RADIATION TRAPPING FORCES ON MICROSPHERES WITH OPTICAL TWEEZERS

Axial trapping forces exerted on microspheres are predicted using a Ga ussian beam electromagnetic field model and a ray-optics model, and co mpared with experimental measurements. Ray-optics predicts a maximum t rapping efficiency Q = -0.14 for optically trapped polystyrene microsp heres in water, compared to a measured value of -0.12 +/- 0.014 for 10 mum diam microspheres. When the microspheres are composed of amorphou s silica, the predicted ray-optics Q decreases to -0.11, compared to a Q = -0.034 predicted by the electromagnetic field model, and a measur ed value of -0.012 +/- 0.001 for 1 mum diam microspheres. These result s indicate that the two models have applicability in two different siz e regimes, and thus, are complementary.