Picometer scale differential imaging of a terahertz-field-driven topological phase transition

Light-induced phase transitions in complex materials are promising for enabling ultrafast control of electronic and photonic properties in emerging technologies. Among these materials, WTe₂—a type-II Weyl semimetal candidate—stands out due to its topologically protected states hosting massless fermions. In this study, we demonstrate a topological phase transition on the surface layer of WTe₂, triggered by ultrafast terahertz fields that are enhanced by an atomically sharp tip and resonantly interact with an interlayer shear mode at its apex. Using atomically resolved imaging and hybrid-level density functional theory calculations, we find that this transition involves a small, single-digit picometer shift of the top atomic layer. Our calculations show that the interlayer coupling and symmetry breaking in WTe₂ are critical for this stable-to-metastable topological transition, which is related to one of the material’s two surface terminations. Changes in the band structure, specifically in the electron and hole pockets, are confirmed via tunneling spectroscopy, pointing to a reversible annihilation of topological surface states. This terahertz-driven, ultrafast modulation of topological properties highlights a pathway toward dynamic control of electronic states in topological materials.