Abstract

When light-matter interaction is described in dipole approximation, second order non-linear processes are parity forbidden in centrosymmetric media. Yet, second harmonic generation in reflection from cubic crystals was observed already two decades ago¹. This is possible because quadrupole interaction is not restricted by the same parity arguments, and in the surface region of cubic media the bulk symmetry is broken and second order dipole interaction becomes allowed. Obviously, the latter is extremely surface sensitive and therefore well suited for in situ surface analysis² provided it is possible to separate it from quadrupole contributions of the bulk.

© 1988 Optical Society of America