Direct characterisation of a pure evanescent focus of a high numerical aperture objective lens

In this paper, intensity distributions of the tightly focused evanescent field generated by a centre blocked high numerical aperture (NA=1.65) objective lens are investigated by a scanning near-field optical microscope The pure focused evanescent field is mapped and a splitting phenomenon of the focal spot along the direction of polarization, caused by depolarization, is observed. The measurement result is compared with the calculation based on vectorial diffraction theory; a good match has been found. The decaying nature of the focused evanescent wave also shows a good agreement with the theoretical predication, indicating that the field is purely evanescent and does not contain significant contribution from the propagating component.