Real-time sub-wavelength imaging of surface waves with nonlinear near-field optical microscopy

Imaging evanescent waves is of crucial importance for sub-wavelength-scale investigation of various phenomena. However, frequently used techniques for near-field imaging require either a strong perturbation of the field, long acquisition times or complex electron-based tools. Here, we introduce nonlinear near-field optical microscopy (NNOM), which is capable of real-time evanescent wave imaging by nonlinear wave mixing while using only standard optical components. As a proof-of-concept, we present non-perturbative, single-shot mapping of evanescent plasmonic patterns, utilizing the nonlinearity of the host metal, and monitor in real time the externally controlled changes to the patterns. We further demonstrate the ability to extract the full field information—the amplitude and phase of all electric-field components—in a polarization-sensitive, spin-selective manner. This simple and highly tunable technique could be extended to deep sub-wavelength imaging of polaritons in two-dimensional materials or other nanophotonic guided modes, for swift photonic device characterization and optimized light−matter interactions.