coherently control the lattice vibrations with a pair of THz pulses. Our work demonstrates a versatile, table-top tool to probe and control phonon dynamics in a range of systems, particularly at surfaces and interfaces.