Femtosecond Structural Dynamics inVO2during an Ultrafast Solid-Solid Phase Transition

Abstract

Femtosecond x-ray and visible pulses were used to probe structural and electronic dynamics during an optically driven, solid-solid phase transition in VO(2). For high interband electronic excitation (approximately 5 x 10(21) cm(-3)), a subpicosecond transformation into the high-T, rutile phase of the material is observed, simultaneous with an insulator-to-metal transition. The fast time scale observed suggests that, in this regime, the structural transition may not be thermally initiated.

Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO2

“…First, for the lowest fluence investigated we see a transition time of t f ¼ 40 ± 8 fs, shorter than the previously observed dynamics in any VO 2 sample, as discussed further below. Second, we observe a dramatic, up to threefold increase in transition time with increasing fluence (b, inset), in contrast to a decrease observed in previous work on polycrystalline films 3,6 . Third, we see the transition time t f decrease with increasing sample temperature (Fig.…”

Ultrafast changes in lattice symmetry probed by coherent phonons

“…The interpretation of the ultrafast structural phase transition in VO 2 that results from these measurements differs from the previously proposed mechanism, in which the timescale was said to be dictated by the phonon period of the monoclinic phase 6 . On the contrary, at high fluences we observe a complete loss of the coherent motion associated with the monoclinic phase and a drastic change in the response of the excited state, demonstrating a change in symmetry of the lattice potential that is promptly driven by the laser pulse.…”

“…The increased symmetry reduces the number of Raman active phonon modes from 18 in the M 1 phase to 4 in the R-phase, without any significant mode softening near the transition temperature 18 . The same structural transition has been induced on the ultrafast timescale by exciting M 1 -phase VO 2 at room temperature with an intense 800 nm pump laser, using a pump fluence greater than F th~7 mJ cm − 2 , and observed by femtosecond X-ray 6,19 and electron diffraction 7 , as well as through changes in the optical 20,21 and electrical 22,23 properties. These experiments show that the complete transformation to the R-phase, after some initial fast dynamics, is a slow process, taking hundreds of picoseconds to complete.…”

“…However, unlike bismuth, the transition in VO 2 involves complex changes in multiple phonons and results in a final state that is also an ordered solid. It should be emphasized that the establishment of the R-phase with long-range order is a slow process, occurring over tens of picoseconds 7 , that takes much longer than the observed loss in phonon modes 6,7,19 . The state of VO 2 immediately after photoexcitation is far from equilibrium, and the lattice potential is likely to continue to evolve to that of the R-phase during the thermalization process 30,31 .…”

“…Changes in ionic positions are observed by changes to these peaks and, by extending this technique to the time domain, dynamical phase transitions can be clearly identified [5][6][7][8] . However, the equivalent analysis of the time-resolved changes in the phonon spectrum during a time-dependent phase transition has received less attention.…”