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Recent discoveries of correlated phases in two-dimensional quantum materials have created a strong demand for optical techniques capable of resolving electronic and structural features at the atomic scale. Near-field microscopy (SNOM) breaks the diffraction limit by coupling light to a sharp metallic tip, but achieving the atomic-scale optical resolution remains a formidable challenge. Recent advances in lightwave-driven scanning tunneling microscopy have opened a new route: intense terahertz and mid-infrared fields can drive tunneling currents through an atomically confined junction. However, detecting the associated optical emission in a SNOM-style readout remains challenging because the emitting volume is extremely small. Here, we show that even continuous-wave MIR radiation from a commercial quantum cascade laser can induce tunneling currents strong enough for direct optical detection [1].
Using picometer-stable qPlus-based tip-sample control under UHV and cryogenic conditions, we detect a strongly localized optical response near an Au(111) surface, marked by a rapid increase in scattered amplitude and a pronounced optical phase shift at tunneling distances. Lateral scans across monoatomic gold steps reveal optical contrast confined to the ångström scale and closely correlated with the simultaneously measured tunneling current. These signatures closely resemble recent pulsed-THz results, where radiation was emitted from AC tunneling currents driven by optical near fields, enabling all-optical subcycle microscopy on atomic length scales [2]. Together, these results establish continuous-wave near-field optical tunneling emission as a practical route toward atomic-scale optical microscopy with widely accessible tabletop light sources.
[1] F. Schiegl et al., Nano Letters 26, 1689–1696 (2026)
[2] T. Siday et al., Nature 629, 329–334 (2024)