Reversible Saturable Optical Fluorescence Transition
RESOLFT stands for a general switching principle: the switching mechanism does no longer need to be purely electronically, but can include switching mechanisms like for example conformational changes of a molecule. Strictly speaking, the STED method is one possible implementation of the RESOLFT concept.
The approach is similar as shown for STED – a beam scanning setup with overlapping beams of the excitation and the switching beam.
Depending on the implementation of the switch, typical saturation intensities and typical switching times are characteristic – e.g. conformational changes feature much lower switching intensities, while the switching time is much longer compared to electronical states (as for STED).
Shown below is an exemplary implementation of a RESOLFT-switch via light induced chemical reaction which generates the highly fluorescent dye from the colorless and non-fluorescent precursor.
The implementation is similar as shown for STED – a beam scanning setup with overlapping beams of excitation and switching beams. In a STED setup, the switching beam would be denoted as STED beam. The phaseplate is responsible for redistributing the switching light in the focal plane, i.e. for creating the donut.
RESOLFT features theoretically unlimited resolution (see equation from the STED section), since only the molecule in the center of the focus is left in the on-state.
The detector only needs to see at minimum one photon to detect individual molecule (clusters) on the nanoscale resolution.