Direct two-photon creation of a biexciton

The nonlinear absorption coefficient for the two-photon creation of biexciton in CuCl is calculated in the approximation of one intermediate, optically active excitonic state. The two-band model with realistic band structure at the band extrema is considered. The exciton–biexciton dipole transition momentum matrix element is expressed by the functional of the exciton and biexciton envelopes. In the numerical calculations the envelope of the Hylleraas–Ore type, modified by Brinkman et al., is used. Our calculated value of the absorption coefficient is about two orders of magnitude smaller than that estimated previously by Hanamura, and agrees better with the experimental value reported by Gale and Mysyrowicz.