Splitting of the focal spot of a high numerical-aperture objective in free space

Reported in this letter is a phenomenon that the focal spot of a high numerical-aperture objective in free space can split into two spots if a ring beam is used for illumination. Diffraction by a high numerical-aperture objective results in a depolarization such that the diffracted field in the focal region includes not only a component with the same polarization as the incident beam, but also orthogonal and longitudinal components. The use of a ring beam enhances the relative contribution from the longitudinal component. As a result, a single focal spot splits into two spots along the incident polarization direction. It is revealed theoretically that for an objective of given numerical aperture there is a threshold of the central obstruction size of a ring beam for the appearance of a two-peak focus. © 2002 American Institute of Physics.