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MANUFACTURING with short pulse lasers
LASER-INDUCED NANOSTRUCTURING
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Jörn BONSE 1, *, Eric RAHNER 2, Heike VOSS 1, Klaus SOKOLOWSKI-TINTEN 3, Stephan GRÄF 2
1
Bundesanstalt für Materialforschung und-prüfung( BAM), Berlin, Germany
2
Friedrich Schiller University Jena, Jena, Germany
3
University of Duisburg-Essen, Duisburg, Germany * joern. bonse @ bam. de
The laser-induced fabrication of nanostructures with feature sizes below the optical diffraction limit is possible for almost any material for arbitrary sample geometries and dimensions by exploiting nonlinear excitations or optical near-field interactions. This overview highlights historical milestones, explains the underlying physical processes and associated challenges, and discusses current and future trends in the field of ultrafast laser nanostructuring.
https:// doi. org / 10.1051 / photon / 202613748
Laser-Induced Nanostructuring( LIN) represents one of the thriving industrial demands of the past years [ 1 ]. It appears simple and appealing to flexibly functionalize either tiny sample spots or largearea surfaces in a single, contactless, aseptic, and cost-effective processing step that can be even conducted under ambient conditions. This expectation is seeded by the enormous developments in laser technology over the past decades, featuring a Moore-law like scaling through an output power-doubling of Ultrashort Pulsed Lasers( UPLs) about every two years. Currently, UPLs emitting in the near-infrared( NIR) or visible spectral range are commercially available at the 10 kW average power level, featuring MHz to GHz pulse repetition rates, while providing µ J to mJ pulse energies. Combined with state-of-the-art scanner technology, this puts areal laser processing rates towards the m 2 / s at sight [ 2 ].
Unfortunately, the generation of nanoscale structures by simply focussing a laser beam onto a surface or into the volume of a workpiece has to overcome Abbe’ s fundamental optical diffraction limit for far-field applications. This limit defines a minimum achievable focal spot diameter of about half of the radiation wavelength λ. However, ultrashort laser pulses are reaching enormous peak intensities, which helps to enable LIN.
Figure 1 summarizes how UPLs and their interplay with specific physical effects enable LIN with feature sizes below the classical optical diffraction limit. This setting leads to two fundamental approaches. The first is based on near-field effects. Optical near-fields typically occur as evanescent fields in the vicinity of interfaces( known from the effect of total reflection). Through their exponential decay, they rapidly vanish on a length scale of typically 10 nm only. Moreover, in the vicinity of sharp topographical features, optical near-fields can exhibit field amplitudes enhanced by several orders of magnitude. Under certain conditions, the enhancement can be further amplified via resonant excitation channels, such as Plasmons.
The second approach towards LIN is based on nonlinear absorption: Several( m) photons are simultaneously absorbed from a laser beam
48 www. photoniques. com I Photoniques 137