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

Ultra-confined optical fields are of great importance in fundamental optics and optical technologies. The extreme field confinement in ultra-small nanostructures presents significant challenges in direct near-field characterization. Conventional scanning near-field optical microscopy encounters difficulties in characterizing sub-10-nm confined light fields due to significant disturbances of the optical field caused by the probe. Here, by employing a high spatial-resolved photoemission electron microscopy (PEEM), we succeeded in imaging the ultra-confined near fields of a nanoslit mode in a coupled nanowire pair (CNP) with weak disturbance for the first time and demonstrating a quasi-three-dimensional field distribution of the nanoslit mode. We also show that a PEEM image can identify fabrication defects that are influential to the confined field but are imperceptible to many other means. These results open an opportunity for weak-disturbance characterization of ultra-confined optical near fields, which is an essential step toward future optical devices or technology relying on ultra-confined light.