Abstract

The traditional approach for calculating nonlinear optical signals is based on performing response theory of the material system under study with respect to the external laser (electric) fields. The optical response is then characterized completely by the eigenstates of the material system with instantaneous Coulomb interactions. For a molecular crystal, these eigenstates are the Coulomb excitons and the optical response may be expressed in terms of the exciton energies scattermg rates, and transition dipoles. This picture, however, neglects the transverse internal radiation field (retardation)[1], which is a quantum degree of freedom and cannot be switched off experimentally. Consequently, in low temperature pure crystals with sufficiently high density of oscillator strength, the proper eigenmodes are those of the coupled material-radiation system: polaritons [2-4]. The polaritons are coupled to the external fields through the boundary conditions on the crystal’s surface and in a proper description of the optical experiment, we should consider the response of the polariton system to these external fields [1]. The optical signal then contains characteristics of polaritons rather than excitons. Recently, such explicit signatures of retardation have received much experimental and theoretical attention [3-7].

© 1990 Optical Society of America