FOCUS
WAVEFRONT control
of freedom for laser processing. Combined wavefront and pulse-front shaping allows the position of maximum intensity to move along the propagation axis with a programmable velocity.
This concept leads to the flying or sliding focus, for which the intensity maximum can propagate at subluminal or even apparently superluminal velocities— without, of course, transporting information or energy faster than light. Controlling this trajectory provides a new way of managing the interaction between the incoming pulse and the plasma it generates( see Figure 3 c-d). In particular, an appropriately designed moving focus can mitigate ionization-induced refraction and maintain a localized high-intensity region [ 8 ].
The next generation of beam shaping may therefore no longer consist in defining only where light is focused, but also when and how the focus propagates through matter. Combining adaptive algorithms, artificial intelligence, high-power modulators and spatio-temporal control could ultimately provide a much finer localization of energy deposition, opening particularly exciting perspectives for three-dimensional micro- and nanoprocessing.
CONCLUSION Spatial beam shaping has become a key enabling technology for ultrafast laser processing, allowing the available laser power to be redistributed in space for higher throughput, improved flexibility and access to new processing geometries. Recent progress in wavefront modulators, optimization algorithms and artificial intelligence is steadily improving shaping fidelity and efficiency, while high-power handling remains a major technological challenge. Beyond purely spatial control, emerging spatio-temporal approaches such as flying-focus techniques open new perspectives for controlling energy deposition in three dimensions and at ultrafast timescales.
Figure 3.( a) High-fidelity beam shaping assisted by convolutional neural networks( CNNs), illustrated by the generation of a complex butterfly intensity pattern.( b) Structure of a thermo-optically addressed spatial light modulator( TOA-SLM), a promising architecture for increased laser power handling.( c) Concept of combined wavefront and pulse-front control using an SLM and a deformable mirror( DM), enabling control of the position and velocity of the intensity maximum— the flying focus.( d) Control of the flying-focus trajectory relative to the laser-induced free-electron plasma, opening new possibilities for mitigating plasma-induced beam distortions and localizing energy deposition.( a) adapted from [ 6 ] under Optica Open Access Publishing Agreement,( b) adapted from [ 7 ] under Optica Open Access Publishing Agreement,( c) adapted from with permission from Sun, Bangshan, Patrick S. Salter, et Martin J. Booth. Optics Express 23, nᵒ 15( 2015): 19348‐57 with the permission of Optical Society of America,( d) adapted from [ 8 ], with the permission of AIP Publishing.
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