Weak-disturbance imaging and characterization of ultra-confined optical near fields

The optical field with sub-nanometer confinement can greatly enhance light-matter interactions at nanoscale (e.g., enabling forbidden atomic transitions) and push extreme optical technologies (e.g., cutting chemical bonds at single-molecule level). Recently, ultra-confined optical fields with sub-10-nm or even sub-1-nm levels in metal and dielectric nanostructures have been reported. More recently, based on coherent oscillation of polarized bound electrons around a 1-nm-width slit in a coupled nanowire pair (CNP), a nanoslit waveguiding mode supported by the CNP offers a sub-nm-confined optical field.

 

However, as such a field exists only in the near field of nanostructures (e.g., few-nanometer decay length) with ultra-small feature sizes (e.g., 1-nm-width slit), its direct imaging that is critical to its characterization and applications, remains challenging. Generally, when the confinement goes down to the sub-nm level, conventional scanning near-field optical microscope (SNOM) will significantly influence the original field due to the huge disturbance caused by a probe (Fig. 1a), making the characterization totally ineffective. By imaging optical-field-excited electrons (photon in, electron out), photoemission electron microscopy (PEEM) has almost no disturbance to the original optical field as the density of induced photoelectrons is far lower than that of the polarized electrons (Fig. 1b), which is crucial for the near-field characterization for ultra-confined optical fields.

 

In a new paper published in Light: Science & Applications, a team of scientists, led by Prof. Limin Tong from Zhejiang University, China, and Prof. Guowei Lyu and Dr. Yaolong Li from Peaking University, China, using the weak-disturbance and high spatial-resolved imaging ability of a PEEM, demonstrated near-field imaging and characterization of ultra-confined optical near fields in nanoslits for the first time. They show that, a PEEM image can identify fabrication defects that are influential to the confined field but are imperceptible to many other means.

 

Experimentally, when exciting ZnO CNPs with a beam of femtosecond pulses, incident light couples into the waveguiding modes in the CNP waveguide mainly from the end faces of the CNPs (Fig. 2). They observed a characteristic standing-wave pattern emerges at the central slit of the ZnO CNP in a typical PEEM image (Fig. 3). The consistency of experimental and calculated results confirms that the interference pattern is derived from the near-field nanoslit mode.

 

The central electric field of the nanoslit mode is strongly correlated with the morphology and optical properties of the material around the slit. However, conventional morphological characterization methods (e.g., SEM and TEM) struggle to identify the defects of the CNP samples. In PEEM characterization, minimal imperfections in the CNP can affect the PE intensity. For example, despite the difficulty in discerning differences in the morphology of the two slits of a typical CNP sample in the SEM image, the PEEM image exhibits a significant contrast in the PE intensity of the two slits (Fig. 4).

 

“Compared with many other morphological techniques that are difficult or inadequate for characterizing atomic-scale optical fields, PEEM holds a promising prospect as a powerful tool for characterizing extremely confined optical fields with minimal disturbance.” the scientists forecast.

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