Optical tweezers can be used to measure forces in the order of picoNewtons. Much like a spring, the approximate force that the laser beam applies to an object is proportional to the object’s displacement from the center of the beam. A known trap stiffness is often required to make reliable force measurements. The trap stiffness can be calculated using several experimental methods such as escape force, drag force, equipartition, step response and power spectrum methods. In this paper, drag force method was used experimentally to measure trap stiffness for polystyrene particle in three environments with different refractive index and report refractive index’ effect in trap stiffness. 

Keywords: Optical tweezers, trapping, trap stiffness, drag force method

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