Spin response in nonlocal nonlinear optics

A novel effect of the electron spin in nonlinear optics is predicted. Starting from the Pauli Hamiltonian, we present the influence of the electron spin on the nonlinear optical response of a homogeneous electron gas in the low temperature limit. It is shown that the longitudinal second-harmonic response which results from a transverse incoming field differs significantly from the usual paramagnetic response when spin effects are included. The calculation is done both with and without screening