Modern nano-optics and nanophotonics rely heavily on the precise formation of nanostructured light fields. Accurate and deterministic light field formation and characterization are indispensable for device operation as well as for revealing the underlying physical mechanisms involved. Despite a significant progress made in detection of scattered light with extremely high precision down to 1 nm resolution, there are only a limited number of techniques for direct subwavelength light mapping which do not rely on measurements of light scattering, fluorescence, or non-linear light conversion. Hence, techniques for direct conversion of light to electrical signals with precise and non-destructive imaging of nanoscale light would be of great benefit. Here, we report a nanoscale light field imaging approach based on photodetection with a p-n junction that is induced and moved inside a graphene probe by gate voltage, formed by a set of external electrodes. The spatial resolution of this electrical scanning technique is determined by p-n junction width, reaching ~ 20 nm. The developed approach is demonstrated with mapping the electric field distribution of a plasmonic slot-waveguide at telecom wavelengths. Our method provides a non-invasive nanoscale light field imaging that ensures extremely high spatial resolution and precision.
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