Ultrafast Insulator-Metal Phase Transition in VO2 Studied by Multiterahertz Spectroscopy

TL;DR

This study uses multiterahertz spectroscopy to investigate the ultrafast insulator-metal transition in VO2, revealing detailed dynamics of lattice and electronic responses and their coupling during the phase change.

Contribution

It provides new insights into the ultrafast dynamics of VO2's phase transition, including temperature-dependent critical fluence and phonon-electron coupling mechanisms.

Findings

Critical fluence increases as temperature decreases.

Observation of phonon frequency modulation due to anharmonic coupling.

Detection of quadratic electronic response modulation at twice the phonon frequency.

Abstract

The ultrafast photoinduced insulator-metal transition in VO2 is studied at different temperatures and excitation fluences using multi-THz probe pulses. The spectrally resolved mid-infrared response allows us to trace separately the dynamics of lattice and electronic degrees of freedom with a time resolution of 40 fs. The critical fluence of the optical pump pulse which drives the system into a long-lived metallic state is found to increase with decreasing temperature. Under all measurement conditions we observe a modulation of the eigenfrequencies of the optical phonon modes induced by their anharmonic coupling to the coherent wave packet motion of V-V dimers at 6.1 THz. Furthermore, we find a weak quadratic coupling of the electronic response to the coherent dimer oscillation resulting in a modulation of the electronic conductivity at twice the frequency of the wave packet motion. The findings are discussed in the framework of a qualitative model based on an approximation of local photoexcitation of the vanadium dimers from the insulating state.