Large Optical Nonlinearity Due to Exciton State in PbI₂

Optical nonlinearity is studied in a PbI₂ evaporated film under the pump and probe configuration using two tunable nanosecond pulsed 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. This Rayleigh-type nonlinearity is neither due to hole burning effect nor thermal grating effect, but due to excitonic population oscillation. The maximum nonlinear absorption amounts to —2 × 10⁵ cm/MW which corresponds to a third-order nonlinear susceptibility of —10–⁴ esu. Such a large nonlinearity can be explained if the nonlinear interaction takes place over many unit cells, which comes from coherent expanse of an exciton in the crystal.