Influence of atomic-scale defects on coherent phonon excitations by THz near fields in an STM.

Coherent phonons describe the collective, ultrafast motion of atoms and play a central role in light-induced structural dynamics. Here, we use terahertz (THz) scanning tunneling microscopy to excite and detect coherent phonons in semiconducting 2H-MoTe2 and resolve how their excitation is influenced by atomic-scale defects. In a THz pump-probe scheme, we observe long-lived oscillatory signals that we assign to out-of-plane breathing and in-plane shear modes, which are both dipole forbidden in the bulk. The relative excitation strength of these modes varies near defects, indicating that tip-induced local band bending modulates the coupling to the THz field. This defect-tunable coupling offers previously unidentified opportunities to control selective excitation of vibrational modes at the nanoscale.