AI-Designed Silicon Cavities Boost Light Emission from Atom-Thin Semiconductor Tenfold

A team of researchers at Purdue University and Harvard University has demonstrated that computer-optimized silicon nanostructures can dramatically amplify the light emitted by an atomically thin semiconductor, achieving enhancements that rival plasmonic metal antennas while avoiding their most fundamental drawback: energy lost as heat. The work, published in Light: Science & Applications, shows that topology-optimized dielectric cavities fabricated with CMOS-compatible processes can concentrate light into volumes thousands of times smaller than a cubic wavelength of light, and that when a monolayer of tungsten diselenide (WSe2) is draped over these structures, its excitonic emission brightens by roughly an order of magnitude.

The central challenge the researchers set out to solve is a familiar one in nanophotonics. Two-dimensional semiconductors such as WSe2 host tightly bound electron-hole pairs called excitons, which possess large oscillator strengths and make these materials attractive for photodetectors, light emitters, valleytronics, and quantum technologies. Yet because the material is only one atom thick, its overlap with free-space light is vanishingly small. Conventional dielectric resonators, including photonic crystals, microring resonators, and distributed Bragg reflector cavities, confine light only to diffraction-limited volumes, which limits how strongly the cavity field can interact with the monolayer. Plasmonic structures can squeeze light into nanoscale gaps, but the ohmic losses in metals reabsorb photons and degrade the coherence that quantum applications demand.

The Purdue-Harvard collaboration turned to photonic inverse design, specifically adjoint-based topology optimization, to escape this trade-off. In this approach, a computer algorithm iteratively reshapes the distribution of silicon and air within a metasurface unit cell to maximize the electric-field intensity at a target point on the surface. Each optimization cycle consists of a forward electromagnetic simulation, an adjoint simulation driven by a virtual dipole at the target location, and a gradient-based update of a continuous material-density field. Gaussian filtering and a progressively sharpened Heaviside projection push the design toward binary, manufacturable patterns, with roughly 200 to 250 iterations required before the figure of merit saturates. Strikingly, regardless of the random starting point, the algorithm consistently converged on a bowtie-like gap at the center of each unit cell, suggesting that this geometry represents a global optimum for field concentration.

The resulting cavities were fabricated from 100 to 200 nanometer-thick silicon layers on sapphire substrates using electron beam lithography and reactive ion etching. Simulations of the ideal designs predicted local field-intensity enhancements exceeding 50-fold relative to the incident field, with an effective mode volume of approximately 30 by 30 by 100 cubic nanometers, corresponding to roughly one 2500th of a cubic wavelength. The optical hotspots sit directly beneath where the WSe2 monolayer would rest, ensuring deterministic coupling between the excitons and the confined cavity mode. Because the cavities are supported on a substrate rather than suspended as membranes, they are mechanically robust and well suited to the scalable integration of van der Waals materials.

Experimentally, the team transferred large hexagonal boron nitride-encapsulated WSe2 monolayers, roughly 40 by 30 micrometers in area, onto arrays of these cavities using a gold-assisted exfoliation and transfer technique. Each flake was positioned so that part of it lay on the cavity array and part on adjacent unpatterned silicon, enabling a direct on-chip comparison between cavity-coupled and reference emission. Raman spectroscopy confirmed that strain induced by the underlying pattern was negligible, with peak shifts below one wavenumber. Photoluminescence maps then revealed bright, reproducible emission from the cavity regions: raw enhancements of approximately 6-fold, 14-fold, and, for the best-performing array, nearly 28-fold were measured across three different cavity designs, with a consistent average of about tenfold. When normalized to the tiny effective hotspot area, this corresponds to a per-area enhancement on the order of one thousand.

Numerical simulations that incorporated realistic material absorption and the actual shapes extracted from scanning electron microscope images helped explain why the measured enhancement fell short of the ideal 50-fold prediction. The small feature sizes and dense patterning amplify proximity effects during electron beam lithography, and etching introduces roughness that scatters light and degrades field localization. Loading the cavity with the WSe2 and boron nitride stack further broadens and shifts the resonance. When all of these imperfections were included, the simulated near-field enhancement dropped to approximately tenfold, in close agreement with the experimental photoluminescence results and the observed lifetime shortening. The simulations also showed that emitters placed in the hotspot radiate highly directional light, with much of the power falling within the collection cone of a standard high-numerical-aperture microscope objective, whereas emitters on flat silicon radiate mostly at grazing angles.

Perhaps the most intriguing findings came from time-resolved photoluminescence measurements. Using femtosecond pulsed excitation, the researchers found that exciton decay on the cavities became strongly non-exponential at all pump powers, while decay on the flat reference regions remained nearly exponential except at the highest powers. Fitting the decay curves with a mixed model combining a stretched exponential and a single exponential revealed a stretching parameter of roughly 0.38 on the cavity, very close to the value of one third expected for two-dimensional dipole-dipole energy transfer between excitons. Off the cavity, the stretching parameter started near 0.8 at low power and decreased as excitation increased. The team also measured an approximately eightfold shortening of the exciton lifetime on the cavity, consistent with Purcell-enhanced radiative decay.

The power dependence of the decay dynamics pointed to cavity-mediated exciton-exciton interactions as a dominant mechanism. A biexcitonic recombination model best described the shortening of the effective decay time with pump power, and the extracted biexcitonic recombination coefficient on the cavity was more than an order of magnitude, about 33 times, larger than off the cavity. By concentrating optical near fields into nanoscale hotspots, the cavities increase the local exciton density and strengthen resonant dipole-dipole interactions, effectively amplifying many-body excitonic phenomena that are normally difficult to access. Saturation measurements reinforced this picture: emission on the cavity saturated at 20 microwatts of pump power, five times lower than the 100 microwatts required off the cavity, indicating substantially enhanced excitonic nonlinearities.

Low-temperature spectroscopy added further richness. Between 5 and 200 kelvin, the cavity-coupled monolayers displayed a well-resolved manifold of excitonic states, including bright excitons, trions, biexcitons, and defect-bound excitons, with noticeably narrower linewidths on the cavity than on the flat substrate. The presence of trions, which normally require deliberate gate-controlled doping, revealed unintentional electron doping introduced during the gold-assisted transfer, at densities estimated between ten to the eleventh and ten to the twelfth per square centimeter. Notably, defect-bound exciton emission, which persisted even to room temperature, exhibited unexpectedly short lifetimes on the cavity, roughly half those of the bright excitons, and about twice shorter on the cavity than off it at every detection wavelength.

The researchers emphasize that the reported raw enhancement values are the most physically meaningful metric for practical applications, even though normalized figures are conventionally used to compare across studies. Compared against previously reported plasmonic enhancements of WSe2 emission, the dielectric platform is competitive while preserving coherence and enabling efficient light extraction. The work establishes topology-optimized silicon-on-sapphire cavities as a versatile, CMOS-compatible alternative to plasmonics for controlling exciton-photon and exciton-exciton interactions, and the authors anticipate that the platform will accelerate the integration of van der Waals semiconductors into scalable quantum photonics, opening routes toward cavity-enabled quantum many-body physics, valley-selective emission, and unprecedented excitonic nonlinearities, with a detailed study of enhanced two-photon nonlinearities planned as a follow-up.

Subject of Research: Topology-optimized dielectric nanocavities enhancing excitonic light emission and exciton-exciton interactions in monolayer WSe2

Article Title: Topology-optimized dielectric cavities for enhanced excitonic light emission from WSe2

Article References: Matthiessen, O., Triplett, B., Yesilyurt, O., Cassara, D., Pagadala, K., Yang, M. M., Llacsahuanga Allcca, A. E., Ather, H., Fruhling, C., Solanki, A. B., Chen, Y. P., Alaeian, H., Kildishev, A. V., Shalaev, V. M., Capasso, F., Boltasseva, A., & Mkhitaryan, V. (2026). Topology-optimized dielectric cavities for enhanced excitonic light emission from WSe2. Light: Science & Applications, 15(1), Article 387. https://doi.org/10.1038/s41377-026-02436-w

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02436-w

Keywords: WSe2, topology optimization, dielectric cavities, nanophotonics, excitons, photoluminescence, Purcell enhancement, two-dimensional semiconductors, inverse design, quantum photonics, silicon photonics, light-matter coupling