Atomic-Scale Optical Microscopy with Continuous-Wave Mid-Infrared Radiation.

Understanding matter at the most fundamental level requires optical microscopy with ever-higher spatial resolution. Scanning near-field optical microscopy (SNOM) has enabled important advances, circumventing the diffraction limit of light by confining it to the apex of a sharp metallic tip. However, the mesoscopic tip geometry restricts the spatial resolution to the nanometer scale. Here, using a conventional tabletop continuous-wave mid-infrared laser and intensity-based detection we observe optical signals modulated on Ångstrom length scales, consistent with light emission from atomically confined tunneling currents. The emergence of near-field optical tunneling emission (NOTE) ─ considered a strong-field excitation process ─ under continuous-wave driving is remarkable, as it typically requires ultrashort high-intensity laser pulses. Further, we find that anharmonic tip oscillation can influence the signal and propose strategies to mitigate this effect. Our findings enable the use of this tunneling-mediated contrast mechanism with standard optical setups, establishing a pathway to optical imaging with unprecedented resolution.