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Using a conventional tabletop mid-infrared laser, we observed optical signals modulated on Ångström length scales. These signatures stem from light emission from tunneling currents flowing between the most frontier atom of a metallic tip and a sample surface and are driven by the oscillating electric field of the laser light.
This tunneling-mediated contrast emerges under relatively weak continuous-wave excitation, although such strong-field effects are typically associated with ultrashort, high-intensity laser pulses.
These results open a pathway toward optical imaging with unprecedented spatial resolution using standard optical setups. The results were obtained in close collaboration with the group of Tom Siday (University of Birmingham) and have been published in Nano Letters.