In the low exciton density regime, we study the optical nonlinearity in the exciton resonant region in ZnSe grown by MBE on a GaAs substrate. At low temperature, excitons in bulk ZnSe largely contribute to enhance the nonlinearity. Using reflection type polarization spectroscopy with weak picosecond pulses, we evaluate third-order susceptibility and obtain huge value of χ(3), over 10-2 esu. From a degenerate four-wave mixing experiment with different polarization combinations, it is found that this huge nonlinearity originates from the exciton-exciton interaction.
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