Over the past decade, breakthroughs in nanofabrication technology have enabled the structuring of various materials down to 10-nanometer or even atomic scale, ushering nanophotonic research into a new frontier: deep nanoscale optics. At this scale, light-matter interaction can reach an unprecedented regime of strength, unlocking immense potential for new physics discoveries and technology breakthroughs. Mapping light fields and local density of optical states (LDOS) at single-digit nanometer resolution is instrumental for advancing both fundamentals and applications in this exciting field and beyond. In particular, LDOS governs key processes such as spontaneous emission, light scattering, van der Waals interactions, and nanoscale heat transfer, yet it remains inaccessible with conventional optical microscopy techniques, such as scanning near-field optical microscopy (SNOM).
In a recent paper published in eLight , a team of scientists, led by Xue-Wen Chen, Jianwei Tang (Huazhong University of Science and Technology, China), and Haiyan Qin (Zhejiang University, China), demonstrated an optical imaging modality called scanning-exciton optical nanoscopy (SEON), to robustly map the nanoscopic light fields and LDOS simultaneously around nanostructures. The technique employs ultra photostable single quantum dots grafted on the apex of a 50 nm-diameter silica tip as a robust scanning quantum sensor. The excitons in the quantum dot are generated and decay at the rates proportional to the local light intensity and LDOS, respectively. Consequently, SEON provides correlative mappings of the light intensity and LDOS near photonic and plasmonic nanostructures with a spatial resolution of few nanometers—an achievement beyond the reach of existing techniques.
The quantum dot grafted on the scanning tip features an average size of 6.6 nm (including a 3 nm CdSe core where the exciton recombination occurs), long-term photostability in air with negligible fluorescence blinking, near-unity quantum efficiency, narrow emission spectrum, monoexponential exciton relaxation dynamics, 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.
The research team first validated the dual-parameter imaging capability of SEON using single gold nanospheres as a model system, whose optical responses are well-known theoretically. By scanning the quantum dot (also excitons) over a gold nanosphere, they demonstrated that SEON can produce illumination-dependent light intensity and illumination-independent LDOS distributions, which are perfectly aligned with the theoretical predictions. In particular, the intensity mapping captures the subtle interference effects of the illumination and the scattering, showcasing the high fidelity of the imaging modality. Their current experiments demonstrated a spatial resolution of approximately 4 nm in both the vertical and lateral directions.
The authors next applied SEON to reveal the intricate coupling physics in a plasmonic trimer consisting of three nearly touching gold nanospheres. The results highlighted SEON’s unique ability to disentangle rich nanoscale physics, including multiple scatterings and their interference with the incident illumination, as well as the enhancement and quenching of spontaneous emission of an emitter. “This level of mechanistic interpretability is unattainable with single-parameter sensing techniques,” Chen added.
Finally, the authors performed correlative dual-parameter mapping on a waveguide-interfaced photonic-crystal nanocavity. The measured fluorescence intensity map reflects the modulation of fluorescence coupling efficiency by the resonant cavity mode, while the fluorescence decay rate map reveals the combined regulation of the LDOS by both the resonant mode and the local dielectric environment. The experimental results agree well with simulations and show excellent reproducibility, confirming that SEON maintains high fidelity and robustness even for large-scale integrated 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.
eLight
Scanning-exciton optical nanoscopy using a single quantum dot