Atomically thin semiconductors such as tungsten disulfide (WS₂) are promising materials for future photonic technologies. Despite being only a single layer of atoms thick, they host tightly bound excitons—pairs of electrons and holes that interact strongly with light—and can efficiently generate new colors of light through nonlinear optical processes such as second-harmonic generation. These properties make them attractive for quantum optics, sensing, and on-chip light sources. At the same time, their extreme thinness imposes a basic limitation: there is very little material for light to interact with. As a result, emission and frequency conversion are often weak unless the surrounding photonic environment is carefully engineered.
A study published in Advanced Photonics introduces a new way to address this challenge by reshaping not the two-dimensional material itself, but the space beneath it. The researchers demonstrate a hybrid platform in which a monolayer of WS₂ is placed on top of nanoscale air cavities, known as Mie voids, carved into a high-index crystal of bismuth telluride (Bi₂Te₃). The work shows that these voids can strongly enhance light emission and nonlinear optical signals, while also allowing direct visualization of localized optical modes.
Turning empty space into a resonator
Conventional dielectric nanoresonators trap light inside solid materials such as silicon. While effective in many settings, this approach concentrates optical fields away from the surface, where atomically thin materials reside. It also performs poorly when the host material absorbs light, which damps resonances and reduces field strength.
Mie voids operate on a different principle. Instead of confining light inside solid matter, they trap it inside subwavelength air cavities carved into a material with a very high refractive index. Strong reflection at the air–dielectric boundary keeps the light circulating within the void. As a result, the optical field is concentrated in air and near the top surface of the cavity—exactly where an overlaid monolayer sits.
This "inverted" confinement geometry offers several advantages. The field enhancement is naturally accessible to surface-bound materials, the resonant wavelength can be tuned by adjusting cavity geometry, and the approach remains effective even when the host material is strongly absorptive. Bi₂Te₃, which would be a poor choice for conventional resonators, turns out to be well suited for void-based designs.
Designing and fabricating the heterostructure
Guided by full-wave electromagnetic simulations, the researchers designed void geometries that support a simple dipolar resonance aligned with the main emission feature of WS₂, known as the A-exciton. By adjusting the radius and depth of the cavities, they could tune both the resonance wavelength and the vertical position of the optical mode.
The voids were fabricated by focused ion beam milling directly into thick, mechanically exfoliated Bi₂Te₃ flakes. The cavities were spaced far enough apart to act as isolated resonators rather than as a coupled array. A continuous monolayer of WS₂ was then transferred across the patterned surface, covering resonant voids, off-resonant voids, and flat regions of the substrate. This layout ensured that differences in optical response could be attributed to the cavity geometry rather than variations in material quality or excitation conditions.
Optical reflection measurements confirmed that the resonances behaved as predicted. Increasing the cavity size produced a smooth redshift of the resonance, while varying the depth at fixed radius caused both spectral shifts and a gradual migration of the optical mode deeper into the cavity. Importantly, the resonances remained well defined even away from the optimal geometry, indicating strong tolerance to fabrication imperfections.
Enhancing light emission from WS₂
To probe how the cavities affect emission, the team measured photoluminescence from WS₂ under laser excitation while systematically varying the void depth. When the cavity resonance was aligned with the WS₂ emission band, the photoluminescence intensity increased by about a factor of 20 compared with the most off-resonant cavity.
Crucially, simulations showed no significant field enhancement at the excitation wavelength, and measurements performed with different pump wavelengths confirmed that the strongest emission always occurred at the same cavity depth. This ruled out enhanced absorption of the incoming light as the main cause. Instead, the results point to emission-side effects: the resonant void increases the local optical density of states and improves how efficiently emitted light escapes the structure.
Because the WS₂ monolayer was continuous, the researchers could directly compare emission from resonant cavities, non-resonant cavities, and flat substrate regions under identical conditions. This made clear that the observed contrast was governed by emission-resonant mode engineering rather than by differences in the monolayer itself.
Probing nonlinear optics and visualizing modes
The researchers extended the approach to nonlinear optics by scaling the cavity geometry so that the dipolar resonance shifted into the near-infrared. Under resonant excitation, the second-harmonic signal from WS₂ increased by about 25 times compared with non-resonant cavities. The signal showed a clear spectral peak when the pump wavelength was tuned through the cavity resonance.
Beyond enhancement, the system offers a distinctive capability: direct spatial mapping of localized optical modes. Far-field images of the second-harmonic signal revealed bright, well-defined hotspots above individual voids. As the pump wavelength or cavity depth was varied, these hotspots moved across the array in a predictable way, reflecting the engineered dispersion of the resonant modes. This provided a real-space view of how optical fields evolve inside single resonators, without the need for near-field probes.
A flexible platform for atom-thin photonics
By combining tunable optical enhancement with deterministic spatial control in a van der Waals–compatible architecture, Mie-void heterostructures offer a new route for working with atomically thin materials. The approach avoids reliance on large periodic metasurfaces and works even in highly absorptive materials that are difficult to use in conventional photonics.
This platform could support future studies of nonlinear light generation, surface-enhanced sensing, and spatially programmable photonic devices based on two-dimensional semiconductors. More broadly, the work highlights how carefully shaping empty space can be just as important as choosing the right material when engineering light–matter interactions at the nanoscale.
For details, see the original Gold Open Access article by Z. Lu et al., " Light–matter interaction in van der Waals heterostructures with Mie voids ," Adv. Photon. 8(2), 026002 (2026), doi: 10.1117/1.AP.8.2.026002
