Spectral dependence of photorefractive effect in nitrogen-doped silica

Photoinduced variations in the refractive index of nitrogen-doped silica are directly measured in the wavelength range from 350 to 2500 nm. It is found that the change in the refractive index under the action of 193 nm laser radiation significantly increases in the long-wavelength region and amounts to ~10^–3 at λ =2.5 μm (for the energy fluence ~10⁴ J cm^–2). This means that the nature of the photorefractive effect in nitrogen-doped silica is related to photostructural transformations affecting the phonon part of the optical absorption spectrum.