  The desire to exert active optical control over matter is a unifying theme
across multiple scientific disciplines, as exemplified by all-optical magnetic
switching, light-induced metastable or exotic phases of solids and the coherent
control of chemical reactions. Typically, these approaches dynamically steer a
system towards states or reaction products far from equilibrium. In solids,
metal-insulator transitions are an important target for optical manipulation,
offering dramatic and ultrafast changes of the electronic and lattice
properties. In this context, essential questions concern the role of coherence
in the efficiencies and thresholds of such transitions. Here, we demonstrate
coherent vibrational control over a metal-insulator structural phase transition
in a quasi-one-dimensional solid-state surface system. An optical double-pulse
excitation scheme is used to drive the system from the insulating to a
metastable metallic state, and the corresponding structural changes are
monitored by ultrafast low-energy electron diffraction. We observe strong
oscillations in the switching efficiency as a function of the double-pulse
delay, revealing the importance of vibrational coherence in two key structural
modes governing the transition on a femtosecond timescale. This mode-selective
coherent control of solids and surfaces could open new routes to switching
chemical and physical functionalities, facilitated by metastable and
non-equilibrium states.
