Mid-infrared free electron laser induced periodic surface structures on semiconductors

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.