The three-dimensional trap stiffness of optical tweezers formed with high-NA micromirrors is investigated by back-focal-plane interferometry and power spectrum analysis. Normalized stiffness values of κxy/Ptrap=1.2(μN/m)/mW and κz/Ptrap=0.52(μN/m)/mW in the transverse and axial directions, respectively, have been measured for polystyrene spheres with a radius of 1.03 μm. Compared with high-NA microscope objectives, micromirrors achieve much better trapping performances, particularly in the axial direction.
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