Study on particle size dependence of axial trapping efficiency. 
An optical tweezers system using laser beams with a Gaussian intensity profile and doughnut intensity profiles made by hollow core optical fiber and axicon lenses, respectively, was constructed.  The axial trapping efficiencies for the three intensity profiles were measured and compared with each other.  The particle size dependence of axial trapping efficiencies in the range of the particle diameter from 1 to 20 μm were analyzed by using the modified ray optics model [Appl.  Opt.33, 1735 (1994)].