Topology-Enabled Switchable Unidirectional Radiative Band in a Bilayer Photonic Crystal

TL;DR

This work addresses the problem of achieving robust, controllable directional emission from open photonic systems. It introduces a hetero-bilayer photonic crystal where non-Hermitian hybridization of symmetry-protected resonances is described by a $2\times2$ temporal coupled-mode theory Hamiltonian $H$, including interlayer coupling $W$, phase delays $\\Phi$, and radiative rates $\\gamma_U$, $\\gamma_L$, and identifies a Friedrich–Wintgen BIC (FW-BIC) framework together with a radiation-asymmetry (RA) pseudo-polarization vortex of topological charge $q=-1$ that governs emission direction. The main results show near-unity radiation asymmetry $F_{asy}$ across broad spectral and momentum ranges, with a switchable emission direction achieved by perturbing the synthetic parameter space via environmental refractive index, a capability validated by both numerical models and experimental samples (A,B,C). The work demonstrates a scalable platform for robust directional emitters and topological sensing, with implications for reconfigurable metasurfaces and potentially unidirectional lasing in high-$Q$ modes.

Abstract

Controlling how an open photonic system exchanges energy with its environment-and in particular how it radiates into the far field-is a cornerstone of non-Hermitian wave physics and a key enabler for directional photonic functionalities. Here, we propose a new route to robust unidirectional emission based on the non-Hermitian hybridization of resonances localized in spatially separated layers of a hetero-bilayer photonic crystal. By tailoring the interlayer coupling, we engineer hybrid photonc bands that exhibit strong unidirectional radiation across a broad spectral and momentum range while maintaining theoretically high quality factors. This asymmetric emission is organized by a topological vortex in a pseudo-polarization field defined from the front/back intensity imbalance, which endows the directionality with robustness against perturbations. We further show that, by tuning the surrounding refractive index, this singularity can be displaced in parameter space, enabling reversible switching of the emission direction and a reconfigurable far-field response. This framework opens perspectives for topological photonic sensing and for directional and switchable light sources, including unidirectional lasing supported by high-quality-factor modes.

Figures (7)

• Figure 1: (a) Conceptual diagram of the hetero bilayer PhC with asymmetric emission. (b-c) Dependence of quality factor ($Q$) and asymmetry factor ($F_\mathrm{asy}$) (top panel) and pseudo-polarization angle (bottom panel) on the propagation phases in the bilayer PhC system. The results are analytically calculated from the simplified $2\times2$ TCMT model. $\Phi_{\mathrm{U}}$ and $\Phi_{\mathrm{L}}$ represent the phase factor caused by upper and lower PhC slabs, respectively. $\Phi$ is the phase factor induced by the interlayer gap.
• Figure 2: (a-b) Bonding and antibonding mode behavior as a function of $t_{\mathrm{Si}}$ (a) and $t_{\mathrm{gap}}$ (b) for $k_x = 0.001\times2\pi/a$. Top: band energy. Middle: radiative decay rate. Bottom: RA factor. Solid lines denote the analytical calculation (i.e., ana.) results, and scattered dots denote the numerical simulation (i.e., num.) results. (c-e) Mapping of normalized radiative decay rate (c), RA factor $F_{\mathrm{asy}}$ (d), and pseudo-polarization angle $\varphi_{\mathrm{RA}}$ (e) for the bonding mode in the parameter space for $k_x = 0.001\times2\pi/a$. The square and circle labels in (d) represent parameters of the sample A and B, respectively in the experiments. In all simulations, the PhC is consist of a square lattice of air holes with a period $a =$ 445nm and hole diameter $d =$ 300nm.
• Figure 3: (a) SEM images (left: top-view; right: tilted view) of the fabricated bilayer PhC. False colors represent different layers: purple –$\mathrm{Si}$; gray –$\mathrm{SiO_2}$; yellow –$\mathrm{TiO_2}$. The scale bar is 500nm. (b-c) Measured momentum-resolved PL spectra of the sample A (b) and sample B (c). Left: front-side. Right: back-side. (d-e) Numerical and analytical simulated band RA of the EQ-BIC bonding mode in sample-A (d) and sample-B (e). The black arrow indicates the RA factor at $k_x = 0.04\times2a$. (f-g) Comparison of the PL spectra collected from front and back sides at $k_x = 0.04\times2a$, from sample A (f) and sample B (g). (h) Emission directionality of EQ-BIC bonding mode at normalized angular frequency $\sim0.605$ from sample-A (blue) and sample-B (red), after subtracting the contributions from larger-angle leaky mode emissions. Specifically, sample A features $t_{\mathrm{gap}}=144nm$ and $t_{\mathrm{Si}}=24nm$, while sample B has $t_{\mathrm{gap}}=130nm$ and $t_{\mathrm{Si}}=32nm$.
• Figure 4: (a) Schematic diagram of a tunable radiation-asymmetry device. By changing the surrounding solution, the refractive indices of the superstrate and nanoholes are adjusted. (b) Mapping of the RA factor in the parameter space under varying RI. The singularity shifts within the parameter space as the the solution RI changes. The red (blue) circle indicates the tunable sample in an oil (PMMA) environment. (c,d) Measured angle-resolved PL spectra of the tunable sample in the oil (c) and in PMMA (d). Left: front side. Right: back side. (e,f) Comparison of front-side and back-side PL spectra at a specific momentum ($k_x=0.03\times 2\pi/a$), measured in oil (e) and in PMMA (f). Sample C features $t_{\mathrm{gap}}=144nm$ and $t_{\mathrm{Si}}=26nm$
• Figure 5: Schematic of the bilayer PhC slabs and the four radiation ports used in the coupled-mode description.