Ion- and plasma-assisted deposition has been extensively used for the fabrication of high-performance optical films with dense and smooth microstructures that are essential for applications such as low-loss and environmentally stable optics. {\mathrm{SiO}}_2 is a well-known amorphous material suitable for energetic deposition. {\mathrm{SiO}}_2 single layers and {\mathrm{SiO}}_2 -based single-cavity narrow-bandpass filters were prepared by plasma-ion-assisted deposition. The refractive index and film thickness were determined by variable-angle spectroscopic ellipsometry. The high compressive stress of the densified film was correlated to increased packing density. The center wavelength shift of the narrow-bandpass filters as a function of sample-temperature as well as high-temperature annealing was determined via spectral transmission measurement. Structural relaxation of the densified {\mathrm{SiO}}_2 films was observed from the variation of the refractive index and physical thickness for the single layers and the center wavelength shift for the narrow-bandpass filters, suggesting elastic and plastic deformation of the densified films corresponding to a reversible and an irreversible center wavelength shift, respectively.