We use broadband reflectivity and phonon spectroscopy to probe the photoinduced structural phase transition in VO2. At room temperature, vanadium dioxide is an insulator with a monoclinic crystal structure consisting of dimerized chains of vanadium ions. On heating to above approximately 60 °C the material becomes metallic with a rutile structure where the dimerization of the lattice is lost.
The same phase transition can be driven optically using light pulses from a laser. This has enabled the study and control of the properties of VO2 on the ultrafast timescale. In this work, we study the structural phase transition using coherent phonons as a probe of the lattice potential. A laser pulse can generate a force on a lattice via absorption. If this force is applied quickly, because the laser pulse is short, then the lattice will ‘ring’ coherently in terms of the normal modes (phonons) of the system. This ringing can be observed optically as a periodic modulation of the reflectivity.
By studying this ringing as a function of excitation strength we could show that, above a critical fluence, the material stops ringing, indicating that the light pulse changes the phonon spectrum, and hence lattice potential energy surface, on the ultrafast timescale.
Left: Coherent phonons modulating the broadband reflectivity below threshold. Middle: Excitation above the phase transition threshold produces no coherent motion. Right: Schematic of the phase transition. The material starts in the low temperature (L.T) monoclinic phase with an optical phonon mode. The laser pulse arrives at t=0, which changes the potential energy and hence phonon modes, before the system thermalizes into the high temperature (H.T) rutile phase.
D. Wegkamp, M. Meyer, C. Richter, M. Wolf, and J. Stähler
Photoinduced work function modifications and their effect on photoelectron spectroscopy
Appl. Phys. Lett. 103 151603 (2013) (publisher version) (free full text)
S. Wall, L. Foglia, D. Wegkamp, K. Appavoo, J. Nag, R. F. Haglund, J. Stähler, M. Wolf
Tracking the evolution of the electronic and structural properties of VO2 during the ultrafast photoinduced insulator-metal transition
Phys. Rev. B 87 115126 (2013) (publisher version) (free full text)
S. Wall, D. Wegkamp, L. Foglia, K. Appavoo, J. Nag, R. F. Haglund, J. Stähler, M. Wolf
Ultrafast changes in lattice symmetry probed by coherent phonons
Nat. Commun., 3, 721 (2012), (publisher version) (free full text)
Schneller Wandel im Kristall
Ultrafast sonograms shed new light on rapid phase transitions