  The real-time dynamics of interacting electrons out of equilibrium contains
detailed microscopic information about electronically correlated materials,
which can be read out with time-resolved optical spectroscopy. The reflectivity
that is typically measured in pump-probe experiments is related to the
nonequilibrium optical conductivity. We show how to express this quantity in
terms of real-time Green functions using dynamical mean-field theory. As an
application we study the electrical response of the Falicov-Kimball model
during the ultrafast buildup of the gapped phase at large interaction.
