High Quality Content by WIKIPEDIA articles! Surface second harmonic generation is a method for probing interfaces in atomic and molecular systems. In second harmonic generation (SHG), the light frequency is doubled, essentially converting two photons of the original beam of energy E into a single photon of energy 2E as it interacts with noncentrosymmetric media. Surface second harmonic generation is a special case of SHG where the second beam is generated because of a break of symmetry caused by an interface. Since symmetry is only disrupted in the first (occasionally second and third) atomic or molecular layer of a system, properties of the second harmonic signal give us information about the first atomic or molecular layers only. Surface SHG is possible even for materials which do not exhibit SHG in the bulk.