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A design for focusing an incoming light into an ultrasmall spot with a long depth of focus is proposed. We use a phase plate with three concentric regions to modulate the incoming light from radial polarization into outward-inward-outward polarization. The analysis shows that the design is more suitable for a lens with a high numerical aperture because much energy is concentrated in the main lobe of the focus spot.
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Sun et al. (2003) studied this question.
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