The physics is older than the laser itself. In 1929, mathematicians John von Neumann and Eugene Wigner showed that quantum mechanics admits localized states whose energies lie inside a continuum of propagating solutions — a result long dismissed as a curiosity. Optics eventually turned that curiosity into a design tool. A bound state in the continuum is an optical mode whose frequency sits above the light line, the boundary beyond which a flat structure should radiate into free space, yet which remains perfectly dark. The trick is interference. In a symmetry-protected BIC, the mode’s radiation pattern decouples from the outside world by symmetry alone; in a Friedrich–Wintgen BIC, two leaky channels annihilate each other through destructive interference. On a dispersion diagram, the quality factor — the Q factor, which counts how many oscillations a mode survives before its energy drains away — diverges as radiation vanishes. Real devices never reach infinity. But break the symmetry slightly, offsetting a pair of nanobars by a few nanometers, and the BIC becomes a quasi-BIC: finite in Q, enormous by ordinary standards, with linewidths that can shrink to fractions of a nanometer.

The platform of choice for such states is the metasurface: a two-dimensional lattice of subwavelength resonators — pillars, disks or bars of high-index dielectrics such as silicon, titanium dioxide or gallium arsenide — patterned into a film thinner than the wavelength of light it controls. By adjusting the geometry of every resonator, designers can bend beams, shape wavefronts and, crucially, engineer collective resonances with prescribed radiation patterns. Quasi-BIC metasurfaces typically take the form of paired nanobars or broken-symmetry nanodisks, in which a deliberate geometric asymmetry meters the leakage like a valve. In recent years, such structures have delivered quality factors in the tens of thousands, field enhancements that supercharge nonlinear frequency conversion, biosensors responsive to vanishingly thin molecular layers, and miniature lasers. Every one of those demonstrations, however, shared a common crutch: optical excitation, with an external laser supplying energy from above. Elegant in the laboratory, it is a poor match for real devices, where the ideal light source simply turns on with a wire.

That is where atomically thin semiconductors enter. Crystals such as molybdenum disulfide, tungsten disulfide, molybdenum diselenide and tungsten diselenide — the transition metal dichalcogenides — behave as lackluster, indirect-bandgap emitters in bulk form. Shaved to a single atomic layer, roughly seven angstroms thick, they transform: the band gap becomes direct and the material lights up. What emits are excitons, electron–hole pairs bound so tightly — by hundreds of millielectronvolts — that they survive at room temperature, and whose oscillator strength is astonishing for so flimsy a film: near its exciton resonance, a monolayer can absorb on the order of a tenth of the light falling upon it. Researchers have coaxed electroluminescence from these monolayers using p–n junctions, electrostatic gating and tunneling contacts, effectively building the thinnest light-emitting diodes imaginable, and passivation of defects has pushed their brightness steadily upward. The stubborn problems are extraction and direction. Much of the generated light escapes at grazing angles, is lost to non-radiative recombination, or is smeared across broad, featureless linewidths. A naked monolayer, in short, makes photons but squanders most of them.

The new study proposes to marry the two platforms at the point where each is strongest. In a quasi-BIC metasurface, the electromagnetic field concentrates into intense hotspots within each unit cell, exactly where the near field of the resonant mode peaks. Place an electrically driven monolayer there, the authors argue, and the semiconductor’s spontaneous emission couples into the resonance through the Purcell effect, which accelerates emission in proportion to the local field intensity divided by the mode volume. Because a quasi-BIC combines a diffraction-limited mode volume with a quality factor that can climb into the thousands, the enhancement can be dramatic: excitons that would ordinarily dribble photons in all directions instead funnel their energy into a single, sharply defined resonance. That mode, in turn, radiates in a beam-like, vertically directed pattern with a narrow linewidth and a polarization fixed by geometry rather than chance. The injected current — delivered through contacts, gates or junctions — becomes the switch that brings the dark state to life. The quasi-BIC is not merely decorated by the semiconductor; it is activated by it.

The analysis reaches beyond a simple intensity boost. Electrostatic gates can shift a monolayer’s exciton energy through the quantum-confined Stark effect, allowing the emitter to be tuned into — or out of — resonance with the quasi-BIC, dialing the coupling up and down with a voltage. The study examines the regimes that follow. In weak coupling, the resonance amplifies emission and narrows the spectrum. In strong coupling, when the coherent exchange of energy between excitons and photons outpaces the losses of both, the two hybridize into exciton–polaritons, part matter and part light, which quasi-BIC architectures can sustain at unusually low thresholds. The work also confronts the awkward arithmetic of hybrid devices head-on: the very material that lights the mode also loads it. Atomic layers absorb, scatter and dephase, and the authors map how much optical loss the semiconductor imposes on the resonance, and how detuning, oscillator strength and geometry must be balanced so that the quality factor survives the partnership.

The quest is not merely academic. Sharp resonances are the currency of nanophotonics, and quasi-BICs are its sharpest coins; the field’s long-standing frustration has been that its best resonances could not be plugged in. Optical pumping hard-codes the excitation geometry, imposes a thermal burden, and ties the source to bulky equipment, which is precisely why so many spectacular BIC demonstrations have remained laboratory marvels rather than components. Feeding the resonance with electrons, rather than photons, dissolves those constraints at a stroke: currents are the standard currency of chip technology, they can be modulated at high speed, and they scale to arrays with the ease of any other wired device. The open question — whether the atomically thin semiconductors that dominate two-dimensional-material photonics could shoulder the task — is the one the study takes up.

The payoff, if the engineering holds, would be a class of light sources that ordinary laser diodes struggle to imitate. Because the resonance is fed electrically, there is no optical pump to damage the sample, no pump spot to define the emitting region, and no high-power optics to keep aligned. Narrow, directional, polarization-pure emission is precisely what optical interconnects demand, where every wasted photon becomes heat, and what LiDAR-style ranging rewards, where beam quality translates directly into resolution. Arrays of such pixels could be addressed individually, each one a voltage-tuned, line-narrowed emitter on a chip. Modulators and tunable filters could borrow the same trick, steering a razor-sharp resonance across a spectrum with a gate voltage. The physics also runs in reverse for sensing: a quasi-BIC resonance whose sharp shift betrays a molecule landing in its hotspot is among the most sensitive refractive-index probes known, and making that resonance electrically active would fold source and sensor into a single, self-contained instrument.

Quantum optics stands to gain as well. Monolayers of tungsten diselenide host single-photon emitters — atomically localized defects and strain traps that release photons one at a time — which are coveted for quantum communication but notoriously hard to collect efficiently. Anchoring such emitters to a quasi-BIC hotspot would both brighten them and pour their emission into a narrow, well-defined optical mode, tackling two of the chief obstacles to practical single-photon sources. Nonlinear optics could benefit too: the giant fields that quasi-BICs confine are already known to supercharge frequency conversion, and an electrically fed version would build that enhancement into an active, chip-scale source. The vertical, beam-like radiation of quasi-BIC modes also makes the platform a natural partner for integrated photonics, where light must enter waveguides with minimal loss. And because a metasurface is a planar structure defined by lithography, the road to mass manufacture looks, in principle, more like the road to a processor than the road to a laboratory laser.

None of this is a foregone conclusion, and the study is candid about the distance between principle and device. A monolayer must be transferred over nanostructured resonators with near-atomic registration; electrical contacts add resistance and optical loss; non-radiative defects, thermal loading and the intrinsic linewidth of the exciton all threaten to wash out the very sharpness that makes quasi-BICs worthwhile. Encapsulation in hexagonal boron nitride, cleaner crystal growth and gentler transfer chemistry are among the remedies the field is already pursuing. What the work contributes is a coherent physical map of the terrain: which couplings matter, where the losses bite, and how the pieces must be balanced for electricity — not a laser — to do the switching. Bound states in the continuum began as a mathematical curiosity nearly a century ago. If the vision set out here holds, they may complete their journey as working elements in the thinnest light sources ever built: dark states, at last, with somewhere to go.

A groundbreaking study, published in Advanced Photonics, has unveiled a novel hybrid photonic platform that circumvents the limitations of conventional approaches by focusing on restructuring the nanoscale space beneath the 2D semiconductor itself. The research team engineered a heterostructure where a WS₂ monolayer is delicately placed over arrays of Mie void resonators—subwavelength air cavities intricately milled into a bismuth telluride (Bi₂Te₃) substrate, a material known for its high refractive index. This design harnesses a new paradigm of light confinement inside air-filled voids, strongly enhancing emission and nonlinear response from WS₂, while allowing unprecedented visualization of localized optical modes through far-field imaging techniques.

Traditional dielectric nanoresonators typically trap light within solid materials such as silicon, often resulting in the optical fields being concentrated deep inside the resonator’s bulk. This configuration inherently limits interaction with ultrathin materials placed at the surface and suffers performance degradation when the host material exhibits absorption losses, which reduce the quality of resonances and field confinement. In stark contrast, Mie void resonators exploit the high refractive index contrast at the air-dielectric boundary to confine light inside nanoscale air cavities. These subwavelength voids sustain circulating electromagnetic fields that are tightly confined near the surface where the WS₂ monolayer resides, effectively transforming “empty space” into a highly effective resonant cavity for light-matter coupling.

This unique “inverted” resonator geometry confers several vital advantages: the optical field enhancement is naturally accessible to the atomically thin WS₂ layer positioned atop the void; the resonant wavelengths can be finely tuned by modifying cavity dimensions; and the resonators maintain robust performance even though Bi₂Te₃ exhibits significant optical absorption. Notably, materials like Bi₂Te₃, typically unsuitable for conventional photonic resonators due to absorption losses, become ideal substrates in this void-based system, opening pathways to harness a wider class of materials for integrated photonics.

The research team utilized full-wave electromagnetic simulations to meticulously optimize the void resonator geometries. Their goal was to align a dipolar resonance mode with the main photoluminescence signature of WS₂, known as the A-exciton at visible wavelengths. By engineering cavity radius and depth, they tuned both the spectral position and vertical distribution of the resonant optical mode. The cavities were precision-fabricated into mechanically exfoliated Bi₂Te₃ flakes using focused ion beam milling, maintaining enough spatial separation to ensure each void functioned as an individual resonator rather than part of a coupled photonic crystal array. A continuous WS₂ monolayer was then transferred across the patterned surface to cover resonant voids, non-resonant voids, and flat regions of the substrate alike, ensuring direct comparative analysis of emission influenced solely by cavity geometry instead of material inhomogeneity.

Optical characterization through reflectance spectroscopy demonstrated that the resonance wavelengths shifted systematically with changes in cavity size, showing a predictable redshift as void radii increased and spectral shifts as cavity depth varied. The resonances exhibited remarkable tolerance to minor fabrication imperfections, remaining well-defined outside strict optimum geometries, which highlights the platform’s robustness and feasibility for scalable nanophotonic devices. This validates the precision of simulation models used and underscores the practical potential for real-world applications where device fabrication may encounter variability.

Photoluminescence measurements revealed a striking amplification of light emission from WS₂ when the dipolar resonance spectrally overlapped with the monolayer’s intrinsic emission band. Specifically, resonance alignment led to an approximately twenty-fold increase in photoluminescence intensity compared to cavities tuned far off resonance. Intriguingly, the enhancement did not arise from increased absorption of the excitation laser light, as simulations and control measurements with varied pump wavelengths showed negligible field amplification at the excitation frequencies. Instead, emission-side effects predominated: the resonant voids augmented the local density of optical states accessible to excitons and improved photon extraction efficiency, thereby significantly boosting emission yield.

The continuous nature of the WS₂ sheet over the substrate was pivotal, enabling a direct head-to-head comparison of photoluminescence from resonant Mie voids, non-resonant voids, and flat Bi₂Te₃ areas under identical experimental conditions. This setup eliminated variability caused by differing material quality or laser excitation conditions, conclusively attributing the enhanced photoluminescence to resonant photonic mode engineering. This demonstration of controlled enhancement marks a critical step toward practical photonic devices where modulation of optical response relies on structural design rather than material alteration.

Extending their approach to the nonlinear optical regime, the researchers adjusted the cavity geometry to bring the dipolar resonance into the near-infrared wavelength range, resonant with the fundamental excitation in second-harmonic generation experiments. Here too, the hybrid platform excelled, with second-harmonic signals from WS₂ increasing by approximately twenty-five times compared to off-resonant cavities. The nonlinear emission intensity exhibited a sharp spectral peak when the pump laser was tuned through the resonant frequency, underscoring the precision and efficacy of the Mie void resonators in enhancing nonlinear photonic processes with atomically thin materials.

Beyond quantifying intensity enhancements, the system’s design revealed a striking capability: near real-time, far-field visualization of localized optical modes within individual Mie void resonators. Using the spatially resolved second-harmonic emission, the researchers observed bright, well-defined photoluminescence “hotspots” precisely matching the void positions. Remarkably, by varying pump wavelength or cavity depth, these hotspots migrated predictably across the array, offering a direct, intuitive window into the spatial dynamics and dispersion of localized electromagnetic modes. This ability to map optical fields in real space without invasive near-field scanning techniques provides a powerful diagnostic tool for nanophotonics.

With its unique combination of resonant mode tunability, robust field enhancement, and direct spatial control, the Mie-void heterostructure platform stands poised to advance studies in nonlinear light generation, enhanced optical sensing, and spatially programmable photonic devices leveraging van der Waals atomic layers. Crucially, the approach circumvents reliance on large-scale metasurfaces with periodic patterns, offering a nanoscale engineering route that remains effective even with strongly absorbing substrates. This opens vast opportunities to integrate emerging 2D materials into complex photonic architectures previously limited by material constraints and coupling inefficiencies.

More broadly, this work redefines the conventional paradigm of photonic device design by focusing on engineering the “empty” spaces beneath atomically thin layers rather than modifying the 2D materials themselves. By sculpting nanoscale voids that support deeply subwavelength resonant modes in air, researchers can dramatically amplify light-matter interactions and introduce spatial programmability directly into heterostructure platforms. This insight could accelerate the development of next-generation photonic technologies that are both highly efficient and versatile, spanning applications from quantum information processing to ultrafast programmable optical networks.

As the photonics community pursues ever-thinner, more compact, and more efficient light sources and sensors, innovations like Mie void resonators represent powerful new tools. This research bridges theoretical modeling, precision nanofabrication, and sophisticated optical characterization to transform fundamental understanding into practical device concepts. Future work will undoubtedly explore integrating other transition metal dichalcogenides and layered materials with tailored void geometries, expanding functionality to cover broader spectral ranges and complex optical phenomena, heralding a new era in two-dimensional material photonics.

Subject of Research: Not applicable

Article Title: Light–matter interaction in van der Waals heterostructures with Mie voids

News Publication Date: 14-Feb-2026

Web References:
https://www.spiedigitallibrary.org/journals/advanced-photonics/volume-8/issue-02/026002/Lightmatter-interaction-in-van-der-Waals-heterostructures-with-Mie-voids/10.1117/1.AP.8.2.026002.full

References:
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

Image Credits: Zhuoyuan Lu (Australian National University)

Keywords

Semiconductors, Applied optics, Photonic nanostructures, Two-dimensional materials, Tungsten disulfide, Mie void resonators, Van der Waals heterostructures, Nonlinear optics, Light emission enhancement, Nanoscale photonics, Bismuth telluride, Second-harmonic generation