Fluorescence modulation for selective recovery of desired fluorescence signals has to date required careful fluorophore selection combined with repeated optical recovery from long-lived photoinduced dark states. Adapting an all-optical scheme, modulated Stimulated Emission Depletion generalizes such modulation schemes by eliminating the need for dark state residence by directly optically depopulating the emissive state at any externally applied frequency. Using two overlapped Gaussian laser spots with the depletion beam being intensity-modulated, fluorescence modulation is readily achieved with a depletion ratio governed by the intensity of the depleting laser. Selective image recovery of otherwise unmodulatable fluorophore signals is directly achieved through this all-optical modulation, and common STED-degrading multiphoton-excited background is readily discriminated against. Both beads and dyes in solution as well as fluorophores bound within fixed cells are readily imaged in this manner.
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