The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photo-induced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, in order to indentify when and how fast a phase transition occurs, methods that can probe the symmetry change in the time domain are required. We show that a time dependent change in symmetry of the lattice potential can probe changes in the coherent phonon spectrum, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO2 and show that, above the phase transition threshold, photo-excitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes the potential symmetry before ionic rearrangement has occurred.
The original publication is available by link DOI: 10.1038/ncomms1719