Pump-probe excitation spectroscopy of insulating cuprates

We examine the transient optical response of optically thick single crystals of the insulating cuprates La$_2$CuO$_4$, YBa$_2$Cu$_3$O$_6$, and Sr$_2$CuO$_2$Cl$_2$, as a function of probe wavelength, pump excitation wavelength, and pump fluence. At pump-probe time delay $t>1$ ps, the transient reflectance spectrum mimics a change in temperature, while for $t<1$ ps we observe a non-thermal response for which this description fails. The pump-probe signal saturates at a characteristic fluence that depends on pump wavelength. In all three materials, the saturation fluence reaches a minimum at a pump photon energy 0.6 eV above the optical absorption peak associated with the charge transfer gap. We associate both the pump-probe spectrum and its saturation behavior with the thermalization of the initial photoexcited state, and argue that the saturation spectrum indicates a relaxation bottleneck just above the charge transfer gap.