Direct observation of a pure focused evanescent field of a high numerical aperture objective lens by scanning near-field optical microscopy

Intensity distributions of a tightly focused evanescent field generated by a center blocked high numerical aperture (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 not only on the interface, where the evanescent field is generated, but also in the parallel planes away from the interface. The decaying nature of the focused evanescent field shows good agreement with the theoretical predication, indicating that the field is purely evanescent and does not contain a significant contribution from the propagating component. It is found in our experiment that the light coupling efficiencies of the longitudinal polarization component Ez and the transverse polarization component Ex to the fiber probe differ by a factor of 3.