Observation of impact ionization in vanadium dioxide

Pump-probe optical spectroscopy was used to investigate the possibility of charge carrier multiplication by impact ionization in a 100~nm film of VO$_{\mathrm{2}}$ in the M$_{\mathrm{1}}$ insulating phase. The film was excited by pump pulses with energies above and below twice the band gap energy and observed with two different probe wavelengths. The transient reflectivities of the film were then compared. We observed an enhancement of the reflectivity for the higher energy pump pulses near zero delay compared to the reflectivity for the lower energy pump pulses for both probe wavelengths. Additionally, we identified and described multiple timescales within the charge dynamics. This experiment demonstrated that impact ionization acts as a carrier multiplication process in this prototypical strongly-correlated system.