Local investigation of electronically-driven non-thermalsurface restructuring

强而短的激光脉冲是一种有趣的工具，可以根据实际应用的需要，在辐照目标的表层产生纳米级的结构变化，从而定制表面特性。短脉冲激光辐照具有使材料进入高度非平衡状态的能力。因此，它可以深入了解极端条件下的材料行为。在亚皮秒时间尺度下，发现低于热熔阈值的半导体表面的超短辐射会破坏晶体秩序。这已经从理论上解释了声子模式的软化由于电子耗尽在近表面区域。在相同的条件下，理论预测贵金属的声子模式会变硬。它们的熔化温度相应的倍增意味着这类材料没有非热熔化。相比之下，我们最近发现fs激光照射会产生表面空位和附着原子，这与理论预测不一致。该项目的目标是利用我们独特的装置，允许将fs激光束直接耦合到低温扫描隧道显微镜的隧道结中，系统地研究不同材料表面和方向的这种现象，最终了解基于微观信息的金属非热熔化现象。我们的研究结果将为金属表面对超短激光照射的响应提供微观的见解。

Intense short laser pulses are an intriguing tool for tailoring surface properties by producing nanoscale structural changes in the surface layer of an irradiated target to the needs of practical applications. Short-pulse laser irradiation has the ability to bring material into a highly non-equilibrium state. It thus provides insight into material behaviour under extreme conditions. Ultra-short radiation of semi-conductor surfaces below the thermal melting threshold was found to destroy the crystal order on the sub-picosecond time-scale. This has been explained theoretically by a softening of the phonon modes due to electron depletion in the near surface region. Under the same conditions, theory predicts a hardening of the phonon modes for noble metals. The corresponding multiplication of their melting temperature would imply no non-thermal melting for this class of materials. In contrast, we recently discovered that fs-laser irradiation creates surface vacancies and adatoms, at odds with the theoretical prediction. The goal of this project is to use our unique set-up that allows to couple a fs-laser beam directly into the tunneling junction of a low-temperature scanning tunneling microscope to investigate this phenomenon systematically for surfaces of different materials and orientation to eventually understand the phenomenon of non-thermal melting of metals based on microscopic information. Our results will give microscopic insight into the response of metal surfaces to ultra-short laser irradiation.