Frequency down-conversion of terahertz waves at optically induced temporal boundaries in GaAs waveguides2024 · 記述言語： 英語 掲載種別： 研究論文（学術雑誌） 出版者・発行元： Walter de Gruyter GmbH Abstract In this study, the frequency down-conversion of terahertz waves is analytically and experimentally demonstrated at the temporal boundaries within a GaAs waveguide. The temporal boundary is established by photoexciting the top surface of the waveguide, thereby instantaneously increasing its electrical conductivity. This photoexcited waveguide supports a transverse electromagnetic (TEM) mode with a frequency lower than those of the transverse magnetic (TM) modes present in the original waveguide. At the temporal boundary, the incident TM mode couples with the TEM mode, resulting in frequency down-conversion. Subtracting the propagation loss from the frequency-converted components indicates that the frequency conversion occurs with an efficiency consistent with the analytical predictions. The propagation loss is primarily due to ohmic loss, caused by the finite electrical conductivity of the photoexcited region. Given that the frequency of transverse electric modes is up-converted at the temporal boundary, our findings suggest that the direction of frequency conversion (upward or downward) can be controlled by manipulating the incident polarization. The polarization-dependent frequency conversion in waveguides holds significant potential for applications in devices designed for the interconversion of terahertz signals across various frequency channels. This capability is instrumental in the development of frequency-division-multiplexed terahertz wave communication systems, thereby enabling high data transfer rates. DOI： 10.1515/nanoph-2024-0010 researchmap その他リンク： https://www.degruyter.com/document/doi/10.1515/nanoph-2024-0010/pdf
Observation of transient aspects of self-sustained oscillations and the role of parallel capacitance in VO2-based planar devices2024 · 記述言語： 英語 掲載種別： 研究論文（学術雑誌） 出版者・発行元： AIP Publishing Self-sustained electrical oscillations (SEOs) in VO2 films have attracted considerable attention owing to their potential to emulate spiking pulses in neuromorphic circuits. However, triggering stable SEOs and the controlling oscillation frequency remain challenging because the details of circuit operation with VO2-based devices are not yet well understood. In this study, we propose a method to observe SEOs stably in a VO2-based planar device with Au/Ti facing electrodes by introducing a 50 Hz sinusoidal voltage from a curve tracer. The transient aspects, including the onset and collapse of the SEO, were captured, providing clarity on the oscillation frequency range and circuit conditions for the SEOs, which are closely correlated with the device temperature. It became clear that the parallel capacitance not only determined the oscillation frequency but also controlled the current through VO2 just after the insulator–metal transition of VO2, playing a role in triggering stable oscillations. We also successfully observed the transient aspects from in-phase to anti-phase synchronized oscillations in the coupled oscillations. This study advances the experimental procedures and applications of SEOs in VO2-based planar devices. DOI： 10.1063/5.0211327 researchmap
Pure moving optical media consisting of magnetochiral metasurfaces2024 · 記述言語： 英語 掲載種別： 研究論文（学術雑誌） 出版者・発行元： Optica Publishing Group We report on numerical studies of microwave bianisotropies by magnetochiral (MCh) metasurfaces consisting of double Z-type gammadions with perpendicularly magnetized substrates. The metasurfaces’ effective polarizability tensor, extracted from calculated reflection and transmission coefficients, has components of the non-reciprocal moving-type bianisotropy as well as the reciprocal chiral-type bianisotropy and non-reciprocal magneto-optical (MO) effect. The combination of the metasurfaces with contraposition MCh metasurfaces cancels both the chiral-type bianisotropy and MO effect, resulting in pure moving optical media. An achieved perfect transmission with a phase difference of π realizes an ideal gyrator for arbitrary spatial and polarization modes. DOI： 10.1364/ome.535115 researchmap その他リンク： https://opg.optica.org/viewmedia.cfm?URI=ome-14-11-2499&seq=0
