Radiation trapping forces on microspheres with optical tweezers

Axial trapping forces exerted on microspheres are predicted using a Gaussian beam electromagnetic field model and a ray-optics model, and compared with experimental measurements. Ray-optics predicts a maximum trapping efficiency Q= −0.14 for optically trapped polystyrene microspheres in water, compared to a measured value of −0.12 ± 0.014 for 10 μm diam microspheres. When the microspheres are composed of amorphous silica, the predicted ray-optics Q decreases to −0.11, compared to a Q = −0.034 predicted by the electromagnetic field model, and a measured value of −0.012 ± 0.001 for 1 μm diam microspheres. These results indicate that the two models have applicability in two different size regimes, and thus, are complementary.