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
In this paper, we demonstrate a time-reversal methodology to create diffraction-limited optical focal spot with arbitrarily oriented magnetic dipolar field component using a 4pi microscopic configuration. Through combining the magnetic dipole radiation pattern and the Richards–Wolf vectorial diffraction method, the required illuminations at the pupil plane of a 4pi focusing configuration for the reconstruction of magnetic dipole focal field are found analytically. In general, the calculated pupil field is a complex optical field with amplitude, phase and polarization variations within the cross section. Such required pupil fields can be experimentally generated with the recently develop Vectorial Optical Field Generator. Furthermore, the orientation of the magnetic field component within the doughnut shape focal field can be rotated arbitrarily by modulating the pupil field distribution carefully while maintaining the diffraction-limited focal spot size. These unique focal field distributions are expected to exhibit novel phenomena when interact with various type of structured-materials. These interactions may find important applications in super-resolution microscopy, particle trapping and manipulation, materials characterization, as well as three-dimensional high-density optical storage.