Off-resonant Fano-enhanced single-molecule-resolution imaging with a CW source

Apertureless scanning near-field optical microscopy (a-SNOM) is typically limited to ∼10 nm resolution by the tip apex size. We numerically demonstrate that ∼1 nm resolution can be achieved under continuous-wave (CW) illumination by exploiting Fano path interference. A defect center that naturally forms at the apex of a metal-coated AFM tip acts as a quantum object and induces Fano interference, forcing a stronger but normally off-resonant plasmonic mode (597 nm) to operate effectively on resonance at the driving wavelength (520 nm). Because this interference occurs only beneath the defect, an ∼1 nm wide defect-dependent contribution is formed inside the broader apex hotspot; the full metallic hotspot is not reduced to 1 nm. Using this off-resonant Fano-enhanced field, we numerically resolve two closely spaced model objects based on exact three-dimensional Maxwell simulations.