In a News & Views commentary on a recent study published in the journal Light: Science & Applications, Bowen Wang, Qian Chen, and Chao Zuo addressed the fundamental challenge in modern photonics: determining the ultimate limit of light confinement and the ability to observe such highly confined light without causing disturbance. Advances in nanofabrication, particularly using metallic and dielectric nanostructures, have enabled optical field confinement down to sub-10-nm and even sub-1-nm scales, which is critical for applications across nonlinear optics, super-resolution microscopy, and next-generation photonic devices. The ability to "witness" these ultra-confined fields with minimal interaction is both paradoxical and essential for advancing the understanding of nanoscale light–matter interactions.
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Background
Achieving extreme optical field confinement faces different challenges depending on the material used. While metallic plasmonic modes suffer pronounced momentum mismatch and material damage when fields are confined to extremely small scales, dielectric nanostructures offer an alternative. Dielectrics achieve extreme optical field confinement, potentially at sub-nm scales, by exploiting the coherent oscillation of polarized bound electrons under low-loss conditions. A key example is the coupled nanowire pair (CNP) structure, where sub-nm optical field confinement has been demonstrated in a narrow, approximately 1-nm-wide slit. However, characterizing these fragile, ultra-confined fields poses a significant problem, as even minimal detector interaction can perturb the near-field distribution. Conventional scanning near-field optical microscopy (SNOM) can reach resolutions of about 10 nm, but achieving finer resolution requires reducing the probe-sample distance to a point where significant disturbance is unavoidable.
Studies Highlighted in this Review
This News & Views article discusses a recent study that employed high-spatial-resolution Photoemission Electron Microscopy (PEEM) to achieve, for the first time, weak-disturbance imaging of the ultra-confined nanoslit mode in a CNP. The experimental work was carried out by a team led by Professor Limin Tong at Zhejiang University, in collaboration with researchers from Peking University. PEEM is particularly well suited for this purpose as it operates by exploiting the photoelectric effect, whereby incident photons generate emitted electrons from the sample surface. This non-contact approach offers high spatial resolution and sensitivity while introducing minimal perturbation to the original optical field, making it an ideal platform for characterizing otherwise inaccessible ultra-confined near fields.
A femtosecond laser beam, vertically polarized, was focused onto the CNP to successfully excite the TE0-like nanoslit mode, characterized by its electric field being concentrated within the slit. PEEM data indicate a strong standing-wave pattern localized at the midpoint of the CNP gap, generated by coherent interference between the nanoslit resonance and light guided along the nanowires. Numerical simulations indicated strong electromagnetic confinement within the slit, with FWHM values of approximately 0.4 nm along the y direction and 4 nm along the z direction. Although the instrumental resolution of the PEEM system broadened the measured profile along the y-axis to about 40 nm, the overall measured mode shape and effective wavelength showed a close match with theoretical predictions.
Furthermore, the research team performed quasi-three-dimensional (3D) imaging of the nanoslit mode by adjusting the focal plane of the PEEM system along the z-axis. As the focal plane was moved up from the ITO substrate towards the CNP's mid-plane, the contrast of the vertically polarized nanoslit mode reached its maximum, effectively pinpointing the highly localized hotspot within the central slit. Scanning through the focal plane yielded direct experimental confirmation of the quasi-three-dimensional spatial profile of the nanoslit mode. The study also demonstrated PEEM's high sensitivity to subtle structural variations in the CNP, such as slight increases in slit width or non-uniform alignment, which significantly alter photoemission intensity but are often difficult to detect using conventional characterization methods like SEM or TEM.
Discussion
The work establishes PEEM as a powerful tool for probing extreme optical fields, primarily due to its non-perturbative detection capability and high surface sensitivity. Despite the current resolution limitation of PEEM (∼40 nm), which restricts the direct visualization of sub-nm features and the retrieval of phase or vectorial information, its potential for characterizing ultra-confined fields is substantial.
Promising directions include integrating techniques such as multi-angle illumination or focal-plane scanning to enhance axial (z-axis) resolution, enabling more faithful 3D reconstruction of light-field distributions, even within bulk samples. Leveraging femtosecond time-resolved PEEM (TR-PEEM) is also suggested for dynamically capturing the transient light–matter interaction mechanisms, observing the formation, evolution, and annihilation of extreme optical fields. Finally, combining PEEM with computational imaging techniques, such as physics-based optical field inversion and deep learning-driven field reconstruction, is expected to help surpass intrinsic resolution and information-dimensionality limits, potentially allowing for the recovery of complex physical quantities like nonlinear responses and excitation lifetimes.
Conclusion
Future developments outlined in the News & Views article aim to transform PEEM from a tool focused solely on "field intensity mapping" into a platform for "computational multidimensional sensing". These collective advances are essential for deepening the understanding of nanoscale light–matter interactions and providing the necessary characterization capabilities for developing next-generation high-performance photonic devices, sensors, and optical information chips.
Journal Reference
Wang B., Chen Q., Zuo C. (2026). Seeing without touching: weak-disturbance imaging and characterization of ultra-confined optical near fields. Light Science & Applications 15, 40 (2026). DOI: 10.1038/s41377-025-02110-7, https://www.nature.com/articles/s41377-025-02110-7