Light induced transient dynamics of the charge transfer insulator La${}_2$CuO${}_4$

We investigate the transient dynamics of La${}_2$CuO${}_4$, the parent compound of the Lanthanum based cuprate high temperature superconductors, upon photo excitation with UV photons across the charge transfer (CT) gap of 2.1 eV. The resulting transient state is studied for extracting the time-evolution of the broadband complex dielectric function in the spectral range of 0.5 - 2.6 eV. Experiments are performed as a function of the excitation density over several orders of magnitude, up to 0.1 absorbed photons/Cu-atom. Modeling the changes in the complex dielectric function with the (induced) Drude - Lorentz model reveals a pronounced renormalization of the CT gap, accompanied by the light-induced mid-gap absorption, resembling the evolution of optical properties by chemical doping. The data provide strong constraints on the possible photogenerated free carrier (Drude) response. We demonstrate, that even at the highest excitation densities, where in the case of comparable chemical doping a metallic state is realized, photodoping results in a negligible density of free carriers, underscoring the underlying Mott physics.