MANY-PHOTON EFFECTS IN INELASTIC LIGHT-SCATTERING

The usual approach to second harmonic generation (SHG) is based on low-order perturbation theory and neglects the changes in the electronic states that are induced by the exciting beam. To see what happens when this approximation fails, we propose a new formalism, based on the method of excitation amplitudes, to set up a theory of the SHG response and other inelastic light scattering phenomena. We develop the quantum theory in the dipole approximation and show that the inclusion of electron-photon coupling to all orders leads to new features. Strong exciting fields produce a nonnegligible SHG response in highly polarisable centro-symmetric systems, and the emitted spectrum shows a dynamical Stark effect.