A simple model resembling those used in scanning tunneling microscopy is proposed for an optical probe tip. The tip is considered as a pointlike source (detector) displaced from the apex in the direction towards or from the sample. The exact position of this point depends on the shape of electric field lines within the gap and is a crucial factor determining the probe imaging properties. For various probes, both the aperture and apertureless ones, a general expression relating
the lateral resolution with the corrugation amplitude, the gap width, and the tip (the aperture) radius is obtained.
Gleb S. Zhdanov,
"Tunneling and spatial resolution in near-field optics", Proc. SPIE 5506, Nonresonant Laser-Matter Interaction (NLMI-11), (15 July 2004); doi: 10.1117/12.580274; https://doi.org/10.1117/12.580274