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Femtosecond optical tweezers for in-situ control of two-photon fluorescence

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Abstract

We perform a comparison of optical tweezing using continuous wave (cw) and femtosecond lasers. Measurement of the relative Q-values in the femtosecond and cw regimes shows that femtosecond optical tweezers are just as effective as cw optical tweezers. We also demonstrate simultaneous optical tweezing and in-situ control of two-photon fluorescence (at 400nm) from dye-doped polymer microspheres. By switching the 800 nm tweezing laser source between femtosecond and cw regimes, we turned the fluorescent signal from the tweezed particle on and off while maintaining an equivalent tweezing action. Femtosecond lasers can thus be used for optical tweezing and simultaneously utilized to induce nonlinear multi-photon processes such as two-photon excitation or even photoporation.

©2004 Optical Society of America

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Figures (5)

Fig. 1.
Fig. 1. Experimental set-up for the femtosecond and cw optical tweezers
Fig. 2.
Fig. 2. (a) Horizontal and (b) vertical profile of the output beam from the titanium-sapphire laser in both cw (non-modelocked) and femtosecond (modelocked) operations.
Fig. 3.
Fig. 3. Excitation and emission characteristics of the blue fluorescent polymer microspheres [www.dukescientific.com]
Fig. 4.
Fig. 4. Logarithm of average tweezing power versus logarithm of fluorescence intensity from tweezed polymer microspheres. The straight line y=2x describes the quadratic nature of the two-photon absorption process.
Fig. 5.
Fig. 5. (2.6MB) Movie (monochrome) of simultaneous two-photon excitation fluorescence and femtosecond optical tweezing of dye-doped 1.28 µm polymer microspheres. While tweezing of the same microsphere is maintained, the fluorescent signal is turned off by simply switching the titanium-sapphire source laser from the femtosecond regime to the cw regime. This procedure is entirely reversible (3.5MB version).

Tables (1)

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Table 1. Comparison of Q-values between cw and femtosecond optical tweezers for 1.28 µm spheres

Equations (1)

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Q = 3 π η d v k c n P twz
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