It is known that the several-times irradiation of a short pulse laser with fluence near the ablation threshold on material induces self-organization of fine periodic surface structures (so called LIPSS: laser induced periodic surface structure) with periodic interspaces Ʌ smaller than the laser wavelength ʎ. In particular, LIPSS such as Ʌ ≪ ʎ are called “fine LIPSS”. And they are attracting much attention as a method of processing for structures finer than the diffraction limit of laser light. However, their formation mechanism has not been clarified yet. To clarify the formation mechanism of LIPSS, in-situ observation is an effective way. However, the measurement of the small periodic interspaces with fast time scale (comparable to the laser pulse duration) are the difficulties. On a way to in-situ observation, we have reported the LIPSS formation on semiconductors irradiated by mid-infrared Free-Electron Laser (MIR-FEL). Their periodic interspaces are several micrometers and therefore can be measured by visible light. In this presentation, we compared the features of LIPSSs by MIR-FEL to the several models applied to LIPSSs by near-infrared or visible light lasers. Through this discussion, we estimated the temporal resolution needed for in-situ observation.
We present the study of optical and spectral properties of radiation-induced stable point defects, known as color centers (CCs), in lithium fluoride (LiF) for the detection of 10 keV XFEL beam at Spring-8 Angstrom Compact free electron LAser (SACLA) in Japan. A thick LiF crystal was irradiated in four spots with 10 keV XFEL beam (pulse duration = 10 fs) with different number of accumulated shots. After irradiation the colored-LiF spots were characterized with an optical microscope in fluorescence mode and their photoluminescence intensity and spectra were analyzed.
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