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Ultrafast observation of nano-scale dynamics on the surface of metal
Expect to clarify the initial process of laser processing
Jun. 17 , 2010
Joint research group of Japan Atomic Energy Agency (JAEA, President Toshio Okazaki), Institute of Solid State Physics (ISSP) and University of Tokushima has succeeded to develop an x-ray laser interferometer, which enables us to observe transient deformation of metal surface with 1 nm depth-resolution and 10 ps time-resolution.
Soft x-ray interferometer has been a promising tool to observe nano-scale surface deformation of materials, however due to poor reflectivity of the soft x-ray mirrors, the efficiency of the interferometer under the conventional optical arrangement was too low for probing the surface dynamics so far. In order to overcome this difficulty, the joint research group employed "double Lloyd's mirror", which is a combination of two grazing incidence mirrors with having very small inclination angle each other. The schematic of the interferometer is shown in Fig, 1. The use of "double Lloyd's mirror" brought about drastic improvement in the efficiency of the soft x-ray interferometer, and by the combination of this interferometer with 10 ps-duration soft x-ray laser at a wavelength of 13.9 nm developed in JAEA, and we firstly succeeded to observe the dynamics of deformation of metal surface irradiated by an infrared laser pulse.
Figure 2 shows temporal evolution of the deformation of the platinum surface irradiated by a 100 fs-duration infrared laser pulse. After the irradiation, very small but substantial deformation starts at around 25 ps. At around 50 ps, apparent expansion of the surface is shown, and the height of the peak reaches 30 nm. And finally the central portion of the expanded region blows off, leaving the crater on the surface. This is the first observation of the initial stage of laser processing or laser welding, and the result provides useful information to understand the fundamental processes in the laser-matter interaction. In addition to it, it is noted that the present soft x-ray laser interferometer has potential to be powerful tool to observe the dynamics in phase transition of substances and the formation of high performance thin film.
Fig. 1. Schematic of soft x-ray laser interferometer
Fig. 2. Temporal evolution of soft x-ray laser interferogram of the platinum surface deformed by an ultra-short infrared pumping pulse.
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