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Designing chromatic optical retarder stacks for segmented next‐generation easySTED phase plates

Fluorescence nanoscopy methods based on the RESOLFT principle, such as beam‐scanning STED nanoscopy, require the co‐alignment of optical beams for molecular state (on/off) switching and fluorescence excitation. The complexity and stability of the beam alignment can be drastically simplified and improved by using a single‐mode fibre as the sole light source for all required laser beams. This in turn then requires a chromatic optical element for shaping the off‐switching beam into a focal‐plane donut while simultaneously leaving the focal intensity distributions at other wavelengths shaped as regular focal spots. Here we describe novel designs of such so‐called ‘easySTED phase plates’ and provide a rationale how to find the desired spectral signature for combinations of multiple wavelengths.