  Multicolor excitation is at the core of many fluorescence spectroscopy
techniques such as PIE-FRET, ALEX-FRET, or FCCS. However, the influence of the
multiple laser excitations on the dye photophysics is often overlooked. Here,
we show that green laser pulses can surprisingly quench the fluorescence of
common red dyes Alexa Fluor 647 and Atto 647N, even when the conditions leading
to photobleaching are avoided. The physical origin of this phenomenon is
discussed via a long-lived dark state and/or photorefractive effects. These
observations are important to avoid measurement artifacts as both the
fluorophore concentration and the fluorescence brightness are affected.
