Ultrafast changes in lattice symmetry probed by coherent phonons

TL;DR

This paper demonstrates an all-optical method to probe ultrafast lattice symmetry changes during photoinduced phase transitions using coherent phonons, revealing non-thermal pathways in VO2.

Contribution

It introduces a novel time-resolved optical technique to detect symmetry changes via coherent phonon spectra during ultrafast phase transitions.

Findings

Photoinduced lattice potential changes occur on an ultrafast timescale.

Loss of equilibrium phonon modes indicates symmetry change.

Evidence of a non-thermal pathway in VO2 phase transition.

Abstract

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, in order to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. We show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, 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, photoexcitation 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 its symmetry before ionic rearrangement has occurred.