As is well known, when using lasers to cut metal materials, the equipment mainly releases an ultra-high density laser beam. This laser beam shines on the metal material through a spot, causing the metal to melt, vaporize, or reach its ignition point rapidly. At the same time, a high-speed airflow coaxial with the beam quickly blows away the molten or burning material, thus forming a kerf.
However, a recent study in the United States has challenged our previous understanding. A research team from the SLAC National Accelerator Laboratory of the U.S. Department of Energy recently revealed through experiments that gold exhibits some unique behaviors different from those previously observed when exposed to high-energy laser pulses.
Experiments show that some materials (such as silicon) decompose rapidly under the excitation of high-energy lasers. However, under certain specific conditions, metals such as gold become more robust rather than melt when subjected to strong laser pulses.
This phenomenon is primarily attributed to changes in phonon behavior and adjustments in the vibrational modes of gold atoms. This contrasts sharply with semiconductor materials, which often become unstable and melt when exposed to strong lasers during processing.
In fact, simulations have demonstrated the possibility of this phenomenon for decades, a phenomenon known as phonon hardening. Now, researchers at the U.S. Department of Energy's SLAC National Accelerator Laboratory have revealed this phonon hardening using SLAC's Linear Accelerator Coherent Light Source (LCLS).
In their "Material Under Extreme Conditions" lab, they aimed optical laser pulses at gold films and then used ultrafast X-ray pulses from LCLS to take atomic-level snapshots of the material's reaction, capturing atomic-level images of the gold films' response to optical laser pulses under extreme experimental conditions.
By meticulously observing subtle changes and precisely capturing the instantaneous increase in the phonon energy of gold atoms, they were able to explore the world of gold atoms from a high-resolution perspective, providing concrete and conclusive evidence for the phonon hardening phenomenon.
Researchers have discovered that when gold absorbs extremely high-energy laser pulses, the bonding forces between gold atoms are significantly enhanced. This change leads to an increase in atomic vibration frequency, which may in turn affect the melting point and thermal reactivity properties of gold.
This experiment resolved a long-standing problem regarding ultrafast excitation of metals and demonstrated that powerful lasers can completely alter the lattice response. Furthermore, the experimental confirmation of theoretical predictions also indirectly shows the astonishing extent to which the SLAC linear accelerator coherent light source (LCLS) can measure these phenomena, opening up new possibilities for the future of materials science research.
Of course, similar phenomena may occur in other metals, such as copper, platinum, and aluminum. Future research may also reveal how metals should cope with harsh environments, which will help create materials with greater resilience. From the perspective of laser processing and materials manufacturing, understanding these two processes at the atomic level could lead to another round of technological and material innovation.
