Ultrafast switching to an insulating-like metastable state by amplitudon excitation of a charge density wave

In correlated electron materials, multiple electronic phases may appear next to each other in their phase diagram, and these can be tuned, for example, by applying static pressure or chemical doping(1-3). These perturbations modify the subtle balance between the electron transfer energy and Coulomb repulsion between electrons. It is, therefore, tempting to explore whether new states of matter can be accessed through the direct tuning of their order parameters, for example, by driving a collective mode of the emergent phase. Here we demonstrate that the direct excitation of the amplitude mode of a charge density wave (amplitudon) by an intense terahertz pulse in a layered transition metal dichalcogenide compound, namely, 3R-Ta1+xSe2, leads to the appearance of an insulating-like metastable state. The formation dynamics of the metastable phase manifest in the opening of a gap in the optical conductivity spectrum, and we show that they synchronize with an oscillation of the amplitudon. This indicates the intimate interplay between the order parameters of the equilibrium charge density wave and the metastable states.