4Pi mirror based focusing

Creating an isotropic, diffraction limited excitation spot is beneficial for several fields in science, e.g. 3D confocal imaging or quantum-optics experiments with single atoms. One method to obtain such a focal field distribution is 4Pi microscopy. High resolution imaging techniques such as STED and RESOLFT-type microscopy also benefit from 4Pi focusing since the depletion beam has a steeper slope. Conventional 4Pi microscopes with two opposing high NA objective lenses often struggle with technical difficulties such as precise coalignment of the objective lenses or the need for phase compensation in multi-wavelength setups. Furthermore, axial side lobes are often a major problem. Here we present the experimental characterization of a novel 4Pi objective based on a parabolic mirror. Since it is based on a single reflective element, it is inherently free of chromatic aberrations, has a nearly side lobe free axial Point Spread Function (PSF) and is mechanically stable. We characterize the focusing properties measuring the excitation PSF with a single trapped Yb+ ion as an intensity sensor and discuss further improvements using aberration-correction.