We have demonstrated a new approach of focal field engineering that generates a three-dimensional diffraction limited focal spot with flattop and uniform intensity structure. The required input field at the pupil plane of 4pi microscope objectives to generate the desired focal field is analytically calculated by collecting the radiations of electric and magnetic dipoles oscillating at the focal point with an appropriate orientation. By using the input field at the pupil plane as an illumination source to the system and reversing the propagation, focusing the input field leads to the desired focal spot in the focal volume. The designed three-dimensional flattop focal spot is diffraction limited and possesses a uniform and smooth intensity distribution over a cross section area of 0.725λ diameter and FWHM=0.9625λ. This kind of focal fields is required by many optical applications, such as, nanofabrication, laser machining, particle trapping and acceleration, and many more.
Keywords: 4pi microscope, focal field, polarization, laser beam shaping, diffraction theory
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