Enhancement of axial force of optical tweezers by utilizing a circular stop at the back focal plane of the objective

Optical tweezers are indispensable force spectroscopes. The trap stiffness and the linear force range of the instrument determine the working force range of the instrument. Here we show both theoretically and experimentally that utilizing a circular obstruction at the back focal plane of the objective can significantly increase the maximum linear force. For instance, utilizing a disk with an obstruction ratio of 0.773 could increase the maximum linear force by a factor of similar to 39 when a 3.4 mu m polystyrene bead is trapped. We also show that this simple beam shaping method can significantly improve the maximum applicable force per unit power of the laser entering the objective lens. (c) 2018 Optical Society of America