MANUFACTURING with short pulse lasers FOCUS fs-laser processed metal-ion doped titania films that were subjected to scan-processing [ 8 ]. Here, the coherent scattering and interference leads to a 3D spatially patterned deposition of the laser pulse energy that, in turn, promotes the local precipitation and formation of plasmonic metallic nanoparticles( NP), see Fig. 2( d. I). These NP, being periodically spaced and arranged in different layers, allow the generation of macroscopically visible structural color effects and can serve to encode view-point dependent images as security tags( Fig. 2( d. II)) [ 9 ].
Figure 3. Time-resolved scattering experiment performed at the FEL FLASH in Hamburg [ 1,10 ]. The scattering patterns are recorded by a CCD camera that is synchronized with the fs-XUV probe pulses.( Adapted from [ 10 ], © 2024 under Creative Commons BY 4.0 license. Retrieved from https:// doi. org / 10.1002 / lpor. 202300912).
ADVANCED CHARACTERISATION OF NANOSTRUCTURES A problem that immediately emerges with the successful realization of LIN is that conventional optical far-field characterization methods – such as optical microscopy – are no longer applicable for in-situ evaluation of the LIN results. Hence, either slow scanning probe or near-field optical characterization techniques, or farfield spectral ensemble measurements are required. The latter refer, e. g., to optical scattering from NP, where the scattered light is spectrally resolved and analysed in the far-field. Alternatively, shorter wavelength electromagnetic radiation may be applied, pushing the required photon energies into the XUV- or X-ray spectral range.
In this direction, the development of short wavelength Free Electron Lasers( XFELs) has opened up exciting new possibilities since these sources combine an extremely high tuneable photon flux with spatial coherence and pulse durations in the fs- or even as-range. The potential of such short wavelength and ultrashort-pulsed radiation sources for studying the multi-scale processes transiently manifesting in laser processing was recently reviewed in [ 10 ]. XFELs allow, for example, the in-situ investigation of laser-driven structure formation at extreme scales in space and time via fs-pump-probe small-angle X-ray scattering( fs-SAXS) or even grazing-incidence small-angle X-ray scattering( fs-GISAXS). This covers the entire spatial range from atomic scales, over the nano- and meso- towards the micro-scale, while simultaneously covering temporal observation ranges up to ms, probed with sub-ps temporal resolution. Apart from clarifying the dynamics of ultrafast phase transitions or the formation mechanisms of the HSFL [ 7 ] featuring ≈100 nm spatial period only, also the disintegration of a laser-molten surface layer into voids and NP during fs laser ablation can be successfully revealed [ 11 ].
Figure 3 exemplifies a time-resolved single-pulse scattering pumpprobe experiment( ps optical pump with λ = 523 nm / fs XUV probe with λ = 13.5 nm) performed in transmission geometry on a 100 nm thin silicon film at the FLASH FEL at DESY( Hamburg, Germany) [ 1,10 ]. A sequence of pump-probe far-field scattering patterns acquired at different delay times Δt allow to record in a stroboscopic fashion the spatial frequency distributions of the laser-induced nanostructures. For example, the characteristic doublearc feature arranged symmetrically in the vertical direction on pattern # 3( Δt ≈ 1 ns) represents the transient signature of classical LSFL(“ wavelength ripples”) formed on laser-molten silicon and featuring spatial periods of ≈ 300 nm, here.
PROCESSING OF LIPSS AND THE CONTROL OF REGULARITY In the following, we will focus on LIPSS as an example of LIN with a tremendous technological and industrial potential, having surface functionalization applications in the fields of optics, tribology, and medicine. Figure 4 visualizes LSFL and HSFL created on the surface of titanium by LIN in a laser beam scanning approach with an industrial laser system( 1030 nm, 860fs, 10 kHz). For both types of LIPSS, the left panel shows an SEM image with the same magnification, while the right panel displays a two-dimensional Fast Fourier Transform, representing the spatial frequency distributions of the corresponding spatial domain. LSFL are formed perpendicular to the linear laser beam polarisation indicated by the double-arrows, while the HSFL are rotated by 90 °.
The LSFL have a most frequent period of Λ^ = 773 nm, with standard deviation of ΔΛ^ = ± 44 nm. For the much smaller HSFL, values of Λ^ ± ΔΛ^ =( 87 ± 12) nm can be deduced, i. e., < λ / 10. Comparing the 2D-FFTs of both types of LIPSS, it becomes clear that their signatures differ remarkably: While the LSFL are represented by a pair of sickle-shaped, radially asymmetric arcs, the HSFL appear as a pair of broad, radially symmetric“ clouds”. The ratio of the spread of the periods to the most frequent spatial
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