[16p-P1-1~21] 3.9 Terahertz technologies

[16p-P1-17] Characterization of focusing of sub-THz wave using a dielectric aperture and design of metal structures for superfocusing

〇Osamu Morikawa¹, Kohji Yamamoto², Kazuyoshi Kurihara², Fumiyoshi Kuwashima³, Masahiko Tani² (1.JCGA, 2.Univ. Fukui, 3.FUT)

Keywords:superfocusing, aperture, THz-TDS

Focusing of the sub-THz radiation can be assisted by tapered slab transmission lines (STL) and the beam diameter can be smaller than the diffraction limit, which is favorable for observation of nonlinear effect or spectroscopy of samples with small amount. For characterization of the focusing, some method has been reported such as scattering probe method and EO sampling. However, those method, where the time delay in the THz time-domain measurement system is fixed, sometimes produces data with signal overshoot which is difficult to interpret. Focusing of the sub-THz radiation can also be characterized by measuring the aperture transmission, which produces simpler data. The beam can be regarded as tightly focused if the transmission of the small aperture is high. In the case that the tapered STL is used to assist the focusing and a metal aperture is used for characterization of focusing, unexpected effects may appear because the combination of the aperture metal plate and the tip of the tapered STL may form transmission lines. Therefore we previously proposed using insulator apertures. However, the analysis method was partly unclear. The sum of signal components caused by the beams transmitting the hole part and the frame part was not equal to that caused by the total beam. In this study, we employed a calculation which considers the phases of the signal components and observed that the problem was solved. In addition, we consider the optimized design of the tapered STL for focusing the sub-THz radiation. The coupling efficiency between the incident beam and TEM mode of the tapered STL with various taper angles will be calculated.