Observation of ultrafast impact ionization in diamond driven by mid-infrared femtosecond pulses

We report on the observation of ultrafast impact ionization in monocrystalline diamond driven by high-intensity mid-infrared femtosecond laser pulses. The measurements are based on monitoring the excited carrier population during and after the interaction of the pre-excited sample with a strong infrared pulse by transient transmission spectroscopy and photoluminescence measurements. A twofold increase in the initial carrier population due to impact ionization is observed with the peak infrared intensity of 2.5 TW/cm2. The experimental results are supported by numerical simulations of electron dynamics using time-dependent density functional theory, which show that the electrons in the conduction band reach the energy threshold for impact ionization during the interaction with the infrared pulse.