arXiv subjects

Taiki Yoda

Publications and source records attributed to Taiki Yoda.

14 recordsLinked to original sources

arXiv2026

Transverse spin texture in optical non-Hermitian skin modes

In structured electromagnetic fields, polarization textures are often closely linked to the spatial variation of the energy flow. However, this familiar picture has been established mainly for lossless and isotropic settings, and concrete examples showing how it is modified in media with gain and loss remain limited. Here, we demonstrate that optical skin modes associated with the non-Hermitian skin effect (NHSE) carry a finite transverse circular-polarization texture and further show that the accompanying in-plane electric-field spin texture deviates from the familiar lossless spin-flow picture. Using exact TE mode solutions, we separate the common exponential skin envelope from the oscillatory component. This decomposition shows that the circular-polarization texture is not generated by the skin envelope itself but by the oscillatory interference component modified by non-Hermiticity. It also reveals a handedness bias and a reshaped spatial relation between circularity and intensity. Finite-element calculations confirm that these features remain robust in loss-biased anisotropic media. These results show that gain and loss provide additional freedom for engineering electric-field spin textures beyond conventional lossless photonic settings.

physics.optics↗

arXiv2026

Fine Structures of Berry Curvature and Unquantized Valley Chern Numbers in Valley Photonic Crystals

Valley photonics has emerged as a promising platform in topological photonic systems, yet the topological nature of valley-dependent phenomena remains unsettled. Theoretically, inter-valley scattering may occur with structural imperfections, and global Chern numbers vanish due to time-reversal symmetry. As a result, valley-dependent topology is locally defined around K(K') points in the half-Brillouin zone (HBZ). While half-integer valley Chern numbers have been widely assumed, their quantization and topological validity remain controversial. Here, we systematically investigate a continuous spectrum of valley photonic crystal designs by evaluating their Berry curvatures, valley Chern numbers, and angular momenta. We show that valley Chern numbers are generically unquan-tized and instead form a continuous spectrum varying with structural parameters. We further reveal previously unexplored fine structures in the Berry curvature distribution in momentum space. The unquantized valley Chern numbers are attributed to inter- and intra-valley cancellation of Berry curvature, highlighting the absence of a protecting mechanism for quantization. Our results call for a reassessment of valley-dependent topology and provide a more rigorous framework for interpreting valley-related photonic phenomena.

physics.optics↗

arXiv2025

Observation of non-Hermitian point gap in photonic crystals

Non-Hermitian point gap (NHPG) is a unique phenomenon in non-Hermitian systems and induces non-Hermitian skin effect (NHSE). In photonic crystals, NHPG and the NHSE have previously been explored mainly through material loss, where the typically low $Q$ factors make direct observation of complex frequencies challenging. Here, we demonstrate the direct experimental observation of an NHPG by using a radiation-loss-based non-Hermitian photonic crystal. Radiation loss can be engineered through structural design, enabling control of the imaginary part of the complex frequency and allowing relatively high $Q$ factors. This approach is compatible with widely used absorption-free silicon-slab photonic crystals. We developed a measurement system that can measure photonic bands along arbitrary lines in $k$-space. Our measurements demonstrated direct observation of the NHPG in photonic crystals, and the reversal of non-Hermitian topology through the flip of loop rotation in a complex plane. Our platform, which requires neither gain media nor synthetic dimensions, establishes radiation-loss engineering as a simple and versatile route for photonic functionality using an NHSE in nanophotonic systems.

physics.optics↗

arXiv2025

Propagation and circulating modes of reciprocal non-Hermitian skin effect

The non-Hermitian skin effect (NHSE) is a novel localization phenomenon, in which all bulk states in a non-Hermitian system under certain conditions are localized at the edge of the system. Conventionally, most studies of NHSE have dealt with discrete lattice systems with non-reciprocal couplings. However in recent years, NHSE in a reciprocal two-dimensional continuous medium, such as photonic crystal systems, has also been reported. In particular, we have previously shown that NHSE also occurs in two-dimensional uniform media. In such two-dimensional systems, skin modes propagate in a direction perpendicular to the localization direction, and especially, they have the property of propagating in only one direction. In this paper, we show numerically an intriguing scattering phenomenon: when a scatterer is placed in the path of a skin mode, the scattering causes the skin mode to hop between opposing edges. In addition, we propose a new method of generating circulating modes with orbital angular momentum using this scattering phenomenon. Our work paves the way for new applications of NHSE as micro-sized optical devices manipulating or generating OAM.

physics.optics↗

arXiv2025

High Transmission in 120-degree Sharp Bends of Inversion-symmetric and Inversion-asymmetric Photonic Crystal Waveguides

Bending loss is one of the serious problems for constructing nanophotonic integrated circuits. Recently, many works reported that valley photonic crystals (VPhCs) enable significantly high transmission via 120-degree sharp bends. However, it is unclear whether the high bend-transmission results directly from the valley-photonic effects, which are based on the breaking of inversion symmetry. In this study, we conduct a series of comparative numerical and experimental investigations of bend-transmission in various triangular PhCs with and without inversion symmetry and reveal that the high bend-transmission is solely determined by the domain-wall configuration and independent of the existence of the inversion symmetry. Preliminary analysis of the polarization distribution indicates that high bend-transmissions are closely related to the appearance of local topological polarization singularities near the bending section. Our work demonstrates that high transmission can be achieved in a much wider family of PhC waveguides, which may provide novel designs for low-loss nanophotonic integrated circuits with enhanced flexibility and a new understanding of the nature of valley-photonics

physics.optics↗

arXiv2024

EP restoration and fast-light edge states in photonic crystal waveguide with glide and time reversal symmetry

Exceptional points (EPs) in the propagation states give rise to the emergence of intriguing properties with the divergence of the group velocity. However, there have been no experimental reports due to the necessity of maintaining high levels of fabrication precision and the requisite high group velocity contrast. In our study, we propose a design of photonic crystal waveguide with glide and time reversal symmetry, and derive an effective Hamiltonian for edge states to realize fast-light edge states. We adopt a systematic method to generate EPs in edge states by introducing non-Hermitian perturbations to Dirac points guaranteed by glide symmetry, which ensures that EP modes are free from out-of-plane radiation losses. Then, our study reveals the conditions for the exact EP restoration and provides an analytical solution to offset the EP smoothing due to symmetry breaking, which drastically reduces the group velocity contrast. A good symmetry property of the photonic crystal waveguide allows us to derive the effective Hamiltonian as a simple form, and the EPs can be restored by adjusting the real part of the permittivity. Furthermore, we design a feasible photonic crystal slab waveguide incorporating graphene as the absorbing material, and numerically demonstrate a group velocity reaching $v_g = 3.3c$ near the EP, which is up to 25 times that of the original structure. Thanks to the short periodicity of photonic crystals, it's possible to reach the speed of light in vacuum with group velocity contrasts on the order of one digit. Our study paves an innovative way to manipulate the group velocity of light.

physics.optics↗

arXiv2024

Photonic topological phase transition induced by material phase transition

Photonic topological insulators (PTIs) have been proposed as an analogy to topological insulators in electronic systems. In particular, two-dimensional PTIs have gained attention for the integrated circuit applications. However, controlling the topological phase after fabrication is difficult because the photonic topology requires the built-in specific structures. This study experimentally demonstrates the band inversion in two-dimensional PTI induced by the phase transition of deliberately-designed nanopatterns of a phase-change material, Ge2Sb2Te5 (GST), which indicates the first observation of the photonic topological phase transition with changes in the Chern number. This approach allows us to directly alter the topological invariants, which is achieved by symmetry-breaking perturbation through GST nanopatterns with different symmetry from original PTI. The success of our scheme is attributed to the ultrafine lithographic alignment technologies of GST nanopatterns. These results demonstrate to control photonic topological properties in a reconfigurable manner, providing an insight into new possibilities for reconfigurable photonic processing circuits.

physics.optics↗

arXiv2024

Non-Bloch band theory of generalized eigenvalue problems

Waves in a variety of fields in physics, such as mechanics, optics, spintronics, and nonlinear systems, obey generalized eigenvalue equations. To study non-Hermitian physics of those systems, in this paper, we construct a non-Bloch band theory of generalized eigenvalue problems. Specifically, we show that eigenvalues of a transfer matrix lead to a certain condition imposed on the generalized Brillouin zone, which allows us to develop a theory to calculate the continuum bands. As a concrete example, we examine the non-Hermitian skin effect of photonic crystals composed of chiral metamaterials by invoking our theoretical framework. When the medium has circularly polarized eigenmodes, we find that each eigenmode localizes at either of the edges, depending on whether it is left- or right-circularly polarized. In contrast, when the medium only has linearly polarized eigenmodes, every eigenmode localizes to the edge of the same side independent of its polarization. We demonstrate that the localization lengths of those eigenmodes can be determined from the chiral parameters and eigenfrequencies of the photonic crystal.

cond-mat.mes-hall↗

arXiv2023

Vector beam generation from standing hollow GaN nanowire lasers on sapphire substrate

We fabricated GaN based hollow nanowires standing upright on a sapphire substrate by the sublimation method and found that they exhibit laser oscillation at room temperature. These very long, hollow, nano-sized structures cannot be fabricated by other means. Furthermore, we determined the condition under which the fundamental mode is azimuthally polarized by investigating the dispersion of the hollow structure. Examination of the measured emission properties indicates that the hollow nanowire operates as a topological, vector-beam, light source.

physics.optics↗

arXiv2023

Optical non-Hermitian skin effect in two-dimensional uniform media

The non-Hermitian skin effect (NHSE) is a novel localization phenomenon in certain non-Hermitian systems with gain and/or loss. Most of previous works study the non-Hermitian skin effect in periodic systems. However, electromagnetic waves often propagate within uniform materials without periodic modulation, and it has not been clear whether the optical NHSE occurs in uniform media such as bulk materials and electromagnetic metamaterials. Here we establish the theory of the optical NHSE in non-Hermitian anisotropic media. We show that the NHSE occurs even in uniform media with appropriate anisotropy and material loss. The localization of non-Hermitian skin modes are completely determined by an effective gauge potential caused by the anisotropy of a dielectric tensor. On the basis of the theory, we propose subwavelength multilayer metamaterials as a novel platform for the optical NHSE. We also propose a new concept of stationarily-excited skin modes whose frequencies are forced to be real in non-Hermitian systems. We find that the NHSE occurs even under the condition that the frequency is forced to be real, which implies that the NHSE we propose is observable under stationary excitation. Our work presents a general theory of the NHSE in homogeneous systems, and pave the way to realize the optical NHSE in bulk materials and metamaterials.

physics.optics↗

arXiv2022

Non-Hermitian waves in a continuous periodic model and application to photonic crystals

In some non-Hermitian systems, the eigenstates in the bulk are localized at the boundaries of the systems. This is called the non-Hermitian skin effect, and it has been studied mostly in discrete systems. In the present work, we study the non-Hermitian skin effect in a continuous periodic model. In a one-dimensional system, we show that the localization lengths are equal for all the eigenstates. Moreover, the localization length and the eigenspectra in a large system are independent of the types of open boundary conditions. These properties are also found in a non-Hermitian photonic crystal. Such remarkable behaviors in a continuous periodic model can be explained in terms of the non-Bloch band theory. By constructing the generalized Brillouin zone for a complex Bloch wave number, we derive the localization length and the eigenspectra under an open boundary condition. Furthermore we show that the generalized Brillouin zone also has various physical properties, such as bulk-edge correspondence.

cond-mat.mes-hall↗

arXiv2020

Generation and Annihilation of Topologically-Protected Bound States in the Continuum and Circularly-Polarized States by Symmetry Breaking

We demonstrate by breaking the $C_{6}$ symmetry for higher-order at-$Γ$ bound states in the continuum (BICs) with topological charge $-2$ in photonic crystals, (i) deterministic generation of off-$Γ$ BICs from the at-$Γ$ BIC, and (ii) a variety of pair-creation and annihilation processes of circularly-polarized states with opposite topological charge and same handedness. These processes are well explained by the conservation of handedness-wise topological charges, showing topologically robustness of these phenomena. The results indicate a new control of topological aspect of far-field polarization vectors.

physics.optics↗

arXiv2018

Orbital Edelstein effect as a condensed-matter analog of solenoid

We theoretically study current-induced orbital magnetization in a chiral crystal. This phenomenon is an orbital version of the Edelstein effect. We propose an analogy between the current-induced orbital magnetization and an Ampère field in a solenoid in classical electrodynamics. In order to quantify this effect, we define a dimensionless parameter from the response coefficient relating a current density with an orbital magnetization. This dimensionless parameter can be regarded as a number of turns within a unit cell when the crystal is regarded as a solenoid, and it represents how "chiral" the crystal is. By focusing on the dimensionless parameter, one can design band structure which realizes induction of large orbital magnetization. In particular, a Weyl semimetal with all the Weyl nodes close to the Fermi energy can have a large value of this dimensionless parameter, which can exceed that of a classical solenoid.

cond-mat.mes-hall↗

arXiv2015

Current-induced Orbital and Spin Magnetizations in Crystals with Helical Structure

We theoretically show that in a crystal with a helical lattice structure, orbital and spin magnetizations along a helical axis are induced by an electric current along the helical axis. We propose a simple tight-binding model for calculations, and the results can be generalized to any helical crystals. The induced magnetizations are opposite for right-handed and left-handed helices. The current-induced spin magnetization along the helical axis comes from a radial spin texture on the Fermi surface. This is in sharp contrast to Rashba systems where the induced spin magnetization is perpendicular to the applied current.

cond-mat.mes-hall↗