Ultrafast switching of optical properties in a doped semiconductor by intense femtosecond laser pulses

The rapid switching of materials when excited by ultrashort pulses of light is central for many optical technologies, and in particular to the developing field of time-varying metamaterials. These out-of-equilibrium interactions are difficult to capture with traditional theoretical models. Here we combine experiments and theory to unravel different regimes of interactions and a response saturation for a 44 fs, near-infrared pump pulse exciting a switchable doped semiconductor indium tin oxide thin film target. We model this process as a change in plasma frequency due to the excitation of hot electrons in a non-parabolic conduction band, which increases their effective mass. Our calculations show that saturation at high pump intensities arises because the pump heavily depopulates electrons from below the Fermi level. Excellent agreement with values extracted from experimental data confirms our model. For lower pump intensities, a two-temperature model is consistent with our data, but at higher intensities, it is apparent that other processes are at work, which we attribute to Auger transitions from the valence band, which introduce complex structure into the response, due to non-equilibrium rearrangement of energy between electrons and holes.