Giant-Oscillator-Strength Effect on Excitonic Optical Nonlinearities Due to Localization

The optical nonlinearity ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ of excitons localized by disorder or impurity has a large value as a consequence of the double multiplication of the giant oscillator strength due to exciton localization because the ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ process involves the double repetition of the excitonic transition. The importance of the exciton correlation in the coherent nonlinear optical process is pointed out for the first time. The sign of ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ relative to ${\chi }^{\mathrm{}\left(1\right)\mathrm{}}$ gives important information about the character of the exciton correlation.