Subcycle optical nanoscopy with atomic-scale resolution

For centuries, optical microscopy has expanded the limits of human perception, revealing the microscopic world in ever greater detail. Yet, resolving ultrafast dynamics simultaneously on atomic length and subcycle time scales has remained beyond reach. In this work, I demonstrate an all-optical microscopy technique which achieves simultaneous atomic-scale spatial and subcycle temporal resolution by harnessing light emission from ultrafast tunnelling currents confined to the apex of an atomically sharp metallic tip. To breach the limits of ultrafast subcycle near-field microscopy, we developed a novel experimental platform that combines a megahertz-repetition-rate, high-power terahertz source with a cryogenic, ultra-high-vacuum scanning probe microscope. This setup enables picometre-precise tuning of the tunnelling junction and subcycle field-resolved detection of the light scattered from the tip–sample junction. In this regime, we observe a novel near-field signal that is in phase with the vector potential of the driving field and decays on atomic length scales. Supported by semiclassical and ab initio simulations, we attribute this signal to light emission from subcycle tunnelling currents driven by the ultrafast lightwave bias — a process we term Near-field Optical Tunnelling Emission (NOTE). We showcase the capabilities of NOTE by performing all-optical imaging and subcycle spectroscopy with atomic-scale resolution. Finally, we retrieve subcycle atomic-scale tunnelling currents directly in the time domain, even within the band gap of a semiconducting quantum material. To extend the capabilities to pump–probe studies of ultrafast phenomena — such as exciton formation and phase transitions in quantum materials — we developed a tunable optical pump source based on optical parametric amplification at up to 8.26 MHz. Together with the novel NOTE mechanism, this opens a pathway toward visualising a wide array of quantum dynamics in space and time, offering new opportunities to unravel the inner workings of condensed matter on its most fundamental scales.