arXiv subjects

Shupeng Xu

Publications and source records attributed to Shupeng Xu.

5 recordsLinked to original sources

arXiv2026

Geometric Engineering of Flat Bands in a Single-layer Photonic Graphene

Photonic flat bands offer significant potential for strong light-matter interactions, nonlinear optics, and sensing thanks to their localization of light and high density of states. However, realizing these flat bands typically requires intricate fabrication, perfect alignment and/or specialized geometries, and a general design strategy is missing. In this work, we demonstrate a simple yet versatile strategy to engineer radiative flat bands above the light line, using only a single-layer honeycomb photonic crystal slab. By applying a density wave like geometric perturbation-a spatially periodic displacement of the lattice air holes-we couple intrinsic flat band states from below the light cone into the radiative continuum. This structural modulation creates a highly anisotropic band structure that exhibits linear, Dirac-like dispersion in one direction and nearly flat dispersion in the orthogonal direction, forming an extended van Hove singularity at band extrema. Furthermore, by tuning the Fourier components of the modulation, we can manipulate the Dirac mass term to realize band inversion and switch between two topologically distinct phases. As an application, we demonstrate a Jackiw-Rebbi interface state positioned at the junction of two domains with opposite Dirac mass, that also shows flat band dispersion along the interface. This density-wave perturbation approach provides a conceptually clear and fabrication friendly platform for programming complex photonic band dispersions, opening new avenues for both topological photonics and practical flat-band optoelectronic devices.

physics.optics↗

arXiv2025

General Framework for Twisted Bilayer Photonic Crystal with Interlayer Coupling and Far-Field Response

We develop a general theory for twisted bilayer photonic crystals that takes into account both far-field response and near-field coupling. The theory is based on the framework of a generalized Rayleigh-Schr\"odinger perturbation theory for non-Hermitian Hamiltonians. A universal form for interlayer coupling is derived, which relates the hopping strength to the Fourier transforms of the Wannier functions in the single layer photonic crystal. For low energy states at the K point in hexagonal lattices, the interlayer coupling reduces to that in the Bistritzer-MacDonald model for graphene. As an example, we study a twisted bilayer photonic crystal slab with air holes arranged in a honeycomb lattice in each layer. The first order solution of our model predicts a four-fold band splitting in the far-field spectrum compared to the single-layer case, which is confirmed by numerical simulations. Moreover, our theory reveals that for low energy states at K points, scattering towards the {\Gamma} point via the moir\'e potential is suppressed. Based on our theory, we propose a wide-angle, high-Q tunable flat band cavity by combining the bilayer at a large twist angle with a Brillouinzone-folding perturbation within each layer. The cavity behaves like a collection of quasi-bound states in the continuum with a divergent density of states, with potential applications in nonlinear optics, lasing and quantum optics.

physics.optics↗

arXiv2024

Strong Chirality Suppression in 1-D correlated Weyl Semimetal (TaSe4)2I

The interaction of light with correlated Weyl semimetals (WSMs) provides a unique platform for exploring non-equilibrium phases and fundamental properties such as chirality. Here, we investigate the structural chirality of (TaSe4)2I, a correlated WSM, under weak optical pumping using Circular Photogalvanic Effect (CPGE) measurements and Raman spectroscopy. Surprisingly, we find that there is a loss of chirality in (TaSe4)2I above a threshold light intensity. We suggest that the loss of chirality is due to an optically driven phase transition into an achiral structure distinct from the ground state. This structural transformation is supported by fluence-dependent Raman spectra, revealing a new peak at low pump fluences that disappears above the threshold fluence. The loss of chirality even at low optical powers suggests that the system quickly transitions into a non WSM phase, and also highlights the importance of considering light-induced structural interactions in understanding the behavior of correlated systems. These studies showcase that even low excitation powers can be used to control the properties of correlated topological systems, opening up new avenues for low power optical devices.

cond-mat.mtrl-sci↗

arXiv2024

Simple realization of a fragile topological lattice with quasi flat-bands in a microcavity array

Topological flat bands (TFBs) are increasingly recognized as an important paradigm to study topological effects in the context of strong correlation physics. As a representative example, recently it has been theoretically proposed that the topological non-triviality offers a unique contribution to flat-band superconductivity, which can potentially lead to a higher critical temperature of superconductivity phase transition. Nevertheless, the topological effects within flat bands in bosonic systems, specifically in the context of Bose-Einstein condensation (BEC), are less explored. It has been shown theoretically that non-trivial topological and geometric properties will also have a significant influence in bosonic condensates as well. However, potential experimental realizations have not been extensively studied yet. In this work, we introduce a simple photonic lattice from coupled Kagome and triangular lattices designed based on topological quantum chemistry theory, which supports topologically nontrivial quasi-flat bands. Besides band representation analysis, the non-triviality of these quasi-flat bands is also confirmed by Wilson loop spectra which exhibit winding features. We further discuss the corresponding experimental realization in a microcavity array for future study supporting the potential extension to condensed exciton-polaritons. Notably, we showed that the inevitable in-plane longitudinal-transverse polarization splitting in optical microcavities will not hinder the construction of topological quasi-flat bands. This work acts as an initial step to experimentally explore the physical consequence of non-trivial topology and quantum geometry in quasi-flat bands in bosonic systems, offering potential channels for its direct observation.

cond-mat.mes-hall↗

arXiv2023

Absence of topological protection of the interface states in $\mathbb{Z}_2$ photonic crystals

Inspired from electronic systems, topological photonics aims to engineer new optical devices with robust properties. In many cases, the ideas from topological phases protected by internal symmetries in fermionic systems are extended to those protected by crystalline symmetries. One such popular photonic crystal model was proposed by Wu and Hu in 2015 for realizing a bosonic $\mathbb{Z}_2$ topological crystalline insulator with robust topological edge states, which led to intense theoretical and experimental studies. However, rigorous relationship between the bulk topology and edge properties for this model, which is central to evaluating its advantage over traditional photonic designs, has never been established. In this work we revisit the expanded and shrunken honeycomb lattice structures proposed by Wu and Hu by using topological quantum chemistry tools and show that they are topologically trivial in the sense that symmetric, localized Wannier functions can be constructed. We show that the $\mathbb{Z}$ and $\mathbb{Z}_2$ type classification of the Wu-Hu model are equivalent to the $C_2T$ protected Euler class and the second Stiefel-Whitney class respectively, with the latter characterizing the full valence bands of Wu-Hu model indicating only a higher order topological insulator (HOTI) phase. We show that the Wu-Hu interface states can be gapped by a uniform topology preserving $C_6$ and $T$ symmetric perturbation, which demonstrates the trivial nature of the interface. Our results reveals that topology is not a necessary condition for the reported helical edge states in many photonics systems and opens new possibilities for interface engineering that may not be constrained to require topological designs.

cond-mat.mtrl-sci↗