arXiv subjects

Fumiaki Miyamaru

Publications and source records attributed to Fumiaki Miyamaru.

9 recordsLinked to original sources

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↗

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

Energy loss of terahertz electromagnetic waves by nano-sized connections in near-self-complementary metallic checkerboard patterns

The design of a self-complementary metallic checkerboard pattern achieves broadband, dispersion-less, and maximized absorption, concentrating in the deep subwavelength resistive connections between squares, without any theoretical limitation on the energy absorbing area. Here, we experimentally and numerically investigate the electromagnetic response in the limit of extremely small connections. We show that finite conductivity and randomness in a near-self-complementary checkerboard pattern plays a crucial role in producing a frequency-independent energy loss in the terahertz frequency region. Here metals behave like an almost perfect conductor. When the checkerboard pattern approaches the perfect self-complementary pattern, the perfect conductor approximation spontaneously breaks down, owing to the finite conductivity at the nano- scale connection, leading to broadband absorption. It is also shown that the random connections between metallic squares also lead to broadband and maximized energy loss through scattering loss, similar to finite conductivity.

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

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↗

arXiv2016

Supersymmetric correspondence in spectra on a graph and its line graph: From circuit theory to spoof plasmons on metallic lattices

We investigate the supersymmetry (SUSY) structures for inductor-capacitor circuit networks on a simple regular graph and its line graph. We show that their eigenspectra must coincide (except, possibly, for the highest eigenfrequency) due to SUSY, which is derived from the topological nature of the circuits. To observe this spectra correspondence in the high-frequency range, we study spoof plasmons on metallic hexagonal and kagomé lattices. The band correspondence between them is predicted by a simulation. Using terahertz time-domain spectroscopy, we demonstrate the band correspondence of fabricated metallic hexagonal and kagomé lattices.

physics.optics↗