Publication | Open Access
Effect of radial polarization and apodization on spot size under tight focusing conditions
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2008
Year
Tight Focusing ConditionsEngineeringBeam OpticOphthalmologyMicroscopyOptical PropertiesInner AnnulusRadial Polarization IlluminationApodized AperturesOptic DesignClassical OpticsSpot SizeGeometrical OpticGeometrical AberrationOptical SystemsMedicinePolarization ImagingRadial Polarization
The study investigates how polarization and aperture geometry affect the focal spot size of a high‑numerical‑aperture aplanatic lens. The authors analyze high‑NA annular apertures, determining inner annulus sizes that mark the transition to smaller spot sizes with radial polarization, and examine how transverse and longitudinal field components influence spot size differently for circular versus radial illumination. They find that for clear apertures linear or circular polarization yields smaller spots than radial, that a very thin annulus makes radial polarization approach scalar theory predictions while linear and circular deviate, that the longitudinal component of radial illumination produces the narrowest spots across many geometries, and that tight focusing at dielectric interfaces can be mitigated or exploited to further reduce spot size.
We study the effect of polarization and aperture geometry on the focal spot size of a high numerical aperture (NA) aplanatic lens. We show that for a clear aperture geometry, illuminating the lens by linear or circular polarization is preferable over radial polarization for spot size reduction applications. For annular aperture and objective lenses of 0.85 NA and above we give the sizes of the inner annulus which constitute the transition points to a state where the radial polarization illumination gives smaller spot size. We analyze the evolution, the profile and the effect of transverse and longitudinal field components in the focal plane, and show that they play an opposite role on the spot size in the cases of circular and radial polarization illumination. We show that in the limit of a very thin annulus the radial polarization approaches the prediction of the scalar theory at high NA, whereas the linear and circular polarizations deviate from it. We verify that the longitudinal component generated by radially polarized illumination produces the narrowest spot size for wide range of geometries. Finally, we discuss the effects of tight focusing on a dielectric interface and provide some ideas for circumventing the effects of the interface and even utilize them for spot size reduction.