Fabrication of sub-wavelength surface ripples and in-volume nanostructures by fs-laser induced selective etching

Continuous sub-wavelength ripples on the surface of sapphire and fused silica have been produced not only in one dimension but also in two dimensions by scanning several tracks with an offset. This results in the formation of an extended grating. By investigation of cross-sections of sub-wavelength ripples a surprising high aspect ratio of larger than 10 is observed in the case of fused silica and a aspect ratio of about 2.5 in the case of sapphire. Using in-volume selective laser etching hollow nanoplanes (∼200 nm in width, 1 mm in length) and nano-channels (∼100 nm in diameter) are obtained in sapphire. By scanning in the volume of sapphire and fused silica, structures very similar to the surface sub-wavelength ripples are observed in the volume. A common feed-back mechanism for both the formation of coherent sub-wavelength surface ripples and nanoplanes in the volume is discussed.