Giant optical nonlinearity due to biexcitons in CuCl

Abstract
A lower limit for the value of χ(3) associated with the two-photon resonance of the Γ1 biexcitons in CuCl at 1.6 K is found to be 5.4×10−5 esu. The upper bound of the two-photon resonance width is ΔE=0.02 meV. Broadening of the resonance occurs for light intensities I0>103 W/cm2.