Dark excitons: Scientists make previously hidden states of light shine 300,000 times brighter

In the world of ultra thin semiconductors, strange forms of light and matter mingle in ways that feel almost mysterious. One of the most intriguing examples is the dark exciton, a fragile pairing of an electron and the empty space it leaves behind.

These pairs form when light touches an atomically thin material such as tungsten diselenide, yet they rarely make their presence known. Their spins do not line up correctly for normal light emission, so they stay quiet, linger longer, and tempt researchers with their promise for future quantum and photonic devices.

The quiet nature of these excitons has made them hard to study. They barely respond to common optical tools, and earlier attempts to make them shine often produced confusing signals that looked like something else. Scientists disagreed about whether they were seeing true dark excitons or unwanted artifacts. The field needed a clearer way forward.

A team from the City University of New York and the University of Texas at Austin tackled this challenge with a structure small enough to fit inside a virus. They crafted an optical cavity made of gold nanocubes set above a single sheet of tungsten diselenide. Between them sat thin layers of boron nitride, which guarded the semiconductor from unwanted charges and kept its spectral lines sharp.

The cavity itself was less than 15 nanometers thick. That tiny gap funneled light into a strong vertical electric field, the exact orientation needed to excite spin forbidden dark excitons. By shaping the cavity around this vertical field, the researchers created a place where hidden states could finally speak.

When they cooled their samples to cryogenic temperatures, the effect was immediate. Spots on the sample that sat beneath 90 nanometer gold cubes suddenly lit up with new spectral peaks that had never been seen before.

Nine razor thin resonances emerged from the semiconductor once the cavity was in place. They came from spin forbidden dark excitons with out of plane dipole moments, now made visible by the intense field in the nanogap. These peaks had extremely narrow linewidths, narrower than those of bright excitons. Their lifetimes stretched to nearly half a nanosecond. The team confirmed that the peaks were not caused by defects, single photon emitters, or charged byproducts that had confused earlier studies.

When they compared the strength of the cavity enhanced dark states to ordinary bright excitons, the numbers were astounding. In one sample, the strongest dark exciton was more than 1,400 times brighter than the bright exciton. In another, it was around 2,700 times brighter.

Scaling these measurements to the size of the nanocubes, the researchers calculated enhancement factors reaching 300,000 times brighter. According to the authors, this is among the highest photoluminescence boosts seen in gold based nanocavities for any light emitting material.

With the dark excitons now visible, the team turned to magnetic fields to see how these states behaved. When a magnetic field was applied perpendicular to the material, each dark exciton split into two lines. The energy difference between them matched the expected Zeeman splitting for spin forbidden excitons in tungsten based semiconductors. All nine new peaks shared the same g factor of 9.3, a signature value already known for this class of excitons.

A magnetic field applied in the plane of the material produced a different effect. The new cavity enhanced dark excitons weakened as the field grew stronger, while conventional dark excitons, normally too faint to notice, brightened. This tug of war showed that the new excitons were a distinct family shaped by the cavity itself rather than ordinary dark states brought into view.

The team also learned that they could turn these excitons on and off with tiny changes in electrical charge. By using a gold film and a graphene sheet as gates, they nudged the semiconductor’s Fermi level and watched the dark excitons respond. The main peak vanished outside a narrow doping window, while the dark trion state flipped between negative and positive versions with only a small voltage shift.

The findings, published in Nature Photonics, give researchers a direct way to brighten and control excitons that have remained hidden since the rise of two dimensional semiconductors.

Andrea Alù, the study’s principal investigator and founding director of the Photonics Initiative at the CUNY Advanced Science Research Center, said the work proves that light matter states once out of reach can now be accessed and steered at will. First author Jiamin Quan called the discovery only the beginning of a much larger effort to uncover long overlooked states in atomically thin materials.

This work also resolves a years long debate. Past studies using plasmonic structures often produced signals that looked like dark excitons but were later found to be charged states formed by transfer of electrons between metals and semiconductors. The new structure avoids that problem by using clean boron nitride spacers that protect the material and keep the excitons in their natural form.

The ability to brighten dark excitons with extreme precision could help build faster and more efficient photonic chips. These long lived states are appealing for quantum communication because they resist noise from the environment.

The possibility of switching them with small electric signals may lead to nanoscale modulators and sensors that work at low power.

The methods shown here could also be extended to moiré materials and other advanced semiconductor platforms, where engineered dark excitons may form the basis of tunable quantum surfaces for imaging, information transport, and secure communication.

Research findings are available online in the journal Nature Photonics.

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