LINEAR AND NONLINEAR-OPTICAL PROPERTIES OF SURFACES AND SPATIALLY NONLOCAL POTENTIALS

We have developed a theory of the random phase approximation (RPA) opt ical response of surfaces and interfaces that, in the supercell approx imation, takes into account the possible non-locality in the effective potential. One can greatly simplify the theory by decomposing the per turbing electric field into an almost spatially constant external fiel d and an induced z-dependent part. A self-consistent solution of Maxwe ll equations allows us to express the first and second order in the ex ternal field z-dependent densities of polarization by means of ''effec tive'' first and second order z-dependent susceptibilities for which a pproximated expressions are given.