MANUFACTURING with short pulse lasers FOCUS in the sample material, with a transition probability scaling with I m( I: local intensity). Hence, for a Gaussian laser beam, the profile of nonlinearly excited electrons radially narrows( depending on m) and is confined in the center of the laser beam. Thus, the interaction of high-intensity laser beams with wide bandgap dielectrics can confine the deposited optical energy to the nm-scale.
However, the exploitation of locally confined or enhanced optical absorption for LIN is only successful if the deposited optical energy remains spatially confined until it is transferred to the lattice. This becomes neatly possible for UPLs since their duration is shorter than the typical electron-phonon relaxation times that determine how fast a material can be heated. As consequence of this energy confinement during the laser pulse absorption and the subsequent transfer to the lattice, the so-called heat-affected zone( HAZ) surrounding the initially irradiated material volume is minimized. For sub-picosecond pulse durations, it extends typically a few tens to hundreds of nanometers into the surrounding.
Finally, UPL-induced damage of irradiated materials is often very deterministic with sharply defined threshold intensities / fluences. If a laser with a Gaussian beam profile is then operated just above the damage threshold, sub-micrometric damage sites can be created in the center of the focussed spot. The need to work very precisely at a peak fluence close to the damage threshold, in turn, imposes rather strict requirements on the pulse-to-pulse stability of the laser. Here, major advances have been made thanks to solid-state laser technologies, which play a key role in the latest generations of UPLs.
In brief, for rendering LIN possible, the ultrashort pulse duration is essential as it translates to high peak intensities and enables energy confinement through nonlinear or near-field effects, along with sharp fluence thresholds for material modifications, and a controlled material response via a minimized HAZ.
LASER-INDUCED SURFACE AND VOLUME NANOSTRUCTURING EXPERIMENTAL IMPLEMENTATIONS OF LIN Since the turn of the millennium, enormous progress has been made by demonstrating LIN through different experimental implementations. At the same time, significant advances have been achieved in the scientific understanding and control of the physical and chemical effects, as well as in the improvement of engineering aspects. Figure 2 provides a collage highlighting some selected LIN results. The experimental methods can be categorized in surface processing( left panels) and volume processing( right panels). Within each of those methods, LIN can be realized either through direct focussing( top panels) allowing a punctual nanoscale material modification or sculpturing, or it can be realized through matter self-ordering effects( bottom panels). The latter are enabled via coherent optical scattering and matter redistribution in the spatially extended focal regions upon loose focussing often realized along with laser beam scanning. Joglekar et al. [ 3 ] demonstrated
Figure 1. Physical effects and technologies for jointly enabling LIN. the concept of combining direct focussing of a NIR fs-laser beam with a high numerical aperture( NA) microscope objective onto the surface with nonlinear absorption effects in a glass sample( Fig. 2( a. I)). With a Gaussian focal spot diameter of 2w 0 = 0.41 µ m they obtained surface crater diameters of a few tens of nanometers only. Belloni et al. [ 4 ] also used direct focussing but employed a beam-shaping spatial light modulator and interference for creating a doughnut-shaped first-order Bessel beam at the rear-surface of a sapphire crystal. Utilizing a single Ti: sapphire fs-laser pulse, they were able to eject a nano-pillar-like, still crystalline sapphire rod of less than 1 µ m in diameter with a length of ≈10 µ m( Fig.( 2a. II)). Juodkazis et al. [ 5 ] focussed single Ti: sapphire fs-laser pulses tightly into the volume of the sapphire for generating bulk-confined micro-explosions( Fig. 2( b. I)). Crosssectional inspection by scanning electron microscopy( SEM) revealed the formation of a tear-drop-shaped nano-void with a diameter of a few hundred nm, surrounded by a sub- µ m shell of amorphous material embedded in a locally defective crystal. When reducing the pulse energy and increasing the number of laser pulses applied per tightly focussed laser spot, void-less local material modifications can be generated in the volume of the laser-irradiated material. This can be used, e. g., for writing optical waveguides in glasses, or for 3D additive manufacturing on the nanoscale by using two-photon-polymerization in suitable organic liquids. Cao et al. [ 6 ] used this sub-ablative NIR fs-laser bulk scan processing strategy at high pulse repetition rates( 500 kHz) to locally crystallize a lithium niobate-silicate multi-component glass, visualized in Fig. 2( b. II) in cross-section by SEM and electron backscatter diffraction for different laser processing conditions. Loose focussing with low NA optics also provides the possibility of LIN at the surface and in the volume of solids. At first glance, this appears counterintuitive since
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