Ultrafast photo-induced dynamics across the metal-insulator transition of VO₂

¹ Department of Physics and Center for Advanced Nanoscience, University of California, San Diego La Jolla, CA 92093, USA
² Materials Science and Engineering Program, University of California, San Diego - La Jolla, CA 92093, USA
³ Materials Sciences Division, Lawrence Berkeley National Laboratory - Berkeley, CA 94720, USA
⁴ Department of Physics, Universidad de los Andes - Bogotá 111711, Colombia
⁵ Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University - Tel Aviv 69978, Israel
⁶ School of Chemistry, the Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University Tel Aviv 69978, Israel

^(a) jgramirez@uniandes.edu.co

Received: 21 November 2016
Accepted: 5 June 2017

Abstract

The transient reflectivity of VO₂ films across the metal-insulator transition clearly shows that with low-fluence excitation, when insulating domains are dominant, energy transfer from the optically excited electrons to the lattice is not instantaneous, but precedes the superheating-driven expansion of the metallic domains. This implies that the phase transition in the coexistence regime is lattice-, not electronically-driven, at weak laser excitation. The superheated phonons provide the latent heat required for the propagation of the optically-induced phase transition. For VO₂ this transition path is significantly different from what has been reported in the strong-excitation regime. We also observe a slow-down of the superheating-driven expansion of the metallic domains around the metal-insulator transition, which is possibly due to the competition among several co-existing phases, or an emergent critical-like behavior.