  In vanadium dioxide, the interplay between coherent lattice transformation
and electronic correlation drives an insulator-to-metal transition (IMT). This
phase commutation can be triggered by temperature, pressure, doping or
deposition of optical energy. Here we demonstrate that an atomically-strong
terahertz electric field initiates a metastable ultrafast IMT in vanadium
dioxide without a concomitant lattice transformation. The free-space terahertz
field acts as off-resonant excitation with photon energy below the lattice
phonons and the interband transitions. Differently from optical and infrared
excitation, terahertz interaction leads to a full IMT by interband Zener
tunneling with a negligible entropy deposition. In previous experiments the
temporal dynamics of IMT in VO2 could be only indirectly inferred. We
disentangle the electronic and lattice contributions to the IMT on a
sub-picosecond timescale. Near the critical temperature the IMT becomes
dissipative and the terahertz field concludes the lattice-assisted metallic
nucleation initiated by heating. The method of strong-field induced phase
transition presented here is applicable to a wide class of strongly correlated
systems and will enable the discovery of novel metastable phases.
