Multiphoton fluorescence excitation in continuous-wave infrared optical traps.

The multiphoton fluorescence excitation observed in acontinuous-wave (cw) single-beam gradient force optical trap(optical tweezers) is reported for latex beads labeled with ayellow-green fluorescent dye (BODIPY). The fluorescenceemission spectra of the yellow-green beads trapped and excited by thesame 1064-nm laser light are identical to the spectra excited by the365-nm UV light. The influence of the numerical aperture of theobjective on the slope of the log-log power-dependence has beendemonstrated for BODIPY-Oil solution under cw and pulsed-laserconditions. The possibility that three-photon excitation processoccurs is discussed within the context of a dog-bone saturationmodel. Other possibilities for the observed fluorescence excitationhave been discussed.