  Strong second harmonic generation (SHG) in silicon nitride has been
extensively studied-among others, in terms of laser-induced SHG enhancement in
Si3N4 waveguides. This enhancement has been ascribed to the all-optical poling
induced by the coherent photogalvanic effect. Yet, an analogous process for
Si3N4 thin films has not been reported. Our article reports on the observation
of laser-induced 3-fold SHG enhancement in Si3N4 thin films. The observed
enhancement has many features similar to all-optical poling, such as highly
nonlinear power dependence, cumulative effect, or connection to the Si3N4-Si
interface. However, identical experiments for low-oxygen silicon oxynitride
thin films lead to complex behavior, including laser-induced SHG reduction. By
a thorough experimental study, the observed results were ascribed to
heat-induced SHG variation caused by multiphoton absorption. Such behavior
indicates that the origin of optically-induced SHG enhancement in SiOxNy-Si
structures can be a complex interplay of various phenomena.
