We propose an approach to obtain a diffraction-limited spherical focal spot in 4Pi microscopy. By combining the dipole antenna radiation pattern and the Richards–Wolf vectorial diffraction method, an input field at a pupil plane of aplanatic objective lenses for generating a spherical spot can be found analytically by solving the inverse problem. The required field at the pupil plane is found to be a radial polarization with spatial amplitude modulation. With spatial phase modulation, two identical spherical spots with diffraction-limited size and variable distance along the optical axis can also be obtained.
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