**5. Conclusions**

A 2D imaging method is proposed in this study, based on a meta-structure with a scanning defect, using a metal hole array (MHA) and a conductive probe (a needle-like conductor) in the millimeter-wave range. In order to validate the proposed method, a theoretical model of SSPP generation on an MHA was introduced, numerical electromagnetic analyses of localized distortions of the electric fields on and around the MHA were conducted, and one- and two-dimensional imaging experiments using conductive and dialectic samples verified the theoretical predictions.

In Section 2, the dispersion relation of SSPPs in the case where the MHA has rectangular holes with oblique incidence of sampling waves was derived, which indicated the physical phenomenon of SSPP generation on an MHA. In Section 3, with the use of HFSS, the transmission and reflection properties of an MHA were analyzed when millimeter waves were injected into the structure, and this analysis implied that SSPP modes exist under the cut-off frequency in our MHA model. In addition, the electromagnetic distributions around an MHA with an inserted conductive probe were compared with those of an MHA without the probe, confirming the difference in the electric distributions around the MHA in the two cases. Specifically, insertion of a conductive probe into an MHA can be useful for localized electromagnetic distortion. In Section 4, using an MHA and a conductive probe, a one-dimensional experiment was performed and confirmed the frequency dependency of the detected signal intensity in the range of 78–86 GHz. This showed that a wave at 83 GHz can be

monitored, producing the most intense level of detected signals. Then, two-dimensional imaging experiments with conductive samples (copper sticks) and dielectric samples (an alumina stick and DMSO (liquid)) were performed with the use of optimal waves at 83 GHz. Consequently, it was confirmed that the proposed measurement system allows the detection of the positions of conductive and dielectric samples by comparing the intensity levels of reflected signals with and without the samples. Finally, we demonstrated a biomedical diagnosis in the case of a rat lung by using the system. The result shows that although there is slight signal misdetection, two-dimensional dialectic responses of the biomedical sample can be obtained using the proposed method. Therefore, the proposed method has the potential for use in the two-dimensional imaging of permittivity distribution, such as in the biomedical task of localized tumor detection.

**Author Contributions:** G.I. performed the experiments and theoretical analyses shown here and wrote the paper by drawing the figures. O.S. conceived of and designed the research activities, as well as wrote the paper. Y.H. accelerated the study by presenting the technical issues in the medical point of view.

**Funding:** This research was partly funded by Project for Kyoto Bio-industry Creation & Support, and by JSPS Kakenhi with gran<sup>t</sup> number 18H03690.

**Acknowledgments:** The author thanks Y. Nishio and A. Iwai at Kyoto University for their fruitful comments on this study.

**Conflicts of Interest:** The authors declare no conflict of interest.
