Coherent control of a surface structural phase transition

Active optical control over matter is desirable in many scientific disciplines, with prominent examples in all-optical magnetic switching(1,2), light-induced metastable or exotic phases of solids(3-8) and the coherent control of chemical reactions(9,10). Typically, these approaches dynamically steer a system towards states or reaction products far from equilibrium. In solids, metal-to-insulator transitions are an important target for optical manipulation, offering ultrafast changes of the electronic(4) and lattice(11-16) properties. The impact of coherences on the efficiencies and thresholds of such transitions, however, remains a largely open subject. Here, we demonstrate coherent control over a metal-insulator structural phase transition in a quasi-one-dimensional solid-state surface system. A femtosecond double-pulse excitation scheme(17-20) is used to switch the system from the insulating to a metastable metallic state, and the corresponding structural changes are monitored by ultrafast low-energy electron diffraction(21,22). To govern the transition, we harness vibrational coherence in key structural modes connecting both phases, and observe delay-dependent oscillations in the double-pulse switching efficiency. Mode-selective coherent control of solids and surfaces could open new routesto switching chemical and physical functionalities, enabled by metastable and non-equilibrium states.