Silicon nitrides are synthesized by ion-assisted deposition with only one coating material and a nitrogen-ion-beam source. All the SiN(x) films are amorphous and mechanically strong. A wide range of refractive indices from 3.43 to 1.72 at a wavelength of 1550 nm is obtained. Near-IR antireflection coating and a bandpass filter based on the multilayers of SiN(x) and Si are demonstrated.