Focus-Splitting of the High NA Objective Lens with Ring Beam

INTEREST GROUP STREAM: Australian Optical Society
TITLE: Splitting of the focal spot of a high numerical-aperture objective Lens in free space
AUTHORS: James W. M. Chon, Xiasong Gan, and Min Gu
AFFILIATION: Centre for Micro-Photonics,
School of Biophysical Sciences and Electrical Engineering,
Swinburne University of Technology,
P. O. Box 218, 3122, Australia
Reported in this paper is a new phenomenon that the focal spot of a high numericalaperture 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 nature that the
diffracted field in the focal region includes 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.