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Toshihiro Nakanishi

Publications and source records attributed to Toshihiro Nakanishi.

At least 19 recordsLinked to original sources
arXiv2026

Microwave One-way Transparency by Large Synthetic Motion of Magnetochiral Polaritons in Metamolecules

We observe microwave nonreciprocal one-way transparency via ultrastrongly-coupled magnetochiral polaritons (MChPs) in a metamolecule at room temperature. The experimental results using MCh metamolecules with simultaneous breaking of time-reversal and space-inversion symmetries are reproduced by numerical simulations. Based on effective polarizability tensor analyses, we verify massive synthetic motion of MChPs as an origin of the one-way transparency. This study paves a way to hybrid quantum systems and synthetic gauge fields using metamaterials.

physics.optics↗
arXiv2024

Ultrastrongly-coupled and Directionally-nonreciprocal Magnon-polaritons in Magnetochiral Metamolecules

We experimentally demonstrate magnon-polaritons with ultrastrong coupling and directional nonreciprocity in a metamolecule lacking time-reversal and space-inversion symmetries at room temperature. These experimental results are reproduced well via numerical simulations and theoretical consideration. Ultrastrong coupling is due to a direct interaction of magnons in the magnetic meta-atom with microwave photons confined in the chiral meta-atom as a resonator. Our results reveal a crucial step in identifying deepstrongly-coupled and optically-moving magnon-polaritons for hybrid quantum systems, synthetic gauge fields, and quasi-particle ``chemistry'' using metamaterials.

physics.app-ph↗
arXiv2024

Extension of Babinet's relations to reflective metasurfaces: Application to the simultaneous control of wavefront and polarization

We extend Babinet's relations, which are originally derived for transmissive metasurfaces, to reflective metasurfaces embedding planar metallic structures in a substrate containing a reflection mirror. To verify the extended Babinet's relations, we investigate two types of reflective metasurfaces embedding self-complementary structures, which produce $π$-phase difference between two orthogonal linear polarizations under specific conditions required for the extended Babinet's relations. Both theoretical and experimental studies demonstrate a metasurface-based half-wave plate with a reflective metasurface including single self-complementary structures in the terahertz regions. In addition, we study a reflective metasurface with embedded self-complementary structures with phase gradient in reflection, and demonstrate simultaneous implementation of anomalous reflection and polarization conversion with high efficiency.

physics.optics↗
arXiv2024

Pure moving optical media consisting of magnetochiral metasurfaces

We report 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. Combination of the metasurfaces with contraposition MCh metasurfaces cancels both the chiral-type bianisotropy and MO effect, resulting in pure moving optical media. Achieved perfect transmission with phase difference of $π$ realizes an ideal gyrator for arbitrary spatial and polarization modes.

physics.class-ph↗
arXiv2023

Storage and retrieval of electromagnetic waves in a metasurface based on bound states in the continuum by conductivity modulation

In this study, we develop a time-varying metasurface based on the bound states in the continuum (BIC) with variable conductors, to store electromagnetic waves. The storage and retrieval of electromagnetic waves are demonstrated numerically through dynamic switching between quasi-BIC and BIC states by modulating the variable conductors. The storage efficiency exhibits oscillatory behavior with respect to the timing of storage and retrieval. These behaviors can be attributed to the interference of a resonant mode and a static mode that is formed by direct current. In addition, the storage efficiency of a single-layer metasurface can reach 35% under ideal conditions.

physics.optics↗
arXiv2022

Hidden Symmetry Protection and Topology in Surface Maxwell Waves

Since the latter half of the 20th century, the use of metal in optics has become a promising plasmonics field for controlling light at a deep subwavelength scale. Surface plasmon polaritons localized on metal surfaces are crucial in plasmonics. However, despite the long history of plasmonics, the underlying mechanism producing the surface waves is not fully understood. This study unveils the hidden symmetry protection that ensures the existence of degenerated electric zero modes. These zero modes are identified as physical origins of surface plasmon polaritons, and similar zero modes can be directly excited at a temporal boundary. In real space, the zero modes possess vector-field rotation related to surface impedance. Focusing on the surface impedance, we prove the bulk-edge correspondence, which guarantees the existence of surface plasmon polaritons even with nonuniformity. Lastly, we extract the underlying physics in the topological transition between metal and dielectric material using a minimal circuit model with duality. The transition is considered the crossover between electric and magnetic zero modes.

physics.optics↗
arXiv2022

Dynamic quarter-wave metasurface for efficient helicity inversion of polarization beyond the single-layer conversion limit

Terahertz chiral sensing and polarization-multiplexing communication demand subwavelength devices that dynamically invert polarization helicity. Metasurfaces can enhance anisotropy and fine tunability at subwavelength scales for this purpose. Although metasurfaces enabling deep modulation between orthogonal polarizations have been designed, they suffer from low conversion efficiencies. In this study, it is shown that the efficiency of conversion from linear to circular polarization by conventional single-layer transmissive metasurfaces cannot exceed a fundamental limit. A dynamic reflective metasurface free from this limitation is then proposed. The device includes multilayer structures working as a terahertz quarter-wave plate with switchable slow and fast axes. A phase transition of vanadium dioxide induces the necessary structural transformation of the metallic patterns. A practical fabrication method, based on the room-temperature bonding technique of sapphires, is presented. Dynamic helicity inversion is demonstrated at 0.90 THz, with a conversion efficiency of over 80 % that is beyond the fundamental limit of single-layer transmissive metasurfaces (65 %) and more than four times greater than that of previously reported devices.

physics.optics↗
arXiv2021

Direct measurement of ultrafast temporal wavefunctions

The large capacity and robustness of information encoding in the temporal mode of photons is important in quantum information processing, in which characterizing temporal quantum states with high usability and time resolution is essential. We propose and demonstrate a direct measurement method of temporal complex wavefunctions for weak light at a single-photon level with subpicosecond time resolution. Our direct measurement is realized by ultrafast metrology of the interference between the light under test and self-generated monochromatic reference light; no external reference light or complicated post-processing algorithms are required. Hence, this method is versatile and potentially widely applicable for temporal state characterization.

quant-ph↗
arXiv2020

Broadband operation of active terahertz quarter-wave plate achieved with vanadium-dioxide-based metasurface switchable by current injection

We demonstrate the broadband operation of a switchable terahertz quarter-wave plate achieved with an active metasurface employing vanadium dioxide. For this purpose, we utilize anisotropically deformed checkerboard structures, which present broadband characteristics compatible with deep modulation. Moreover, the metasurface is integrated with a current injection circuit to achieve state switching; this injection circuit can also be employed to monitor the electric state of vanadium dioxide. We estimate the Stokes parameters derived from the experimental transmission spectra of the fabricated metasurface and confirm the helicity switching of circularly polarized waves near a designed frequency of 0.66THz. The relative bandwidth is evaluated as 0.52, which is 4.2 times broader than that in a previous study.

physics.optics↗
arXiv2018

A framework for measuring weak values without weak interactions and its diagrammatic representation

Weak values are typically obtained experimentally by performing weak measurements, which involve weak interactions between the measured system and a probe. However, the determination of weak values does not necessarily require weak measurements, and several methods without weak system-probe interactions have been developed previously. In this work, a framework for measuring weak values is proposed to describe the relationship between various weak measurement techniques in a unified manner. This framework, which uses a probe-controlled system transformation instead of the weak system-probe interaction, improves the understanding of the currently used weak value measurement methods. Furthermore, a diagrammatic representation of the proposed framework is introduced to intuitively identify the complex values obtained in each measurement system. By using this diagram, a new method for measuring weak values with a desired function can be systematically derived. As an example, a scan-free and more efficient direct measurement method of wavefunctions than the conventional techniques using weak measurements is developed.

quant-ph↗
arXiv2018

Storage and retrieval of electromagnetic waves using electromagnetically induced transparency in a nonlinear metamaterial

We investigate the storage and retrieval of electromagnetic waves using a nonlinear metamaterial, analogous to the electromagnetically induced transparency (EIT) observed in atomic systems. We experimentally demonstrate the storage of the electromagnetic wave by reducing an auxiliary "control" wave; the stored wave is then released by recovering the control wave. We also confirm that the metamaterial can store and reproduce the phase distribution of the original input wave. These effects confirm a remarkable analogy between the metamaterial and an atomic EIT medium.

physics.app-ph↗
arXiv2018

Reconfigurable terahertz quarter-wave plate for helicity switching based on Babinet inversion of anisotropic checkerboard metasurface

Dynamic helicity switching by utilizing metasurfaces is challenging because it requires deep modulation of polarization states. To realize such helicity switching, this paper proposes a dynamic metasurface functioning as a switchable quarter-wave plate, the fast axis of which can be dynamically rotated by $90^\circ$. The device is based on the critical transition of an anisotropic metallic checkerboard, which realizes the deep modulation and simultaneous design of the switchable states. After verifying the functionality of the ideally designed device in a simulation, we tune its structural parameters to realize practical experiments in the terahertz frequency range. By evaluating the fabricated sample with vanadium dioxide, the conductivity of which can be controlled by temperature, its dynamic helicity switching function is demonstrated.

physics.optics↗
arXiv2017

Dynamically Babinet-invertible metasurface: a capacitive-inductive reconfigurable filter for terahertz waves using vanadium-dioxide metal-insulator transition

This paper proposes a reconfigurable planar metamaterial that can be switched between capacitive and inductive responses using local changes in the electrical conductivity of its constituent material. The proposed device is based on Babinet's principle and exploits the singular electromagnetic responses of metallic checkerboard structures, which are dependent on the local electrical conductivity. Utilizing the heating-induced metal-insulator transition of vanadium dioxide ($\mathrm{VO}_2$), the proposed metamaterial is designed to compensate for the effect of the substrate and is experimentally characterized in the terahertz regime. This reconfigurable metamaterial can be utilized as a switchable filter and as a switchable phase shifter for terahertz waves.

physics.optics↗
arXiv2017

Broadband and energy-concentrating terahertz coherent perfect absorber based on a self-complementary metasurface

We demonstrate that a self-complementary checkerboard-like metasurface works as a broadband coherent perfect absorber (CPA) when symmetrically illuminated by two counter-propagating incident waves. A theoretical analysis based on wave interference and results of numerical simulations of the proposed metasurface are provided. In addition, we experimentally demonstrate the proposed CPA in the terahertz regime by using a time-domain spectroscopy technique. We observe that the metasurface can work as a CPA below its lowest diffraction frequency. The size of the absorptive areas of the proposed CPA can be much smaller than the incident wavelength. Unlike conventional CPAs, the presented one simultaneously achieves the broadband operation and energy concentration of electromagnetic waves at the deep-subwavelength scale.

physics.optics↗
arXiv2017

Freestanding transparent terahertz half-wave plate using subwavelength cut-wire pairs

We designed a cut-wire-pair metasurface that works as a transparent terahertz half-wave plate, by matching the electric and magnetic resonances of the structure. Due to the impedance matching nature of the resonances, a large transmission phase shift between the orthogonal polarizations was achieved, while permitting a high transmission. The electric and magnetic responses of the proposed structure were confirmed by evaluating the electric admittance and magnetic impedance. The structure was fabricated on a flexible film and its helicity-conversion function in the terahertz frequency range was experimentally demonstrated. The thickness of the device is less than 1/10 of the working vacuum wavelength, and a high amplitude helicity conversion rate over 80% was achieved. Finally, using simulations, we demonstrate the feasibility of the gradually rotating cut-wire-pair array in terahertz wave-front control.

physics.optics↗
arXiv2016

Observation of Fano resonance using a coupled resonator metamaterial composed of meta-atoms arranged by double periodicity

We studied the transmission characteristics of a planar metamaterial consisting of an array of electric split-ring resonators (eSRRs) with double periodicity. Because of coupling between different resonant modes induced by different lattice periods in metamaterials with double periodicity, the appearance of Fano resonance can be expected in the same manner as that in the case of coupled classical oscillators. We fabricated complementary eSRRs and verified that a Fano-like spectral shape appeared in the transmission spectra of eSRRs with double periodicity in the terahertz region.

physics.optics↗
arXiv2016

Anisotropic Babinet-invertible metasurfaces to realize transmission-reflection switching for orthogonal polarizations of light

The electromagnetic properties of an extremely thin metallic checkerboard drastically change from resonant reflection (transmission) to resonant transmission (reflection) when the local electrical conductivity at the interconnection points of the checkerboard is switched. To date, such critical transitions of metasurfaces have been applied only when they have 4-fold rotational symmetry, and their application to polarization control, which requires anisotropy, has been unexplored. To overcome this applicability limitation and open up new pathways for dynamic deep-subwavelength polarization control by utilizing critical transitions of checkerboard-like metasurfaces, we introduce a universal class of anisotropic Babinet-invertible metasurfaces enabling transmission-reflection switching for each orthogonally polarized wave. As an application of anisotropic Babinet-invertible metasurfaces, we experimentally realize a reconfigurable terahertz polarizer whose transmitting axis can be dynamically rotated by $90^\circ$.

physics.optics↗