Ultrafast changes in lattice symmetry probed by coherent phonons
S. Wall, D. Wegkamp, L. Foglia, K. Appavoo*, J.Nag*, R. F. Haglund*, J. Stähler, M. Wolf.
Nat. Commun., 3, 721, DOI:10.1038/ncomms1719
The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a
photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when
and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show
that a timedependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an
alloptical probe of structural transitions. We examine the photoinduced structural phase transition in VO_{2} and show that, above the phase
transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibriumphase
phonon modes occurs promptly, indicating a nonthermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice
potential changes its symmetry before ionic rearrangement has occurred.
*Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 372351807, USA

