Phonon-Enabled Collective Strong Coupling of Color Centers in a Plasmonic Cavity

To understand the work described here, one must first picture the building blocks of a future quantum internet. Scientists are searching for tiny, reliable sources of light that can carry information as individual particles, known as photons. These sources, often called quantum emitters, need to be bright and stable, ideally working at room temperature rather than requiring the extreme cold of a laboratory freezer. A promising candidate for this role is found in hexagonal boron nitride, a material that looks like a single layer of graphite but possesses unique atomic defects. These defects act as tiny light bulbs that glow brightly. However, these light bulbs do not just emit light; they also vibrate with the material they are embedded in. These vibrations, known as phonons, usually scatter energy and make the light messy. The challenge has been to harness these vibrations rather than fight them, and to make many of these tiny light bulbs talk to each other through a shared container of light, a process known as strong coupling.

In a recent study, researchers at Kiel University in Germany have demonstrated a way to make these vibrating light sources interact strongly with a container of light, even at room temperature. They placed flakes of hexagonal boron nitride onto a gold surface that had been patterned with tiny holes. These holes act as plasmonic void cavities, which are essentially microscopic traps that confine light into a very small space, making the light intensity extremely high. The team used two different methods to shine energy into these flakes: a beam of electrons and a beam of laser light. By doing so, they could observe how the light emitted by the defects changed when they were inside the trap compared to when they were outside.

The researchers discovered that the light emitted by the defects did not simply get brighter or dimmer; it fundamentally changed its character. Inside the gold cavity, the light from one specific type of defect, which naturally glows blue, began to mix with the light trapped in the hole. This mixing created a new, hybrid state of light and matter. The most striking evidence of this was a splitting of the light's color. Instead of a single peak of light at a specific wavelength, the researchers observed two distinct peaks separated by a measurable gap. When they used an electron beam to excite the material, this gap measured 36.24 meV. When they used laser light, the gap was slightly smaller, at 24.09 meV. This splitting is the hallmark of strong coupling, a state where the energy swaps back and forth between the light source and the cavity faster than it can be lost to the environment.

What makes this finding particularly surprising is that the light source responsible for this strong interaction was not the main, direct glow of the defect. Instead, it was a sideband of light created by the defect's vibrations. The defect emits a primary color, but it also emits fainter colors shifted by the energy of its vibrations. The researchers found that one of these vibrational colors, specifically the fourth one in the sequence for the blue-emitting defect, was the one that locked into the cavity. This is significant because it shows that vibrations, often seen as a nuisance that ruins quantum effects, can actually serve as a bridge to create strong connections.

The study also revealed a fascinating selectivity in how these interactions occur. The researchers were working with a material that contained two different types of light-emitting defects. One type glowed blue at 443 nanometers, and the other glowed green at 537 nanometers. Even though the green emitter's main color overlapped perfectly with the cavity's natural frequency, it remained weakly coupled and did not show any splitting. It was as if the cavity ignored the green light source entirely. In contrast, the blue emitter, through its vibrational sideband, formed a strong bond with the cavity. This suggests that the specific way a defect vibrates determines whether it can join this strong interaction, regardless of how well its main color matches the cavity.

To confirm that this was a collective effect involving many emitters rather than just one lucky defect, the team ran computer simulations. They modeled a network of hundreds of these emitters interacting with the cavity. The simulations showed that the strong splitting only appeared when a large number of emitters were present and acting together. If the number of emitters was reduced, the splitting disappeared. This indicates that the strength of the interaction comes from the combined effort of many emitters working in unison, a phenomenon known as collective strong coupling. The electron beam used in the experiment was particularly effective at exciting many emitters at once, which is why the splitting was larger in the electron measurements than in the laser measurements, where the excitation was more selective.

The researchers also explored how the thickness of the material and the depth of the defects influenced the results. By varying the energy of the electron beam, they could probe different depths within the thin flakes. They found that the blue-emitting defects were located near the surface of the material, while the green-emitting defects were distributed throughout the volume. This difference in location explained why the blue defects, being closer to the surface, could interact more effectively with the light trapped in the cavity just below them. The green defects, being deeper inside, were further away from the strongest part of the light field and thus remained in a weak coupling state.

Through a combination of electron microscopy, laser spectroscopy, and detailed computer modeling, the study provides a clear picture of how light, matter, and vibrations can be orchestrated to create new quantum states. The work demonstrates that by using the vibrational sidebands of defects, scientists can access strong coupling regimes that might otherwise be impossible. This approach offers a new pathway for controlling light at the nanoscale, showing that the very vibrations that usually disrupt quantum systems can be used to build them. The findings suggest that future quantum devices could be designed to utilize these phonon-assisted pathways, allowing for robust interactions between light and matter without the need for extreme cooling.