Absolute negative conductivity of electrons after ultrafast photoexcitation

We consider transient conductivity of electrons with partially inverted distribution created in the conduction band after an ultrafast interband photogeneration. The time-dependent photoconductivity is calculated for GaAs and InSb semiconductors taking into account the retardation of the response due to electron momentum relaxation. It is found that the absolute negative conductivity takes place in the case of electron excitation near the optical phonon energy and can persist over several nanoseconds in InSb at low temperatures. The existence of the nonlocal negative conductivity causes a peculiar behavior of the circuit response due to retardation effects and a finite capacitance of the sample where the electrons are excited.