Abstract

It is pointed out that third order nonlinear susceptibility, χ⁽³⁾, shows very large value at the exciton resonance because of the macroscopic transition dipole moment of excitons.^[1] When the excitons are harmonic Bosons, exciton polariton states are the eigen modes of the crystal. The Hamiltonian of the exciton–photon system has already been diagonalized and thus we have no intensity dependence of the optical responses. In the actual system, however, excitons are not harmonic. The anharmonicity due to the interaction between excitons modifies the energy levels of the system as shown in Fig. 1. In the third order nonlinear processes, the relevant energy levels are up to 2-exciton states. The difference between the energy of two interacting excitons and that of noninteracting two excitons, denoted as ω_int, is a measure of anharmonicity. The biexciton state is a typical manifestation of negative coint such as the case of CuCl. Int this report, we present the several experimental evidences of resonant coherent optical nonlinearity in such anharmonic excitonic systems. We applied high resolution two-photon spectroscopy and time resolved four wave mixing in bulk CuCl crystals and ZnSe films grown on GaAs substrate by MBE method.

© 1992 IQEC