  Second-harmonic generation (SHG) is a second-order nonlinear optical process
that is not allowed in media with inversion sym-metry. However, due to the
broken symmetry at the surface, surface SHG still occurs, but is generally
small. We experimentally investi-gate the surface SHG in periodic stacks of
alternating, subwave-length dielectric layers, which have a large number of
surfaces, thus enhancing surface SHG considerably. To this end, multilayer
stacks of SiO2/TiO2 were grown by Plasma Enhanced Atomic Layer Deposition
(PEALD) on fused silica substrates. With this technique individual layers of a
thickness of less than 2 nm can be fabricated. We experimentally show that
under large angles of incidence (> 20 degrees) there is substantial SHG, well
beyond the level, which can be observed from simple interfaces. We perform this
experiment for samples with different periods and thickness of SiO2/TiO2 and
our results are in agreement with theoretical calculations.
