Direct observation of tip-gap interactions in THz scattering-type scanning near-field optical microscopy

Gaps in metal structures are accompanied by strong field enhancement (FE) and localized electromagnetic effects. However, completely different characteristics are presented when the microgaps and nanogaps in metallic structures are imaged using THz s-SNOM. We find that the near-field signal profile measured across the gap varies significantly with the ratio of the tip apex radius to the gap width. Increased (reduced) near-FE and a two-peak (one-peak) line profile are observed with microgaps (nanogaps). This phenomenon is caused by strong tip-gap interactions. In our full-wave simulations, we obtained near-field images of gap structures numerically, which were clearly reproduced practically.