Ultrafast laser-induced morphological transformations

  • PDF / 661,993 Bytes
  • 6 Pages / 585 x 783 pts Page_size
  • 112 Downloads / 195 Views

DOWNLOAD

REPORT


duction Ultrashort (femtosecond [fs]) laser pulses are a unique tool for materials processing, as their duration is shorter than the time required to transfer absorbed optical energy, via electron– phonon coupling, from the electronic system of the solid to its lattice. Hence the laser-pulse energy remains localized during the interaction and does not spread via diffusion into the area surrounding the irradiated region. The fs laser thus offers increased precision for material modification or ablation1,2 accompanied by a reduced heat-affected zone of only a few hundred nanometers.3 This confined heat-affected zone is more than an order of magnitude smaller than that typically reported for irradiation with longer, nano-, micro-, and millisecond laser-pulse durations.4 From a physical point of view, the ultrafast interaction provides the means to drive the electronic system of the solid into a strong nonequilibrium state, triggering ultrafast phenomena such as bandgap collapse in semiconductors,5 nonthermal melting,6–8 Coulomb explosion in a near-surface layer depleted of bonding electrons,9 and the excitation of collective electron modes (e.g., surface plasmon polaritons [SPPs]).10–12 Even if initiated by an ultrafast process, the subsequent material relaxation strongly affects the surface morphology as it can occur via a plethora of processes, including thermal melting, hydrodynamic melt flows, vaporization, phase explosion,

structural modifications (amorphization/crystallization), or chemical reactions with the ambient environment (e.g., oxidation).4,13–15 Moreover, repeated exposure of the same irradiated spot transforms roughened surfaces via interpulse feedback phenomena toward characteristic surface morphologies such as nanometric laser-induced periodic surface structures (LIPSSs),16,17 or micrometer-sized grooves and conical spikes.18,19 The feedback mechanism during repetitive treatments selects specific spatial periods of the roughness distribution that can better absorb radiation. In other words, the first pulse generates a rough surface that facilitates the coupling of energy for the subsequent laser pulses.

Contour shaping of surfaces Single-step contactless three-dimensional (3D) machining of contours on an arbitrary workpiece with nanometer precision has attracted the interest of scientific researchers and industrial engineers for decades. Many different processes have been developed for this,4,20 of which direct focusing of laser radiation is the most prominent. Using this technique along with high numerical aperture optics and taking advantage of nonlinear absorption in dielectrics at peak intensities >1013 W/cm2, surface craters and grooves of less than 50 nm were demonstrated using Nd:glass 800-fs-laser pulses at 1053 nm wavelength.21 Such high lateral resolution is achieved through the use of

Michael J. Abere, Sandia National Laboratories, USA; [email protected] and [email protected] Minlin Zhong, Laser Materials Processing Research Centre, School of Material Science & Engineering, Tsinghua University