Integration of 2D Materials in Radial van der Waals Heterostructure Metasurfaces

TL;DR

This work demonstrates a compact, polarization-invariant radial qBIC metasurface in low-index hBN and its integration with a WS$_2$ monolayer to modulate exciton emission. By engineering trapezoidal unit cells and a global scaling approach, the authors achieve high-$Q$ resonances with a footprint of around $4~\mu$m in radius, and $Q$ factors up to about $1300$, far surpassing prior radial designs in hBN. Angle-resolved PL and k-space measurements reveal a ladder of ring eigenmodes carrying orbital angular momentum (OAM) that are radiatively accessed via the qBIC, with spectral overlap and momentum-space features signaling enhanced exciton–photon coupling. Numerical and experimental results indicate robust, linear-regime light–matter interactions and point to potential strong coupling, exciton localization, valley-emission control, and on-chip, momentum-structured emission in 2D vdW heterostructures.

Abstract

Two-dimensional semiconductors, such as monolayer transition metal dichalcogenides (TMDC), exhibit strong excitonic transitions at room temperature and offer a unique platform for exploring light-matter interactions in nanoscale photonic systems. In this work, we demonstrate a compact and polarization-invariant photonic metasurface, fabricated from hexagonal boron-nitride (hBN) and based on radial bound states in the continuum (BIC), which are formed by radially distributed pairs of structurally asymmetric resonators. The metasurface employs multiple symmetry-breaking perturbations to support high quality-(Q-)factor resonances within a footprint smaller than 8 x 8 $μm^2$ - one-sixth of the area of previous approaches. Compared to established hBN metasurface designs, the radial geometry furthermore achieves significantly higher Q-factors with a reduced footprint. By integrating the hBN photonic structure with a WS$_2$ monolayer, we observe enhanced photoluminescence when its resonance is spectrally aligned with the exciton resonance, accompanied by signatures of discrete momentum-space patterns that identify the orbital-angular-momentum-carrying ring eigenmodes. These features persist over a wide range of excitation powers and show minimal linewidth broadening, indicating robust and spatially modulated exciton-photon coupling. This work establishes a scalable approach for generating hybrid photonic-excitonic states with momentum-space structure, offering new opportunities for exciton localization, valley emission, spatially programmable light-matter interaction in two-dimensional material platforms and compact luminescent devices based on 2D material-integrated metasurfaces.

Figures (10)

• Figure 1: Optimization of radial qBICs in hBN. a Radial qBIC structure with diameter 2$\cdot$R fabricated from hBN on a SiO2 substrate. The inset shows the hBN lattice structure. b When moving from a rod-type to a trapezoid-type unit cell (the SEM image shows a fabricated trapezoidal geometry sample with width asymmetry) the fixed parameter is no longer the width of the individual resonators but the gap between them (see Supplementary Note 2), enabling a c 20% increase in Q-factor in numerical simulations. For this, length asymmetry as introduced in previous work is used kuhner2022radial. d The spectral position of the resonance is tuned via a scaling factor applied to all parameters other than height and number of unit cells. e Sum of real parts of in-plane electric fields for radial qBIC in xy- and yz-plane. f Comparison of different asymmetry approaches with corresponding experimental verification, allowing for both Q-factor and signal optimization. g SEM of the fabricated structure revealing slanted sidewalls. h Simulations demonstrating resonance tuning and Q-factor control through the inclusion of slanted sidewalls.
• Figure 2: Radial qBIC in k-space. a (b) Sketch of TE (TM) mode constituent fields in respect to radial qBIC structure for oblique incidence. A phase difference is accumulated in the magnetic (electric) component. c Simulated transmittance dependency for oblique incidence in TE-mode. The resonance trend exhibits a periodically modulated parabolic shape. d Simulated transmittance dependency for oblique incidence in TM-mode. In this case, the resonance trend exhibits a shape constituted by a series of periodically modulated parabolas. e Sketch of the resulting phase difference $\Delta\phi$ at opposing sides of the radial qBIC structure. f Sketch of angled light, launching propagating waves at the respective ring poles with opposing propagation direction, resulting in a standing wave. g E-Field Norm for the first eight eigenmodes (mode number 1 to 8) of the radial qBIC structure. h Spectral position of the respective eigenmodes correlated with (1 - T) summed up over all angles of incidence for both TE and TM.
• Figure 3: Radial qBIC vdW-heterostructure. a Radial qBIC vdW-heterostructure with a mono-atomic layer (ML) of WS2 encapsulated by two bulk layers of hBN. b Respective lattice structures of heterostructure components. c Optical microscope image of final heterostructure. Both the bottom and top layer of hBN are 80 nm thick. d PL microscope image of the heterostructure shows signal from the area of the monolayer. e SEM image of the fabricated structure. f Normalized Transmittance and PL spectra for radial qBIC with scaling factor $S$ = 1.01. Both the excitonic response and a BIC-driven resonant enhancement are visible in the PL spectrum. g Experimental PL maps of radial qBIC vdW-heterostructure close to the excitonic wavelength ($\lambda$ = 620 nm) show BIC-driven enhancement of PL emission.
• Figure 4: k-Space hyperspectral imaging of radial qBIC vdW-heterostructure. Vertical cross-section of the experimental 3D hyperspectral image showing a periodically modulated parabolic dispersion for a TE and b TM. c PL spectrum correlated with 1-T summed up over all angles of incidence for both TE and TM polarizations. d Power-dependent PL of radial qBIC vdW-heterostructure. e Power-dependence of the peak-position and linewidth of three separate local maxima in the PL spectrum.
• Figure S1: Moving from high refractive index dielectric to low refractive-index. Simulated transmittance for differently scaled radial qBIC structures, starting from the silicon design proposed in literature. kuhner2022radiala The transition from a resonator material with a high refractive index ($n = 3.45$, silicon) to a low-index material such as hBN ($n_{x,y} = 2.1, n_{z} = 1.6$ in the investigated spectral range) also entails a reduction in refractive-index contrast between the resonators and the substrate (fused silica, $n = 1.45$). b Therefore, to attain sufficiently strong signal, the dimensions of the ring must first be increased by a factor of approximately 1.8 to recover a qBIC signal around 700 nm for a refractive index similar to that of TiO$_2$ ($n = 2.6$). c To achieve a comparable signal for hBN, the dimensions of the structure must be increased again by a factor of 1.5.