arXiv subjects

Kin Hung Fung

Publications and source records attributed to Kin Hung Fung.

At least 19 recordsLinked to original sources

arXiv2026

Non-Hermitian Skin Effect from Radiative Coupling in a Reciprocal Chiral Medium

We find that long-range radiative coupling through a passive reciprocal chiral medium produces an unusual non-Hermitian skin effect in a chain of electric dipoles. Although the system is reciprocal, the chirality generates polarization-dependent phase accumulation and attenuation, leading to degenerate pairs of skin modes localized at opposite boundaries. We find that the skin mode profile consists of an exponential contribution from an isolated complex-$β$ pole and a longer-range tail proportional to $1/[n(\ln n)^2]$, where $n$ is the distance from the occupied boundary measured in lattice sites. Finite-chain calculations show that part of the OBC spectrum contracts toward the Bloch spectrum, whereas the number of boundary-localized modes remains extensive.

cond-mat.other↗

arXiv2026

Integer-Graded Reciprocal Skin Effect from Internal Rotation

A prominent class of reciprocal non-Hermitian skin effects has a $\Z_{2}$ partner structure, in which a binary label records which of two partners occupies which edge. We show that conserved internal rotation extends this binary structure to an unbounded integer-graded hierarchy. Every angular momentum channel is an exactly solvable asymmetric chain with its own integer point-gap winding, while conjugate channels accumulate at opposite edges so that reciprocity is preserved globally. For conserved channels, the classification contains one integer per conjugate pair, with the number of independent integers determined by the internal multiplet. Half-integer spin realizes the symplectic class AII$^{\dagger}$, whose $\Z_{2}$ index is recovered as the parity of the grading. Conjugate-channel mixing destroys the grading and leaves a critical, scale-free skin effect. A cyclic three-site lattice and a passive array of lossy resonators provide minimal realizations.

cond-mat.other↗

arXiv2026

Scalar-Wave Dispersion in Vectorial Photonic Crystals via Site-Adapted p Orbitals

Electromagnetic waves are intrinsically vectorial and require description via polarization, unlike scalar fields such as acoustic pressure or electronic wavefunctions. In three dimensions, the transversality constraint further prevents any globally smooth transverse-polarization frame at the $Γ$ point, which would apparently rule out a simple scalar band structure for three-dimensional (3D) photonic crystals. We show here that site-adapted $p$-orbitals can realize scalar-wave dispersion: the induced band representation is isomorphic to the scalar elementary band representation up to a one-dimensional character twist, so the symmetry-enforced degeneracies and compatibility relations are the same. We demonstrate this mechanism experimentally in 3D photonic meta-crystals, where the local $p$-orbital axes adapt from site to site according to symmetry. In contrast to a fixed-polarization reduction (e.g., in 2D), our construction preserves site-polarization textures while simultaneously supporting a scalar network with one amplitude per site. Thus, it offers a pathway from vectorial photonic degrees of freedom to scalar band engineering, keeping polarization as an active design knob.

physics.optics↗

arXiv2026

Exact flat bands in a 3D photonic crystal

Photonic flat bands are hard to engineer because Maxwell's equations are vectorial: transversality obstructs the localized scalar-like bases that generate destructive-interference flat bands in tight-binding models. We show that a three-dimensional metallic network of dipolar cavities joined by waveguide channels--a fully vectorial photonic crystal belonging to space group No. 224--hosts an exact scalar sector, carrying exact flat bands. The twelve-band vector problem contains one self-adaptive radial dipole axis per site whose projection is exactly the scalar four-band Hamiltonian of the same network. A microwave-scale coupled-dipole calculation confirms this scalar-vectorial duality. The result is a symmetry-based design rule for scalar-like flat bands in reciprocal vector media.

physics.optics↗

arXiv2026

Demonstration of AI-Assisted Scientific Workflow on Canonical Benchmarks

We present a fully reproducible demonstration of an AI-assisted scientific workflow designed for a broad physics, mathematics, and computer-science readership. The initial project artifact stack was generated from one single user prompt and then reviewed and curated for submission by the human author. Rather than claiming a new scientific discovery, the manuscript uses canonical benchmark problems with exact, manufactured, or independently checkable answers. The analytical component starts from the one-dimensional quantum harmonic oscillator, derives its dimensionless form, and validates finite-difference eigenpairs against exact Hermite-function benchmarks. The numerical partial-differential-equation component solves a heat equation with a known modal solution and a Poisson problem verified by a manufactured solution, with explicit convergence studies. The inverse-modeling component fits synthetic damped-oscillation data by nonlinear least squares and quantifies parametric uncertainty by bootstrap resampling. The computational-science component compares dense and sparse eigensolvers and contrasts direct and iterative sparse linear solvers, with careful interpretation of machine-dependent timing data. Taken together, the results show that contemporary AI can already serve as a useful scientific copilot for derivation, implementation, validation, visualization, and manuscript preparation, provided that each stage is constrained by benchmark theory, explicit verification, and transparent artifacts. The demonstration is therefore relevant not because the underlying science is novel, but because it offers a concrete template for trustworthy AI use in technical research practice.

cond-mat.other↗

arXiv2025

Nonreciprocal Local-Resonance Induced Complex Band Hybridization

We study the complex band hybridization induced by nonreciprocal local resonances in photonic crystals. Composed of trimer unit cells, a two-dimensional (2D) magnetophotonic crystal with an analytically obtainable solution is considered. We find that nonreciprocal spectral gap may appear without nonreciprocal transmission and that the imaginary parts of the complex wavevectors $\text{Im}(\mathbf{k})$ may blow up at resonance to give extreme nonreciprocal transmission. We further show that, for a subwavelegnth lattice, the isolation ratio for the nonreciprocal transmission is determined solely by $\text{Im}(\mathbf{k})$ instead of the extensively studied real part $\text{Re}(\mathbf{k})$. Our finding contradicts the common belief that "spectral nonreciprocity [$ω(\mathbf{k})\neqω(-\mathbf{k})$] always implies nonreciprocal transmission".

physics.optics↗

arXiv2021

Maxwell's demon-like nonreciprocity by non-Hermitian gyrotropic metasurfaces

We show that Maxwell's demon-like nonreciprocity can be supported in a class of non-Hermitian gyrotropic metasurfaces in the linear regime. The proposed metasurface functions as a transmission-only Maxwell's demon operating at a pair of photon energies. Based on multiple scattering theory, we construct a dual-dipole model to explain the underlying mechanism that leads to the antisymmetric nonreciprocal transmission. The results may inspire new designs of compact nonreciprocal devices for photonics.

physics.app-ph↗

arXiv2020

Topological Theory for Perfect Metasurface Isolators

We introduce topological theory of perfect isolation: perfect transmission from one side and total reflection from another side simultaneously. The theory provides an efficient approach for determining whether such a perfect isolation point exists within a finite parameter space. Herein, we demonstrate the theory using an example of a Lorentz non-reciprocal metasurface composed of dimer unit cells. Our theory also suggests that perfect isolation points can annihilate each other through the coalescence of opposite topological charges. Our findings could lead to novel designs for high-performance optical isolators.

physics.optics↗

arXiv2017

Topological interface modes in local resonant acoustic systems

Topological phononic crystals (PCs) are periodic artificial structures which can support nontrivial acoustic topological bands, and their topological properties are linked to the existence of topological edge modes. Most previous studies focused on the topological edge modes in Bragg gaps which are induced by lattice scatterings. While local resonant gaps would be of great use in subwavelength control of acoustic waves, whether it is possible to achieve topological interface states in local resonant gaps is a question. In this article, we study the topological bands near local resonant gaps in a time-reversal symmetric acoustic systems and elaborate the evolution of band structure using a spring-mass model. Our acoustic structure can produce three band gaps in subwavelength region: one originates from local resonance of unit cell and the other two stem from band folding. It is found that the topological interface states can only exist in the band folding induced band gaps but never appear in the local resonant band gap. The numerical simulation perfectly agrees with theoretical results. Our study provides an approach of localizing the subwavelength acoustic wave.

cond-mat.mes-hall↗

arXiv2017

Electrical tunability due to coalescence of exceptional points in parity-time symmetric waveguides

We demonstrate theoretically the electric tunability due to coalescence of exceptional points in PT-symmetric waveguides bounded by imperfect conductive layers. Owing to the competition effect of multimode interaction, multiple exceptional points and PT phase transitions could be attained in such a simple system and their occurrences are strongly dependent on the boundary conductive layers. When the conductive layers become very thin, it is found that the oblique transmittance and reflectance of the same system can be tuned between zero and one by a small change in carrier density. The results may provide an effective method for fast tuning and modulation of optical signals through electrical gating.

physics.optics↗

arXiv2016

Tungsten based Anisotropic Metamaterial as an Ultra-broadband Absorber

The trapped rainbow effect has been mostly found on tapered anisotropic metamaterials (MMs) made of low loss noble metals, such as gold, silver, etc. In this work, we demonstrate that an anisotropic MM waveguide made of high loss metal tungsten can also support the trapped rainbow effect similar to the noble metal based structure. We show theoretically that an array of tungsten/germanium anisotropic nano-cones placed on top of a reflective substrate can absorb light at the wavelength range from 0.3 micrometer to 9 micrometer with an average absorption efficiency approaching 98%. It is found that the excitation of multiple orders of slow-light resonant modes is responsible for the efficient absorption at wavelengths longer than 2 micrometer, and the anti-reflection effect of tapered lossy material gives rise to the near perfect absorption at shorter wavelengths. The absorption spectrum suffers a small dip at around 4.2 micrometer where the first order and second order slow-light modes get overlapped, but we can get rid of this dip if the absorption band edge at long wavelength range is reduced down to 5 micrometer. The parametrical study reflects that the absorption bandwidth is mainly determined by the filling ratio of tungsten as well as the bottom diameter of the nano-cones and the interaction between neighboring nano-cones is quite weak. Our proposal has some potential applications in the areas of solar energy harvesting and thermal emitters.

physics.optics↗

arXiv2016

Anomalous Light Scattering by Topological ${\mathcal{PT}}$-symmetric Particle Arrays

Robust topological edge modes may evolve into complex-frequency modes when a physical system becomes non-Hermitian. We show that, while having negligible forward optical extinction cross section, a conjugate pair of such complex topological edge modes in a non-Hermitian $\mathcal{PT}$-symmetric system can give rise to an anomalous sideway scattering when they are simultaneously excited by a plane wave. We propose a realization of such scattering state in a linear array of subwavelength resonators coated with gain media. The prediction is based on an analytical two-band model and verified by rigorous numerical simulation using multiple-multipole scattering theory. The result suggests an extreme situation where leakage of classical information is unnoticeable to the transmitter and the receiver when such a $\mathcal{PT}$-symmetric unit is inserted into the communication channel.

cond-mat.mes-hall↗

arXiv2016

Simultaneous Multi-frequency Topological Edge Modes between One-dimensional Photonic Crystals

We show theoretically that, in the limit of weak dispersion, one-dimensional (1D) binary centrosymmetric photonic crystals can support topological edge modes in all photonic band gaps. By analyzing their bulk band topology, these "harmonic" topological edge modes can be designed in a way that they exist at all photonic band gaps opened at the center of the Brillouin Zone, or at all gaps opened at the zone boundaries, or both. The results may suggest a new approach to achieve robust multi-frequency coupled modes for applications in nonlinear photonics, such as frequency up-conversion.

cond-mat.mes-hall↗

arXiv2015

Chiral plasmon in gapped Dirac systems

We study the electromagnetic response and surface electromagnetic modes in a generic gapped Dirac material under pumping with circularly polarized light. The valley imbalance due to pumping leads to a net Berry curvature, giving rise to a finite transverse conductivity. We discuss the appearance of nonreciprocal chiral edge modes, their hybridization and waveguiding in a nanoribbon geometry, and giant polarization rotation in nanoribbon arrays.

cond-mat.mes-hall↗

arXiv2015

Formation of Non-reciprocal Bands in Magnetized Diatomic Plasmonic Chains

We show that non-reciprocal bands can be formed in a magnetized periodic chain of spherical plasmonic particles with two particles per unit cell. Simplified form of symmetry operators in dipole approximations are used to demonstrate explicitly the relation between spectral non-reciprocity and broken spatial-temporal symmetries. Due to hybridization among plasmon modes and free photon modes, strong spectral non-reciprocity appears in region slightly below the lightline, where highly directed guiding of energy can be supported. The results may provide a clear guidance on the design of one-way waveguides.

cond-mat.mes-hall↗

arXiv2015

Topological Edge Plasmon Modes between Diatomic Chains of Nanoparticles

We study the topological edge plasmon modes between two "diatomic" chains of identical plasmonic nanoparticles. Zak phase for longitudinal plasmon modes in each chain is calculated analytically by solutions of macroscopic Maxwell's equations for particles in quasi-static dipole approximation. This approximation provides a direct analogy with the Su-Schrieffer-Heeger model such that the eigenvalue is mapped to the frequency dependent inverse-polarizability of the nanoparticles. The edge state frequency is found to be the same as the single-particle resonance frequency, which is insensitive to the separation distances within a unit cell. Finally, full electrodynamic simulations with realistic parameters suggest that the edge plasmon mode can be realized through near-field optical spectroscopy.

cond-mat.mes-hall↗

arXiv2015

Non-reciprocal mu-near-zero mode in PT-symmetric magnetic domains

We find that a new type of non-reciprocal modes exist at an interface between two \emph{parity-time} ($\mathcal{PT}$) symmetric magnetic domains (MDs) near the frequency of zero effective permeability. The new mode is non-propagating and purely magnetic when the two MDs are semi-infinite while it becomes propagating in the finite case. In particular, two pronounced nonreciprocal responses could be observed via the excitation of this mode: one-way optical tunneling for oblique incidence and unidirectional beam shift at normal incidence. When the two MDs system becomes finite in size, it is found that perfect-transmission mode could be achieved if $\mathcal{PT}$-symmetry is maintained. The unique properties of such an unusual mode are investigated by analytical modal calculation as well as numerical simulations. The results suggest a new approach to the design of compact optical isolator.

physics.optics↗

arXiv2015

Tunable light-matter interaction and the role of hyperbolicity in graphene-hBN system

Hexagonal boron nitride (hBN) is a natural hyperbolic material which can also accommodate highly dispersive surface phonon-polariton modes. In this paper, we examine theoretically the mid-infrared optical properties of graphene-hBN heterostructures derived from their coupled plasmon-phonon modes. We found that the graphene plasmon couples differently with the phonons of the two Reststrahlen bands, owing to their different hyperbolicity. This also leads to distinctively different interaction between an external quantum emitter and the plasmon-phonon modes in the two bands, leading to substantial modification of its spectrum. The coupling to graphene plasmons allows for additional gate tunability in the Purcell factor, and narrow dips in its emission spectra.

physics.optics↗