Abstract
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.
Highlights
The radially polarized light is gaining growing attention in the last few years mainly due to the theoretical and experimental results that demonstrated its advantages over the well known linear and circular polarizations [1,2,3,4,5,6,7,8,9,10,11,12]
We investigate the effect of polarization and aperture geometry on the spot size at the focal plane of a high numerical aperture (NA) aplanatic lens
We find that if a clear aperture is used, the radial polarization is inferior compared with the linear and circular polarization for spot size reduction applications
Summary
The radially polarized light is gaining growing attention in the last few years mainly due to the theoretical and experimental results that demonstrated its advantages over the well known linear and circular polarizations [1,2,3,4,5,6,7,8,9,10,11,12]. It was shown that when using an annular aperture the radially polarized beam can be focused to a tighter spot compared with a linearly or circularly polarized beam [1, 2] This is mainly attributed to the strong longitudinal component of the field that is generated in the focal plane. The longitudinal component produces by the radially polarized incident beam is discussed in more details in section 5 and is shown to be the best choice for spot size reduction in wide range of NA and annular aperture geometries, assuming that the detection system is capable of distinguishing between the transverse and longitudinal field components.
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