Tiny structures produced by femtosecond laser
Using ultra-short pulse laser irradiation BAM scientists succeeded in producing structures on the surface of titanium that are smaller than 100 nanometres (nm). "This is a milestone for the machining of surfaces found in the technical and biomedical applications", says Jörg Krüger of BAM Federal Institute for Materials Research and Testing. The special thing is that the laser used generates "only" a radiation of a wavelength of 790 nm. "If you normally focus the beam of a laser of this type, diffraction occurs which, according to the laws of classical far-field optics, limits the achievable resolution to about half the wavelength," says his colleague Jörn Bonse. Thus only about 400 nm would actually have been possible. How then were the scientists able to generate these tiny periodic structures on the titanium surface in a directed fashion - we are talking about an order of one tenth of the wavelength?
A look through the electron microscope shows ridges that are somewhat reminiscent of the ripples on the sea floor. These marks are the results of the bombardment of a material with ultrashort laser pulses. A pulse of only 30 femtoseconds duration is sent to the material 50 times in succession, the scientists report in the journal "Applied Physics A"*. This should be imagined as a strobe flash at the disco, only very much shorter. For a femtosecond is an extremely short time period. A femtosecond is 10-15 seconds, for instance 30 femtoseconds compared to a second, are equivalent to 30 seconds in 32 million years.
After the irradiation with the femtosecond laser, the surface is inspected. BAM scientists succeeded, together with colleagues from the Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin, to find the correct parameters: So how many laser pulse are needed and how much energy per unit area are the individual pulses allowed to transfer? "We see these ripple structures just barely above a threshold at which a change occurs in the material," says the physicist Bonse. If energy density is slightly increased, much bigger ripples with periods of a few 100 nm are produced but they were not wanted in this project funded by the German Research Foundation (DFG). Structures below the 100 nm limit should be researched into.
Finding the right parameter was not easy, but the process itself is relatively straightforward and industry compliant. Carried out in air, an expensive vacuum is not necessary, as it would be the case if lasers that produce a wavelength of well below 200 nm were used. And it is a method which can be performed in one step. The specimen is clamped and exposed to the laser radiation. The pulses are generated in a so-called oscillator, they are amplified in a crystal and then focused with a concave mirror to the sample surface.
So where are the small structures coming from? The scientists are still puzzled over it. The project is not finished yet. There are different approaches, but the process is presently not fully understood. Research over the next two years will study exposing the material to friction tests. Are the structures stable? Do they generate less friction? Which oils can be combined with them? These are just some questions that still need to be clarified. In the biomedical field, the scientists see the processing of implants as an area of application. Through these tiny ripple structures - BAM scientists hope - the processed material could be better absorbed by the body and cells find it easier to settle on the surface. So far, we have focused on the experiments on titanium. "The process is, however, applicable to other materials," says the physicist Jörg Krüger.