arXiv subjects

Issei Takeda

Publications and source records attributed to Issei Takeda.

3 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↗

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↗