A major problem for optical systems is the polarization splitting occurring at any interface in the event of oblique light. Low-index nanostructured silica layers were produced by overcoating an initial organic structure with silica and subsequent removal of the organic constituents. The nanostructured layers can be tailored to achieve defined low effective refractive indices down to 1.05. They can also be stacked together with homogeneous layers to produce broadband antireflective coatings with very low polarization splitting. Especially thin interlayers that separate the low-index structured layers turned out to be useful to optimize the polarization properties.