Programme

I.11 -- Invited, VCTP-51

Date: Thursday, 6 August 2026

Time: 11:10 - 11:50

Non-Hermitian Topological Singularities in Photonic Crystal Slabs

Nguyen Hai Son

(1) CNRS-International-NTU-Thales Research Alliance (CINTRA), IRL 3288, Singapore 637553 (2) Ecole Centrale de Lyon, INSA Lyon, Université Claude Bernard Lyon 1, CPE Lyon, CNRS, Institut des Nanotechnologies de Lyon, UMR5270, Ecully 69130, France

opological photonics has recently emerged as a powerful paradigm for controlling light, and when combined with non-Hermitian physics, it opens new routes to engineer radiative coupling, modal degeneracies, and light–matter interactions. In this talk, I will present how leaky eigenmodes in photonic crystal slabs can be rigorously described by a non-Hermitian Hamiltonian that treats guided-mode coupling and radiation losses on equal footing [1]. This framework enables accurate prediction of complex photonic band structures and unveils a rich variety of topological singularities: bound states in the continuum (BICs) associated with polarization singularities in the radiation topology, and exceptional points (EPs) arising from the coalescence of eigenmodes in both frequency and field profile. I will illustrate these concepts through recent experimental realizations, including non-Hermitian band inversion and the proper definition of topological charge for radiation topology[2], the unconventional enhancement of spontaneous emission at exceptional points occurring at the band-inversion transition[3], and the hybridization of topological interfaces formed by inverted band structures[4]. If time allows, I will also discuss optical trapping[5] and lasing action at on-demanded BICs[6,7] enabled by engineered radiation patterns. These results highlight the interplay between topology, non-Hermiticity, and light–matter interaction, and point toward novel photonic devices with unprecedented control over emission and scattering. [1] “Generalized Guided Mode Expansion for Non-Hermitian Resonances in Photonic Crystal Slabs”, Nanophotonics 14, 17, 5229-5250 (2025) [2]” Band Inversion Flips the Winding of Bound States in the Continuum”, arxiv.2211.09884 [3] “Unveiling the Enhancement of Spontaneous Emission at Exceptional Points”, Physical Review Letters 129, 083602 (2022) [4] “Hybridization of Non-Hermitian Topological Interface Modes” Newton 0, (2026)  [5] ”Super Bound States in the Continuum on Photonic Flatbands: Concept, Experimental Realization, and Optical Trapping Demonstration”, Physical Review Letters 132, 173802 (2024) [6] ” Taming Friedrich-Wintgen interference in resonant metasurface: vortex laser emitting at on-demand tilted-angle”, Nano Letters 23, 10, 4152–4159 (2023) [7] ”Room-Temperature Lasing at Flatband Bound States in the Continuum”, ACS Nano 19, 20, 19287–19296 (2025)

Presenter: Nguyen Hai Son