Ultrafast reflectivity change of a vanadium dioxide induced by THz field enhanced by a metallic structure

We studied the transient reflectivity change of vanadium dioxide that is induced by terahertz (THz) pulse excitation, whose electric field is enhanced by a metallic dipole antenna. The tightly focused probe pulse in the near-infrared region allows one to monitor the reflectivity change between the antenna gap and also quantitative evaluation of the field dependence of reflectivity change. The observed steep rising of the dependence shows that the insulator-to-metal transition occurs with the threshold electric field strength at around 2 MV cm−1. Our result is explained by the model taking into account the carrier generation via the Poole–Frenkel effect induced by THz electric field.