Evidence for impact ionization in vanadium dioxide

Pump-probe optical spectroscopy was used to investigate charge carrier multiplication via impact ionization in the M1 insulating phase of VO2. By comparing the transient reflectivities of the film when pumped at less than and then more than twice the band-gap energy, we observed an enhancement of the ultrafast response with the higher energy pump color while the film was still transiently in the insulating phase. We additionally identified multiple timescales within the charge dynamics and analyzed how these changed when the pump and probe wavelengths were varied. This experiment provided evidence that impact ionization acts efficiently as a carrier multiplication process in this prototypical strongly-correlated insulator.