Ultrafast Insulator-Metal Transition in VO2 Driven by High-field THz Excitation

Vanadium dioxide (VO2) is a prime example of a transition metal oxide with a sharp insulator-metal transition (IMT) at 340 K accompanied by a change of the lattice symmetry. A possibility to induce the metallic state in VO2 by electric field has attracted a lot of attention due to its potential applications in optics and high-speed electronics. However, numerous efforts to control the electronic state by applied electric bias have demonstrated that the switching mechanism is governed by resistive heating, dramatically limiting the operation speed. Recent developments in generation of high-field broadband THz transients offer an attractive way to apply extremely high electric fields on ultrashort timescales.
Here, we demonstrate an IMT in VO2 thin films driven by high-field multi-THz transients on a sub-100 fs timescale. Our broadband and phase-stable THz transients with extremely high peak electric fields of up to 17 MV/cm are generated via difference frequency mixing in a GaSe crystal. A typical excitation transient and the induced relative transmission change T/T traced by 8-fs-short near-infrared pulses are shown in Fig. 1(a). An ultrafast decrease of the transmission indicates the THz-driven switching into the metallic state, succeeded by a relatively slow relaxation on longer timescales. Besides that, the lattice dy-namics related to the coherent wave packet motion of the vanadium dimers manifests itself as an oscillation at a frequency of 5.9 THz [Fig. 1(a)]. Our experiments show that the observed non-thermal switching into a metastable metallic state is governed solely by the amplitude of the applied THz field. In contrast to resonant near-infrared excitation below the threshold fluence, no signatures of excitonic self-trapping are observed down to the lowest fluences of the THz excitation.
Our results can be understood as the generation of spatially separated charge pairs and a cooperative transition into a delocalized metallic state by THz field-induced tunneling. The total density of the delo-calized carriers proportional to the increase in optical conductivity Δσ1 at THz frequencies depicted in Fig. 1(b) shows a highly nonlinear dependence on the peak excitation field expected for a many-body tunneling process. We find good agreement with theoretical equation describing pair production in a Mott insulator and determine an electronic correlation length of 2.1 Å.