Photoinduced gap renormalization and many-body recombination in insulating cuprates*

We have studied the pump-probe response of three insulating cuprates as a function of both pump and probe photon energies, time, and fluence. The fluence dependence follows a simple and universal analytical form that includes a characteristic volume scale, which we associate with the interaction volume for a photoexcitation. This characteristic size varies strongly with pump photon energy, with a maximum just above the charge-transfer absorption peak. We also show evidence for ultrafast many-body recombination in the photocarrier kinetics, characterized by an anomalously large Auger coefficient.