Nano structuringCopyright: Fraunhofer ILT
Femtosecond laser radiation allows to create coherent periodic nano structures (ripples) on the surface of various materials and in the volume of transparent materials with a periodicity significantly smaller than the used wavelength.
Periodical nano structures, so-called riffles, are induced by femtosecond laser radiation on the surface of dielectrica. The riffle spacing is much smaller than the used wavelength for structuring (~ 1 µm). Thus these riffles are also called sub-wavelength riffles. By nano structuring the surface of the materials is functionalized so that changes in the wettability of a liquid are induced for example.
Riffles are fabricated perpendicular to the polarization of laser radiation by one-time scanning of the laser focus with a diameter of about 2 µm. Typical dimensions are a width of approx. 125 nm, a spacing of approx. 240 nm and a depth of approx. 100 nm. Periodical riffle structures are coherently continued in two dimensions by scanning with a constant overlap. Thus several millimeter large surfaces are filled with sub-wavelength riffles.
Nano structured surfaces
The contact angle of water on sapphire is doubled due to the structured riffles. By nano structuring the surface becomes hydrophobic. An optical polished surface of sapphire leads to a contact angle of approx. 68° whereas water on a nano structured surface has a contact angle of approx. 134°. The transmission of visible light is slightly decreased by nano structuring.
Potential fields of application are self-cleaning surfaces (lotus effect) in biology, medicine and mechanical engineering.