A powerful new microscopy technique unveils hidden nanoscale light interactions, offering a glimpse into physics that conventional tools cannot resolve.

Over the past ten years, advances in nanofabrication have made it possible to shape materials at scales as small as 10 nanometers and even down to individual atoms. These capabilities have pushed nanophotonics into a new domain often described as deep nanoscale optics.

At such extremely small scales, interactions between light and matter become much stronger than previously observed. This opens the door to discovering new physical phenomena and developing advanced technologies. Precisely mapping light fields and the local density of optical states (LDOS) at resolutions of just a few nanometers is essential for progress in both fundamental research and practical applications.

LDOS plays a critical role in processes such as spontaneous emission, light scattering, van der Waals interactions, and heat transfer at the nanoscale. Despite its importance, it has remained out of reach for standard optical imaging methods, including scanning near-field optical microscopy (SNOM).

A New Imaging Modality: SEON

In a recent study published in eLight, researchers led by Xue-Wen Chen and Jianwei Tang (Huazhong University of Science and Technology, China), along with Haiyan Qin (Zhejiang University, China), introduced a new imaging approach called scanning-exciton optical nanoscopy (SEON).

This method allows scientists to map both nanoscale light fields and LDOS at the same time around tiny structures. It relies on highly stable single quantum dots attached to the tip of a silica probe measuring 50 nm in diameter, which acts as a sensitive scanning detector.

Within the quantum dot, excitons are generated and decay at rates linked to local light intensity and LDOS. By tracking these changes, SEON produces paired maps of both properties near photonic and plasmonic structures, reaching spatial resolutions of only a few nanometers, beyond what current techniques can achieve.

The quantum dot used in this setup measures about 6.6 nm (including a 3 nm CdSe core where exciton recombination occurs). It remains stable over long periods in air, shows minimal fluorescence blinking, and operates with near-perfect quantum efficiency. It also features a narrow emission spectrum, consistent exciton relaxation behavior, and a signal-to-background ratio as high as 55.

“These outstanding properties possessed by the quantum probe ensure the high resolution, robustness, and fidelity of our SEON technique,” said Xue-Wen Chen.

Validation with Gold Nanospheres

To test the system, the team examined single gold nanospheres, which are well understood from a theoretical standpoint.

By scanning the quantum dot, which carries excitons, across the surface of a nanosphere, the researchers showed that SEON can generate maps of light intensity that depend on illumination, alongside LDOS maps that remain unaffected by it. These results closely matched theoretical expectations.

The measurements also captured subtle interference patterns between incoming light and scattered waves, demonstrating the method’s precision. The experiments achieved a spatial resolution of about 4 nm in both vertical and horizontal directions.

Probing Complex Plasmonic Systems

The researchers then applied SEON to a more complex arrangement known as a plasmonic trimer, made up of three nearly touching gold nanospheres.

This experiment revealed SEON’s ability to separate and analyze multiple nanoscale interactions, including repeated scattering events and their interplay with incoming light. It also provided insight into how spontaneous emission can be either enhanced or suppressed.

“This level of mechanistic interpretability is unattainable with single-parameter sensing techniques,” Chen added.

Mapping Photonic-Crystal Nanocavities

In a final demonstration, the team used SEON to study a waveguide-connected photonic-crystal nanocavity. The resulting fluorescence intensity map showed how the resonant cavity mode affects coupling efficiency, while the fluorescence decay rate map revealed how both the cavity mode and the surrounding material influence LDOS.

The findings were consistent with simulation results and showed strong reproducibility, confirming that SEON remains accurate and reliable even in more complex photonic systems.

“To the best of our knowledge, this work represents the first optical mapping of the LDOS for a photonic-crystal nanocavity,” Chen stated.

“Our SEON technique bridges the gap between surface morphology and far-field optical response, and thus establishes a foundational platform for exploring and scrutinizing light-matter interactions at the deep nanoscale, with wide-ranging implications for functional nanomaterials, quantum optics, integrated photonics, and nanoplasmonics,” said Chen.

“An immediate extension of the technique would be to develop a reflection-mode SEON, i.e., excitation and collection through the same tapered fiber, which will extend the applicability of the current technique for non-transparent samples. Further extensions could integrate multicolor QD probes for wavelength-multiplexed LDOS mapping, or combine this technique with ultrafast spectroscopy to resolve dynamical processes in quantum materials,” Chen envisioned.