Motivated by the prospect of chiral-mode control in compact photonic systems, we analyze discrete coupled single-mode resonators. Using the minimal three-resonator model, we show that an infinitesimal complex onsite perturbation near a Hermitian diabolic point (DP) induces chiral-mode selection, governed by what we term an asymptotic exceptional point (AEP). Here, an AEP denotes a Hermitian DP equipped with a non-Hermitian perturbation that induces an asymptotically defective effective Hamiltonian. The eigenvectors coalesce in the asymptotic limit toward the DP, although the Hamiltonian at the point itself remains diagonalizable. Operationally, this AEP response realizes chirality switching from an achiral state to a chiral state. The associated eigenvalue response exhibits the anomalous fractional-power scaling Δλ ∝ε^3/2, distinct from the square-root response of an ordinary exceptional point (EP). We further show that, in a broader two-parameter perturbation space, ordinary EPs lie on exceptional-line branches that meet at the AEP. A finitebias control sweep crosses these branches at an EP pair, enabling chirality reversal between opposite chiral states. The central message is therefore that the AEP organizes two related routes for chirality switching: direct switching from an achiral state to a chiral state via the AEP, and switching between opposite chiral states via an EP pair in the vicinity of the AEP. Within a finite-resolution averaging model, these two operating points exhibit different practical performance characteristics, and under sufficiently high control resolution, the AEP operating point can become more favorable than the EP-pair operating point, suggesting a route toward compact and low-energy chiral photonic devices.

We experimentally integrated highly transparent Sb 2 Se 3 into silicon photonic crystal nanocavities and waveguides, and demonstrated large-range, non-volatile wavelength tuning and on-demand cavity formation while maintaining a high Q factor.

Photonic topological insulators (PTIs) provide robust platforms for light manipulation; however, achieving reconfigurable control of their topological properties without compromising performance remains a significant challenge. Although phase-change materials (PCMs) offer large refractive index modulation and are widely used in commercial applications such as optical data storage, conventional materials like Ge2Sb2Te5 (GST) exhibit significant optical absorption in the crystalline state. This severe absorption limits their applicability in transmissive photonic devices such as PTIs, where high transparency is essential. Here, we address this limitation by integrating the ultralow-loss PCM antimony triselenide (Sb2Se3) with a silicon-based two-dimensional PTI. We demonstrate, for the first time, the submicron-scale selective patterning of Sb2Se3 on a photonic crystal, realizing a topological phase transition triggered by the switching of the material state. Capitalizing on the transparency of Sb2Se3 in both its amorphous and crystalline states, we maintain a high Q-factor on the order of 103, representing an order-of-magnitude improvement over previous GST-based devices. This work resolves the absorption-loss bottleneck in reconfigurable PTIs, paving the way for practical, low-loss, and tunable topological photonic devices.

A non-Hermitian point gap (NHPG) is a unique phenomenon in non-Hermitian systems and induces a non-Hermitian skin effect (NHSE). In photonic crystals, NHPG and 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 showed complex-frequency loops of the NHPG in photonic crystals, and the reversal of non-Hermitian topology through the flip of the loop rotation in a complex plane. These results establish radiation-loss engineering as a practical route to directly measuring and controlling complex photonic band structures and bulk point-gap topology in nanophotonic systems without gain media or nonreciprocity.

Aperiodic systems, such as quasiperiodic structures, exhibit properties distinct from those of periodic structures. In 2023, Smith et al. discovered an aperiodic structure based on a single tile shape that can tessellate the plane only aperiodically, known as an aperiodic monotile. Here, by using this monotile tiling, we propose a quasiperiodic structure which possesses perfect threefold rotational (C3) symmetry but lacks mirror symmetry. Diffraction experiments reveal quasicrystalline properties of the proposed aperiodic structure through observation of clear Bragg peaks and independence of illumination position. Furthermore, we observe chiral properties including pinwheel-like diffraction patterns and unconventional circular-polarization-dependent behavior, which is absent in conventional quasiperiodic structures with mirror symmetry. These findings establish chiral quasiperiodic structures as a platform for studying aperiodic systems beyond traditional quasicrystals, broadening the study of nonperiodic structures.

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.

Valley photonic crystals enable valley-dependent transport and chirality-selective emission, but incorporating wavelength-scale localization remains challenging. Existing valley-photonic-crystal cavities rely on finite defects or local lattice modifications that require structure-specific optimization and offer limited continuous control. Here, we theoretically and experimentally demonstrate two-dimensional nanocavity confinement using two orthogonal domain walls in a glide-symmetric valley photonic crystal. A valley domain wall confines the guided interface mode transversely, while an SSH-like domain wall localizes it longitudinally. Starting from a glide-symmetry-protected Dirac point in a bearded-interface waveguide, controlled displacements of adjacent triangular holes open a topological gap in the continuous guided-mode dispersion. The displacement amplitude ΔR tunes the gap, mode volume, and intrinsic radiative Q factor. Implemented in a silicon photonic-crystal slab, the structure exhibits localized resonances within the topological mode gap and systematic spectral tuning with ΔR. The maximum measured loaded Q factor is 1.2×10^4. This approach enables continuously tunable, high-Q nanocavities integrated into topological waveguide networks for compact resonant devices and enhanced light–matter interactions.

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.

Exceptional points (EPs) in propagation states induce intriguing properties as the group velocities. However, superluminal propagation near EPs is challenging due to the smoothing caused by symmetry breaking. In this study, we propose a photonic crystal waveguide design with glide and time-reversal symmetry and derive an effective Hamiltonian 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. We identify the conditions for exact EP restoration and provide an analytical solution to offset the EP smoothing due to symmetry breaking, which significantly reduces the group velocity contrast. A glide symmetry of the photonic crystal waveguide allows us to derive the effective Hamiltonian in 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 vg = 3.3c near the EP, which is up to 25 times that of the original structure. The proposed setup based on non-Hermitian photonics paves the way for the realization of superluminal devices in nanophotonics. In addition, the method for restoring EPs also addresses challenges in the phenomena arising from non-Hermitian properties.

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

We experimentally observed a non-Hermitian point gap using photonic crystal slabs. The point gap is induced by a radiation loss of the slabs, and we discuss its topological relationship to non-Hermitian skin effects. © 2024 The Author(s)

We propose and numerically demonstrate that at-Γ high-Q chiral modes can be obtained from BIC by controlling the symmetry of non-Hermitian hybrid photonic crystals with selectively-patterned absorptive materials, paving the way for circularly polarized emission.

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.

Non-Hermitian skin effect (NHSE), which is a localization phenomenon in non-Hermitian systems, has recently been of great interest. Most previous works in various systems studied periodic systems. However, electromagnetic waves do not feel the periodicity of a system if the wavelength is sufficiently long, and it has not been clear that electromagnetic waves exhibit the localization caused by the non-Hermiticity itself even in uniform systems without the periodicity. Here, we establish the theory of the optical NHSE in non-Hermitian uniform media and show that the optical NHSE occurs even in uniform media. We apply our theory to subwavelength metamaterials and demonstrate the existence of a non-Hermitian skin mode in a multilayer metamaterial. Finally, we propose a concept of a stationarily excited skin mode to describe a lossy system under stationary excitation with real excitation frequency.

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.

We investigate transmission properties via sharp bends of various triangular- and square-lattice photonic-crystal waveguides including valley-photonic waveguides, and have found that high transmission appears for some type of bands and interface configurations, irrespective of the inversion symmetry. This phenomenon is governed by bright circularly-polarized singular points around the bending interface.

We have discovered that optical media showing the reciprocal non-Hermitian skin effect can generate circulating modes with orbital angular momentum. We examine this intriguing mechanism for various boundaries including loss-biased slab structures with finite thickness. © 2024 The Author(s)

We demonstrated the band inversion on topological photonic crystal using the transparent phase change material $\text{Sb}_{2}\text{Se}_{3}$, which paves the way to realize the reconfigurable photonic topological circuits with low energy loss. © 2024 The Author(s)

Our proposal utilizes glide and time-reversal symmetric photonic crystal waveguides to achieve anomalous mode dispersion. We also theoretically demonstrate the way to restore fast-light edge states even in gain- or loss-biased scenarios.

It has been well established that photonic crystal nanocavities with wavelength sized mode volume enable various integrable photonic devices with extremely small consumption energy and small footprint. In this study, we explore the possibility of non-volatile functionalities employing photonic crystal nanocavities and phase change material, Ge 2 Sb 2 Te 5 (GST). Recently, non-volatile photonic devices based on GST have attracted significant interest and are expected to enable energy-efficient photonic processing, especially for optical computing. However, the device size and the area of GST in previous studies have been rather large. Here, we propose and fabricate Si photonic crystal nanocavities on which submicron-square GST patterns are selectively loaded. Because of the strong light confinement, extremely small area of GST is sufficient to manipulate the cavity mode. We have succeeded to fabricate 30-nm-thick and several-100nm-square GST blocks patterned at the center of photonic crystal cavity with a high alignment accuracy. We confirmed that the resonant wavelength and Q-factor of cavity modes are controlled by the phase change of GST. Moreover, cavity formation controlled by submicron-sized GST is also demonstrated by GST-loaded photonic-crystal line-defect waveguides. Our approach in which we place sub-micron-sized GST inside a photonic crystal nanocavity is promising for realizing extremely energy-efficient non-volatile integrable photonic devices, such as switches, modulators, memories, and reconfigurable novel devices.