In the Rapid Research Letter on p. 242 ff., titania nanotubes are proposed as building blocks of focusing elements. The design of photonic crystal (PC) - in particular concave - lenses on the basis of metallized nanotubes is found to result in excellent focusing of visible electromagnetic radiation. The lenses prove to be tolerant to disorder in the spatial arrangement of the nanotubes. Moreover, some disorder is even shown to improve the focusing properties.

The cover picture is an artist's view of a concave photonic crystal lens made from metallized titania nanotubes. A beam incident to the concave PC lens is focused in a very narrow spot showing features of super-resolution.
The first author, Vladimir Sergentu, is a research scientist at the Institute of Applied Physics of the Academy of Sciences of Moldova working on the design of PC based optical elements.