Silicon nitride (SiNx) is an important material for on-chip waveguides and resonators due to its tunable refractive index
and low optical loss at the visible and near-infrared wavelengths. In this work, we report our results on second-harmonic
generation from SiNx thin films at the fundamental wavelength of 1064 nm. The SiNx thin films with the thicknesses
between 100 nm and 1500 nm were prepared on fused silica substrates by plasma enhanced chemical vapor deposition.
Strong SHG signal was observed from SiNx films, with the absolute levels significantly higher than those from typical
dielectric surfaces. The second-order properties of the samples were fully characterized by second-harmonic generation
as a function of the state of polarization of the fundamental field. The polarization dependent SHG indicates that the
SiNx films possess in-plane isotropy and polar order along the surface normal. The strong second-order nonlinear
response from the SiNx films has great potential applications in the on-chip nanophotonic devices.