Line width of the biexciton states in PbI2 estimated from the analysis of reflection-type four-wave mixing spectra

We investigated the reflection-type four-wave mixed (FWM) light in the resonance energy region of excitons and biexcitons of PbI2 at 10 K. It is found that FWM spectra show the structures due to both exciton and biexciton nonlinearities and depend significantly on the polarization state of incident light. The experimental results are qualitatively explained by the four-level model which takes into account a biexciton level as well as an exciton one. It is found that the FWM spectra reflect the line width of biexciton state. The line-width of the biexciton state is estimated to be 0.3 meV. The oscillator strength of the biexcitons relative to that of excitons (|μme/μeg|2) is evaluated to be ≈2.76×103±1.05×102.