May 11, 2010
High-speed femtosecond laser induced nanostructuring
Researchers at the Max Born Institute for Nonlinear Optics and Short-Pulse Spectroscopy in Germany (MBI) have obtained a remarkable increase in the processing speed of femtosecond laser-induced surface structuring. Until now, self-organized nanostructures on extended areas were obtained by sample translation through Gaussian focal spots and multiple scans. With line-shaped foci, larger areas can be structured in a shorter time to speed up the fabrication of functional devices.
Tailoring materials on the nanometre scale can enable specific interactions with light fields (for example, the case of black silicon) or with other materials such as fluids (for example, superwetting). Nanostructured functional surfaces are of interest in numerous applications, including biosensing, solar-light conversion, optoelectronics and microfluidics.
It is well known that the complex interplay of light-induced plasmons with polarization-dependent scattering and wave interference can lead to self-organized regular surface patterning of nearly all kinds of material. Grating periods near the laser wavelength or slightly below are observed (wavelength ripples). The high photon density of focused femtosecond pulses, however, opens nonlinear excitation channels even in materials with high bandgap energies. As a result of such multiphoton processes, structures with periods far below the laser wavelength can appear (sub-wavelength ripples).
For large-area structuring, the samples to be treated are typically moved along a scanning path through a fixed laser spot. The coherent linking of nanostructures is best along the parts of linear translation, but can be complicated from line scan to line scan.
Extending the interaction zone
The MBI team has demonstrated that high-speed fabrication of nanostructures on extended areas is possible without any multi-scan technology if the circular spot is replaced by a linear one, which can be generated easily by a cylindrical lens or mirror. The new technology not only enables the zone of interaction to be extended from a line to a plane, but also enhances the structural coherence.
In the first experiments, areas of up to a few millimetres-squared were processed. Encouragingly, the size of the line focus is only limited by the available laser power. Further improvements are expected if the intensity profile of the focal line is transformed into a high-quality flat-top profile.
Under optimized conditions with respect to pulse energy, number of pulses and sample velocity, ripples with periods down to 80 nm were obtained in rutile-type TiO2 crystals with a Ti:sapphire laser (pulse energy 3 mJ, pulse duration 40 fs) converted by second harmonic generation to deliver wavelengths around 400 nm.
These structures are promising candidates for applications in photocatalysis and micromechanics. The approach is universal and can be transferred to almost any solid material including dielectric, semiconducting or metallic surfaces.
Full results are available in the journal Nanotechnology.
About the author
Dr Susanta Kumar Das works as a postdoc in the group of Dr Ruediger Grunwald at the Max Born Institute (MBI), Berlin, Germany. His main research interests are the study of mechanisms of laser-induced periodic nanostructuring, the nonlinear spectroscopy of nanorods and related photonic applications. Besides nonlinear nanophysics, the team in the division of Prof. Thomas Elsasser develops new techniques for adaptively shaping and diagnosing ultrashort wavepackets in space and time. The work was supported by DFG under contract number 1782/12-1.