Material science using THz-FEL

The free electron laser in the terahertz region (THz-FEL) at the Institute of Scientific and Industrial Research (ISIR) in Osaka University is based on the stimulated radiation from a compressed high-energy electron pulses. The natures of FEL are, short pulse, high intensity, variable wavelength, monochromatic, polarization, and coherency in time and space. THz and/or FIR is the electromagnetic wave locating on the boundary between radio waves and photons. It behaves as a high-frequency oscillating electric field and also as low-energy photons against for the electrons in materials. Thus, the THz-FEL can be used not only as a brilliant probe light source for spectroscopy but also as a pump light source that enables novel excitations in materials. Recently, nonlinear interactions between semiconductors and the intense THz wave from FEL (frequency: 2.5-7.5 THz, electric field: >3 MV/cm) have been investigated [1]. Under irradiation of focused THz-FEL, a periodic structure so called LIPSS was observed on the surface of semiconductor Si as an irreversible phenomenon. The periodicity of LIPSS by THz-FEL is ~1/25 of the wavelength, which is small enough to exceed the diffraction limit. A scaling law was found between the LIPSS periodicity and the pulse number, which also might be seen in case of LIPSSs with other materials, by other wavelengths (e.g. NIR). These results possibly be understood in a framework of self-organized criticality phenomena shown in nonequilibrium open system [2]. As another irradiation experiments, we will touch in the talk about a remarkable performance in processing of biomolecular aggregate structure.
[1] A. Irizawa, S. Suga, T. Nagashima, A. Higashiya, M. Hashida, and S. Sakabe, Appl. Phys. Lett. 111, 251602 (2017).
[2] N. Gregoire and I. Prigogine, Self-Organization in Nonequilibrium Systems ( Wiley, New York, 1977), Vol. 191977.