Local investigation of electronically-driven non-thermalsurface restructuring

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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.