Excitonic optical nonlinearity in PbI₂

Optical nonlinearity is studied in PbI₂ evaporated film under pump and probe configuration using two tunable nanosecond pulsed dye lasers. A change of the transmitted probe intensity is observed when the pump frequency is equal to the probe one and is in the exciton region. The spectral shape is dispersive if the probe frequency is off the exciton energy, but is peak-like at the resonance, where the nonlinear absorption coefficient amounts to -2×10⁵ cm/MW which corresponds to a third-order nonlinear susceptability of -1×10^-4 esu. This Rayleigh-type structure is not due to the hole burning effect because it totally disappears when the polarization of the two beams is crossed. It is explained by the oscillation of the exciton population due to the coherent interaction between pump and probe laser beams.